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Title: Climate and Time in their Geological Relations - A Theory of Secular Changes of the Earth's Climate
Author: Croll, James
Language: English
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CLIMATE AND TIME
[Illustration: FRONTISPIECE
N. WINTER SOLSTICE IN APHELION.
N. WINTER SOLSTICE IN PERIHELION.
W. & A. K. Johnston, Edinb^r. and London.]
CLIMATE AND TIME
_IN THEIR GEOLOGICAL RELATIONS_
A THEORY OF
SECULAR CHANGES OF THE EARTH’S CLIMATE
BY JAMES CROLL
OF H.M. GEOLOGICAL SURVEY OF SCOTLAND
LONDON
DALDY, ISBISTER, & CO.
56, LUDGATE HILL
1875
LONDON:
PRINTED BY VIRTUE AND CO.,
CITY ROAD.
PREFACE.
In the following pages I have endeavoured to give a full and concise
statement of the facts and arguments adduced in support of the theory
of Secular Changes of the Earth’s Climate. Considerable portions of
the volume have already appeared in substance as separate papers in
the Philosophical Magazine and other journals during the past ten or
twelve years. The theory, especially in as far as it relates to the
cause of the glacial epoch, appears to be gradually gaining acceptance
with geologists. This, doubtless, is owing to the greatly increased
and constantly increasing knowledge of the drift-phenomena, which has
induced the almost general conviction that a climate such as that of
the glacial epoch could only have resulted from cosmical causes.
Considerable attention has been devoted to objections, and to the
removal of slight misapprehensions, which have naturally arisen in
regard to a subject comparatively new and, in many respects, complex,
and beset with formidable difficulties.
I have studiously avoided introducing anything of a hypothetical
character. All the conclusions are based either on known facts or
admitted physical principles. In short, the aim of the work, as will be
shown in the introductory chapter, is to prove that secular changes of
climate follow, as a necessary effect, from admitted physical agencies,
and that these changes, in as far as the past climatic condition of
the globe is concerned, fully meet the demand of the geologist.
The volume, though not intended as a popular treatise, will be found,
I trust, to be perfectly plain and intelligible even to readers not
familiar with physical science.
I avail myself of this opportunity of expressing my obligations to my
colleagues, Mr. James Geikie, Mr. Robert L. Jack, Mr. Robert Etheridge,
jun., and also to Mr. James Paton, of the Edinburgh Museum of Science
and Art, for their valuable assistance rendered while these pages were
passing through the press. To the kindness of Mr. James Bennie I am
indebted for the copious index at the end of the volume, as well as
for many of the facts relating to the glacial deposits of the West of
Scotland.
JAMES CROLL.
EDINBURGH, _March, 1875_.
CONTENTS.
CHAPTER I.
INTRODUCTION.
PAGE
The Fundamental Problem of Geology.—Geology a Dynamical
Science.—The Nature of a Geological Principle.—Theories
of Geological Climate.—Geological Climate dependent
on Astronomical Causes.—An Important Consideration
overlooked.—Abstract of the Line of Argument pursued in the
Volume 1
CHAPTER II.
OCEAN-CURRENTS IN RELATION TO THE DISTRIBUTION OF HEAT OVER
THE GLOBE.
The absolute Heating-power of Ocean-currents.—Volume of
the Gulf-stream.—Absolute Amount of Heat conveyed by
it.—Greater Portion of Moisture in Inter-tropical Regions
falls as Rain in those Regions.—Land along the Equator
tends to lower the Temperature of the Globe.—Influence
of Gulf-stream on Climate of Europe.—Temperature of
Space.—Radiation of a Particle.—Professor Dove on Normal
Temperature.—Temperature of Equator and Poles in the Absence
of Ocean-currents.—Temperature of London, how much due to
Ocean-currents 23
CHAPTER III.
OCEAN-CURRENTS IN RELATION TO THE DISTRIBUTION OF HEAT OVER THE
GLOBE.—(_Continued._)
Influence of the Gulf-stream on the Climate of the Arctic
Regions.—Absolute Amount of Heat received by the Arctic
Regions from the Sun.—Influence of Ocean-currents shown by
another Method.—Temperature of a Globe all Water or all
Land according to Professor J. D. Forbes.—An important
Consideration overlooked.—Without Ocean-currents the
Globe would not be habitable.—Conclusions not affected by
Imperfection of Data 45
CHAPTER IV.
OUTLINE OE THE PHYSICAL AGENCIES WHICH LEAD TO SECULAR
CHANGES OF CLIMATE.
Eccentricity of the Earth’s Orbit; its Effect on
Climate.—Glacial Epoch not the direct Result of an
Increase of Eccentricity.—An important Consideration
overlooked.—Change of Eccentricity affects Climate only
indirectly.—Agencies which are brought into Operation
by an Increase of Eccentricity.—How an Accumulation
of Snow is produced.—The Effect of Snow on the Summer
Temperature.—Reason of the Low Summer Temperature of Polar
Regions.—Deflection of Ocean-currents the chief Cause of
Secular Changes of Climate.—How the foregoing Causes deflect
Ocean-currents.—Nearness of the Sun in Perigee a Cause of
the Accumulation of Ice.—A remarkable Circumstance regarding
the Causes which lead to Secular Changes of Climate.—The
primary Cause an Increase of Eccentricity.—Mean Temperature
of whole Earth should be greater in Aphelion than in
Perihelion.—Professor Tyndall on the Glacial Epoch.—A
general Reduction of Temperature will not produce a Glacial
Epoch.—Objection from the present Condition of the Planet
Mars 54
CHAPTER V.
REASON WHY THE SOUTHERN HEMISPHERE IS COLDER THAN THE NORTHERN.
Adhémar’s Explanation.—Adhémar’s Theory founded upon a physical
Mistake in regard to Radiation.—Professor J. D. Forbes on
Underground Temperature.—Generally accepted Explanation.—Low
Temperature of Southern Hemisphere attributed to
Preponderance of Sea.—Heat transferred from Southern to
Northern Hemisphere by Ocean-current the true Explanation.—A
large Portion of the Heat of the Gulf-stream derived from the
Southern Hemisphere 81
CHAPTER VI.
EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC
CIRCULATION.—LIEUT. MAURY’S THEORY.
Introduction.—Ocean-currents, according to Maury, due to
Difference of Specific Gravity.—Difference of Specific
Gravity resulting from Difference of Temperature.—Difference
of Specific Gravity resulting from Difference of
Saltness.—Maury’s two Causes neutralize each other.—How,
according to him, Difference in Saltness acts as a Cause 95
CHAPTER VII.
EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC
CIRCULATION.—LIEUT. MAURY’S THEORY.—(_Continued._)
Methods of determining the Question.—The Force resulting from
Difference of Specific Gravity.—Sir John Herschel’s Estimate
of the Force.—Maximum Density of Sea-Water.—Rate of Decrease
of Temperature of Ocean at Equator.—The actual Amount of
Force resulting from Difference of Specific Gravity.—M.
Dubuat’s Experiments 115
CHAPTER VIII.
EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC
CIRCULATION.—DR. CARPENTER’S THEORY.
Gulf-stream according to Dr. Carpenter not due to Difference of
Specific Gravity.—Facts to be Explained.—The Explanation of
the Facts.—The Explanation hypothetical.—The Cause assigned
for the hypothetical Mode of Circulation.—Under Currents
account for all the Facts better than the Gravitation
Hypothesis.—Known Condition of the Ocean inconsistent with
that Hypothesis 122
CHAPTER IX.
EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC
CIRCULATION.—THE MECHANICS OF DR. CARPENTER’S THEORY.
Experimental Illustration of the Theory.—The Force exerted by
Gravity.—Work performed by Gravity.—Circulation not by
Convection.—Circulation depends on Difference in Density
of the Equatorial and Polar Columns.—Absolute Amount of
Work which can be performed by Gravity.—How Underflow is
produced.—How Vertical Descent at the Poles and Ascent at
the Equator is produced.—The Gibraltar Current.—Mistake in
Mechanics concerning it.—The Baltic Current 145
CHAPTER X.
EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC
CIRCULATION.—DR. CARPENTER’S THEORY.—OBJECTIONS CONSIDERED.
_Modus Operandi_ of the Matter.—Polar Cold considered by Dr.
Carpenter the _Primum Mobile_.—Supposed Influence of
Heat derived from the Earth’s Crust.—Circulation without
Difference of Level.—A Confusion of Ideas in Reference to the
supposed Agency of Polar Cold.—M. Dubuat’s Experiments.—A
Begging of the Question at Issue.—Pressure as a Cause of
Circulation 172
CHAPTER XI.
THE INADEQUACY OF THE GRAVITATION THEORY PROVED BY ANOTHER
METHOD.
Quantity of Heat which can be conveyed by the General Oceanic
Circulation trifling.—Tendency in the Advocates of the
Gravitation Theory to under-estimate the Volume of the
Gulf-stream.—Volume of the Stream as determined by the
_Challenger_.—Immense Volume of Warm Water discovered by
Captain Nares.—Condition of North Atlantic inconsistent with
the Gravitation Theory.—Dr. Carpenter’s Estimate of the
Thermal Work of the Gulf-stream 191
CHAPTER XII.
MR. A. G. FINDLAY’S OBJECTIONS CONSIDERED.
Mr. Findlay’s Estimate of the Volume of the Gulf-stream.—Mean
Temperature of a Cross Section less than Mean Temperature
of Stream.—Reason of such Diversity of Opinion regarding
Ocean-currents.—More rigid Method of Investigation necessary 203
CHAPTER XIII.
THE WIND THEORY OF OCEANIC CIRCULATION.
Ocean-Currents not due alone to the Trade-winds.—An Objection
by Maury.—Trade-winds do not explain the Great Antarctic
Current.—Ocean-currents due to the System of Winds.—The
System of Currents agrees with the System of the
Winds.—Chart showing the Agreement between the System
of Currents and System of Winds.—Cause of the Gibraltar
Current.—North Atlantic an immense Whirlpool.—Theory of Under
Currents.—Difficulty regarding Under Currents obviated.—Work
performed by the Wind in impelling the Water forward.—The
_Challenger’s_ crucial Test of the Wind and Gravitation
Theories.—North Atlantic above the Level of Equator.—Thermal
Condition of the Southern Ocean irreconcilable with the
Gravitation Theory 210
CHAPTER XIV.
THE WIND THEORY OF OCEANIC CIRCULATION IN RELATION TO CHANGE
OF CLIMATE.
Direction of Currents depends on Direction of the Winds.—Causes
which affect the Direction of Currents will affect
Climate.—How Change of Eccentricity affects the Mode
of Distribution of the Winds.—Mutual Reaction of Cause
and Effect.—Displacement of the Great Equatorial
Current.—Displacement of the Median Line between the Trades,
and its Effect on Currents.—Ocean-currents in Relation to the
Distribution of Plants and Animals.—Alternate Cold and Warm
Periods in North and South.—Mr. Darwin’s Views quoted.—How
Glaciers at the Equator may be accounted for.—Migration
across the Equator 226
CHAPTER XV.
WARM INTER-GLACIAL PERIODS.
Alternate Cold and Warm Periods.—Warm Inter-glacial Periods
a Test of Theories.—Reason why their Occurrence has not
been hitherto recognised.—Instances of Warm Inter-glacial
Periods.—Dranse, Dürnten, Hoxne, Chapelhall, Craiglockhart,
Leith Walk, Redhall Quarry, Beith, Crofthead, Kilmaurs,
Sweden, Ohio, Cromer, Mundesley, &c., &c.—Cave and River
Deposits.—Occurrence of Arctic and Warm Animals in some Beds
accounted for.—Mr. Boyd Dawkins’s Objections.—Occurrence
of Southern Shells in Glacial Deposits.—Evidence of Warm
Inter-glacial Periods from Mineral Borings.—Striated
Pavements.—Reason why Inter-glacial Land-surfaces are so rare 236
CHAPTER XVI.
WARM INTER-GLACIAL PERIODS IN ARCTIC REGIONS.
Cold Periods best marked in Temperate, and Warm Periods
in Arctic, Regions.—State of Arctic Regions during
Glacial Period.—Effects of Removal of Ice from Arctic
Regions.—Ocean-currents; Influence on Arctic Climate.—Reason
why Remains of Inter-glacial Period are rare in Arctic
Regions.—Remains of Ancient Forests in Banks’s Land, Prince
Patrick’s Island, &c.—Opinions of Sir R. Murchison, Captain
Osborn, and Professor Haughton.—Tree dug up by Sir E. Belcher
in lat. 75° N. 258
CHAPTER XVII.
FORMER GLACIAL EPOCHS.—REASON OF THE IMPERFECTION OF
GEOLOGICAL RECORDS IN REFERENCE TO THEM.
Two Reasons why so little is known of Glacial Epochs.—Evidence
of Glaciation to be found on Land-surfaces.—Where are all
our ancient Land-surfaces?—The stratified Rocks consist
of a Series of old Sea-bottoms.—Transformation of a
Land-surface into a Sea-bottom obliterates all Traces of
Glaciation.—Why so little remains of the Boulder Clays of
former Glacial Epochs.—Records of the Glacial Epoch are fast
disappearing.—Icebergs do not striate the Sea-bottom.—Mr.
Campbell’s Observations on the Coast of Labrador.—Amount
of Material transported by Icebergs much exaggerated.—Mr.
Packard on the Glacial Phenomena of Labrador.—Boulder Clay
the Product of Land-ice.—Palæontological Evidence.—Paucity of
Life characteristic of a Glacial Period.—Warm Periods better
represented by Organic Remains than cold.—Why the Climate
of the Tertiary Period was supposed to be warmer than the
present.—Mr. James Geikie on the Defects of Palæontological
Evidence.—Conclusion 266
CHAPTER XVIII.
FORMER GLACIAL EPOCHS; GEOLOGICAL EVIDENCE OF.
Cambrian Conglomerate of Islay and North-west of
Scotland.—Ice-action in Ayrshire and Wigtownshire
during Silurian Period.—Silurian Limestones in Arctic
Legions.—Professor Ramsay on Ice-action during Old
Red Sandstone Period.—Warm Climate in Arctic Regions
during Old Red Sandstone Period.—Professor Geikie and
Mr. James Geikie on a Glacial Conglomerate of Lower
Carboniferous Age.—Professor Haughton and Professor Dawson
on Evidence of Ice-action during Coal Period.—Mr. W. T.
Blanford on Glaciation in India during Carboniferous
Period.—Carboniferous Formations of Arctic Regions.—Professor
Ramsay on Permian Glaciers.—Permian Conglomerate in
Arran.—Professor Hull on Boulder Clay of Permian Age.—Permian
Boulder Clay of Natal.—Oolitic Boulder Conglomerate in
Sutherlandshire.—-Warm Climate in North Greenland during
Oolitic Period.—Mr. Godwin-Austen on Ice-action during
Cretaceous Period.—Glacial Conglomerates of Eocene Age in the
Alps.—M. Gastaldi on the Ice-transported Limestone Blocks of
the Superga.—Professor Heer on the Climate of North Greenland
during Miocene Period 292
CHAPTER XIX.
GEOLOGICAL TIME.—PROBABLE DATE OF THE GLACIAL EPOCH.
Geological Time measurable from Astronomical Data.—M. Leverrier’s
Formulæ.—Tables of Eccentricity for 3,000,000 Years in the
Past and 1,000,000 Years in the Future.—How the Tables have
been computed.—Why the Glacial Epoch is more recent than had
been supposed.—Figures convey a very inadequate Conception
of immense Duration.—Mode of representing a Million of
Years.—Probable Date of the Glacial Epoch 311
CHAPTER XX.
GEOLOGICAL TIME.—METHOD OF MEASURING THE RATE OF SUBAËRIAL
DENUDATION.
Rate of Subaërial Denudation a Measure of Time.—Rate determined
from Sediment of the Mississippi.—Amount of Sediment carried
down by the Mississippi; by the Ganges.—Professor Geikie on
Modern Denudation.—Professor Geikie on the Amount of Sediment
conveyed by European Rivers.—Rate at which the Surface of
the Globe is being denuded.—Alfred Tylor on the Sediment
of the Mississippi.—The Law which determines the Rate of
Denudation.—The Globe becoming less oblate.—Carrying Power
of our River Systems the true Measure of Denudation.—Marine
Denudation, trifling in comparison to Subaërial.—Previous
Methods of measuring Geological Time.—Circumstances which
show the recent Date of the Glacial Epoch.—Professor Ramsay
on Geological Time 329
CHAPTER XXI.
THE PROBABLE AGE AND ORIGIN OF THE SUN.
Gravitation Theory.—Amount of Heat emitted by the Sun.—Meteoric
Theory.—Helmholtz’s Condensation Theory.—Confusion of
Ideas.—Gravitation not the chief Source of the Sun’s
Heat.—Original Heat.—Source of Original Heat.—Original Heat
derived from Motion in Space.—Conclusion as to Date of
Glacial Epoch.—False Analogy.—Probable Date of Eocene and
Miocene Periods 346
CHAPTER XXII.
A METHOD OF DETERMINING THE MEAN THICKNESS OF THE SEDIMENTARY
ROCKS OF THE GLOBE.
Prevailing Methods defective.—Maximum Thickness of British
Rocks.—Three Elements in the Question.—Professor Huxley
on the Rate of Deposition.—Thickness of Sedimentary Rocks
enormously over-estimated.—Observed Thickness no Measure of
mean Thickness.—Deposition of Sediment principally along
Sea-margin.—Mistaken Inference regarding the Absence of a
Formation.—Immense Antiquity of existing Oceans 360
CHAPTER XXIII.
THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE
OF THE LAND DURING THE GLACIAL EPOCH.
Displacement of the Earth’s Centre of Gravity by Polar
Ice-cap.—Simple Method of estimating Amount of
Displacement.—Note by Sir W. Thomson on foregoing
Method.—Difference between Continental Ice and
a Glacier.—Probable Thickness of the Antarctic
Ice-cap.—Probable Thickness of Greenland Ice-sheet.—The
Icebergs of the Southern Ocean.—Inadequate Conceptions
regarding the Magnitude of Continental Ice 368
CHAPTER XXIV.
THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE OF THE
LAND DURING THE GLACIAL EPOCH.—(_Continued._)
Extent of Submergence from Displacement of Earth’s Centre
of Gravity.—Circumstances which show that the Glacial
Submergence resulted from Displacement of the Earth’s
Centre of Gravity.—Agreement between Theory and Observed
Facts.—Sir Charles Lyell on submerged Areas during
Tertiary Period.—Oscillations of Sea-Level in Relation to
Distribution.—Extent of Submergence on the Hypothesis that
the Earth is fluid in the Interior 387
CHAPTER XXV.
THE INFLUENCE OF THE OBLIQUITY OF THE ECLIPTIC ON CLIMATE
AND ON THE LEVEL OF THE SEA.
The direct Effect of Change of Obliquity on Climate.—Mr.
Stockwell on the maximum Change of Obliquity.—How Obliquity
affects the Distribution of Heat over the Globe.—Increase of
Obliquity diminishes the Heat at the Equator and increases
that at the Poles.—Influence of Change of Obliquity on the
Level of the Sea.—When the Obliquity was last at its superior
Limit.—Probable Date of the 25-foot raised Beach.—Probable
Extent of Rise of Sea-level resulting from Increase of
Obliquity.—Lieutenant-Colonel Drayson’s and Mr. Belt’s
Theories.—Sir Charles Lyell on Influence of Obliquity 398
CHAPTER XXVI.
COAL AN INTER-GLACIAL FORMATION.
Climate of Coal Period Inter-glacial in Character.—Coal Plants
indicate an Equable, not a Tropical Climate.—Conditions
necessary for Preservation of Coal Plants.—Oscillations
of Sea-level necessarily implied.—Why our Coal-fields
contain more than One Coal-seam.—Time required to form a
Bed of Coal.—Why Coal Strata contain so little evidence of
Ice-action.—Land Flat during Coal Period.—Leading Idea of the
Theory.—Carboniferous Limestones 420
CHAPTER XXVII.
PATH OF THE ICE-SHEET IN NORTH-WESTERN EUROPE AND ITS RELATIONS
TO THE BOULDER CLAY OF CAITHNESS.
Character of Caithness Boulder Clay.—Theories of the Origin
of the Caithness Clay.—Mr. Jamieson’s Theory.—Mr. C. W.
Peach’s Theory.—The proposed Theory.—Thickness of Scottish
Ice-sheet.—Pentlands striated on their Summits.—Scandinavian
Ice-sheet.—North Sea filled with Land-ice.—Great Baltic
Glacier.—Jutland and Denmark crossed by Ice.—Sir R.
Murchison’s Observations.—Orkney, Shetland, and Faroe Islands
striated across.—Loess accounted for.—Professor Geikie’s
Suggestion.—Professor Geikie and B. N. Peach’s Observations
on East Coast of Caithness.—Evidence from Chalk Flints and
Oolitic Fossils in Boulder Clay 435
CHAPTER XXVIII.
NORTH OF ENGLAND ICE-SHEET, AND TRANSPORT OF WASTDALE CRAG
BLOCKS.
Transport of Blocks; Theories of.—Evidence of Continental
Ice.—Pennine Range probably striated on Summit.—Glacial
Drift in Centre of England.—Mr. Lacy on Drift of Cotteswold
Hills.—England probably crossed by Land-ice.—Mr. Jack’s
Suggestion.—Shedding of Ice North and South.—South of England
Ice-sheet.—Glaciation of West Somerset.—Why Ice-markings are
so rare in South of England.—Form of Contortion produced by
Land-ice 456
CHAPTER XXIX.
EVIDENCE FROM BURIED RIVER CHANNELS OF A CONTINENTAL PERIOD
IN BRITAIN.
Remarks on the Drift Deposits.—Examination of Drift
by Borings.—Buried River Channel from Kilsyth to
Grangemouth.—Channels not excavated by Sea nor by
Ice.—Section of buried Channel at Grangemouth.—Mr. Milne
Home’s Theory.—German Ocean dry Land.—Buried River Channel
from Kilsyth to the Clyde.—Journal of Borings.—Marine
Origin of the Drift Deposits.—Evidence of Inter-glacial
Periods.—Oscillations of Sea-Level.—Other buried River
Channels 466
CHAPTER XXX.
THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.—THEORIES OF
GLACIER-MOTION.
Why the Question of Glacier-motion has been found to be so
difficult.—The Regelation Theory.—It accounts for the
Continuity of a Glacier, but not for its Motion.—Gravitation
proved by Canon Moseley insufficient to shear the Ice
of a Glacier.—Mr. Matthew’s Experiment.—No Parallel
between the bending of an Ice Plank and the shearing
of a Glacier.—Mr. Ball’s Objection to Canon Moseley’s
Experiment.—Canon Moseley’s Method of determining the Unit
of Shear.—Defect of Method.—Motion of a Glacier in some
Way dependent on Heat.—Canon Moseley’s Theory.—Objections
to his Theory.—Professor James Thomson’s Theory.—This
Theory fails to explain Glacier-motion.—De Saussure and
Hopkins’s “Sliding” Theories.—M. Charpentier’s “Dilatation”
Theory.—Important Element in the Theory 495
CHAPTER XXXI.
THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.—THE MOLECULAR
THEORY.
Present State of the Question.—Heat necessary to the Motion of
a Glacier.—Ice does not shear in the Solid State.—Motion
of a Glacier _molecular_.—How Heat is transmitted through
Ice.—Momentary Loss of Shearing Force.—The _Rationale_
of Regelation.—The Origin of “Crevasses.”—Effects of
Tension.—Modification of Theory.—Fluid Molecules crystallize
in Interstices.—Expansive Force of crystallizing Molecules
a Cause of Motion.—Internal molecular Pressure the chief
Moving Power.—How Ice can excavate a Rock Basin.—How Ice can
ascend a Slope.—How deep River Valleys are striated across.—A
remarkable Example in the Valley of the Tay.—How Boulders can
he carried from a lower to a higher Level 514
APPENDIX.
I. Opinions expressed previous to 1864 regarding the
Influence of the Eccentricity of the Earth’s Orbit on
Climate 528
II. On the Nature of Heat-Vibrations 544
III. On the Reason why the Difference of Reading between a
Thermometer exposed to direct Sunshine and One Shaded
diminishes as we ascend in the Atmosphere 547
IV. Remarks on Mr. J. Y. Buchanan’s Theory of the Vertical
Distribution of Temperature of the Ocean 550
V. On the Cause of the Cooling Effect produced on Solids by
Tension 552
VI. The Cause of Regelation 554
VII. List of Papers which have appeared in Dr. A. Petermann’s
_Geographische Mittheilungen_ relating to the Gulf-stream
and Thermal Condition of the Arctic Regions 556
VIII. List of Papers by the Author to which Reference is made in
this Volume 560
INDEX 563
LIST OF PLATES.
EARTH’S ORBIT WHEN ECCENTRICITY IS AT ITS SUPERIOR
LIMIT _Frontispiece._
PLATE
I. SHOWING AGREEMENT BETWEEN THE SYSTEM OF OCEAN-CURRENTS
AND WINDS _To face page_ 212
II. SHOWING HOW OPPOSING CURRENTS INTERSECT EACH OTHER 219
III. SECTION OF MID-ATLANTIC 222
IV. DIAGRAM REPRESENTING THE VARIATIONS OF ECCENTRICITY OF THE
EARTH’S ORBIT 313
V. SHOWING PROBABLE PATH OF THE ICE IN NORTH-WESTERN EUROPE 449
VI. SHOWING PATH OF ICE ACROSS CAITHNESS 453
VII. MAP OF THE MIDLAND VALLEY (SCOTLAND), SHOWING BURIED RIVER
CHANNELS 471
CHAPTER I.
INTRODUCTION.
The Fundamental Problem of Geology.—Geology a Dynamical
Science.—The Nature of a Geological Principle.—Theories
of Geological Climate.—Geological Climate dependent
on Astronomical Causes.—An Important Consideration
overlooked.—Abstract of the Line of Argument pursued in the
Volume.
_The Fundamental Problem of Geology._—The investigation of the
successive changes and modifications which the earth’s crust has
undergone during past ages is the province of geology. It will be
at once admitted that an acquaintance with the agencies by means of
which those successive changes and modifications were effected, is of
paramount importance to the geologist. What, then, are those agencies?
Although volcanic and other subterranean eruptions, earthquakes,
upheavals, and subsidences of the land have taken place in all ages,
yet no truth is now better established than that it is not by these
convulsions and cataclysms of nature that those great changes were
effected. It was rather by the ordinary agencies that we see every day
at work around us, such as rain, rivers, heat and cold, frost and snow.
The valleys were not produced by violent dislocations, nor the hills
by sudden upheavals, but were actually carved out of the solid rock,
silently and gently, by the agencies to which we have referred. “The
tools,” to quote the words of Professor Geikie, “by which this great
work has been done are of the simplest and most every-day order—the
air, rain, frosts, springs, brooks, rivers, glaciers, icebergs, and the
sea. These tools have been at work from the earliest times of which
any geological record has been preserved. Indeed, it is out of the
accumulated chips and dust which they have made, afterwards hardened
into solid rock and upheaved, that the very framework of our continents
has been formed.”[1]
It will be observed—and this is the point requiring particular
attention—that the agencies referred to are the ordinary meteorological
or climatic agencies. In fact, it is these agencies which constitute
climate. The various peculiarities or modifications of climate result
from a preponderance of one or more of these agencies over the rest.
When heat, for example, predominates, we have a hot or tropical
climate. When cold and frost predominate, we have a rigorous or arctic
climate. With moisture in excess, we have a damp and rainy climate;
and so on. But this is not all. These climatic agencies are not only
the factors which carved out the rocky face of the globe into hill
and dale, and spread over the whole a mantle of soil; but by them are
determined the character of the _flora_ and _fauna_ which exist on
that soil. The flora and fauna of a district are determined mainly by
the character of the climate, and not by the nature of the soil, or
the conformation of the ground. It is from difference of climate that
tropical life differs so much from arctic, and both these from the life
of temperate regions. It is climate, and climate alone, that causes
the orange and the vine to blossom, and the olive to flourish, in the
south, but denies them to the north, of Europe. It is climate, and
climate alone, that enables the forest tree to grow on the plain, but
not on the mountain top; that causes wheat and barley to flourish on
the mainland of Scotland, but not on the steppes of Siberia.
Again, if we compare flat countries with mountainous, highlands with
lowlands, or islands with continents, we shall find that difference of
climatic conditions is the chief reason why life in the one differs
so much from life in the other. And if we turn to the sea we find
that organic life is there as much under the domain of climate as on
the land, only the conditions are much less complex. For in the case
of the sea, difference in the temperature of the water may be said
to constitute almost the only difference of climatic conditions.
If there is one fact more clearly brought out than another by the
recent deep-sea explorations, it is this, that nothing exercises so
much influence on organic life in the ocean as the temperature of the
water. In fact, so much is this the case, that warm zones were found
to be almost equivalent to zones of life. It was found that even the
enormous pressure at the bottom of the ocean does not exercise so much
influence on life as the temperature of the water. There are few, I
presume, who reflect on the subject that will not readily admit that,
whether as regards the great physical changes which are taking place on
the surface of our globe, or as regards the growth and distribution of
plant and animal life, the ordinary climatic agents are the real agents
at work, and that, compared with them, all other agencies sink into
insignificance.
It will also be admitted that what holds true of the present holds
equally true of the past. Climatic agents are not only now the most
important and influential; they have been so during all past geological
ages. They were so during the Cainozoic as much as during the present;
and there is no reason for supposing they were otherwise during the
remoter Mesozoic and Palæozoic epochs. They have been the principal
factors concerned in that long succession of events and changes which
have taken place since the time of the solidification of the earth’s
crust. The stratified rocks of the globe contain all the records which
now remain of their action, and it is the special duty of the geologist
to investigate and read those records. It will be at once admitted that
in order to a proper understanding of the events embodied in these
records, an acquaintance with the agencies by which they were produced
is of the utmost importance. In fact, it is only by this means that we
can hope to arrive at their rational explanation. A knowledge of the
agents, and of the laws of their operations, is, in all the physical
sciences, the means by which we arrive at a rational comprehension
of the effects produced. If we have before us some complex and
intricate effects which have been produced by heat, or by light, or by
electricity, &c., in order to understand them we must make ourselves
acquainted with the agents by which they were produced and the laws of
their action. If the effects to be considered be, for example, those of
heat, then we must make ourselves acquainted with this agent and its
laws. If they be of electricity, then a knowledge of electricity and
its laws becomes requisite.
This is no mere arbitrary mode of procedure which may be adopted in
one science and rejected in another. It is in reality a necessity of
thought arising out of the very constitution of our intellect; for the
objective law of the agent is the conception by means of which the
effects are subjectively united in a rational unity. We may describe,
arrange, and classify the effects as we may, but without a knowledge of
the laws of the agent we can have no rational unity. We have not got
the higher conception by which they can be _comprehended_. It is this
relationship between the effects and the laws of the agent, a knowledge
of which really constitutes a science. We might examine, arrange, and
describe for a thousand years the effects produced by heat, and still
we should have no science of heat unless we had a knowledge of the
laws of that agent. The effects would never be seen to be necessarily
connected with anything known to us; we could not connect them with
any rational principle from which they could be deduced _à priori_.
The same remarks hold, of course, equally true of all sciences, in
which the things to be considered stand in the relationship of cause
and effect. Geology is no exception. It is not like systematic botany,
a mere science of classification. It has to explain and account for
effects produced; and these effects can no more be explained without
a knowledge of the laws of the agents which produced them, than can
the effects of heat without a knowledge of the laws of heat. The only
distinction between geology and heat, light, electricity, &c., is,
that in geology the effects to be explained have almost all occurred
already, whereas in these other sciences effects actually taking place
have to be explained. But this distinction is of no importance to
our present purpose, for effects which have already occurred can no
more be explained without a knowledge of the laws of the agent which
produced them than can effects which are in the act of occurring. It
is, moreover, not strictly true that all the effects to be explained
by the geologist are already past. It falls within the scope of his
science to account for the changes which are at present taking place on
the earth’s crust.
No amount of description, arrangement, and classification, however
perfect or accurate, of the facts which come under the eye of the
geologist can ever constitute a science of geology any more than a
description and classification of the effects of heat could constitute
a science of heat. This will, no doubt, be admitted by every one who
reflects upon the subject, and it will be maintained that geology,
like every other science, must possess principles applicable to the
facts. But here confusion and misconception will arise unless there be
distinct and definite ideas as to what ought to constitute a geological
principle. It is not every statement or rule that may apply to a great
many facts, which will constitute a geological principle. A geological
principle must bear the same characteristics as the principles of those
sciences to which we have referred. What, then, is the nature of the
principles of light, heat, electricity, &c.? The principles of heat
are the laws of heat. The principles of electricity are the laws of
electricity. And these laws are nothing more nor less than the ways
according to which these agents produce their effects. The principles
of geology are therefore the laws of geology. But the laws of geology
must be simply the laws of the geological agents, or, in other words,
the methods by which they produce their effects. Any other so-called
principle can be nothing more than an empirical rule, adopted for
convenience. Possessing no rationality in itself, it cannot be justly
regarded as a principle. In order to rationality the principle must be
either resolvable into, or logically deducible from, the laws of the
agents. Unless it possess this quality we cannot give the explanation
_à priori_.
The reason of all this is perfectly obvious. The things to be explained
are effects; and the relationship between cause and effect affords the
subjective connection between the principle and the explanation. The
explanation follows from the principle simply as the effect results
from the laws of the agent or cause.
_Theories of Geological Climate._—We have already seen that the
geological agents are chiefly the ordinary climatic agents.
Consequently, the main principles of geology must be the laws of the
climatic agents, or some logical deductions from them. It therefore
follows that, in order to a purely scientific geology, the grand
problem must be one of geological climate. It is through geological
climate that we can hope to arrive ultimately at principles which will
afford a rational explanation of the multifarious facts which have
been accumulating during the past century. The facts of geology are
as essential to the establishment of the principles, as the facts of
heat, light, and electricity are essential to the establishment of the
principles of these sciences. A theory of geological climate devised
without reference to the facts would be about as worthless as a theory
of heat or of electricity devised without reference to the facts of
these sciences.
It has all along been an admitted opinion among geologists that the
climatic condition of our globe has not, during past ages, been
uniformly the same as at present. For a long time it was supposed that
during the Cambrian, Silurian, and other early geological periods, the
climate of our globe was much hotter than now, and that ever since
it has been gradually becoming cooler. And this high temperature of
Palæozoic ages was generally referred to the influence of the earth’s
internal heat. It has, however, been proved by Sir William Thomson[2]
that the general climate of our globe could not have been sensibly
affected by internal heat at any time more than ten thousand years
after the commencement of the solidification of the surface. This
physicist has proved that the present influence of internal heat on
the temperature amounts to about only 1/75th of a degree. Not only
is the theory of internal heat now generally abandoned, but it is
admitted that we have no good geological evidence that climate was much
hotter during Palæozoic ages than now; and much less, that it has been
becoming _uniformly_ colder.
The great discovery of the glacial epoch, and more lately that of a
mild and temperate condition of climate extending during the Miocene
and other periods to North Greenland, have introduced a complete
revolution of ideas in reference to geological climate. Those
discoveries showed that our globe has not only undergone changes of
climate, but changes of the most extraordinary character. They showed
that at one time not only an arctic condition of climate prevailed in
our island, but that the greater part of the temperate region down
to comparatively low latitudes was buried under ice, while at other
periods Greenland and the Arctic regions, probably up to the North
Pole, were not only free from ice, but were covered with a rich and
luxuriant vegetation.
To account for these extraordinary changes of climate has generally
been regarded as the most difficult and perplexing problem which has
fallen to the lot of the geologist. Some have attempted to explain
them by assuming a displacement of the earth’s axis of rotation in
consequence of the uprising of large mountain masses on some part
of the earth’s surface. But it has been shown by Professor Airy,[3]
Sir William Thomson,[4] and others, that the earth’s equatorial
protuberance is such that no geological change on its surface could
ever possibly alter the position of the axis of rotation to an extent
which could at all sensibly affect climate. Others, again, have tried
to explain the change of climate by supposing, with Poisson, that the
earth during its past geological history may have passed through hotter
and colder parts of space. This is not a very satisfactory hypothesis.
There is no doubt a difference in the quantity of force in the form of
heat passing through different parts of space; but space itself is not
a substance which can possibly be either cold or hot. If, therefore,
we were to adopt this hypothesis, we must assume that the earth during
the hot periods must have been in the vicinity of some other great
source of heat and light besides the sun. But the proximity of a
mass of such magnitude as would be sufficient to affect to any great
extent the earth’s climate would, by its gravity, seriously disarrange
the mechanism of our solar system. Consequently, if our solar system
had ever, during any former period of its history, really come into
the vicinity of such a mass, the orbits of the planets ought at the
present day to afford some evidence of it. But again, in order to
account for a cold period, such as the glacial epoch, we have to assume
that the earth must have come into the vicinity of a cold body.[5]
But recent discoveries in regard to inter-glacial periods are wholly
irreconcilable with this theory.
A change in the obliquity of the ecliptic has frequently been, and
still is, appealed to as an explanation of geological climate. This
theory appears, however, to be beset by a twofold objection: (1), it
can be shown from celestial mechanics, that the variations in the
obliquity of the ecliptic must always have been so small that they
could not materially affect the climatic condition of the globe; and
(2), even admitting that the obliquity could change to an indefinite
extent, it can be shown[6] that no increase or decrease, however great,
could possibly account for either the glacial epoch or a warm temperate
condition of climate in polar regions.
The theory that the sun is a variable star, and that the glacial
epochs of the geologists may correspond to periods of decrease in the
sun’s heat, has lately been advanced. This theory is also open to two
objections: (1), a general diminution of heat[7] never could produce
a glacial epoch; and (2), even if it could, it would not explain
inter-glacial periods.
The only other theory on the subject worthy of notice is that
of Sir Charles Lyell. Those extraordinary changes of climate are,
according to his theory, attributed to differences in the distribution
of land and water. Sir Charles concludes that, were the land all
collected round the poles, while the equatorial zones were occupied by
the ocean, the general temperature would be lowered to an extent that
would account for the glacial epoch. And, on the other hand, were the
land all collected along the equator, while the polar regions were
covered with sea, this would raise the temperature of the globe to an
enormous extent. It will be shown in subsequent chapters that this
theory does not duly take into account the prodigious influence exerted
on climate by means of the heat conveyed from equatorial to temperate
and polar regions by means of ocean-currents. In Chapters II. and III.
I have endeavoured to prove (1), that were it not for the heat conveyed
from equatorial to temperate and polar regions by this means, the
thermal condition of the globe would be totally different from what it
is at present; and (2), that the effect of placing all the land along
the equator would be diametrically the opposite of that which Sir
Charles supposes.
But supposing that difference in the distribution of land and water
would produce the effects attributed to it, nevertheless it would not
account for those extraordinary changes of climate which have occurred
during geological epochs. Take, for example, the glacial epoch.
Geologists almost all agree that little or no change has taken place
in the relative distribution of sea and land since that _epoch_. All
our main continents and islands not only existed then as they do now,
but every year is adding to the amount of evidence which goes to show
that so recent, geologically considered, is the glacial epoch that the
very contour of the surface was pretty much the same then as it is at
the present day. But this is not all; for even should we assume (1),
that a difference in the distribution of sea and land would produce the
effects referred to, and (2), that we had good geological evidence to
show that at a very recent period a form of distribution existed which
would produce the necessary glacial conditions, still the glacial
epoch would not be explained, for the phenomena of warm inter-glacial
periods would completely upset the theory.
_Geological Climate depending on Astronomical Causes._—For a good many
years past, an impression has been gradually gaining ground amongst
geologists that the glacial epoch, as well as the extraordinary
condition of climate which prevailed in arctic regions during the
Miocene and other periods, must some way or other have resulted from
a cosmical cause; but all seemed at a loss to conjecture what that
cause could possibly be. It was apparent that the cosmical cause must
be sought for in the relations of our earth to the sun; but a change
in the obliquity of the ecliptic and the eccentricity of the earth’s
orbit are the only changes from which any sensible effect on climate
could possibly be expected to result. It was shown, however, by Laplace
that the change of obliquity was confined within so narrow limits that
it has scarcely ever been appealed to as a cause seriously affecting
climate. The only remaining cause to which appeal could be made was
the change in the eccentricity of the earth’s orbit—precession of the
equinoxes without eccentricity producing, of course, no effect whatever
on climate. Upwards of forty years ago Sir John Herschel and a few
other astronomers directed their attention to the consideration of this
cause, but the result arrived at was adverse to the supposition that
change of eccentricity could greatly affect the climate of our globe.
As some misapprehension seems to prevail with reference to this, I
would take the liberty of briefly adverting to the history of the
matter,—referring the reader to the Appendix for fuller details.
About the beginning of the century some writers attributed the lower
temperature of the southern hemisphere to the fact that the sun remains
about seven days less on that hemisphere than on the northern; their
view being that the southern hemisphere on this account receives
seven days less heat than the northern. Sir Charles Lyell, in the
first edition of his “Principles,” published in 1830, refers to this
as a cause which might produce some slight effect on climate. Sir
Charles’s remarks seem to have directed Sir John Herschel’s attention
to the subject, for in the latter part of the same year he read a
paper before the Geological Society on the astronomical causes which
may influence geological phenomena, in which, after pointing out the
mistake into which Sir Charles had been led in concluding that the
southern hemisphere receives less heat than the northern, he considers
the question as to whether geological climate could be influenced by
changes in the eccentricity of the earth’s orbit. He did not appear at
the time to have been aware of the conclusions arrived at by Lagrange
regarding the superior limit of the eccentricity of the earth’s orbit;
but he came to the conclusion that possibly the climate of our globe
may have been affected by variations in the eccentricity of its orbit.
“An amount of variation,” he says, “which we need not hesitate to
admit (at least provisionally) as a possible one, may be productive
of considerable diversity of climate, and may operate during great
periods of time either to mitigate or to exaggerate the difference of
winter and summer temperatures, so as to produce alternately in the
same latitude of either hemisphere a perpetual spring, or the extreme
vicissitudes of a burning summer and a rigorous winter.”
This opinion, however, was unfortunately to a great extent nullified
by the statement which shortly afterwards appeared in his “Treatise
on Astronomy,” and also in the “Outlines of Astronomy,” to the effect
that the elliptic form of the earth’s orbit has but a very trifling
influence in producing variation of temperature corresponding to the
sun’s distance; the reason being that whatever may be the ellipticity
of the orbit, it follows that equal amounts of heat are received
from the sun in passing over equal angles round it, in whatever part
of the ellipse those angles may be situated. Those angles will of
course be described in unequal times, but the greater proximity of
the sun exactly compensates for the more rapid description, and thus
an equilibrium of heat is maintained. The sun, for example, is much
nearer the earth when he is over the southern hemisphere than he is
when over the northern; but the southern hemisphere does not on this
account receive more heat than the northern; for, owing to the greater
velocity of the earth when nearest the sun, the sun does not remain
so long on the southern hemisphere as he does on the northern. These
two effects so exactly counterbalance each other that, whatever be
the extent of the eccentricity, the total amount of heat reaching
both hemispheres is the same. And he considered that this beautiful
compensating principle would protect the climate of our globe from
being seriously affected by an increase in the eccentricity of its
orbit, unless the extent of that increase was very great.
“Were it not,” he says, “for this, the eccentricity of the orbit
would materially influence the transition of seasons. The fluctuation
of distance amounts to nearly 1/30th of its mean quantity, and
consequently the fluctuation in the sun’s direct heating power to
double this, or 1/15th of the whole. Now the perihelion of the orbit is
situated nearly at the place of the northern winter solstice; so that,
were it not for the compensation we have just described, the effect
would be to exaggerate the difference of summer and winter in the
southern hemisphere, and to moderate it in the northern; thus producing
a more violent alternation of climate in the one hemisphere, and an
approach to perpetual spring in the other. As it is, however, no such
inequality subsists, but an equal and impartial distribution of heat
and light is accorded to both.”[8]
Herschel’s opinion was shortly afterwards adopted and advocated by
Arago[9] and by Humboldt.[10]
Arago, for example, states that so little is the climate of our globe
affected by the eccentricity of its orbit, that even were the orbit to
become as eccentric as that of the planet Pallas (that is, as great as
0·24), “still this would not alter in any appreciable manner the mean
thermometrical state of the globe.”
This idea, supported by these great authorities, got possession of the
public mind; and ever since it has been almost universally regarded
as settled that the great changes of climate indicated by geological
phenomena could not have resulted from any change in the relation of
the earth to the sun.
There is, however, one effect that was not regarded as compensated. The
total amount of heat received by the earth is inversely proportional
to the minor axis of its orbit; and it follows, therefore, that the
greater the eccentricity, the greater is the total amount of heat
received by the earth. On this account it was concluded that an
increase of eccentricity would tend to a certain extent to produce a
warmer climate.
All those conclusions to which I refer, arrived at by astronomers, are
perfectly legitimate so far as the direct effects of eccentricity are
concerned; and it was quite natural, and, in fact, proper to conclude
that there was nothing in the mere increase of eccentricity that could
produce a glacial epoch. How unnatural would it have been to have
concluded that an increase in the quantity of heat received from the
sun should lower the temperature, and cover the country with snow and
ice! Neither would excessively cold winters, followed by excessively
hot summers, produce a glacial epoch. To assert, therefore, that the
purely astronomical causes could produce such an effect would be simply
absurd.
_Important Consideration overlooked._—The important fact, however, was
overlooked that, although the glacial epoch could not result _directly_
from an increase of eccentricity, it might nevertheless do so
_indirectly_. Although an increase of eccentricity could have no direct
tendency to lower the temperature and cover our country with ice, yet
it might bring into operation physical agents which would produce this
effect.
If, instead of endeavouring to trace a direct connection between a high
condition of eccentricity and a glacial condition of climate, we turn
our attention to the consideration of what are the physical effects
which result from an increase of eccentricity, we shall find that a
host of physical agencies are brought into operation, the combined
effect of which is to lower to a very great extent the temperature of
the hemisphere whose winters occur in aphelion, and to raise to nearly
as great an extent the temperature of the opposite hemisphere, whose
winters of course occur in perihelion. Until attention was directed to
those physical circumstances to which I refer, it was impossible that
the true cause of the glacial epoch could have been discovered; and,
moreover, many of the indirect and physical effects, which in reality
were those that brought about the glacial epoch, could not, in the
nature of things, have been known previously to recent discoveries in
the science of heat.
The consideration and discussion of those various physical agencies are
the chief aim of the following pages.
_Abstract of the Line of Argument pursued in this Volume._—I shall
now proceed to give a brief abstract of the line of argument pursued
in this volume. But as a considerable portion of it is devoted to the
consideration of objections and difficulties bearing either directly
or indirectly on the theory, it will be necessary to point out what
those difficulties are, how they arose, and the methods which have been
adopted to overcome them.
Chapter IV. contains an outline of the physical agencies affecting
climate which are brought into operation by an increase of
eccentricity. By far the most important of all those agencies, and the
one which mainly brought about the glacial epoch, is the _Deflection_
of Ocean-Currents. The consideration of the indirect physical
connection between a high state of eccentricity and the deflection
of ocean-currents, and also the enormous influence on climate which
results from this deflection constitute not only the most important
part of the subject, but the one beset with the greatest amount of
difficulties.
The difficulties besetting this part of the theory arise mainly from
the imperfect state of our knowledge, (1st) with reference to the
absolute amount of heat transferred from equatorial to temperate and
polar regions by means of ocean-currents and the influence which the
heat thus transferred has on the distribution of temperature on the
earth’s surface; and (2nd) in connection with the physical cause of
ocean circulation.
In Chapters II. and III. I have entered at considerable length into
the consideration of the effects of ocean currents on the distribution
of heat over the globe. The only current of which anything like
an accurate estimate of volume and temperature has been made is
the Gulf-stream. In reference to this stream we have a means of
determining in absolute measure the quantity of heat conveyed by it.
On the necessary computation being made, it is found that the amount
transferred by the Gulf-stream from equatorial regions into the North
Atlantic is enormously greater than was ever anticipated, amounting
to no less than one-fifth part of the entire heat possessed by the
North Atlantic. This striking fact casts a new light on the question
of the distribution of heat over the globe. It will be seen that to
such an extent is the temperature of the equatorial regions lowered,
and that of high temperate, and polar regions raised, by means of ocean
currents, that were they to cease, and each latitude to depend solely
on the heat received directly from the sun, only a very small portion
of the globe would be habitable by the present order of beings. This
being the case, it becomes obvious to what an extent the deflection
of ocean currents must affect temperature. For example, were the
Gulf-stream stopped, and the heat conveyed by it deflected into the
Southern Ocean, how enormously would this tend to lower the temperature
of the northern hemisphere, and raise the temperature south of the
equator.
Chapters VI., VII., VIII., IX., X., and XIII., are devoted to the
consideration of the physical cause of oceanic circulation. This has
been found to be the most difficult and perplexing part of the whole
inquiry. The difficulties mainly arise from the great diversity of
opinion and confusion of ideas prevailing in regard to the mechanics
of the subject. There are two theories propounded to account for
oceanic circulation; the one which may be called the _Wind_ theory, and
the other the _Gravitation_ theory; and this diversity of opinion and
confusion of ideas prevail in connection with both theories. As the
question of the cause of oceanic circulation has not only a direct and
important bearing on the subject of the present volume, but is further
one of much general interest, I have entered somewhat fully into the
matter.
The Gravitation theories may be divided into two classes. The first of
these attributes the Gulf-stream and other sensible currents of the
ocean to difference of specific gravity, resulting from difference
of temperature between the sea in equatorial and polar regions. The
leading advocate of this theory was the late Lieutenant Maury, who
brought it so much into prominence in his interesting book on the
“Physical Geography of the Sea.” The other class does not admit that
the sensible currents of the ocean can be produced by difference of
specific gravity; but they maintain that difference of temperature
between the sea in equatorial and polar regions produces a general
movement of the upper portion of the sea from the equator to the
poles, and a counter-movement of the under portion from the poles
to the equator. This form of the gravitation theory has been ably
and zealously advocated by Dr. Carpenter, who may be regarded as
its representative. The Wind theories also divide into two classes.
According to the one ocean currents are caused and maintained by the
impulse of the trade-winds, while according to the other they are
due not to the impulse of the trade-winds alone, but to that of the
prevailing winds of the globe, regarded as a general system. The former
of these is the one generally accepted; the latter is that advocated in
the present volume.
The relations which these theories bear to the question of secular
change of climate, will be found stated at length in Chapter VI. It
will, however, be better to state here in a few words what those
relations are. When the eccentricity of the earth’s orbit attains a
high value, the hemisphere, whose winter solstice occurs in aphelion,
has, for reasons which are explained in Chapter IV., its temperature
lowered, while that of the opposite hemisphere is raised. Let us
suppose the northern hemisphere to be the cold one, and the southern
the warm one. The difference of temperature between the equator and
the North Pole will then be greater than between the equator and the
South Pole; according, therefore, to theory, the trades of the northern
hemisphere will be stronger than those of the southern, and will
consequently blow across the equator to some distance on the southern
hemisphere. This state of things will tend to deflect equatorial
currents southwards, impelling the warm water of the equatorial regions
more into the southern or warm hemisphere than into the northern or
cold hemisphere. The tendency of all this will be to exaggerate the
difference of temperature already existing between the two hemispheres.
If, on the other hand, the great ocean currents which convey the warm
equatorial waters to temperate and polar regions be not produced by
the impulse of the winds, but by difference of temperature, as Maury
maintains, then in the case above supposed the equatorial waters would
be deflected more into the northern or cold hemisphere than into the
southern or warm hemisphere, because the difference of temperature
between the equator and the poles would be greater on the cold than
on the warm hemisphere. This, of course, would tend to neutralize or
counteract that difference of temperature between the two hemispheres
which had been previously produced by eccentricity. In short, this
theory of circulation would effectually prevent eccentricity from
seriously affecting climate.
Chapters VI. and VII. have been devoted to an examination of this form
of the gravitation theory.
The above remarks apply equally to Dr. Carpenter’s form of the theory;
for according to a doctrine of General Oceanic Circulation resulting
from difference of specific gravity between the water at the equator
and at the poles, the equatorial water will be carried more to the
cold than to the warm hemisphere. It is perfectly true that a belief
in a general oceanic circulation may be held quite consistently with
the theory of secular changes of climate, provided it be admitted
that not this general circulation but ocean currents are the great
agency employed in distributing heat over the globe. The advocates of
the theory, however, admit no such thing, but regard ocean currents
as of secondary importance. It may be stated that the existence of
this general ocean circulation has never been detected by actual
observation. It is simply assumed in order to account for certain
facts, and it is asserted that such a circulation must take place as
a physical necessity. I freely admit that were it not that the warm
water of equatorial regions is being constantly carried off by means
of ocean currents such as the Gulf-stream, it would accumulate till,
in order to restoration of equilibrium, such a general movement as is
supposed would be generated. But it will be shown that the warm water
in equatorial regions is being drained off so rapidly by ocean currents
that the actual density of an equatorial column differs so little
from that of a polar column that the force of gravity resulting from
that difference is so infinitesimal that it is doubtful whether it is
sufficient to produce sensible motion. I have also shown in Chapter
VIII. that all the facts which this theory is designed to explain are
not only explained by the wind theory, but are deducible from it as
necessary consequences. In Chapter XI. it is proved, by contrasting
the quantity of heat conveyed by ocean currents from inter-tropical to
temperate and polar regions with such an amount as could possibly be
conveyed by means of a general oceanic circulation, that the latter
sinks into insignificance before the former. In Chapters X. and XII.
the various objections which have been advanced by Dr. Carpenter and
Mr. Findlay are discussed at considerable length, and in Chapter IX.
I have entered somewhat minutely into an examination of the mechanics
of the gravitation theory. A statement of the wind theory is given in
Chapter XIII.; and in Chapter XIV. is shown the relation of this theory
to the theory of Secular changes of climate. This terminates the part
of the inquiry relating to oceanic circulation.
We now come to the _crucial test_ of the theories respecting the cause
of the glacial epoch, viz., Warm Inter-glacial Periods. In Chapters
XV. and XVI. I have given a statement of the geological facts which
go to prove that that long epoch known as the Glacial was not one
of continuous cold, but consisted of a succession of cold and warm
periods. This condition of things is utterly inexplicable on every
theory of the cause of the glacial epoch which has hitherto been
advanced; but, according to the physical theory of secular changes of
climate under consideration, it follows as a necessary consequence.
In fact, the amount of geological evidence which has already been
accumulated in reference to inter-glacial periods may now be regarded
as perfectly sufficient to establish the truth of that theory.
If the glacial epoch resulted from some accidental distribution of sea
and land, then there may or may not have been more than one glacial
epoch, but if it resulted from the cause which we have assigned, then
there must have been during the geological history of the globe a
succession of glacial epochs corresponding to the secular variations
in the eccentricity of the earth’s orbit. A belief in the existence
of recurring glacial epochs has been steadily gaining ground for many
years past. I have, in Chapter XVIII., given at some length the facts
on which this belief rests. It is true that the geological evidence of
glacial epochs in prior ages is meagre in comparison with that of the
glacial epoch of Post-tertiary times; but there is a reason for this in
the nature of geological evidence itself. Chapter XVII. deals with the
geological records of former glacial epochs, showing that they are not
only imperfect, but that there is good reason why they should be so,
and that the imperfection of the records in reference to them cannot be
advanced as an argument against their existence.
If the glacial epoch resulted from a high condition of eccentricity, we
have not only a means of determining the positive date of that epoch,
but we have also a means of determining geological time in absolute
measure. For if the glacial epochs of prior ages correspond to periods
of high eccentricity, then the intervals between those periods of high
eccentricity become the measure of the intervals between the glacial
epochs. The researches of Lagrange and Leverrier into the secular
variations of the elements of the orbits of the planets enable us
to determine with tolerable accuracy the values of the eccentricity
of the earth’s orbit for, at least, four millions of years past and
future. With the view of determining those values, I several years
ago computed from Leverrier’s formula the eccentricity of the earth’s
orbit and longitude of the perihelion, at intervals of ten thousand and
fifty thousand years during a period of three millions of years in the
past, and one million of years in the future. The tables containing
these values will be found in Chapter XIX. These tables not only give
us the date of the glacial epoch, but they afford, as will be seen
from Chapter XXI., evidence as to the probable date of the Eocene and
Miocene periods.
Ten years ago, when the theory was first advanced, it was beset by
a very formidable difficulty, arising from the opinions which then
prevailed in reference to geological time. One or two glacial epochs in
the course of a million of years was a conclusion which at that time
scarcely any geologist would admit, and most would have felt inclined
to have placed the last glacial epoch at least one million of years
back. But then if we assume that the glacial epoch was due to a high
state of eccentricity, we should be compelled to admit of at least two
glacial epochs during that lapse of time. It was the modern doctrine
that the great changes undergone by the earth’s crust were produced,
not by convulsions of nature, but by the slow and almost imperceptible
action, of rain, rivers, snow, frost, ice, &c., which impressed so
strongly on the mind of the geologist the vast duration of geological
periods. When it was considered that the rocky face of our globe had
been carved into hills and dales, and ultimately worn down to the
sea-level by means of those apparently trifling agents, not only once
or twice, but many times, during past ages, it was not surprising that
the views entertained by geologists regarding the immense antiquity of
our globe should not have harmonised with the deductions of physical
science on the subject. It had been shown by Sir William Thomson and
others, from physical considerations relating to the age of the sun’s
heat and the secular cooling of our globe, that the geological history
of our earth’s crust must be limited to a period of something like
one hundred millions of years. But these speculations had but little
weight when pitted against the stern and undeniable facts of subaërial
denudation. How, then, were the two to be reconciled? Was it the
physicist who had under-estimated geological time, or the geologist
who had over-estimated it? Few familiar with modern physics, and who
have given special attention to the subject, would admit that the sun
could have been dissipating his heat at the present enormous rate for
a period much beyond one hundred millions of years. The probability
was that the amount of work performed on the earth’s crust by the
denuding agents in a period so immense as a million of years was, for
reasons stated in Chapter XX., very much under-estimated. But the
difficulty was how to prove this. How was it possible to measure the
rate of operation of agents so numerous and diversified acting with
such extreme slowness and irregularity over so immense areas? In other
words, how was it possible to measure the rate of subaërial denudation?
Pondering over this problem about ten years ago, an extremely simple
and obvious method of solving it suggested itself to my mind. This
method—the details of which will be found in Chapter XX.—showed that
the rate of subaërial denudation is enormously greater than had been
supposed. The method is now pretty generally accepted, and the result
has already been to bring about a complete reconciliation between
physics and geology in reference to time.
Chapter XXI. contains an account of the gravitation theories of the
origin of the sun’s heat. The energy possessed by the sun is generally
supposed to have been derived from gravitation, combustion being
totally inadequate as a source. But something more than gravitation
is required before we can account for even one hundred millions of
years’ heat. Gravitation could not supply even one-half that amount.
There must be some other and greater source than that of gravitation.
There is, however, as is indicated, an obvious source from which far
more energy may have been derived than could have been obtained from
gravitation.
The method of determining the rate of subaërial denudation enables us
also to arrive at a rough estimate of the actual mean thickness of the
stratified rocks of the globe. It will be seen from Chapter XXII. that
the mean thickness is far less than is generally supposed.
The physical cause of the submergence of the land during the glacial
epoch, and the influence of change in the obliquity of the ecliptic on
climate, are next considered. In Chapter XXVI. I have given the reasons
which induce me to believe that coal is an inter-glacial formation.
The next two chapters—the one on the path of the ice in north-western
Europe, the other on the north of England ice-sheet—are reprints of
papers which appeared a few years ago in the _Geological Magazine_.
Recent observations have confirmed the truth of the views advanced
in these two chapters, and they are rapidly gaining acceptance among
geologists.
I have given, at the conclusion, a statement of the molecular theory of
glacier motion—a theory which I have been led to modify considerably on
one particular point.
There is one point to which I wish particularly to direct
attention—viz., that I have studiously avoided introducing into the
theories propounded anything of a hypothetical nature. There is not,
so far as I am aware, from beginning to end of this volume, a single
hypothetical element: nowhere have I attempted to give a hypothetical
explanation. The conclusions are in every case derived either from
facts or from what I believe to be admitted principles. In short, I
have aimed to prove that the theory of secular changes of climate
follows, as a necessary consequence, from the admitted principles of
physical science.
CHAPTER II.
OCEANS-CURRENTS IN RELATION TO THE DISTRIBUTION OF HEAT OVER
THE GLOBE.
The absolute Heating-power of Ocean-currents.—Volume of
the Gulf-stream.—Absolute Amount of Heat conveyed by
it.—Greater Portion of Moisture in inter-tropical Regions
falls as Rain in those Regions.—Land along the Equator
tends to lower the Temperature of the Globe.—Influence
of Gulf-stream on Climate of Europe.—Temperature of
Space.—Radiation of a Particle.—Professor Dove on Normal
Temperature.—Temperature of Equator and Poles in the Absence
of Ocean-currents.—Temperature of London, how much due to
Ocean-currents.
_The absolute Heating-power of Ocean-currents._—There is perhaps no
physical agent concerned in the distribution of heat over the surface
of the globe the influence of which has been so much underrated as that
of ocean-currents. This is, no doubt, owing to the fact that although
their surface-temperature, direction, and general influence have
obtained considerable attention, yet little or nothing has been done
towards determining the absolute amount of heat or of cold conveyed by
them or the resulting absolute increase or decrease of temperature.
The modern method of determining the amount of heat-effects in absolute
measure is, doubtless, destined to cast new light on all questions
connected with climate, as it has done, and is still doing, in every
department of physics where energy, under the form of heat, is being
studied. But this method has hardly as yet been attempted in questions
of meteorology; and owing to the complicated nature of the phenomena
with which the meteorologist has generally to deal, its application
will very often prove practically impossible. Nevertheless, it is
particularly suitable to all questions relating to the direct thermal
effects of currents, whatever the nature of these currents may happen
to be.
In the application of the method to an ocean-current, the two most
important elements required as data are the volume of the stream and
its mean temperature. But although we know something of the temperature
of most of the great ocean-currents, yet, with the exception of the
Gulf-stream, little has been ascertained regarding their volume.
The breadth, depth, and temperature of the Gulf-stream have formed the
subject of extensive and accurate observations by the United States
Coast Survey. In the memoirs and charts of that survey cross-sections
of the stream at various places are given, showing its breadth and
depth, and also the temperature of the water from the surface to the
bottom. We are thus enabled to determine with some precision the
mean temperature of the stream. And knowing its mean velocity at any
given section, we have likewise a means of determining the number of
cubic feet of water passing through that section in a given time. But
although we can obtain with tolerable accuracy the mean temperature,
yet observations regarding the velocity of the water at all depths have
unfortunately not been made at any particular section. Consequently we
have no means of estimating as accurately as we could wish the volume
of the current. Nevertheless, since we know the surface-velocity of the
water at places where some of the sections were taken, we are enabled
to make at least a rough estimate of the volume.
From an examination of the published sections, I came to the conclusion
some years ago[11] that the total quantity of water conveyed by the
stream is probably equal to that of a stream fifty miles broad and
1,000 feet deep,[12] flowing at the rate of four miles an hour,
and that the mean temperature of the entire mass of moving water is
not under 65° at the moment of leaving the Gulf. But to prevent the
possibility of any objections being raised on the grounds that I may
have over-estimated the volume of the stream, I shall take the velocity
to be _two_ miles instead of four miles an hour. We are warranted,
I think, in concluding that the stream before it returns from its
northern journey is on an average cooled down to at least 40°,[13]
consequently it loses 25° of heat. Each cubic foot of water, therefore,
in this case carries from the tropics for distribution upwards of
1,158,000 foot-pounds of heat. According to the above estimate of the
size and velocity of the stream, which in Chapter XI. will be shown
to be an under-estimate, 2,787,840,000,000 cubic feet of water are
conveyed from the Gulf per hour, or 66,908,160,000,000 cubic feet
daily. Consequently the total quantity of heat thus transferred per day
amounts to 77,479,650,000,000,000,000 foot-pounds.
This estimate of the volume of the stream is considerably less by
one-half than that given both by Captain Maury and by Sir John
Herschel. Captain Maury considers the Gulf-stream equal to a stream
thirty-two miles broad and 1,200 feet deep, flowing at the rate of five
knots an hour.[14] This gives 6,165,700,000,000 cubic feet per hour
as the quantity of water conveyed by this stream. Sir John Herschel’s
estimate is still greater. He considers it equal to a stream thirty
miles broad and 2,200 feet deep, flowing at the rate of four miles
an hour.[15] This makes the quantity 7,359,900,000,000 cubic feet
per hour. Dr. Colding, in his elaborate memoir on the Gulf-stream,
estimates the volume at 5,760,000,000,000 cubic feet per hour, while
Mr. Laughton’s estimate is nearly double that of mine.
From observations made by Sir John Herschel and by M. Pouillet on the
direct heat of the sun, it is found that, were no heat absorbed by the
atmosphere, about eighty-three foot-pounds per second would fall upon
a square foot of surface placed at right angles to the sun’s rays.[16]
Mr. Meech estimates that the quantity of heat cut off by the atmosphere
is equal to about twenty-two per cent. of the total amount received
from the sun. M. Pouillet estimates the loss at twenty-four per cent.
Taking the former estimate, 64·74 foot-pounds per second will therefore
be the quantity of heat falling on a square foot of the earth’s surface
when the sun is in the zenith. And were the sun to remain stationary in
the zenith for twelve hours, 2,796,768 foot-pounds would fall upon the
surface.
It can be shown that the total amount of heat received upon a unit
surface on the equator, during the twelve hours from sunrise till
sunset at the time of the equinoxes, is to the total amount which
would be received upon that surface, were the sun to remain in the
zenith during those twelve hours, as the diameter of a circle to half
its circumference, or as 1 to 1·5708. It follows, therefore, that
a square foot of surface on the equator receives from the sun at
the time of the equinoxes 1,780,474 foot-pounds daily, and a square
mile 49,636,750,000,000 foot-pounds daily. But this amounts to only
1/1560935th part of the quantity of heat daily conveyed from the
tropics by the Gulf-stream. In other words, the Gulf-stream conveys as
much heat as is received from the sun by 1,560,935 square miles at the
equator. The amount thus conveyed is equal to all the heat which falls
upon the globe within thirty-two miles on each side of the equator.
According to calculations made by Mr. Meech,[17] the annual quantity
of heat received by a unit surface on the frigid zone, taking the
mean of the whole zone, is 5·45/12th of that received at the equator;
consequently the quantity of heat conveyed by the Gulf-stream in one
year is equal to the heat which falls on an average on 3,436,900
square miles of the arctic regions. The frigid zone or arctic regions
contain 8,130,000 square miles. There is actually, therefore, nearly
one-half as much heat transferred from tropical regions by the
Gulf-stream as is received from the sun by the entire arctic regions,
the quantity conveyed from the tropics by the stream to that received
from the sun by the arctic regions being nearly as two to five.
But we have been assuming in our calculations that the percentage of
heat absorbed by the atmosphere is no greater in polar regions than
it is at the equator, which is not the case. If we make due allowance
for the extra amount absorbed in polar regions in consequence of the
obliqueness of the sun’s rays, the total quantity of heat conveyed by
the Gulf-stream will probably be nearly equal to one-half the amount
received from the sun by the entire arctic regions.
If we compare the quantity of heat conveyed by the Gulf-stream with
that conveyed by means of aërial currents, the result is equally
startling. The density of air to that of water is as 1 to 770, and
its specific heat to that of water is as 1 to 4·2; consequently the
same amount of heat that would raise 1 cubic foot of water 1° would
raise 770 cubic feet of air 4°·2, or 3,234 cubic feet 1°. The quantity
of heat conveyed by the Gulf-stream is therefore equal to that which
would be conveyed by a current of air 3,234 times the volume of the
Gulf-stream, at the same temperature and moving with the same velocity.
Taking, as before, the width of the stream at fifty miles, and its
depth at 1,000 feet, and its velocity at two miles an hour, it follows
that, in order to convey an equal amount of heat from the tropics by
means of an aërial current, it would be necessary to have a current
about 1¼ mile deep, and at the temperature of 65°, blowing at the
rate of two miles an hour from every part of the equator over the
northern hemisphere towards the pole. If its velocity were equal to
that of a good sailing-breeze, which Sir John Herschel states to be
about twenty-one miles an hour, the current would require to be above
600 feet deep. A greater quantity of heat is probably conveyed by the
Gulf-stream alone from the tropical to the temperate and arctic regions
than by all the aërial currents which flow from the equator.
We are apt, on the other hand, to over-estimate the amount of the heat
conveyed from tropical regions to us by means of aërial currents. The
only currents which flow from the equatorial regions are the upper
currents, or anti-trades as they are called. But it is not possible
that much heat can be conveyed directly by them. The upper currents of
the trade-winds, even at the equator, are nowhere below the snow-line;
they must therefore lie in a region of which the temperature is
actually below the freezing-point. In fact, if those currents were
warm, they would elevate the snow-line above themselves. The heated air
rising off the hot burning ground at the equator, after ascending a
few miles, becomes exposed to the intense cold of the upper regions of
the atmosphere; it then very soon loses all its heat, and returns from
the equator much colder than it went thither. It is impossible that
we can receive any heat directly from the equatorial regions by means
of aërial currents. It is perfectly true that the south-west wind, to
which we owe so much of our warmth in this country, is a continuation
of the anti-trade; but the heat which this wind brings to us is not
derived from the equatorial regions. This will appear evident, if we
but reflect that, before the upper current descends to the snow-line
after leaving the equator, it must traverse a space of at least 2,000
miles; and to perform this long journey several days will be required.
During all this time the air is in a region below the freezing-point;
and it is perfectly obvious that by the time it begins to descend it
must have acquired the temperature of the region in which it has been
travelling.
If such be the case, it is evident that a wind whose temperature
is below 32° could never warm a country such as ours, where the
temperature does not fall below 38° or 39°. The heat of our south-west
winds is derived, not directly from the equator, but from the warm
water of the Atlantic—in fact, from the Gulf-stream. The upper current
acquires its heat after it descends to the earth. There is one way,
however, whereby heat is indirectly conveyed from the equator by the
anti-trades; that is, in the form of aqueous vapour. In the formation
of one pound of water from aqueous vapour, as Professor Tyndall
strikingly remarks, a quantity of heat is given out sufficient to melt
five pounds of cast iron.[18] It must, however, be borne in mind that
the greater part of the moisture of the south-west and west winds is
derived from the ocean in temperate regions. The upper current receives
the greater part of its moisture after it descends to the earth, whilst
the moisture received at the equator is in great part condensed, and
falls as rain in those regions.
This latter assertion has been so frequently called in question
that I shall give my reasons for making it. According to Dr. Keith
Johnston (“Physical Atlas”) the mean rainfall of the torrid regions
is ninety-six inches per annum, while that of the temperate regions
amounts to only thirty-seven inches. If the greater part of the
moisture of the torrid regions does not fall as rain in those regions,
it must fall as such beyond them. Now the area of the torrid to that
of the two temperate regions is about as 39·3 to 51. Consequently
ninety-six inches of rain spread over the temperate regions would give
seventy-four inches; but this is double the actual rainfall of the
temperate regions. If, again, it were spread over both temperate and
polar regions this would yield sixty-four inches, which, however, is
nearly double the mean rainfall of the temperate and polar regions. If
we add to this the amount of moisture derived from the ocean within
temperate and polar regions, we should have a far greater rainfall for
these latitudes than for the torrid region, and we know, of course,
that it is actually far less. This proves the truth of the assertion
that by far the greater part of the moisture of the torrid regions
falls in those regions as rain. It will hardly do to object that the
above may probably be an over-estimate of the amount of rainfall in
the torrid zone, for it is not at all likely that any error will ever
be found which will affect the general conclusion at which we have
arrived.
Dr. Carpenter, in proof of the small rainfall of the torrid zone,
adduces the case of the Red Sea, where, although evaporation is
excessive, almost no rain falls. But the reason why the vapour raised
from the Red Sea does not fall in that region as rain, is no doubt
owing to the fact that this sea is only a narrow strip of water in a
dry and parched land, the air above which is too greedy of moisture
to admit of the vapour being deposited as rain. Over a wide expanse
of ocean, however, where the air above is kept to a great extent in a
constant state of saturation, the case is totally different.
_Land at the Equator tends to Lower the Temperature of the Globe._—The
foregoing considerations, as well as many others which might be stated,
lead to the conclusion that, in order to raise the mean temperature of
the whole earth, _water_ should be placed along the equator, and not
_land_, as is supposed by Sir Charles Lyell and others. For if land is
placed at the equator, the possibility of conveying the sun’s heat from
the equatorial regions by means of ocean-currents is prevented. The
transference of heat could then be effected only by means of the upper
currents of the trades; for the heat conveyed by _conduction_ along the
solid crust, if any, can have no sensible effect on climate. But these
currents, as we have just seen, are ill-adapted for conveying heat.
The surface of the ground at the equator becomes intensely heated by
the sun’s rays. This causes it to radiate its heat more rapidly into
space than a surface of water heated under the same conditions. Again,
the air in contact with the hot ground becomes also more rapidly
heated than in contact with water, and consequently the ascending
current of air carries off a greater amount of heat. But were the
heat thus carried away transferred by means of the upper currents to
high latitudes and there employed to warm the earth, then it might to
a considerable extent compensate for the absence of ocean-currents,
and in this case land at the equator might be nearly as well adapted
as water for raising the temperature of the whole earth. But such is
not the case; for the heat carried up by the ascending current at the
equator is not employed in warming the earth, but is thrown off into
the cold stellar space above. This ascending current, instead of being
employed in warming the globe, is in reality one of the most effectual
means that the earth has of getting quit of the heat received from the
sun, and of thus maintaining a much lower temperature than it would
otherwise possess. It is in the equatorial regions that the earth loses
as well as gains the greater part of its heat; so that, of all places,
here ought to be placed the substance best adapted for preventing the
dissipation of the earth’s heat into space, in order to raise the
general temperature of the earth. Water, of all substances in nature,
seems to possess this quality to the greatest extent; and, besides, it
is a fluid, and therefore adapted by means of currents to carry the
heat which it receives from the sun to every region of the globe.
These results show (although they have reference to only one stream)
that the general influence of ocean-currents on the distribution of
heat over the surface of the globe must be very great. If the quantity
of heat transferred from equatorial regions by the Gulf-stream
alone is nearly equal to all the heat received from the sun by the
arctic regions, then how enormous must be the quantity conveyed from
equatorial regions by all the ocean-currents together!
_Influence of the Gulf-stream on the Climate of Europe._—In a paper
read before the British Association at Exeter, Mr. A. G. Findlay
objects to the conclusions at which I have arrived in former papers
on the subject, that I have not taken into account the great length
of time that the water requires in order to circulate, and the
interference it has to encounter in its passage.
The objection is, that a stream so comparatively small as the
Gulf-stream, after spreading out over such a large area of the
Atlantic, and moving so slowly across to the shores of Europe, losing
heat all the way, would not be able to produce any very sensible
influence on the climate of Europe.
I am unable to perceive the force of this objection. Why, the very
efficiency of the stream as a heating agent necessarily depends upon
the slowness of its motion. Did the Gulf-stream move as rapidly along
its whole course as it does in the Straits of Florida, it could produce
no sensible effect on the climate of Europe. It does not require much
consideration to perceive this. (1) If the stream during its course
continued narrow, deep, and rapid, it would have little opportunity of
losing its heat, and the water would carry back to the tropics the heat
which it ought to have given off in the temperate and polar regions.
(2) The Gulf-stream does not heat the shores of Europe by direct
radiation. Our island, for example, is not heated by radiation from a
stream of warm water flowing along its shores. The Gulf-stream heats
our island _indirectly_ by heating the winds which blow over it to our
shores.
The anti-trades, or upper return-currents, as we have seen, bring no
heat from the tropical regions. After traversing some 2,000 miles
in a region of extreme cold they descend on the Atlantic as a cold
current, and there absorb the heat and moisture which they carry to
north-eastern Europe. Those aërial currents derive their heat from the
Gulf-stream, or if it is preferred, from the warm water poured into the
Atlantic by the Gulf-stream.
How, then, are these winds heated by the warm water? The air is heated
in two ways, viz., by direct _radiation_ from the water, and by
_contact_ with the water. Now, if the Gulf-stream continued a narrow
and deep current during its entire course similar to what it is at
the Straits of Florida, it could have little or no opportunity of
communicating its heat to the air either by radiation or by contact. If
the stream were only about forty or fifty miles in breadth, the aërial
particles in their passage across it would not be in contact with the
warm water more than an hour or two. Moreover, the number of particles
in contact with the water, owing to the narrowness of the stream,
would be small, and there would therefore be little opportunity for
the air becoming heated by contact. The same also holds true in regard
to radiation. The more we widen the stream and increase its area, the
more we increase its radiating surface; and the greater the radiating
surface, the greater is the quantity of heat thrown off. But this is
not all; the number of aërial particles heated by radiation increases
in proportion to the area of the radiating surface; consequently, the
wider the area over which the waters of the Gulf-stream are spread,
the more effectual will the stream be as a heating agent. And, again,
in order that a very wide area of the Atlantic may be covered with the
warm waters of the stream, slowness of motion is essential.
Mr. Findlay supposes that fully one-half of the Gulf-stream passes into
the south-eastern branch, and that it is only the north-eastern branch
of the current that can be effectual in raising the temperature of
Europe. But it appears to me that it is to this south-eastern portion
of the current, and not to the north-eastern, that we, in this country,
are chiefly indebted for our heat. The south-west winds, to which we
owe our heat, derive their temperature from this south-eastern portion
which flows away in the direction of the Azores. The south-west winds
which blow over the northern portion of the current which flows past
our island up into the arctic seas cannot possibly cross this country,
but will go to heat Norway and northern Europe. The north-eastern
portion of the stream, no doubt, protects us from the ice of Greenland
by warming the north-west winds which come to us from that cold region.
Mr. Buchan, Secretary of the Scottish Meteorological Society, has
shown[19] that in a large tract of the Atlantic between latitudes 20°
and 40° N., the mean pressure of the atmosphere is greater than in any
other place on the globe. To the west of Madeira, between longitude
10° and 40° W., the mean annual pressure amounts to 30·2 inches, while
between Iceland and Spitzbergen it is only 29·6, a lower mean pressure
than is found in any other place on the northern hemisphere. There
must consequently, he concludes, be a general tendency in the air to
flow from the former to the latter place along the earth’s surface.
Now, the air in moving from the lower to the higher latitudes tends
to take a north-easterly direction, and in this case will pass over
our island in its course. This region of high pressure, however,
is situated in the very path of the south-eastern branch of the
Gulf-stream, and consequently the winds blowing therefrom will carry
directly to Britain the heat of the Gulf-stream.
As we shall presently see, it is as essential to the heating of our
island as to that of the southern portion of Europe, that a very large
proportion of the waters of the Gulf-stream should spread over the
surface of the Atlantic and never pass up into the arctic regions.
Even according to Mr. Findlay’s own theory, it is to the south-west
wind, heated by the warm waters of the Atlantic, that we are indebted
for the high temperature of our climate. But he seems to be under the
impression that the Atlantic would be able to supply the necessary
heat independently of the Gulf-stream. This, it seems to me, is the
fundamental error of all those who doubt the efficiency of the stream.
It is a mistake, however, into which one is very apt to fall who does
not adopt the more rigid method of determining heat-results in absolute
measure. When we apply this method, we find that the Atlantic, without
the aid of such a current as the Gulf-stream, would be wholly unable to
supply the necessary amount of heat to the south-west winds.
The quantity of heat conveyed by the Gulf-stream, as we have seen,
is equal to all the heat received from the sun by 1,560,935 square
miles at the equator. The mean annual quantity of heat received from
the sun by the temperate regions per unit surface is to that received
by the equator as 9·08 to 12.[20] Consequently, the quantity of heat
conveyed by the stream is equal to all the heat received from the sun
by 2,062,960 square miles of the temperate regions. The total area of
the Atlantic from the latitude of the Straits of Florida, 200 miles
north of the tropic of Cancer, up to the Arctic Circle, including also
the German Ocean, is about 8,500,000 square miles. In this case the
quantity of heat carried by the Gulf-stream into the Atlantic through
the Straits of Florida, is to that received by this entire area from
the sun as 1 to 4·12, or in round numbers as 1 to 4. It therefore
follows that one-fifth of all the heat possessed by the waters of the
Atlantic over that area, even supposing that they absorb every ray that
falls upon them, is derived from the Gulf-stream. Would those who call
in question the efficiency of the Gulf-stream be willing to admit that
a decrease of one-fourth in the total amount of heat received from the
sun, over the entire area of the Atlantic from within 200 miles of
the tropical zone up to the arctic regions, would not sensibly affect
the climate of northern Europe? If they would not willingly admit
this, why, then, contend that the Gulf-stream does not affect climate?
for the stoppage of the Gulf-stream would deprive the Atlantic of
77,479,650,000,000,000,000 foot-pounds of energy in the form of heat
per day, a quantity equal to one-fourth of all the heat received from
the sun by that area.
How much, then, of the temperature of the south-west winds derived from
the water of the Atlantic is due to the Gulf-stream?
Were the sun extinguished, the temperature over the whole earth
would sink to _nearly_ that of stellar space, which, according to
the investigations of Sir John Herschel[21] and of M. Pouillet,[22]
is not above −239° F. Were the earth possessed of no atmosphere, the
temperature of its surface would sink to exactly that of space, or to
that indicated by a thermometer exposed to no other heat-influence than
that of radiation from the stars. But the presence of the atmospheric
envelope would slightly modify the conditions of things; for the
heat from the stars (which of course constitutes what is called the
temperature of space) would, like the sun’s heat, pass more freely
through the atmosphere than the heat radiated back from the earth, and
there would in consequence of this be an accumulation of heat on the
earth’s surface. The temperature would therefore stand a little higher
than that of space; or, in other words, it would stand a little higher
than it would otherwise do were the earth exposed in space to the
direct radiation of the stars without the atmospheric envelope. But,
for reasons which will presently be stated, we may in the meantime,
till further light is cast upon this matter, take -239° F. as probably
not far from what would be the temperature of the earth’s surface were
the sun extinguished.
Suppose now that we take the mean annual temperature of the Atlantic
at, say, 56°.[23] Then 239° + 56° = 295° represents the number of
degrees of rise due to the heat which it receives. In other words,
it takes all the heat that the Atlantic receives to maintain its
temperature 295° above the temperature of space. Stop the Gulf-stream,
and the Atlantic would be deprived of one-fifth of the heat which
it possesses. Then, if it takes five parts of heat to maintain a
temperature of 295° above that of space, the four parts which would
remain after the stream was stopped would only be able to maintain a
temperature of four-fifths of 295°, or 236° above that of space: the
stoppage of the Gulf-stream would therefore deprive the Atlantic of an
amount of heat which would be sufficient to maintain its temperature
59° above what it would otherwise be, did it depend alone upon the heat
received directly from the sun. It does not, of course, follow that
the Gulf-stream actually maintains the temperature 59° above what it
would otherwise be were there no ocean-currents, because the actual
heating-effect of the stream is neutralized to a very considerable
extent by cold currents from the arctic regions. But 59° of rise
represents its actual power; consequently 59°, minus the lowering
effect of the cold currents, represents the actual rise. What the rise
may amount to at any particular place must be determined by other means.
This method of calculating how much the temperature of the earth’s
surface would rise or fall from an increase or a decrease in the
absolute amount of heat received is that adopted by Sir John Herschel
in his “Outlines of Astronomy,” § 369^a.
About three years ago, in an article in the _Reader_, I endeavoured
to show that this method is not rigidly correct. It has been shown
from the experiments of Dulong and Petit, Dr. Balfour Stewart,
Professor Draper, and others, that the rate at which a body radiates
its heat off into space is not directly proportionate to its absolute
temperature. The rate at which a body loses its heat as its temperature
rises increases more rapidly than the temperature. As a body rises
in temperature the rate at which it radiates off its heat increases;
the _rate_ of this increase, however, is not uniform, but increases
with the temperature. Consequently the temperature is not lowered in
proportion to the decrease of the sun’s heat. But at the comparatively
low temperature with which we have at present to deal, the error
resulting from assuming the decrease of temperature to be proportionate
to the decrease of heat would not be great.
It may be remarked, however, that the experiments referred to were
made on solids; but, from certain results arrived at by Dr. Balfour
Stewart, it would seem that the radiation of a material particle may
be proportionate to its absolute temperature.[24] This physicist found
that the radiation of a thick plate of glass increases more rapidly
than that of a thin plate as the temperature rises, and that, if we go
on continually diminishing the thickness of the plate whose radiation
at different temperatures we are ascertaining, we find that as it grows
thinner and thinner the rate at which it radiates off its heat as its
temperature rises becomes less and less. In other words, as the plate
grows thinner and thinner its rate of radiation becomes more and more
proportionate to its absolute temperature. And we can hardly resist the
conviction that if we could possibly go on diminishing the thickness
of the plate till we reached a film so thin as to embrace but only one
particle in its thickness, its rate of radiation would be proportionate
to its temperature. Dr. Balfour Stewart has very ingeniously suggested
the probable reason why the rate of radiation of thick plates increases
with rise of temperature more rapidly than that of thin. It is this:
all substances are more diathermanous for heat of high temperatures
than for heat of low temperatures. When a body is at a low temperature,
we may suppose that only the exterior rows of particles supply the
radiation, the heat from the interior particles being all stopped by
the exterior ones, the substance being very opaque for heat of low
temperature; while at a high temperature we may imagine that part
of the heat from the interior particles is allowed to pass, thereby
swelling the total radiation. But as the plate becomes thinner and
thinner, the obstructions to interior radiation become less and less,
and as these obstructions are greater for radiation at low temperatures
than for radiation at high temperatures, it necessarily follows that,
by reducing the thickness of the plate, we assist radiation at low
temperatures more than we do at high.
In a gas, where each particle may be assumed to radiate by itself, and
where the particles stand at a considerable distance from one another,
the obstruction to interior radiation must be far less than in a
solid. In this case the rate at which a gas radiates off its heat as
its temperature rises must increase more slowly than that of a solid
substance. In other words, its rate of radiation must correspond more
nearly to its absolute temperature than that of a solid. If this be the
case, a reduction in the amount of heat received from the sun, owing to
an increase of his distance, should tend to produce a greater lowering
effect on the temperature of the air than it does on the temperature of
the solid ground. But as the temperature of our climate is determined
by the temperature of the air, it must follow that the error of
assuming that the decrease of temperature would be proportionate to the
decrease in the intensity of the sun’s heat may not be great.
It may be observed here, although it does not bear directly on this
point, that although the air in a room, for example, or at the earth’s
surface is principally cooled by convection rather than by radiation,
yet it is by radiation alone that the earth’s atmosphere parts with its
heat to stellar space; and this is the chief matter with which we are
at present concerned. Air, like all other gases, is a bad radiator;
and this tends to protect it from being cooled to such an extent as it
would otherwise be, were it a good radiator like solids. True, it is
also a bad absorber; but as it is cooled by radiation into space, and
heated, not altogether by absorption, but to a very large extent by
convection, it on the whole gains its heat more easily than it loses
it, and consequently must stand at a higher temperature than it would
do were it heated by absorption alone.
But, to return; the error of regarding the decrease of temperature
as proportionate to the decrease in the amount of heat received, is
probably neutralized by one of an opposite nature, viz., that of taking
space at too high a temperature; for by so doing we make the result too
small.
We know that absolute zero is at least 493° below the melting-point
of ice. This is 222° below that of space. Consequently, if the heat
derived from the stars is able to maintain a temperature of −239°,
or 222° of absolute temperature, then nearly as much heat is derived
from the stars as from the sun. But if so, why do the stars give so
much heat and so very little light? If the radiation from the stars
could maintain a thermometer 222° above absolute zero, then space must
be far more transparent to heat-rays than to light-rays, or else the
stars give out a great amount of heat, but very little light, neither
of which suppositions is probably true. The probability is, I venture
to presume, that the temperature of space is not very much above
absolute zero. At the time when these investigations into the probable
temperature of space were made, at least as regards the labours of
Pouillet, the modern science of heat had no existence, and little or
nothing was then known with certainty regarding absolute zero. In this
case the whole matter would require to be reconsidered. The result of
such an investigation in all probability would be to assign a lower
temperature to stellar space than −239°.
Taking all these various considerations into account, it is probable
that if we adopt −239° as the temperature of space, we shall not be far
from the truth in assuming that the absolute temperature of a place
above that of space is proportionate to the amount of heat received
from the sun.
We may, therefore, in this case conclude that 59° of rise is probably
not very far from the truth, as representing the influence of the
Gulf-stream. The Gulf-stream, instead of producing little or no effect,
produces an effect far greater than is generally supposed.
Our island has a mean annual temperature of about 12° above the normal
due to its latitude. This excess of temperature has been justly
attributed to the influence of the Gulf-stream. But it is singular
how this excess should have been taken as the measure of the _rise
resulting from the influence of the stream_. These figures only
represent the number of degrees that the mean normal temperature of
our island stands above what is called the normal temperature of the
latitude.
The mode in which Professor Dove constructed his Tables of normal
temperature was as follows:—He took the temperature of thirty-six
equidistant points on every ten degrees of latitude. The mean
temperature of these thirty-six points he calls in each case the
_normal_ temperature of the parallel. The excess above the normal
merely represents how much the stream raises our temperature above
the mean of all places on the same latitude, but it affords us no
information regarding the absolute rise produced. In the Pacific, as
well as in the Atlantic, there are immense masses of water flowing
from the tropical to the temperate regions. Now, unless we know how
much of the normal temperature of a latitude is due to ocean-currents,
and how much to the direct heat of the sun, we could not possibly,
from Professor Dove’s Tables, form the most distant conjecture as
to how much of our temperature is derived from the Gulf-stream. The
overlooking of this fact has led to a general misconception regarding
the positive influence of the Gulf-stream on temperature. The 12°
marked in Tables of normal temperature do not represent the absolute
effect of the stream, but merely show how much the stream raises the
temperature of our country above the mean of all places on the same
latitude. Other places have their temperature raised by ocean-currents
as well as this country; only the Gulf-stream produces a rise of
several degrees over and above that produced by other streams in the
same latitude.
At present there is a difference merely of 80° between the mean
temperature of the equator and the poles;[25] but were each part of the
globe’s surface to depend only upon the direct heat which it receives
from the sun, there ought, according to theory, to be a difference of
more than 200°. The annual quantity of heat received at the equator is
to that received at the poles (supposing the proportionate quantity
absorbed by the atmosphere to be the same in both cases) as 12 to 4·98,
or, say, as 12 to 5. Consequently, if the temperatures of the equator
and the poles be taken as proportionate to the absolute amount of heat
received from the sun, then the temperature of the equator above that
of space must be to that of the poles above that of space as 12 to 5.
What ought, therefore, to be the temperatures of the equator and the
poles, did each place depend solely upon the heat which it receives
directly from the sun? Were all ocean and aërial currents stopped,
so that there could be no transference of heat from one part of the
earth’s surface to another, what ought to be the temperatures of the
equator and the poles? We can at least arrive at a rough estimate
on this point. If we diminish the quantity of warm water conveyed
from the equatorial regions to the temperate and arctic regions, the
temperature of the equator will begin to rise, and that of the poles
to sink. It is probable, however, that this process would affect the
temperature of the poles more than it would that of the equator; for as
the warm water flows from the equator to the poles, the area over which
it is spread becomes less and less. But as the water from the tropics
has to raise the temperature of the temperate regions as well as the
polar, the difference of effect at the equator and poles might not, on
that account, be so very great. Let us take a rough estimate. Say that,
as the temperature of the equator rises one degree, the temperature of
the poles sinks one degree and a half. The mean annual temperature of
the globe is about 58°. The mean temperature of the equator is 80°, and
that of the poles 0°. Let ocean and aërial currents now begin to cease,
the temperature of the equator commences to rise and the temperature
of the poles to sink. For every degree that the temperature of the
equator rises, that of the poles sinks 1½°; and when the currents are
all stopped and each place becomes dependent solely upon the direct
rays of the sun, the mean annual temperature of the equator above that
of space will be to that of the poles, above that of space, as 12 to
5. When this proportion is reached, the equator will be 374° above
that of space, and the poles 156°; for 374 is to 156 as 12 is to 5.
The temperature of space we have seen to be −239°, consequently the
temperature of the equator will in this case be 135°, reckoned from the
zero of the Fahrenheit thermometer, and the poles 83° below zero. The
equator would therefore be 55° warmer than at present, and the poles
83° colder. The difference between the temperature of the equator and
the poles will in this case amount to 218°.
Now, if we take into account the quantity of positive energy in the
form of heat carried by warm currents from the equator to the temperate
and polar regions, and also the quantity of negative energy (cold)
carried by cold currents from the polar regions to the equator, we
shall find that they are sufficient to reduce the difference of
temperature between the poles and the equator from 218° to 80°.
The quantity of heat received in the latitude of London, for example,
is to that received at the equator nearly as 12 to 8. This, according
to theory, should produce a difference of about 125°. The temperature
of the equator above that of space, as we have seen, would be 374°.
Therefore 249° above that of space would represent the temperature
of the latitude of London. This would give 10° as its temperature.
The stoppage of all ocean and aërial currents would thus increase the
difference between the equator and the latitude of London by about
85°. The stoppage of ocean-currents would not be nearly so much felt,
of course, in the latitude of London as at the equator and the poles,
because, as has been already noticed, in all latitudes midway between
the equator and the poles the two sets of currents to a considerable
extent compensate each other—the warm currents from the equator
raise the temperature, while the cold ones from the poles lower it;
but as the warm currents chiefly keep on the surface and the cold
return-currents are principally under-currents, the heating effect very
greatly exceeds the cooling effect. Now, as we have seen, the stoppage
of all currents would raise the temperature of the equator 55°; that
is to say, the rise at the equator alone would increase the difference
of temperature between the equator and that of London by 55°. But the
actual difference, as we have seen, ought to be 85°; consequently the
temperature of London would be lowered 30° by the stoppage of the
currents. For if we raise the temperature of the equator 55° and lower
the temperature of London 30°, we then increase the difference by
85°. The normal temperature of the latitude of London being 40°, the
stoppage of all ocean and aërial currents would thus reduce it to 10°.
But the Gulf-stream raises the actual mean temperature of London 10°
above the normal. Consequently 30° + 10° = 40° represents the actual
rise at London due to the influence of the Gulf-stream over and above
all the lowering effects resulting from arctic currents. On some parts
of the American shores on the latitude of London, the temperature is
10° below the normal. The stoppage of all ocean and aërial currents
would therefore lower the temperature there only 20°.
It is at the equator and the poles that the great system of ocean and
aërial currents produces its maximum effects. The influence becomes
less and less as we recede from those places, and between them there
is a point where the influence of warm currents from the equator and
of cold currents from the poles exactly neutralize each other. At
this point the stoppage of ocean-currents would not sensibly affect
temperature. This point, of course, is not situated on the same
latitude in all meridians, but varies according to the position of the
meridian in relation to land, and ocean-currents, whether cold or hot,
and other circumstances. A line drawn round the globe through these
various points would be very irregular. At one place, such as on the
western side of the Atlantic, where the arctic current predominates,
the neutral line would be deflected towards the equator, while on
the eastern side, where warm currents predominate, the line would be
deflected towards the north. It is a difficult problem to determine the
mean position of this line; it probably lies somewhere not far north of
the tropics.
CHAPTER III.
OCEAN-CURRENTS IN RELATION TO THE DISTRIBUTION OF HEAT OVER THE
GLOBE.—(_Continued._)
Influence of the Gulf-stream on the Climate of the Arctic
Regions.—Absolute Amount of Heat received by the Arctic
Regions from the Sun.—Influence of Ocean-currents shown by
another Method.—Temperature of a Globe all Water or all
Land according to Professor J. D. Forbes.—An important
Consideration overlooked.—Without Ocean-currents the
Globe would not be habitable.—Conclusions not affected by
Imperfection of Data.
_Influence of the Gulf-stream on the Climate of the Arctic
Regions._—Does the Gulf-stream pass into the arctic regions? Are the
seas around Spitzbergen and North Greenland heated by the warm water of
the stream?
Those who deny this nevertheless admit the existence of an arctic
current. They admit that an immense mass of cold water is continually
flowing south from the polar regions around Greenland into the
Atlantic. If it be admitted, then, that a mass of water flows across
the arctic circle from north to south, it must also be admitted that an
equal mass flows across from south to north. It is also evident that
the water crossing from south to north must be warmer than the water
crossing from north to south; for the temperate regions are warmer than
the arctic, and the ocean in temperate regions warmer than the ocean in
the arctic; consequently the current which flows into the arctic seas,
to compensate for the cold arctic current, must be a warmer current.
Is the Gulf-stream this warm current? Does this compensating warm
current proceed from the Atlantic or from the Pacific? If it proceeds
from the Atlantic, it is simply the warm water of the Gulf-stream.
We may call it the warm water of the Atlantic if we choose; but this
cannot materially affect the question at issue, for the heat which
the waters of the Atlantic possess is derived, as we have seen, to
an enormous extent from the water brought from the tropics by the
Gulf-stream. If we deny that the warm compensating current comes from
the Atlantic, then we must assume that it comes from the Pacific. But
if the cold current flows from the arctic regions into the Atlantic,
and the warm compensating current from the Pacific into the arctic
regions, the highest temperature should be found on the Pacific side of
the arctic regions and not on the Atlantic side; the reverse, however,
is the case. In the Atlantic, for example, the 41° isothermal line
reaches to latitude 65°30′, while in the Pacific it nowhere goes beyond
latitude 57°. The 27° isotherm reaches to latitude 75° in the Atlantic,
but in the Pacific it does not pass beyond 64°. And the 14° isotherm
reaches the north of Spitzbergen in latitude 80°, whereas on the
Pacific side of the arctic regions it does not reach to latitude 72°.
On no point of the earth’s surface does the mean annual temperature
rise so high above the normal as in the northern Atlantic, just at
the arctic circle, at a spot believed to be in the middle of the
Gulf-stream. This place is no less than 22°·5 above the normal, while
in the northern Pacific the temperature does not anywhere rise more
than 9° above the normal. These facts prove that the warm current
passes up the Atlantic into the arctic regions and not up the Pacific,
or at least that the larger amount of warm water must pass into the
arctic regions through the Atlantic. In other words, the Gulf-stream is
the warm compensating current. Not only must there be a warm stream,
but one of very considerable magnitude, in order to compensate for the
great amount of cold water that is constantly flowing from the arctic
regions, and also to maintain the temperature of those regions so much
above the temperature of space as they actually are.
No doubt, when the results of the late dredging expedition are
published, they will cast much additional light on the direction and
character of the currents forming the north-eastern branch of the
Gulf-stream.
The average quantity of heat received by the arctic regions as a whole
per unit surface to that received at the equator, as we have already
seen, is as 5·45 to 12, assuming that the percentage of rays cut off by
the atmosphere is the same at both places. In this case the mean annual
temperature of the arctic regions, taken as a whole, would be about
−69°, did those regions depend entirely for their temperature upon the
heat received directly from the sun. But the temperature would not even
reach to this; for the percentage of rays cut off by the atmosphere in
arctic regions is generally believed to be greater than at the equator,
and consequently the actual mean quantity of heat received by the
arctic regions will be less than 5·45−12ths of what is received at the
equator.
In the article on Climate in the “Encyclopædia Britannica” there is
a Table calculated upon the principle that the quantity of heat cut
off is proportionate to the number of aërial particles which the rays
have to encounter before reaching the surface of the earth—that, as
a general rule, if the tracts of the rays follow an arithmetical
progression, the diminished force with which the rays reach the ground
will form a decreasing geometrical progression. According to this Table
about 75 per cent. of the sun’s rays are cut off by the atmosphere
in arctic regions. If 75 per cent. of the rays were cut off by the
atmosphere in arctic regions, then the direct rays of the sun could
not maintain a mean temperature 100° above that of space. But this is
no doubt much too high a percentage for the quantity of heat cut off;
for recent discoveries in regard to the absorption of radiant heat by
gases and vapours prove that Tables computed on this principle must be
incorrect. The researches of Tyndall and Melloni show that when rays
pass through any substance, the absorption is rapid at first: but the
rays are soon “sifted,” as it is called, and they then pass onwards
with but little further obstruction. Still, however, owing to the dense
fogs which prevail in arctic regions, the quantity of heat cut off
must be considerable. If as much as 50 per cent. of the sun’s rays
are cut off by the atmosphere in arctic regions, the amount of heat
received directly from the sun would not be sufficient to maintain a
mean annual temperature of −100°. Consequently the arctic regions must
depend to an enormous extent upon ocean-currents for their temperature.
_Influence of Ocean-currents shown by another Method._—That the
temperature of the arctic regions would sink enormously, and the
temperature of the equator rise enormously, were all ocean-currents
stopped, can be shown by another method—viz., by taking the mean annual
temperature from the equator to the pole along a meridian passing
through the ocean, say, the Atlantic, and comparing it with the mean
annual temperature taken along a meridian passing through a great
continent, say, the Asiatic.
Professor J. D. Forbes, in an interesting memoir,[26] has endeavoured
by this method to determine what would be the temperature of the
equator and the poles were the globe all water or all land. He has
taken the temperature of the two meridians from the tables and charts
of Professor Dove, and ascertained the exact proportion of land and
water on every 10° of latitude from the equator to the poles, with the
view of determining what proportion of the average temperature of the
globe in each parallel is due to the land, and what to the water which
respectively belongs to it. He next endeavours to obtain a formula for
expressing the mean temperature of a given parallel, and thence arrives
at “an approximate answer to the inquiry as to what would have been the
equatorial or polar temperature of the globe, or that of any latitude,
had its surface been entirely composed of land or of water.”
The result at which he arrived is this: that, were the surface of
the globe all water, 71°·7 would be the temperature of the equator,
and 12°·5 the temperature of the poles; and were the surface all
land, 109°·8 would be the temperature of the equator, and −25°·6 the
temperature of the poles.
But in Professor Forbes’s calculations no account whatever is taken
of the influence of currents, whether of water or of air, and the
difference of temperature is attributed wholly to difference of
latitude and the physical properties of land and water in relation to
their powers in absorbing and detaining the sun’s rays, and to the laws
of conduction and of convection which regulate the internal motion of
heat in the one and in the other. He considers that the effects of
currents are all compensatory.
“If a current of hot water,” he says, “moderates the cold of a Lapland
winter, the counter-current, which brings the cold of Greenland to the
shores of the United States, in a great measure restores the balance of
temperature, so far as it is disturbed by this particular influence.
The prevalent winds, in like manner, including the trade-winds, though
they render some portions of continents, on the average, hotter or
colder than others, produce just the contrary effect elsewhere. Each
continent, if it has a cold eastern shore, has likewise a warm western
one; and even local winds have for the most part established laws of
compensation. In a given parallel of latitude all these secondary
causes of local climate may be imagined to be mutually compensatory,
and the outstanding gradation of mean or normal temperature will
mainly depend, 1st, upon the effect of latitude simply; 2nd, on
the distribution of land and water considered in their primary or
_statical_ effect.”
It is singular that a physicist so acute as Professor Forbes should,
in a question such as this, leave out of account the influence of
currents, under the impression that their effects were compensatory.
If there is a constant transference of hot water from the equatorial
regions to the polar, and of cold water from the polar regions to the
equatorial (a thing which Professor Forbes admitted), then there can
only be one place between the equator and the pole where the two sets
of currents compensate each other. At all places on the equatorial
side of this point a cooling effect is the result. Starting from this
neutral point, the preponderance of the cooling effect over the heating
increases as we approach towards the equator, and the preponderance of
the heating effect over the cooling increases as we recede from this
point towards the pole—the cooling effect reaching a maximum at the
equator, and the heating effect a maximum at the pole.
Had Professor Forbes observed this important fact, he would have
seen at once that the low temperature of the land in high latitudes,
in comparison with that of the sea, was no index whatever as to
how much the temperature of those regions would sink were the sea
entirely removed and the surface to become land; for the present
high temperature of the sea is not due wholly to the mere physical
properties of water, but to a great extent is due to the heat brought
by currents from the equator. Now, unless it is known how much of
the absolute temperature of the ocean in those latitudes is due to
currents, we cannot tell how much the removal of the sea would lower
the absolute temperature of those places. Were the sea removed,
the continents in high latitudes would not simply lose the heating
advantages which they presently derive from the mere fact of their
proximity to so much sea, but the removal would, in addition to this,
deprive them of an enormous amount of heat which they at present
receive from the tropics by means of ocean-currents. And, on the other
hand, at the equator, were the sea removed, the continents there
would not simply lose the cooling influences which result from their
proximity to so much water, but, in addition to this, they would have
to endure the scorching effects which would result from the heat which
is at present carried away from the tropics by ocean-currents.
We have already seen that Professor Forbes concluded that the
removal of the sea would raise the mean temperature of the equator
30°, and lower the temperature of the poles 28°; it is therefore
perfectly certain that, had he added to his result the effect due to
ocean-currents, and had he been aware that about one-fifth of all the
heat possessed by the Atlantic is actually derived from the equator by
means of the Gulf-stream, he would have assigned a temperature to the
equator and the poles, of a globe all land, differing not very far from
what I have concluded would be the temperature of those places were all
ocean and aërial currents stopped, and each place to depend solely upon
the heat which it received directly from the sun.
_Without Ocean-currents the Globe would not be habitable._—All these
foregoing considerations show to what an extent the climatic condition
of our globe is due to the thermal influences of ocean-currents.
As regards the northern hemisphere, we have two immense oceans, the
Pacific and the Atlantic, extending from the equator to near the north
pole, or perhaps to the pole altogether. Between these two oceans lie
two great continents, the eastern and the western. Owing to the earth’s
spherical form, far too much heat is received at the equator and far
too little at high latitudes to make the earth a suitable habitation
for sentient beings. The function of these two great oceans is to
remove the heat from the equator and carry it to temperate and polar
regions. Aërial currents could not do this. They might remove the heat
from the equator, but they could not, as we have already seen, carry
it to the temperate and polar regions; for the greater portion of the
heat which aërial currents remove from the equator is dissipated into
stellar space: the ocean alone can convey the heat to distant shores.
But aërial currents have a most important function; for of what avail
would it be, though ocean-currents should carry heat to high latitudes,
if there were no means of distributing the heat thus conveyed over the
land? The function of aërial currents is to do this. Upon this twofold
arrangement depends the thermal condition of the globe. Exclude the
waters of the Pacific and the Atlantic from temperate and polar regions
and place them at the equator, and nothing now existing on the globe
could live in high latitudes.
Were these two great oceans placed beside each other on one side of the
globe, and the two great continents placed beside each other on the
other side, the northern hemisphere would not then be suitable for the
present order of things: the land on the central and on the eastern
side of the united continent would be far too cold.
_The foregoing Conclusions not affected by the Imperfection of the
Data._—The general results at which we have arrived in reference to the
influence of ocean-currents on the climatic condition of the globe are
not affected by the imperfection of the data employed. It is perfectly
true that considerable uncertainty prevails regarding some of the data;
but, after making the fullest allowance for every possible error, the
influence of currents is so enormous that the general conclusion cannot
be materially affected. I can hardly imagine that any one familiar
with the physics of the subject will be likely to think that, owing to
possible errors in the data, the effects have probably been doubled.
Even admitting, however, that this were proved to be the case, still
that would not materially alter the general conclusion at which we
have arrived. The influence of ocean-currents in the distribution of
heat over the surface of the globe would still be admittedly enormous,
whether we concluded that owing to them the present temperature of the
equator is 55° or 27° colder than it would otherwise be, or the poles
83° or 41° hotter than they would be did no currents exist.
Nay, more, suppose we should again halve the result; even in that case
we should have to admit that, owing to ocean-currents, the equator
is about 14° colder and the poles about 21° hotter than they would
otherwise be; in other words, we should have to admit that, were it not
for ocean-currents, the mean temperature of the equator would be about
100° and the mean temperature of the poles about −21°.
If the influence of ocean-currents in reducing the difference between
the temperature of the equator and poles amounted to only a few
degrees, it would of course be needless to put much weight on any
results arrived at by the method of calculation which I have adopted;
but when it is a matter of two hundred degrees, it is not at all likely
that the general results will be very much affected by any errors which
may ever be found in the data.
Objections of a palæontological nature have frequently been urged
against the opinion that our island is much indebted for its mild
climate to the influence of the Gulf-stream; but, from what has already
been stated, it must be apparent that all objections of that nature
are of little avail. The palæontologist may detect, from the character
of the flora and fauna brought up from the sea-bottom by dredging and
other means, the presence of a warm or of a cold current; but this can
never enable him to prove that the temperate and polar regions are
not affected to an enormous extent by warm water conveyed from the
equatorial regions. For anything that palæontology can show to the
contrary, were ocean-currents to cease, the mean annual temperature
of our island might sink below the present midwinter temperature of
Siberia. What would be the thermal condition of our globe were there no
ocean-currents is a question for the physicist; not for the naturalist.
CHAPTER IV.
OUTLINE OF THE PHYSICAL AGENCIES WHICH LEAD TO SECULAR CHANGES
OF CLIMATE.
Eccentricity of the Earth’s Orbit; its Effect on
Climate.—Glacial Epoch not the direct Result of an
Increase of Eccentricity.—An important Consideration
overlooked.—Change of Eccentricity affects Climate only
indirectly.—Agencies which are brought into Operation
by an Increase of Eccentricity.—How an Accumulation
of Snow is produced.—The Effect of Snow on the Summer
Temperature.—Reason of the low Summer Temperature of Polar
Regions.—Deflection of Ocean-currents the chief Cause of
secular Changes of Climate.—How the foregoing Causes deflect
Ocean-currents.—Nearness of the Sun in Perigee a Cause of
the Accumulation of Ice.—A remarkable Circumstance regarding
the Causes which lead to secular Changes of Climate.—The
primary Cause an Increase of Eccentricity.—Mean Temperature
of whole Earth should be greater in Aphelion than in
Perihelion.—Professor Tyndall on the Glacial Epoch.—A
general Reduction of Temperature will not produce a Glacial
Epoch.—Objection from the present Condition of the Planet
Mars.
_Primary cause of Change of Eccentricity of the Earth’s Orbit._—There
are two causes affecting the position of the earth in relation to
the sun, which must, to a very large extent, influence the earth’s
climate; viz., the precession of the equinoxes and the change in the
eccentricity of the earth’s orbit. If we duly examine the combined
influence of these two causes, we shall find that the northern and
southern portions of the globe are subject to an excessively slow
secular change of climate, consisting in a slow periodic change of
alternate warmer and colder cycles.
According to the calculations of Leverrier, the superior limit of the
earth’s eccentricity is 0·07775.[27] The eccentricity is at present
diminishing, and will continue to do so during 23,980 years, from the
year 1800 A.D., when its value will be then ·00314.
The change in the eccentricity of the earth’s orbit may affect
the climate in two different ways; viz., by either increasing or
diminishing the mean annual amount of heat received from the sun, or
by increasing or diminishing the difference between summer and winter
temperature.
Let us consider the former case first. The total quantity of heat
received from the sun during one revolution is inversely proportional
to the minor axis.
The difference of the minor axis of the orbit when at its maximum and
its minimum state of eccentricity is as 997 to 1000. This small amount
of difference cannot therefore sensibly affect the climate. Hence we
must seek for our cause in the second case under consideration.
There is of course as yet some little uncertainty in regard to the
exact mean distance of the sun. I shall, however, in the present volume
assume it to be 91,400,000 miles. When the eccentricity is at its
superior limit, the distance of the sun from the earth, when the latter
is in the aphelion of its orbit, is no less than 98,506,350 miles;
and when in the perihelion it is only 84,293,650 miles. The earth is
therefore 14,212,700 miles further from the sun in the former position
than in the latter. The direct heat of the sun being inversely as the
square of the distance, it follows that the amount of heat received
by the earth when in these two positions will be as 19 to 26. Taking
the present eccentricity to be ·0168, the earth’s distance during
winter, when nearest to the sun, is 89,864,480 miles. Suppose now that,
according to the precession of the equinoxes, winter in our northern
hemisphere should happen when the earth is in the aphelion of its
orbit, at the time when the orbit is at its greatest eccentricity; the
earth would then be 8,641,870 miles further from the sun in winter than
at present. The direct heat of the sun would therefore be one-fifth
less during that season than at present; and in summer one-fifth
greater. This enormous difference would affect the climate to a very
great extent. But if winter under these circumstances should happen
when the earth is in the perihelion of its orbit, the earth would then
be 14,212,700 miles nearer the sun in winter than in summer. In this
case the difference between winter and summer in the latitude of this
country would be almost annihilated. But as the winter in the one
hemisphere corresponds with the summer in the other, it follows that
while the one hemisphere would be enduring the greatest extremes of
summer heat and winter cold, the other would be enjoying a perpetual
summer.
It is quite true that whatever may be the eccentricity of the earth’s
orbit, the two hemispheres must receive equal quantities of heat per
annum; for proximity to the sun is exactly compensated by the effect of
swifter motion—the total amount of heat received from the sun between
the two equinoxes is the same in both halves of the year, whatever the
eccentricity of the earth’s orbit may be. For example, whatever extra
heat the southern hemisphere may at present receive from the sun during
its summer months owing to greater proximity to the sun, is exactly
compensated by a corresponding loss arising from the shortness of the
season; and, on the other hand, whatever deficiency of heat we in the
northern hemisphere may at present have during our summer half year
in consequence of the earth’s distance from the sun, is also exactly
compensated by a corresponding length of season.
It has been shown in the introductory chapter that a simple change in
the sun’s distance would not alone produce a glacial epoch, and that
those physicists who confined their attention to purely astronomical
effects were perfectly correct in affirming that no increase of
eccentricity of the earth’s orbit could account for that epoch. But
the important fact was overlooked that although the glacial epoch
could not result directly from an increase of eccentricity, it might
nevertheless do so indirectly. The glacial epoch, as I hope to show,
was not due directly to an increase in the eccentricity of the earth’s
orbit, but to a number of physical agents that were brought into
operation as a result of an increase.
I shall now proceed to give an outline of what these physical agents
were, how they were brought into operation, and the way in which they
led to the glacial epoch.
When the eccentricity is about its superior limit, the combined
effect of all those causes to which I allude is to lower to a very
great extent the temperature of the hemisphere whose winters occur in
aphelion, and to raise to nearly as great an extent the temperature of
the opposite hemisphere, where winter of course occurs in perihelion.
With the eccentricity at its superior limit and the winter occurring
in the aphelion, the earth would be 8,641,870 miles further from the
sun during that season than at present. The reduction in the amount
of heat received from the sun owing to this increased distance would,
upon the principle we have stated in Chapter II., lower the midwinter
temperature to an enormous extent. In temperate regions the greater
portion of the moisture of the air is at present precipitated in the
form of rain, and the very small portion which falls as snow disappears
in the course of a few weeks at most. But in the circumstances under
consideration, the mean winter temperature would be lowered so much
below the freezing-point that what now falls as rain during that season
would then fall as snow. This is not all; the winters would then not
only be colder than now, but they would also be much longer. At present
the winters are nearly eight days shorter than the summers; but with
the eccentricity at its superior limit and the winter solstice in
aphelion, the length of the winters would exceed that of the summers by
no fewer than thirty-six days. The lowering of the temperature and the
lengthening of the winter would both tend to the same effect, viz., to
increase the amount of snow accumulated during the winter; for, other
things being equal, the larger the snow-accumulating period the greater
the accumulation. I may remark, however, that the absolute quantity
of heat received during winter is not affected by the decrease in the
sun’s heat,[28] for the additional length of the season compensates
for this decrease. As regards the absolute amount of heat received,
increase of the sun’s distance and lengthening of the winter are
compensatory, but not so in regard to the amount of snow accumulated.
The consequence of this state of things would be that, at the
commencement of the short summer, the ground would be covered with the
winter’s accumulation of snow.
Again, the presence of so much snow would lower the summer temperature,
and prevent to a great extent the melting of the snow.
There are three separate ways whereby accumulated masses of snow and
ice tend to lower the summer temperature, viz.:—
_First._ By means of direct radiation. No matter what the intensity of
the sun’s rays may be, the temperature of snow and ice can never rise
above 32°. Hence the presence of snow and ice tends by direct radiation
to lower the temperature of all surrounding bodies to 32°.
In Greenland, a country covered with snow and ice, the pitch has been
seen to melt on the side of a ship exposed to the direct rays of the
sun, while at the same time the surrounding air was far below the
freezing-point; a thermometer exposed to the direct radiation of the
sun has been observed to stand above 100°, while the air surrounding
the instrument was actually 12° below the freezing-point.[29] A similar
experience has been recorded by travellers on the snow-fields of the
Alps.[30]
These results, surprising as they no doubt appear, are what we ought
to expect under the circumstances. The diathermancy of air has been
well established by the researches of Professor Tyndall on radiant
heat. Perfectly dry air seems to be nearly incapable of absorbing
radiant heat. The entire radiation passes through it almost without any
sensible absorption. Consequently the pitch on the side of the ship may
be melted, or the bulb of the thermometer raised to a high temperature
by the direct rays of the sun, while the surrounding air remains
intensely cold. “A joint of meat,” says Professor Tyndall, “might be
roasted before a fire, the air around the joint being cold as ice.”[31]
The air is cooled by _contact_ with the snow-covered ground, but is not
heated by the radiation from the sun.
When the air is humid and charged with aqueous vapour, a similar
cooling effect also takes place, but in a slightly different way. Air
charged with aqueous vapour is a good absorber of radiant heat, but
it can only absorb those rays which agree with it in _period_. It so
happens that rays from snow and ice are, of all others, those which it
absorbs best. The humid air will absorb the total radiation from the
snow and ice, but it will allow the greater part of, if not nearly all,
the sun’s rays to pass unabsorbed. But during the day, when the sun is
shining, the radiation from the snow and ice to the air is negative;
that is, the snow and ice cool the air by radiation. The result is, the
air is cooled by radiation from the snow and ice (or rather, we should
say, _to_ the snow and ice) more rapidly than it is heated by the sun;
and, as a consequence, in a country like Greenland, covered with an
icy mantle, the temperature of the air, even during summer, seldom
rises above the freezing-point. Snow is a good reflector, but as simple
reflection does not change the character of the rays they would not be
absorbed by the air, but would pass into stellar space.
Were it not for the ice, the summers of North Greenland, owing to the
continuance of the sun above the horizon, would be as warm as those of
England; but, instead of this, the Greenland summers are colder than
our winters. Cover India with an ice sheet, and its summers would be
colder than those of England.
_Second._ Another cause of the cooling effect is that the rays which
fall on snow and ice are to a great extent reflected back into
space.[32] But those that are not reflected, but absorbed, do not raise
the temperature, for they disappear in the mechanical work of melting
the ice. The latent heat of ice is about 142° F.; consequently in the
melting of every pound of ice a quantity of heat sufficient to raise
one pound of water 142° disappears, and is completely lost, so far
as temperature is concerned. This quantity of heat is consumed, not
in raising the temperature of the ice, but in the mechanical work of
tearing the molecules separate against the forces of cohesion binding
them together into the solid form. No matter what the intensity of the
sun’s heat may be, the surface of the ground will remain permanently at
32° so long as the snow and ice continue unmelted. [**P1:missing page
number]
_Third._ Snow and ice lower the temperature by chilling the air and
condensing the vapour into thick fogs. The great strength of the sun’s
rays during summer, due to his nearness at that season, would, in the
first place, tend to produce an increased amount of evaporation. But
the presence of snow-clad mountains and an icy sea would chill the
atmosphere and condense the vapour into thick fogs. The thick fogs
and cloudy sky would effectually prevent the sun’s rays from reaching
the earth, and the snow, in consequence, would remain unmelted during
the entire summer. In fact, we have this very condition of things
exemplified in some of the islands of the Southern Ocean at the present
day. Sandwich Land, which is in the same parallel of latitude as the
north of Scotland, is covered with ice and snow the entire summer;
and in the island of South Georgia, which is in the same parallel
as the centre of England, the perpetual snow descends to the very
sea-beach. The following is Captain Cook’s description of this dismal
place:—“We thought it very extraordinary,” he says, “that an island
between the latitudes of 54° and 55° should, in the very height of
summer, be almost wholly covered with frozen snow, in some places many
fathoms deep.... The head of the bay was terminated by ice-cliffs of
considerable height; pieces of which were continually breaking off,
which made a noise like a cannon. Nor were the interior parts of the
country less horrible. The savage rocks raised their lofty summits till
lost in the clouds, and valleys were covered with seemingly perpetual
snow. Not a tree nor a shrub of any size were to be seen. The only
signs of vegetation were a strong-bladed grass growing in tufts, wild
burnet, and a plant-like moss seen on the rocks.... We are inclined to
think that the interior parts, on account of their elevation, never
enjoy heat enough to melt the snow in such quantities as to produce
a river, nor did we find even a stream of fresh water on the whole
coast.”[33]
Captain Sir James Ross found the perpetual snow at the sea-level at
Admiralty Inlet, South Shetland, in lat. 64°; and while near this
place the thermometer in the very middle of summer fell at night to
23° F.; and so rapidly was the young ice forming around the ship that
he began, he says, “to have serious apprehensions of the ships being
frozen in.”[34] At the comparatively low latitude of 59° S., in long.
171° E. (the corresponding latitude of our Orkney Islands), snow was
falling on the longest day, and the surface of the sea at 32°.[35] And
during the month of February (the month corresponding to August in our
hemisphere) there were only three days in which they were not assailed
by snow-showers.[36]
In the Straits of Magellan, in 53° S. lat., where the direct heat of
the sun ought to be as great as in the centre of England, MM. Churrca
and Galcano have seen snow fall in the middle of summer; and though the
day was eighteen hours long, the thermometer seldom rose above 42° or
44°, and never above 51°.[37]
This rigorous condition of climate chiefly results from the rays
of the sun being intercepted by the dense fogs which envelope those
regions during the entire summer; and the fogs again are due to the
air being chilled by the presence of the snow-clad mountains and the
immense masses of floating ice which come from the antarctic seas. The
reduction of the sun’s heat and lengthening of the winter, which would
take place when the eccentricity is near to its superior limit and the
winter in aphelion, would in this country produce a state of things
perhaps as bad as, if not worse than, that which at present exists in
South Georgia and South Shetland.
If we turn our attention to the polar regions, we shall find that
the cooling effects of snow and ice are even still more marked. The
coldness of the summers in polar regions is owing almost solely to this
cause. Captain Scoresby states that, in regard to the arctic regions,
the general obscurity of the atmosphere arising from fogs or clouds is
such that the sun is frequently invisible during several successive
days. At such times, when the sun is near the northern tropic, there is
scarcely any sensible quantity of light from noon till midnight.[38]
“And snow,” he says, “is so common in the arctic regions, that it may
be boldly stated that in nine days out of ten during the months of
April, May, and June more or less falls.”[39]
On the north side of Hudson’s Bay, for example, where the quantity of
floating ice during summer is enormous, and dense fogs prevail, the
mean temperature of June does not rise above the freezing-point, being
actually 13°·5 below the normal temperature; while in some parts of
Asia under the same latitude, where there is comparatively little ice,
the mean temperature of June is as high as 60°.
The mean temperature of Van Rensselaer Harbour, in lat. 78° 37′ N.,
long. 70° 53′ W., was accurately determined from hourly observations
made day and night over a period of two years by Dr. Kane. It was found
to be as follows:—
°
Winter −28·59
Spring −10·59
Summer +33·38
Autumn - 4·03
But although the quantity of heat received from the sun at that
latitude ought to have been greater during the summer than in
England,[40] yet nevertheless the temperature is only 1°·38 above the
freezing-point.
The temperature of Port Bowen, lat. 73° 14′ N., was found to be as
follows:—
°
Winter −25·09
Spring - 5·77
Summer +34·40
Autumn +10·58
Here the summer is only 2°·4 above the freezing-point.
The condition of things in the antarctic regions is even still worse
than in the arctic. Captain Sir James Ross, when between lat. 66° S.
and 77° 5′ S., during the months of January and February, 1841, found
the mean temperature to be only 26°·5; and there were only two days
when it rose even to the freezing-point. When near the ice-barrier on
the 8th of February, 1841, a season of the year equivalent to August
in England, he had the thermometer at 12° at noon; and so rapidly was
the young ice forming around the ships, that it was with difficulty
that he escaped being frozen in for the winter. “Three days later,”
he says, “the thick falling snow prevented our seeing to any distance
before us; the waves as they broke over the ships froze as they fell
on the decks and rigging, and covered our clothes with a thick coating
of ice.”[41] On visiting the barrier next year about the same season,
he again ran the risk of being frozen in. He states that the surface
of the sea presented one unbroken sheet of young ice as far as the eye
could discover from the masthead.
Lieutenant Wilkes, of the American Exploring Expedition, says that the
temperature they experienced in the antarctic regions surprised him,
for they seldom, if ever, had it above 30°, even at midday. Captain
Nares, when in latitude 64°S., between the 13th and 25th February last
(1874), found the mean temperature of the air to be 31°·5; a lower
temperature than is met with in the arctic regions, in August, ten
degrees nearer the pole.[42]
These extraordinarily low temperatures during summer, which we have
just been detailing, were due solely to the presence of snow and ice.
In South Georgia, Sandwich Land, and some other places which we have
noticed, the summers ought to be about as warm as those of England; yet
to such an extent is the air cooled by means of floating ice coming
from the antarctic regions, and the rays of the sun enfeebled by the
dense fogs which prevail, that there is actually not heat sufficient
even in the very middle of summer to melt the snow lying on the
sea-beach.
We read with astonishment that a country in the latitude of England
should in the very middle of summer be covered with snow down to the
sea-shore—the thermometer seldom rising much above the freezing-point.
But we do not consider it so surprising that the summer temperature of
the polar regions should be low, for we are accustomed to regard a low
temperature as the normal condition of things there. We are, however,
mistaken if we suppose that the influence of ice on climate is less
marked at the poles than at such places as South Georgia or Sandwich
Land.
It is true that a low summer temperature is the normal state of
matters in very high latitudes, but it is so only in consequence of
the perpetual presence of snow and ice. When we speak of the normal
temperature of a place we mean, of course, as we have already seen,
the normal temperature under the present condition of things. But
were the ice removed from those regions, our present Tables of normal
summer temperature would be valueless. These Tables give us the normal
June temperature while the ice remains, but they do not afford us the
least idea as to what that temperature would be were the ice removed.
The mere removal of the ice, all things else remaining the same, would
raise the summer temperature enormously. The actual June temperature of
Melville Island, for example, is 37°, and Port Franklin, Nova Zembla,
36°·5; but were the ice removed from the arctic regions, we should
then find that the summer temperature of those places would be about
as high as that of England. This will be evident from the following
considerations:—
The temperature of a place, other things being equal, is proportionate
to the quantity of heat received from the sun. If Greenland receives
per given surface as much heat from the sun as England, its temperature
ought to be as high as that of England. Now, from May 10 till August
3, a period of eighty-five days, the quantity of heat received from
the sun in consequence of his remaining above the horizon is actually
greater at the north pole than at the equator.
Column II. of the following Table, calculated by Mr. Meech,[43]
represents the quantity of heat received from the sun on the 15th of
June at every 10° of latitude. To simplify the Table, I have taken 100
as the unit quantity received at the equator on that day instead of the
unit adopted by Mr. Meech:—
+-----------+---------+-----------+-------------+
| | I. | II. | III. |
| | | | |
| |Latitude.| Quantity | June |
| | | of heat. | temperature.|
+-----------+---------+-----------+-------------+
| | ° | | ° |
|Equator | 0 | 100 | 80·0 |
| | 10 | 111 | 81·1 |
| | 20 | 118 | 81·1 |
| | 30 | 123 | 77·3 |
| | 40 | 125 | 68·0 |
| | 50 | 125 | 58·8 |
| | 60 | 123 | 51·4 |
| | 70 | 127 | 39·2 |
| | 80 | 133 | 30·2 |
|North Pole | 90 | 136 | 27·4 |
+-----------+---------+-----------+-------------+
The calculations are, of course, made upon the supposition that the
quantity of rays cut off in passing through the atmosphere is the
same at the poles as at the equator, which, as we know, is not exactly
the case. But, notwithstanding the extra loss of solar heat in high
latitudes caused by the greater amount of rays that are cut off, still,
if the temperature of the arctic summers were at all proportionate to
the quantity of heat received from the sun, it ought to be very much
higher than it actually is. Column III. represents the actual mean June
temperature, according to Prof. Dove, at the corresponding latitudes.
A comparison of these two columns will show the very great deficiency
of temperature in high latitudes during summer. At the equator, for
example, the quantity of heat received is represented by 100 and the
temperature 80°; while at the pole the temperature is only 27°·4,
although the amount of heat received is 136. This low temperature
during summer, from what has been already shown, is due chiefly to the
presence of snow and ice. If by some means or other we could remove
the snow and ice from the arctic regions, they would then enjoy a
temperate, if not a hot, summer. In Greenland, as we have already seen,
snow falls even in the very middle of summer, more or less, nine days
out of ten; but remove the snow from the northern hemisphere, and a
snow-shower in Greenland during summer would be as great a rarity as it
would be on the plains of India.
Other things being equal, the quantity of solar heat received in
Greenland during summer is considerably greater than in England.
Consequently, were it not for snow and ice, it would enjoy as warm a
climate during summer as that of England. Conversely, let the polar
snow and ice extend to the latitude of England, and the summers of that
country would be as cold as those of Greenland. Our summers would then
be as cold as our winters are at present, and snow in the very middle
of summer would perhaps be as common as rain.
_Mr. Murphy’s Theory._—In a paper read before the Geological Society
by Mr. Murphy[44] he admits that the glacial climate was due to an
increase of eccentricity, but maintains in opposition to me that the
glaciated hemisphere must be that in which the _summer_ occurs in
_aphelion_ during the greatest eccentricity of the earth’s orbit.
I fear that Mr. Murphy must be resting his theory on the mistaken idea
that a summer in aphelion ought to melt less snow and ice than one in
perihelion. It is quite true that the longer summer in aphelion—other
things being equal—is colder than the shorter one in perihelion, but
the quantity of heat received from the sun is the same in both cases.
Consequently the quantity of snow and ice melted ought also to be the
same; for the amount melted is in proportion to the quantity of energy
in the form of heat received.
It is true that with us at present less snow and ice are melted during
a cold summer than during a warm one. But this is not a case in point,
for during a cold summer we have less heat than during a warm summer,
the length of both being the same. The coldness of the summers in
this case is owing chiefly to a portion of the heat which we ought to
receive from the sun being cut off by some obstructing cause.
The reason why we have so little snow, and consequently so little ice,
in temperate regions, is not, as Mr. Murphy seems to suppose, that
the heat of summer melts it all, but that there is so little to melt.
And the reason why we have so little to melt is that, owing to the
warmth of our winters, we have generally rain instead of snow. But
if you increase the eccentricity very much, and place the winter in
perihelion, we should probably have no snow whatever, and, as far as
glaciation is concerned, it would then matter very little what sort of
summer we had.
But it is not correct to say that the perihelion summer of the glacial
epoch must have been hot. There are physical reasons, as we have just
seen, which go to prove that, notwithstanding the nearness of the sun
at that season, the temperature would seldom, if ever, rise much above
the freezing-point.
Besides, Mr. Murphy overlooks the fact that the nearness of the sun
during summer was nearly as essential to the production of the ice, as
we shall shortly see, as his great distance during winter.
We must now proceed to the consideration of an agency which is brought
into operation by the foregoing condition of things, an agency far
more potent than any which has yet come under our notice, viz., the
_Deflection of Ocean-currents_.
_Deflection of Ocean-currents the chief Cause of secular Changes
of Climate._—The enormous extent to which the thermal condition of
the globe is affected by ocean-currents seems to cast new light on
the mystery of geological climate. What, for example, would be the
condition of Europe were the Gulf-stream stopped, and the Atlantic thus
deprived of one-fifth of the absolute amount of heat which it is now
receiving above what it has in virtue of the temperature of space? If
the results just arrived at be at all justifiable, it follows that the
stoppage of the stream would lower the temperature of northern Europe
to an extent that would induce a condition of climate as severe as that
of North Greenland; and were the warm currents of the North Pacific
also at the same time to be stopped, the northern hemisphere would
assuredly be subjected to a state of general glaciation.
Suppose also that the warm currents, having been withdrawn from the
northern hemisphere, should flow into the Southern Ocean: what then
would be the condition of the southern hemisphere? Such a transference
of heat would raise the temperature of the latter hemisphere about
as much as it would lower the temperature of the former. It would
consequently raise the mean temperature of the antarctic regions much
above the freezing-point, and the ice under which those regions are
at present buried would, to a great extent at least, disappear. The
northern hemisphere, thus deprived of the heat from the equator, would
be under a condition of things similar to that which prevailed during
the glacial epoch; while the other hemisphere, receiving the heat from
the equator, would be under a condition of climate similar to what we
know prevailed in the northern hemisphere during a part of the Upper
Miocene period, when North Greenland enjoyed a climate as mild as that
of England at the present day.
This is no mere picture of the imagination, no mere hypothesis devised
to meet a difficult case; for if what has already been stated be not
completely erroneous, all this follows as a necessary consequence from
physical principles. If the warm currents of the equatorial regions
be all deflected into one hemisphere, such must be the condition of
things. How then do the agencies which we have been considering deflect
ocean-currents?
_How the foregoing Causes deflect Ocean-currents._—A high condition
of eccentricity tends, we have seen, to produce an accumulation of
snow and ice on the hemisphere whose winters occur in aphelion. This
accumulation tends in turn to lower the summer temperature, to cut
off the sun’s rays, and so to retard the melting of the snow. In
short, it tends to produce on that hemisphere a state of glaciation.
Exactly opposite effects take place on the other hemisphere, which
has its winter in perihelion. There the shortness of the winters and
the highness of the temperature, owing to the sun’s nearness, combine
to prevent the accumulation of snow. The general result is that the
one hemisphere is cooled and the other heated. This state of things
now brings into play the agencies which lead to the deflection of the
Gulf-stream and other great ocean-currents.
Owing to the great difference between the temperature of the equator
and the poles, there is a constant flow of air from the poles to the
equator. It is to this that the trade-winds owe their existence. Now as
the strength of these winds, as a general rule, will depend upon the
difference of temperature that may exist between the equator and higher
latitudes, it follows that the trades on the cold hemisphere will be
stronger than those on the warm. When the polar and temperate regions
of the one hemisphere are covered to a large extent with snow and ice,
the air, as we have just seen, is kept almost at the freezing-point
during both summer and winter. The trades on that hemisphere will, of
necessity, be exceedingly powerful; while on the other hemisphere,
where there is comparatively little snow and ice, and the air is warm,
the trades will, as a consequence, be weak. Suppose now the northern
hemisphere to be the cold one. The north-east trade-winds of this
hemisphere will far exceed in strength the south-east trade-winds of
the southern hemisphere. The _median-line_ between the trades will
consequently lie to a very considerable distance to the south of
the equator. We have a good example of this at the present day. The
difference of temperature between the two hemispheres at present is
but trifling to what it would be in the case under consideration; yet
we find that the south-east trades of the Atlantic blow with greater
force than the north-east trades, and the result is that the south-east
trades sometimes extend to 10° or 15° N. lat., whereas the north-east
trades seldom blow south of the equator. The effect of the northern
trades blowing across the equator to a great distance will be to impel
the warm water of the tropics over into the Southern Ocean. But this
is not all; not only would the median-line of the trades be shifted
southwards, but the great equatorial currents of the globe would also
be shifted southwards.
Let us now consider how this would affect the Gulf-stream. The South
American continent is shaped somewhat in the form of a triangle, with
one of its angular corners, called Cape St. Roque, pointing eastwards.
The equatorial current of the Atlantic impinges against this corner;
but as the greater portion of the current lies a little to the north
of the corner, it flows westward into the Gulf of Mexico and forms the
Gulf-stream. A considerable portion of the water, however, strikes the
land to the south of the Cape and is deflected along the shores of
Brazil into the Southern Ocean, forming what is known as the Brazilian
current.
Now it is perfectly obvious that the shifting of the equatorial
current of the Atlantic only a few degrees to the south of its present
position—a thing which would certainly take place under the conditions
which we have been detailing—would turn the entire current into the
Brazilian branch, and instead of flowing chiefly into the Gulf of
Mexico as at present, it would all flow into the Southern Ocean, and
the Gulf-stream would consequently be stopped. The stoppage of the
Gulf-stream, combined with all those causes which we have just been
considering, would place Europe under glacial conditions; while, at the
same time, the temperature of the Southern Ocean would, in consequence
of the enormous quantity of warm water received, have its temperature
(already high from other causes) raised enormously.
_Deflection of the Gulf-stream during the Glacial Epoch indicated by
the Difference between the Clyde and Canadian Shell-beds._—That the
glaciation of north-western Europe resulted to a great extent from
the stoppage of the Gulf-stream may, I think, be inferred from a
circumstance pointed out by the Rev. Mr. Crosskey, several years ago,
in a paper read before the Glasgow Geological Society.[45] He showed
that the difference between the glacial shells of Canada and those
now existing in the Gulf of St. Lawrence is much less marked than the
difference between the glacial shells of the Clyde beds and those now
existing in the Firth. And from this he justly infers that the change
of climate in Canada since the glacial epoch has been far less complete
than in Scotland.
The return of the Gulf-stream has raised the mean annual temperature of
our island no less than 15° above the normal, while Canada, deprived of
its influence and exposed to a cold stream from polar regions, has been
kept nearly as much below the normal.
Let us compare the present temperature of the two countries. In making
our comparison we must, of course, compare places on the same latitude.
It will not do, for example, to compare Glasgow with Montreal or
Quebec, places on the latitude of the south of France and north of
Italy. It will be found that the difference of temperature between
the two countries is so enormous as to appear scarcely credible to
those who have not examined the matter. The temperatures have all been
taken from Professor Dove’s work on the “Distribution of Heat over the
Surface of the Globe,” and his Tables published in the Report of the
British Association for 1847.
The mean temperature of Scotland for January is about 38° F., while
in some parts of Labrador, on the same latitude, and all along the
central parts of North America lying to the north of Upper Canada,
it is actually 10°, and in many places 13° below zero. The January
temperature at the Cumberland House, which is situated on the latitude
of the centre of England, is more than 13° below zero. Here is a
difference of no less than 51°. The normal temperature for the month
of January in the latitude of Glasgow, according to Professor Dove, is
10°. Consequently, owing to the influence of the Gulf-stream, we are
28° warmer during that month than we would otherwise be, while vast
tracts of country in America are 23° colder than they should be.
The July temperature of Glasgow is 61°, while on the same latitude
in Labrador and places to the west it is only 49°. Glasgow during
that month is 3° above the normal temperature, while America, owing
to the influence of the cold polar stream, is 9° below it. The mean
annual temperature of Glasgow is nearly 50°, while in America, on the
same latitude, it is only 30°, and in many places as low as 23°. The
mean normal temperature for the whole year is 35°. Our mean annual
temperature is therefore 15° above the normal, and that of America from
5° to 12° below it. The American winters are excessively cold, owing
to the continental character of the climate, and the absence of any
benefit from the Gulf-stream, while the summers, which would otherwise
be warm, are, in the latitude of Glasgow, cooled down to a great extent
by the cold ice from Greenland; and the consequence is, that the mean
annual temperature is about 20° or 27° below that of ours. The mean
annual temperature of the Gulf of St. Lawrence is as low as that of
Lapland or Iceland. It is no wonder, then, that the shells which
flourished in Canada during the glacial epoch have not left the gulf
and the neighbouring seas.
We have good reason to believe that the climate of America during the
glacial epoch was even then somewhat more severe than that of Western
Europe, for the erratics of America extend as far south as latitude
40°, while on the old continent they are not found much beyond latitude
50°. This difference may have resulted from the fact that the western
side of a continent is always warmer than the eastern.
In order to determine whether the cold was as great in America during
the glacial epoch as in Western Europe, we must not compare the fossils
found in the glacial beds about Montreal, for example, with those found
in the Clyde beds, for Montreal lies much further to the south than the
Clyde. The Clyde beds must be compared with those of Labrador, while
the beds of Montreal must be compared with those of the south of France
and the north of Italy, if any are to be found there.
On the whole, it may be concluded that had the Gulf-stream not returned
to our shores at the close of the glacial epoch, and had its place
been supplied by a cold stream from the polar regions, similar to that
which washes the shores of North America, it is highly probable that
nearly every species found in our glacial beds would have had their
representatives flourishing in the British seas at the present day.
It is no doubt true that when we compare the places in which the
Canadian shell-beds referred to by Mr. Crosskey are situated with
places on the same latitude in Europe, the difference of climate
resulting from the influence of the Gulf-stream is not so great as
between Scotland and those places which we have been considering; but
still the difference is sufficiently great to account for why the
change of climate in Canada has been less complete than in Scotland.
And what holds true in regard to the currents of the Atlantic holds
also true, though perhaps not to the same extent, of the currents of
the Pacific.
_Nearness of the Sun in Perigee a Cause of the Accumulation of
Ice._—But there is still another cause which must be noticed:—A strong
under current of air _from_ the north implies an equally strong upper
current _to_ the north. Now if the effect of the under current would
be to impel the warm water at the equator to the south, the effect
of the upper current would be to carry the aqueous vapour formed at
the equator to the north; the upper current, on reaching the snow and
ice of temperate regions, would deposit its moisture in the form of
snow; so that, notwithstanding the great cold of the glacial epoch,
it is probable that the quantity of snow falling in the northern
regions would be enormous. This would be particularly the case during
summer, when the earth would be in the perihelion and the heat at the
equator great. The equator would be the furnace where evaporation would
take place, and the snow and ice of temperate regions would act as a
condenser.
Heat to produce _evaporation_ is just as essential to the accumulation
of snow and ice as cold to produce _condensation_. Now at Midsummer,
on the supposition of the eccentricity being at its superior limit,
the sun would be 8,641,870 miles nearer than at present during that
season. The effect would be that the intensity of the sun’s rays would
be one-fifth greater than now. That is to say, for every five rays
received by the ocean at present, six rays would be received then,
consequently the evaporation during summer would be excessive. But the
ice-covered land would condense the vapour into snow. It would, no
doubt, be during summer that the greatest snowfall would take place. In
fact, the nearness of the sun during that season was as essential to
the production of the glacial epoch as was his distance during winter.
The direct effect of eccentricity is to produce on one of the
hemispheres a long and cold winter. This alone would not lead to a
condition of things so severe as that which we know prevailed during
the glacial epoch. But the snow and ice thus produced would bring into
operation, as we have seen, a host of physical agencies whose combined
efforts would be quite sufficient to do this.
_A remarkable Circumstance regarding those Causes which lead to Secular
Changes of Climate._—There is one remarkable circumstance connected
with those physical causes which deserves special notice. They not only
all lead to one result, viz., an accumulation of snow and ice, but
they react on one another. It is quite a common thing in physics for
the effect to react on the cause. In electricity and magnetism, for
example, cause and effect in almost every case mutually act and react
upon each other. But it is usually, if not universally, the case that
the reaction of the effect tends to weaken the cause. The weakening
influences of this reaction tend to impose a limit on the efficiency
of the cause. But, strange to say, in regard to the physical causes
concerned in the bringing about of the glacial condition of climate,
cause and effect mutually reacted so as to strengthen each other. And
this circumstance had a great deal to do with the extraordinary results
produced.
We have seen that the accumulation of snow and ice on the ground
resulting from the long and cold winters tended to cool the air
and produce fogs which cut off the sun’s rays. The rays thus cut
off diminished the melting power of the sun, and so increased the
accumulation. As the snow and ice continued to accumulate, more and
more of the rays were cut off; and on the other hand, as the rays
continued to be cut off, the _rate_ of accumulation increased, because
the quantity of snow and ice melted became thus annually less and less.
Again, during the long and dreary winters of the glacial epoch the
earth would be radiating off its heat into space. Had the heat thus
lost simply gone to lower the temperature, the lowering of the
temperature would have tended to diminish the rate of loss; but the
necessary result of this was the formation of snow and ice rather than
the lowering of temperature.
And, again, the formation of snow and ice facilitated the rate at which
the earth lost its heat; and on the other hand, the more rapidly the
earth parted with its heat, the more rapidly were the snow and ice
formed.
Further, as the snow and ice accumulated on the one hemisphere, they
at the same time continued to diminish on the other. This tended to
increase the strength of the trade-winds on the cold hemisphere, and
to weaken those on the warm. The effect of this on ocean currents
would be to impel the warm water of the tropics more to the warm
hemisphere than to the cold. Suppose the northern hemisphere to be
the cold one, then as the snow and ice began gradually to accumulate
there, the ocean currents of that hemisphere would begin to decrease in
volume, while those on the southern, or warm, hemisphere, would _pari
passu_ increase. This withdrawal of heat from the northern hemisphere
would tend, of course, to lower the temperature of that hemisphere
and thus favour the accumulation of snow and ice. As the snow and ice
accumulated the ocean currents would decrease, and, on the other hand,
as the ocean currents diminished the snow and ice would accumulate,—the
two effects mutually strengthening each other.
The same must have held true in regard to aërial currents. The more
the polar and temperate regions became covered with snow and ice, the
stronger would become the trades and anti-trades of the hemisphere; and
the stronger those winds became, the greater would be the amount of
moisture transferred from the tropical regions by the anti-trades to
the temperate regions; and on the other hand, the more moisture those
winds brought to temperate regions, the greater would be the quantity
of snow produced.
The same process of mutual action and reaction would take place among
the agencies in operation on the warm hemisphere, only the result
produced would be diametrically opposite of that produced in the cold
hemisphere. On this warm hemisphere action and reaction would tend to
raise the mean temperature and diminish the quantity of snow and ice
existing in temperate and polar regions.
Had it been possible for each of those various physical agents which we
have been considering to produce its direct effects without influencing
the other agents or being influenced by them, its real efficiency in
bringing about either the glacial condition of climate or the warm
condition of climate would not have been so great.
The primary cause that set all those various physical agencies in
operation which brought about the glacial epoch, was a high state of
eccentricity of the earth’s orbit. When the eccentricity is at a
high value, snow and ice begin to accumulate, owing to the increasing
length and coldness of the winter on that hemisphere whose winter
solstice is approaching toward the aphelion. The accumulating snow
then begins to bring into operation all the various agencies which
we have been describing; and, as we have just seen, these, when once
in full operation, mutually aid one another. As the eccentricity
increases century by century, the temperate regions become more and
more covered with snow and ice, first by reason of the continued
increase in the coldness and length of the winters, and secondly,
and chiefly, owing to the continued increase in the potency of those
physical agents which have been called into operation. This glacial
state of things goes on at an increasing rate, and reaches a maximum
when the solstice-point arrives at the aphelion. After the solstice
passes the aphelion, a contrary process commences. The snow and ice
gradually begin to diminish on the cold hemisphere and to make their
appearance on the other hemisphere. The glaciated hemisphere turns, by
degrees, warmer and the warm hemisphere colder, and this continues to
go on for a period of ten or twelve thousand years, until the winter
solstice reaches the perihelion. By this time the conditions of the two
hemispheres have been reversed; the formerly glaciated hemisphere has
now become the warm one, and the warm hemisphere the glaciated. The
transference of the ice from the one hemisphere to the other continues
as long as the eccentricity remains at a high value. This will,
perhaps, be better understood from an inspection of the frontispiece.
_The Mean Temperature of the whole Earth should be greater in Aphelion
than in Perihelion._—When the eccentricity becomes reduced to about
its present value, its influence on climate is but little felt.
It is, however, probable that the present extension of ice on the
southern hemisphere may, to a considerable extent, be the result of
eccentricity. The difference in the climatic conditions of the two
hemispheres is just what should be according to theory:—(1) The mean
temperature of that hemisphere is less than that of the northern.
(2) The winters of the southern hemisphere are colder than those of
the northern. (3) The summers, though occurring in perihelion, are
also comparatively cold; this, as we have seen, is what ought to be
according to theory. (4) The mean temperature of the whole earth is
greater in June, when the earth is in aphelion, than in December, when
it is in perihelion. This, I venture to affirm, is also what ought to
follow according to theory, although this very fact has been adduced
as a proof that eccentricity has at present but little effect on the
climatic condition of our globe.
That the mean temperature of the whole earth would, during the
glacial epoch, be greater when the earth was in aphelion than
when in perihelion will, I think, be apparent from the following
considerations:—When the earth was in the perihelion, the sun would
be over the hemisphere nearly covered with snow and ice. The great
strength of the sun’s rays would in this case have little effect in
raising the temperature; it would be spent in melting the snow and
ice. But when the earth was in the aphelion, the sun would be over the
hemisphere comparatively free, or perhaps wholly free, from snow and
ice. Consequently, though the intensity of the sun’s rays would be less
than when the earth was in perihelion, still it ought to have produced
a higher temperature, because it would be chiefly employed in heating
the ground and not consumed in melting snow and ice.
_Professor Tyndall on the Glacial Epoch._—“So natural,” says Professor
Tyndall, “was the association of ice and cold, that even celebrated
men assumed that all that is needed to produce a great extension of
our glaciers is a diminution of the sun’s temperature. Had they gone
through the foregoing reflections and calculations, they would probably
have demanded _more_ heat instead of less for the production of a
glacial epoch. What they really needed were _condensers_ sufficiently
powerful to congeal the vapour generated by the heat of the sun.” (_The
Forms of Water_, p. 154. See also, to the same effect, _Heat Considered
as a Mode of Motion_, chap. vi.)
I do not know to whom Professor Tyndall here refers, but certainly his
remarks have no application to the theory under consideration, for
according to it, as we have just seen, the ice of the glacial epoch was
about as much due to the nearness of the sun in perigee as to his great
distance in apogee.
There is one theory, however, to which his remarks justly apply, viz.,
the theory that the great changes of climate during geological ages
resulted from the passage of our globe through different temperatures
of space. What Professor Tyndall says shows plainly that the glacial
epoch was not brought about by our earth passing through a cold part
of space. A general reduction of temperature over the whole globe
certainly would not produce a glacial epoch. Suppose the sun were
extinguished and our globe exposed to the temperature of stellar space
(−239° F.), this would certainly freeze the ocean solid from its
surface to its bottom, but it would not cover the land with ice.
Professor Tyndall’s conclusions are, of course, equally conclusive
against Professor Balfour Stewart’s theory, that the glacial epoch may
have resulted from a general diminution in the intensity of the sun’s
heat.
Nevertheless it would be in direct opposition to the well-established
facts of geology to assume that the ice periods of the glacial epoch
were warm periods. We are as certain from palæontological evidence
that the cold was then much greater than now, as we are from physical
evidence that the accumulation of ice was greater than now. Our glacial
shell-beds and remains of the mammoth, the reindeer, and musk-ox, tell
of cold as truly as the markings on the rocks do of ice.
_Objection from the Present Condition of the Planet Mars._—It has been
urged as an objection by Professor Charles Martins[46] and others,
that if a high state of eccentricity could produce a glacial epoch,
the planet Mars ought to be at present under a glacial condition. The
eccentricity of its orbit amounts to 0·09322, and one of its southern
winter solstices is, according to Dr. Oudemans, of Batavia,[47] within
17° 41′ 8″ of aphelion. Consequently, it is supposed that one of the
hemispheres should be in a glacial state and the other free from snow
and ice. But it is believed that the snow accumulates around each pole
during its winter and disappears to a great extent during its summer.
There would be force in this objection were it maintained that
eccentricity alone can produce a glacial condition of climate, but
such is not the case, and there is no good ground for concluding that
those physical agencies which led to the glacial epoch of our globe
exist in the planet Mars. It is perfectly certain that either water
must be different in constitution in that planet from what it is in our
earth, or else its atmospheric envelope must be totally different from
ours. For it is evident from what has been stated in Chapter II., that
were our globe to be removed to the distance of Mars from the sun, the
lowering of the temperature resulting from the decrease in the sun’s
heat would not only destroy every living thing, but would convert the
ocean into solid ice.
But it must be observed that the eccentricity of Mars’ orbit is at
present far from its superior limit of 0·14224, and it may so happen in
the economy of nature that when it approaches to that limit a glacial
condition of things may supervene.
The truth is, however, that very little seems to be known with
certainty regarding the climatic condition of Mars. This is obvious
from the fact that some astronomers believe that the planet possesses
a dense atmosphere which protects it from cold; while others maintain
that its atmosphere is so exceedingly thin that its mean temperature is
below the freezing-point. Some assert that the climatic condition of
Mars resembles very much that of our earth, while others affirm that
its seas are actually frozen solid to the bottom, and the poles covered
with ice thirty or forty miles in thickness. For reasons which will be
explained in the Appendix, Mars, notwithstanding its greater distance
from the sun, may enjoy a climate as warm as that of our earth.
CHAPTER V.
REASON WHY THE SOUTHERN HEMISPHERE IS COLDER THAN THE
NORTHERN.
Adhémar’s Explanation.—Adhémar’s Theory founded upon a physical
Mistake in regard to Radiation.—Professor J. D. Forbes on
Underground Temperature.—Generally accepted Explanation.—Low
Temperature of Southern Hemisphere attributed to
Preponderance of Sea.—Heat transferred from Southern to
Northern Hemisphere by Ocean-current the true Explanation.—A
large Portion of the Heat of the Gulf-stream derived from the
Southern Hemisphere.
_Adhémar’s Explanation._—It has long been known that on the southern
hemisphere the temperature is lower and the accumulation of ice greater
than on the northern. This difference has usually been attributed to
the great preponderance of sea on the southern hemisphere. M. Adhémar,
on the other hand, attempts to explain this difference by referring it
to the difference in the amount of heat lost by the two hemispheres
in consequence of the difference of seven days in the length of their
respective winters. As the northern winter is shorter than the summer,
he concludes that there is an accumulation of heat on that hemisphere,
while, on the other hand, the southern winter being longer than the
summer, there is therefore a loss of heat on the southern hemisphere.
“The south pole,” he says, “loses in one year more heat than it
receives, because the total duration of its night surpasses that of
its day by 168 hours; and the contrary takes place for the north pole.
If, for example, we take for unity the mean quantity of heat which the
sun sends off in one hour, the heat accumulated at the end of the year
at the north pole will be expressed by 168, while the heat lost by the
south pole will be equal to 168 times what the radiation lessens it by
in one hour, so that at the end of the year the difference in the heat
of the two hemispheres will be represented by 336 times what the earth
receives from the sun or loses in an hour by radiation.”[48]
Adhémar supposes that about 10,000 years hence, when our northern
winter will occur in aphelion and the southern in perihelion, the
climatic conditions of the two hemispheres will be reversed; the
ice will melt at the south pole, and the northern hemisphere will
become enveloped in one continuous mass of ice, leagues in thickness,
extending down to temperate regions.
This theory seems to be based upon an erroneous interpretation of a
principle, first pointed out, so far as I am aware, by Humboldt in
his memoir “On Isothermal Lines and Distribution of Heat over the
Globe.”[49] This principle may be stated as follows:—
Although the total quantity of heat received by the earth from the
sun in one revolution is inversely proportional to the minor axis of
the orbit, yet this amount, as was proved by D’Alembert, is equally
distributed between the two hemispheres, whatever the eccentricity may
be. Whatever extra heat the southern hemisphere may at present receive
from the sun daily during its summer months owing to greater proximity
to the sun, is exactly compensated by a corresponding loss arising from
the shortness of the season; and, on the other hand, whatever daily
deficiency of heat we in the northern hemisphere may at present have
during our summer half-year, in consequence of the earth’s distance
from the sun, is also exactly compensated by a corresponding length of
season.
But the surface temperature of our globe depends as much upon the
amount of heat radiated into space as upon the amount derived from the
sun, and it has been thought by some that this compensating principle
holds true only in regard to the latter. In the case of the heat
lost by radiation the reverse is supposed to take place. The southern
hemisphere, it is asserted, has not only a colder winter than the
northern in consequence of the sun’s greater distance, but it has also
a longer winter; and the extra loss of heat from radiation during
winter is not compensated by its nearness to the sun during summer, for
it gains no additional heat from this proximity. And in the same way it
is argued that as our winter in the northern hemisphere, owing to the
less distance of the sun, is not only warmer than that of the southern
hemisphere, but is also at the same time shorter, so our hemisphere
is not cooled to such an extent as the southern. And thus the mean
temperature of the winter half-year, as well as the intensity of the
sun’s heat, is affected by a change in the sun’s distance.
Although I always regarded this cause of Humboldt’s to be utterly
inadequate to produce such effects as those attributed to it by
Adhémar, still, in my earlier papers[50] I stated it to be a _vera
causa_ which ought to produce some sensible effect on climate. But
shortly afterwards on a more careful consideration of the whole
subject, I was led to suspect that the circumstance in question can,
according to theory, produce little or no effect on the climatic
condition of our globe.
As there appears to be a considerable amount of misapprehension in
reference to this point, which forms the basis of Adhémar’s theory, I
may here give it a brief consideration.[51]
The rate at which the earth radiates into space the heat received
from the sun depends upon the temperature of its surface; and the
temperature of its surface (other things being equal) depends upon
the rate at which the heat is received. The greater the rate at which
the earth receives heat from the sun, the greater will therefore be
the rate at which it will lose that heat by radiation. Now the total
quantity of heat received during winter by the southern hemisphere is
exactly equal to that received during winter by the northern. But as
the southern winter is longer than the northern, the rate at which the
heat is received, and consequently the rate of radiation, during that
season must be less on the southern hemisphere than on the northern.
Thus the southern hemisphere loses heat during a longer period than the
northern, and therefore the less rate of radiation (were it not for a
circumstance presently to be noticed) would wholly compensate for the
longer period, and the total quantity of heat lost during winter would
be the same on both hemispheres. The southern summer is shorter than
the northern, but the heat is more intense, and the surface of the
ground kept at a higher temperature; consequently the rate of radiation
into space is greater.
When the rate at which a body receives heat is increased, the
temperature of the body rises till the rate of radiation equals the
rate of absorption, after which equilibrium is restored; and when the
rate of absorption is diminished, the temperature falls till the rate
of radiation equals that of absorption.
But notwithstanding all this, owing to the slow conductivity of the
ground for heat, more heat will pass into it during the longer summer
of aphelion than during the shorter one of perihelion; for the amount
of heat which passes into the ground depends on the length of time
during which the earth is receiving heat, as well as upon the amount
received. In like manner, more heat will pass out of the ground
during the longer winter in aphelion than during the shorter one in
perihelion. Suppose the length of the days on the one hemisphere (say
the northern) to be 23 hours, and the length of the nights, say one
hour; while on the other hemisphere the days are one hour and the
nights 23 hours. Suppose also that the quantity of heat received from
the sun by the southern hemisphere during the day of one hour to be
equal to that received by the northern hemisphere during the day of
23 hours. It is evident that although the surface of the ground on
the southern hemisphere would receive as much heat from the sun during
the short day of one hour as the surface of the northern hemisphere
during the long day of 23 hours, yet, owing to the slow conductivity
of the ground for heat, the amount absorbed would not be nearly so
much on the southern hemisphere as on the northern. The temperature
of the surface during the day, it is true, would be far higher on the
southern hemisphere than on the northern, and consequently the rate
at which the heat would pass into the ground would be greater on that
hemisphere than on the northern; but, notwithstanding the greater rate
of absorption resulting from the high temperature of the surface, it
would not compensate for the shortness of the day. On the other hand,
the surface of the ground on the southern hemisphere would be colder
during the long night of 23 hours than it would be on the northern
during the short night of only one hour; and the low temperature of the
ground would tend to lessen the rate of radiation into space. But the
decrease in the rate of radiation would not compensate fully for the
great length of the night. The general and combined result of all those
causes would be that a slight accumulation of heat would take place on
the northern hemisphere and a slight loss on the southern. But this
loss of heat on the one hemisphere and gain on the other would not go
on accumulating at a uniform rate year by year, as Adhémar supposes.
Of course we are at present simply considering the earth as an absorber
and radiator of heat, without taking into account the effects of
distribution of sea and land and other modifying causes, and are
assuming that everything is the same in both hemispheres, with the
exception that the winter of the one hemisphere is longer than that of
the other.
What, then, is the amount of heat stored up by the one hemisphere and
lost by the other? Is it such an amount as to sensibly affect climate?
The experiments and observations which have been made on underground
temperature afford us a means of making at least a rough estimate of
the amount. And from these it will be seen that the influence of an
excess of seven or eight days in the length of the southern winter over
the northern could hardly produce an effect that would be sensible.
Observations were made at Edinburgh by Professor J. D. Forbes on
three different substances; viz., sandstone, sand, and trap-rock. By
calculation, we find from the data afforded by those observations that
the total quantity of heat accumulated in the ground during the summer
above the mean temperature was as follows:—In the sandstone-rock, a
quantity sufficient to raise the temperature of the rock 1° C. to a
depth of 85 feet 6 inches; in the sand a quantity sufficient to raise
the temperature 1° C. to a depth of 72 feet 6 inches; and in the
trap-rock a quantity only sufficient to raise the temperature 1° C. to
a depth of 61 feet 6 inches.
Taking the specific heat of the sandstone per unit volume, as
determined by Regnault, at ·4623, and that of sand at ·3006, and
trap at ·5283, and reducing all the results to one standard, viz.,
that of water, we find that the quantity of heat stored up in the
sandstone would, if applied to water, raise its temperature 1° C. to
a depth of 39 feet 6 inches; that stored up in the sand would raise
the temperature of the water 1° C. to a depth of 21 feet 8 inches, and
that stored up in the trap would raise the water 1° C. to the depth
of 32 feet 6 inches. We may take the mean of these three results as
representing pretty accurately the quantity stored up in the general
surface of the country. This would be equal to 31 feet 3 inches depth
of water raised 1° C. The quantity of heat lost by radiation during
winter below the mean was found to be about equal to that stored up
during summer.
The total quantity of heat per square foot of surface received by the
equator from sunrise till sunset at the time of the equinoxes, allowing
22 per cent. for the amount cut off in passing through the atmosphere,
is 1,780,474 foot-pounds. In the latitude of Edinburgh about 938,460
foot-pounds per square foot of surface is received, assuming that not
more than 22 per cent. is cut off by the atmosphere. At this rate a
quantity of heat would be received from the sun in two days ten hours
(say, three days) sufficient to raise the temperature of the water 1°
C. to the required depth of 31 feet 3 inches. Consequently the total
quantity of heat stored up during summer in the latitude of Edinburgh
is only equal to what we receive from the sun during three days at the
time of the equinoxes. Three days’ sunshine during the middle of March
or September, if applied to raise the temperature of the ground, would
restore all the heat lost during the entire winter; and another three
days’ sunshine would confer on the ground as much heat as is stored
up during the entire summer. But it must be observed that the total
duration of sunshine in winter is to that of summer in the latitude of
Edinburgh only about as 4 to 7. Here is a difference of two months.
But this is not all; the quantity of heat received during winter is
scarcely one-third of that received during summer; yet, notwithstanding
this enormous difference between summer and winter, the ground during
winter loses only about six days’ sun-heat below the maximum amount
possessed by it in summer.
But if what has already been stated is correct, this loss of heat
sustained by the earth during winter is not chiefly owing to radiation
during the longer absence of the sun, but to the decrease in the
quantity of heat received in consequence of his longer absence combined
with the obliquity of his rays during that season. Now in the case
of the two hemispheres, although the southern winter is longer than
the northern, yet the quantity of heat received by each is the same.
But supposing it held true, which it does not, that the loss of
heat sustained by the earth in winter is as much owing to radiation
resulting from the excess in the length of the winter nights over those
of the summer as to the deficiency of heat received in winter from that
received in summer, three days’ heat would then in this case be the
amount lost by radiation in consequence of this excess in the length of
the winter nights. The total length of the winter nights to those of
the summer is, as we have seen, about as 7 to 4. This is a difference
of nearly 1200 hours. But the excess of the south polar winter over the
north amounts to only about 184 hours. Now if 1200 hours give a loss of
three days’ sun-heat, 184 hours will give a loss of scarcely 5½ hours.
It is no doubt true that the two cases are not exactly analogous; but
it is obvious that any error which can possibly arise from regarding
them as such cannot materially alter the conclusion to which we have
arrived. Supposing the effect were double, or even quadruple, what
we have concluded it to be, still it would not amount to a loss of
two days’ heat, which could certainly have little or no influence on
climate.
But even assuming all the preceding reasoning to be incorrect, and that
the southern hemisphere, in consequence of its longer winter, loses
heat to the extravagant extent of 168 hours, supposed by Adhémar, still
this could not materially affect climate. The climate is influenced
by the mere _temperature_ of the _surface_ of the ground, and not by
the quantity of heat or cold that may be stored up under the surface.
The climate is determined, so far as the ground is concerned, by
the temperature of the surface, and is wholly independent of the
temperature which may exist under the surface. Underground temperature
can only affect climate through the surface. If the surface could,
for example, be kept covered with perpetual snow, we should have a
cold and sterile climate, although the temperature of the ground under
the snow was actually at the boiling-point. Let the ground to a depth
of, say 40 or 50 feet, be deprived of an amount of heat equal to that
received from the sun in 168 hours. This could produce little or no
sensible effect on climate; for, owing to the slow conductivity of the
ground for heat, this loss would not sensibly affect the temperature
of the surface, as it would take several months for the sun’s heat
to penetrate to that depth and restore the lost heat. The cold, if I
may be allowed to use the expression, would come so slowly out to the
surface that its effect in lowering the temperature of the surface
would scarcely be sensible. And, again, if we suppose the 168 hours’
heat to be lost by the mere surface of the ground, the effect would
certainly be sensible, but it would only be so for a few days. We
might in this case have a week’s frozen soil, but that would be all.
Before the air had time to become very sensibly affected by the low
temperature of the surface the frozen soil would be thawed.
The storing up of heat or cold in the ground has in reality very little
to do with climate. Some physicists explain, for example, why the month
of July is warmer than June by referring it to the fact that by the
month of July the ground has become possessed of a larger accumulation
of heat than it possessed in June. This explanation is evidently
erroneous. The ground in July certainly possesses a greater store of
heat than it did in June; but this is not the reason why the former
month is hotter than the latter. July is hotter than June because the
_air_ (not the _ground_) has become possessed of a larger store of
heat than it had in June. Now the air is warmer in July than in June
because, receiving little increase of temperature from the direct rays
of the sun, it is heated chiefly by radiation from the earth and by
contact with its warm surface. Consequently, although the sun’s heat
is greater in June than it is in July, it is near the middle of July
before the air becomes possessed of its maximum store of heat. We
therefore say that July is hotter than June because the air is hotter,
and consequently the temperature in the shade is greater in the former
month than in the latter.
It is therefore, I presume, quite apparent that Adhémar’s theory fails
to explain why the southern hemisphere is colder than the northern.
_The generally accepted Explanation._—The difference in the mean
temperature of the two hemispheres is usually attributed to the
proportion of sea to land in the southern hemisphere and of land to
sea in the northern hemisphere. This, no doubt, will account for the
greater _annual range_ of temperature on the northern hemisphere,
but it seems to me that it will not account for the excess of _mean_
temperature possessed by that hemisphere over the southern.
The general influence of land on climate is to exaggerate the
variation of temperature due to the seasons. On continents the summers
are hotter and the winters colder than on the ocean. The days are
also hotter and the nights colder on land than on sea. This is a
result which follows from the mere physical properties of land and
water, independently of currents, whether of ocean or of air. But it
nevertheless follows, according to theory (and this is a point which
has been overlooked), that the mean annual temperature of the ocean
ought to be greater than that of the land in equatorial regions as
well as in temperate and polar regions. This will appear obvious for
the following reasons:—(1) The ground stores up heat only by the slow
process of conduction, whereas water, by the mobility of its particles
and its transparency for heat-rays, especially those from the sun,
becomes heated to a considerable depth rapidly. The quantity of heat
stored up in the ground is thus comparatively small, while the quantity
stored up in the ocean is great. (2) The air is probably heated more
rapidly by contact with the ground than with the ocean; but, on the
other hand, it is heated far more rapidly by radiation from the ocean
than from the land. The aqueous vapour of the air is to a great extent
diathermanous to radiation from the ground, while it absorbs the
rays from water and thus becomes heated. (3) The air radiates back a
considerable portion of its heat, and the ocean absorbs this radiation
from the air more readily than the ground does. The ocean will not
reflect the heat from the aqueous vapour of the air, but absorbs it,
while the ground does the opposite. Radiation from the air, therefore,
tends more readily to heat the ocean than it does the land. (4) The
aqueous vapour of the air acts as a screen to prevent the loss by
radiation from water, while it allows radiation from the ground to pass
more freely into space; the atmosphere over the ocean consequently
throws back a greater amount of heat than is thrown back by the
atmosphere over the land. The sea in this case has a much greater
difficulty than the land has in getting quit of the heat received from
the sun; in other words, the land tends to lose its heat more rapidly
than the sea. The consequence of all these circumstances is that the
ocean must stand at a higher mean temperature than the land. A state of
equilibrium is never gained until the rate at which a body is receiving
heat is equal to the rate at which it is losing it; but as equal
surfaces of sea and land receive from the sun the same amount of heat,
it therefore follows that, in order that the sea may get quit of its
heat as rapidly as the land, it _must stand at a higher temperature_
than the land. The temperature of the sea must continue to rise till
the amount of heat thrown off into space equals that received from the
sun; when this point is reached, equilibrium is established and the
temperature remains stationary. But, owing to the greater difficulty
that the sea has in getting rid of its heat, the mean temperature
of equilibrium of the ocean must be higher than that of the land;
consequently the mean temperature of the ocean, and also of the air
immediately over it, in tropical regions should be higher than the mean
temperature of the land and the air over it.
The greater portion of the southern hemisphere, however, is occupied by
water, and why then, it may be asked, is this water hemisphere colder
than the land hemisphere? Ought it not also to follow that the sea in
inter-tropical regions should be warmer than the land under the same
parallels; yet, as we know, the reverse is actually found to be the
case. How then is all this to be explained, if the foregoing reasoning
be correct? We find when we examine Professor Dove’s charts of mean
annual temperature, that the ocean in inter-tropical regions has a
mean annual temperature below the normal, and the land a mean annual
temperature above the normal. Both in the Pacific and in the Atlantic
the mean temperature sinks to 2°·3 below the normal, while on the
land it rises 4°·6 above the normal. The explanation in this case is
obviously this: the temperature of the ocean in inter-tropical regions,
as we have already seen, is kept much lower than it would otherwise be
by the enormous amount of _heat_ that is being constantly carried away
from those regions into temperate and polar regions, and of _cold_ that
is being constantly carried from temperate and polar regions to the
tropical regions by means of ocean-currents. The same principle which
explains why the sea in inter-tropical regions has a lower mean annual
temperature than the land, explains also why the southern hemisphere
has a lower mean annual temperature than the northern. The temperature
of the southern hemisphere is lowered by the transference of heat by
means of ocean-currents.
_Heat transferred from the Southern to the Northern Hemisphere by
Ocean-currents the true Explanation._—The great ocean-currents of
the globe take their rise in three immense streams from the Southern
Ocean, which, on reaching the tropical regions, become deflected in
a westerly direction and flow along the southern side of the equator
for thousands of miles. Perhaps more than one half of this mass of
moving water returns into the Southern Ocean without ever crossing the
equator, but the quantity which crosses over to the northern hemisphere
is enormous. This constant flow of water from the southern hemisphere
to the northern in the form of surface currents must be compensated by
_under currents_ of equal magnitude from the northern hemisphere to the
southern. The currents, however, which cross the equator are far higher
in temperature than their compensating under currents; consequently
there is a constant transference of heat from the southern hemisphere
to the northern. Any currents taking their rise in the northern
hemisphere and flowing across into the southern are comparatively
trifling, and the amount of heat transferred by them is also trifling.
There are one or two currents of considerable size, such as the
Brazilian branch of the great equatorial current of the Atlantic, and
a part of the South Equatorial Drift-current of the Pacific, which
cross the equator from north to south; but these cannot be regarded as
northern currents; they are simply southern currents deflected back
after crossing over to the northern hemisphere. The heat which these
currents possess is chiefly obtained on the southern hemisphere before
crossing over to the northern; and although the northern hemisphere may
not gain much heat by means of them, it, on the other hand, does not
lose much, for the heat which they give out in their progress along the
southern hemisphere does not belong to the northern hemisphere.
But, after making the fullest allowance for the amount of heat carried
across the equator from the northern hemisphere to the southern, we
shall find, if we compare the mean temperature of the currents from
south to north with that of the great compensating under currents and
the one or two small surface currents, that the former is very much
higher than the latter. The mean temperature of the water crossing the
equator from south to north is probably not under 65°, that of the
under currents is probably not over 39°. But to the under currents
we must add the surface currents from north to south; and assuming
that this will raise the mean temperature of the entire mass of water
flowing south to, say, 45°, we have still a difference of 20° between
the temperature of the masses flowing north and south. Each cubic
foot of water which crosses the equator will in this case transfer
about 965,000 foot-pounds of heat from the southern hemisphere to the
northern. If we had any means of ascertaining the volume of those great
currents crossing the equator, we should then be able to make a rough
estimate of the total amount of heat transferred from the southern
hemisphere to the northern; but as yet no accurate estimate has been
made on this point. Let us assume, what is probably below the truth,
that the total amount of water crossing the equator is at least double
that of the Gulf-stream as it passes through the Straits of Florida,
which amount we have already found to be equal to 66,908,160,000,000
cubic feet daily. Taking the quantity of heat conveyed by each cubic
foot of water of the Gulf-stream as 1,158,000 foot-pounds, it is
found, as we have seen, that an amount of heat is conveyed by this
current equal to all the heat that falls within 32 miles on each
side of the equator. Then, if each cubic foot of water crossing the
equator transfers 965,000 foot-pounds, and the quantity of water be
double that of the Gulf-stream, it follows that the amount of heat
transferred from the southern hemisphere to the northern is equal to
all the heat falling within 52 miles on each side of the equator, or
equal to all the heat falling on the southern hemisphere within 104
miles of the equator. This quantity taken from the southern hemisphere
and added to the northern will therefore make a difference in the
amount of heat possessed by the two hemispheres equal to all the heat
which falls on the southern hemisphere within somewhat more than 208
miles of the equator.
_A large Portion of the Heat of the Gulf-stream derived from the
Southern Hemisphere._—It can be proved that a very large portion of the
heat conveyed by the Gulf-stream comes from the southern hemisphere.
The proof is as follows:—
If all the heat came from the northern hemisphere, it could only come
from that portion of the Atlantic, Caribbean Sea, and Gulf of Mexico
which lies to the north of the equator. The entire area of these seas,
extending to the Tropic of Cancer, is about 7,700,000 square miles.
But this area is not sufficient to supply the current passing through
the “Narrows” with the necessary heat. Were the heat which passes
through the Straits of Florida derived exclusively from this area, the
following table would then represent the relative quantity per unit
surface possessed by the Atlantic in the three zones, assuming that one
half of the heat of the Gulf-stream passes into the arctic regions and
the other half remains to warm the temperate regions[52]:—
From the equator to the Tropic of Cancer 773
From the Tropic of Cancer to the Arctic Circle 848
From the Arctic Circle to the North Pole 610
These figures show that the Atlantic, from the equator to the Tropic
of Cancer, would be as cold as from the Tropic of Cancer to the North
Pole, were it not that a large proportion of the heat possessed by the
Gulf-stream is derived from the southern hemisphere.
CHAPTER VI.
EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC
CIRCULATION.—LIEUT. MAURY’S THEORY.
Introduction.—Ocean-currents, according to Maury, due to
Difference of Specific Gravity.—Difference of Specific
Gravity resulting from Difference of Temperature.—Difference
of Specific Gravity resulting from Difference of
Saltness.—Maury’s two Causes neutralize each other.—How,
according to him, Difference in Saltness acts as a Cause.
_Introduction._—Few subjects have excited more interest and attention
than the cause of ocean circulation; and yet few are in a more
imperfect and unsatisfactory condition, nor is there any question
regarding which a greater diversity of opinion has prevailed. Our
incomplete acquaintance with the facts relating to the currents of the
ocean and the modes of circulation actually in operation, is no doubt
one reason for this state of things. But doubtless the principal cause
of such diversity of opinion lies in the fact that the question is one
which properly belongs to the domain of physics and mechanics, while
as yet no physicist of note (if we except Dr. Colding, of Copenhagen)
has given, as far as I know, any special attention to the subject. It
is true that in works of meteorology and physical geography reference
is continually made to such eminent physicists as Herschel, Pouillet,
Buff, and others; but when we turn to the writings of these authors we
find merely a few remarks expressive of their opinions on the subject,
and no special discussion or investigation of the matter, nor anything
which could warrant us in concluding that such investigations have ever
been made. At present the question cannot be decided by a reference to
authorities.
The various theories on the subject may be classed under two divisions;
the first of these attributes the motion of the water to the _impulse
of the wind_, and the second to the _force of gravity_ resulting from
difference of density. But even amongst those who adopt the former
theory, it is generally held that the winds are not the sole cause,
but that, to a certain extent at least, difference of specific gravity
contributes to produce motion of the waters. This is a very natural
conclusion; and in the present state of physical geography on this
subject one can hardly be expected to hold any other view.
The supporters of the latter theory may be subdivided into two
classes. The first of these (of which Maury may be regarded as the
representative) attributes the Gulf-stream, and other sensible currents
of the ocean, to difference of specific gravity. The other class (at
present the more popular of the two, and of which Dr. Carpenter may be
considered the representative) denies altogether that such currents can
be produced by difference of specific gravity,[53] and affirms that
there is a general movement of the upper portion of the ocean from the
equator to the poles, and a counter-movement of the under portion from
the poles to the equator. This movement is attributed to difference of
specific gravity between equatorial and polar water, resulting from
difference of temperature.
The widespread popularity of the gravitation theory is no doubt, to a
great extent, owing to the very great prominence given to it by Lieut.
Maury in his interesting and popular work, “The Physical Geography of
the Sea.” Another cause which must have favoured the reception of this
theory is the ease with which it is perceived how, according to it,
circulation of the waters of the ocean is supposed to follow. One has
no difficulty, for example, in perceiving that if the inter-tropical
waters of the ocean are expanded by heat, and the waters around the
poles contracted by cold, the surface of the ocean will stand at a
higher level at the equator than at the poles. Equilibrium being
thus disturbed, the water at the equator will tend to flow towards
the poles as a surface current, and the water at the poles towards
the equator as an under current. This, at first sight, looks well,
especially to those who take but a superficial view of the matter.
We shall examine this theory at some length, for two reasons: 1,
because it lies at the root of a great deal of the confusion and
misconception which have prevailed in regard to the whole subject of
ocean-currents: 2, because, if the theory is correct, it militates
strongly against the physical theory of secular changes of climate
advanced in this volume. We have already seen (Chapter IV.) that
when the eccentricity of the earth’s orbit reaches a high value, a
combination of physical circumstances tends to lower the temperature of
the hemisphere which has its winter solstice in aphelion, and to raise
the temperature of the opposite hemisphere, whose winter solstice will,
of course, be in perihelion. The direct result of this state of things,
as was shown, is to strengthen the force of the trade-winds on the
cold hemisphere, and to weaken their strength on the warm hemisphere:
and this, in turn, we also saw, tends to impel the warm water of
the inter-tropical region on to the warm hemisphere, and to prevent
it, in a very large degree, from passing into the cold hemisphere.
This deflection of the ocean-currents tends to an enormous extent to
increase the difference of temperature previously existing between the
two hemispheres. In other words, the warm and equable condition of the
one hemisphere, and the cold and glacial condition of the other, are,
to a great extent, due to this deflection of ocean-currents. But if
the theory be correct which attributes the motion of ocean-currents to
a difference in density between the sea in inter-tropical and polar
regions, then it follows that these currents (other things being
equal) ought to be stronger on the cold hemisphere than on the warm,
because there is a greater difference of temperature and, consequently,
a greater difference of density, between the polar seas of the cold
hemisphere and the equatorial seas, than between the polar seas of the
warm hemisphere and the equatorial seas. And this being the case,
notwithstanding the influence of the trade-winds of the cold hemisphere
blowing over upon the warm, the currents will, in all probability,
be stronger on the cold hemisphere than on the warm. In other words,
the influence of the powerful trade-winds of the cold hemisphere to
transfer the warm water of the equator to the warm hemisphere will
probably be more than counterbalanced by the tendency of the warm
and buoyant waters of the equator to flow towards the dense and cold
waters around the pole of the cold hemisphere. But if ocean-currents
are due not to difference in specific gravity, but to the influence of
the winds, then it is evident that the waters at the equator will be
impelled, not into the cold hemisphere, but into the warm.
For this reason I have been the more anxious to prove that
inter-tropical heat is conveyed to temperate and polar regions by
ocean-currents, and not by means of any general movement of the ocean
resulting from difference of gravity. I shall therefore on this account
enter more fully into this part of the subject than I otherwise would
have done. Irrespective of all this, however, the important nature of
the whole question, and the very general interest it excites, warrant a
full consideration of the subject.
I shall consider first that form of the gravitation theory advocated
by Maury in his work on the “Physical Geography of the Sea,” which
attributes the motion of the Gulf-stream and other sensible currents
of the ocean to differences of specific gravity. One reason which has
induced me to select Maury’s work is, that it not only contains a much
fuller discussion on the cause of the motion of ocean-currents than is
to be found anywhere else, but also that it has probably passed through
a greater number of editions than any other book of a scientific
character in the English language in the same length of time.
_Examination of Lieut. Maury’s Gravitation Theory._—Although Lieut.
Maury has expounded his views on the cause of ocean-currents at
great length in the various editions of his work, yet it is somewhat
difficult to discover what they really are. This arises chiefly
from the generally confused and sometimes contradictory nature of
his hydrodynamical conceptions. After a repeated perusal of several
editions of his book, the following, I trust, will be found to be a
pretty accurate representation of his theory:—
_Ocean-currents, according to Maury, due to Difference of Specific
Gravity._—Although Maury alludes to a number of causes which, he
thinks, tend to produce currents, yet he deems their influence so
small that, practically, all currents may be referred to difference of
specific gravity.
“If we except,” he says, “the tides, and the partial currents of the
sea, such as those that may be created by the wind, we may lay it down
as a rule that all the currents of the ocean owe their origin to the
differences of specific gravity between sea-water at one place and
sea-water at another; for wherever there is such a difference, whether
it be owing to difference of temperature or to difference of saltness,
&c., it is a difference that disturbs equilibrium, and currents are the
consequence” (§ 467)[54]. To the same effect see §§ 896, 37, 512, 520,
and 537.
Notwithstanding the fact that he is continually referring to difference
of specific gravity as the great cause of currents, it is difficult to
understand in what way he conceives this difference to act as a cause.
Difference of specific gravity between the waters of the ocean at one
place and another can give rise to currents only through the influence
of the earth’s gravity. All currents resulting from difference of
specific gravity can be ultimately resolved into the general principle
that the molecules that are specifically heavier _descend_ and displace
those that are specifically lighter. If, for example, the ocean at the
equator be expanded by heat or by any other cause, it will be forced by
the denser waters in temperate and polar regions to rise so that its
surface shall stand at a higher level than the surface of the ocean in
these regions. The surface of the ocean will become an inclined plane,
sloping from the equator to the poles. Hydro-statically, the ocean,
considered as a mass, will then be in a state of equilibrium; but the
individual molecules will not be in equilibrium. The molecules at the
surface in this case may be regarded as lying on an inclined plane
sloping from the equator down to the poles, and as these molecules
are at liberty to move they will not remain at rest, but will descend
the incline towards the poles. When the waters at the equator are
expanded, or the waters at the poles contracted, gravitation makes, as
it were, a twofold effort to restore equilibrium. It in the first place
sinks the waters at the poles, and raises the waters at the equator,
in order that the two masses may balance each other; but this very
effort of gravitation to restore equilibrium to the mass destroys the
equilibrium of the molecules by disturbing the level of the ocean. It
then, in the second place, endeavours to restore equilibrium to the
molecules by pulling the lighter surface water at the equator down the
incline towards the poles. This tends not only to restore the level
of the ocean, but to bring the lighter water to occupy the surface
and the denser water the bottom of the ocean; and when this is done,
complete equilibrium is restored, both to the mass of the ocean and
to its individual molecules, and all further motion ceases. But if
heat be constantly applied to the waters of the equatorial regions,
and cold to those of the polar regions, and a permanent disturbance of
equilibrium maintained, then the continual effort of gravitation to
restore equilibrium will give rise to a constant current. In this case,
the heat and the cold (the agents which disturb the equilibrium of the
ocean) may be regarded as causes of the current, inasmuch as without
them the current would not exist; but the real efficient cause, that
which impels the water forward, is the force of gravity. But the force
of gravity, as has already been noticed, cannot produce motion (perform
work) unless the thing acted upon _descend_. Descent is implied in
the very conception of a current produced by difference of specific
gravity.
But Maury speaks as if difference of specific gravity could give rise
to a current without any descent.
“It is not necessary,” he says, “to associate with oceanic currents
the idea that they must of necessity, as on land, run from a higher to
a lower level. So far from this being the case, some currents of the
sea actually run up hill, while others run on a level. The Gulf-stream
is of the first class” (§ 403). “The top of the Gulf-stream runs on a
level with the ocean; therefore we know it is not a descending current”
(§ 18). And in § 9 he says that between the Straits of Florida and
Cape Hatteras the waters of the Gulf-stream “are actually forced up an
inclined plane, whose submarine ascent is not less than 10 inches to
the mile.” To the same effect see §§ 25, 59.
It is perfectly true that “it is not necessary to associate with
ocean-currents the idea that they must of necessity, as on land,
run from a higher to a lower level.” But the reason of this is that
ocean-currents do not, like the currents on land, owe their motion to
the force of gravitation. If ocean-currents result from difference of
specific gravity between the waters in tropical and polar regions,
as Maury maintains, then it is necessary to assume that they are
descending currents. Whatever be the cause which may give rise to a
difference of specific gravity, the motion which results from this
difference is due wholly to the force of gravity; but gravity can
produce no motion unless the water _descend_.
This fact must be particularly borne in mind while we are considering
Maury’s theory that currents are the result of difference of specific
gravity.
Ocean-currents, then, according to that writer, owe their existence to
the difference of specific gravity between the waters of inter-tropical
and polar regions. This difference of specific gravity he attributes to
two causes—(1) to difference as to _temperature_, (2) to difference as
to saltness. There are one or two causes of a minor nature affecting
the specific gravity of the sea, to which he alludes; but these two
determine the general result. Let us begin with the consideration of
the first of these two causes, viz.:—
_Difference of Specific Gravity resulting from Difference of
Temperature._—Maury explains his views on this point by means of an
illustration. “Let us now suppose,” he says, “that all the water within
the tropics, to the depth of one hundred fathoms, suddenly becomes oil.
The aqueous equilibrium of the planet would thereby be disturbed, and
a general system of currents and counter currents would be immediately
commenced—the oil, in an unbroken sheet on the surface, running toward
the poles, and the water, in an under current, toward the equator. The
oil is supposed, as it reaches the polar basin, to be reconverted into
water, and the water to become oil as it crosses Cancer and Capricorn,
rising to the surface in inter-tropical regions, and returning as
before” (§ 20). “Now,” he says (§ 22), “do not the cold waters of the
north, and the warm waters of the Gulf, made specifically lighter by
tropical heat, and which we see actually preserving such a system of
counter currents, hold, at least in some degree, the relation of the
supposed water and oil?”
In § 24 he calculates that at the Narrows of Bemini the difference in
weight between the volume of the Gulf-water that crosses a section of
the stream in one second, and an equal volume of water at the ocean
temperature of the latitude, supposing the two volumes to be equally
salt, is fifteen millions of pounds. Consequently the force per second
operating to propel the waters of the Gulf towards the pole would in
this case, he concludes, be the “equilibrating tendency due to fifteen
millions of pounds of water in the latitude of Bemini.” In §§ 511 and
512 he states that the effect of expanding the waters at the torrid
zone by heat, and of contracting the waters at the frigid zone by cold,
is to produce a set of surface-currents of warm and light water from
the equator towards the poles, and another set of under currents of
cooler and heavy water from the poles towards the equator. (See also to
the same effect §§ 513, 514, 896.)
There can be no doubt that his conclusion is that the waters in
inter-tropical regions are expanded by heat, while those in polar
regions are contracted by cold, and that this tends to produce a
surface current from the equator to the poles, and an under current
from the poles to the equator.
“We shall now consider his second great cause of ocean currents, viz.:—
_Difference of Specific Gravity resulting from Difference in Degree of
Saltness._—Maury maintains, and that correctly, that saltness increases
the density of water—that, other things being equal, the saltest water
is the densest. He suggests “that one of the purposes which, in the
grand design, it was probably intended to accomplish by having the sea
salt and not fresh, was to impart to its waters the forces and powers
necessary to make their circulation complete” (§ 495).
Now it is perfectly obvious that if difference in saltness is to
co-operate with difference in temperature in the production of
ocean-currents, the saltest waters, and consequently the densest, must
be in the polar regions, and the waters least salt, and consequently
lightest, must be in equatorial and inter-tropical regions. Were the
saltest waters at the equator, and the freshest at the poles, it would
tend to neutralize the effect due to heat, and, instead of producing
a current, would simply tend to prevent the existence of the currents
which otherwise would result from difference of temperature.
A very considerable portion of his work, however, is devoted to proving
that the waters of equatorial and inter-tropical regions are salter
and heavier than those of the polar regions; and yet, notwithstanding
this, he endeavours to show that this difference in respect to saltness
between the waters of the equatorial and the polar regions is one of
the chief causes, if not the chief cause, of ocean-currents. In fact,
it is for this special end that so much labour is bestowed in proving
that the saltest water is in the equatorial and inter-tropical regions,
and the freshest in the polar.
“In the present state of our knowledge,” he says, “concerning this
wonderful phenomenon (for the Gulf-stream is one of the most marvellous
things in the ocean) we can do little more than conjecture. But we have
two causes in operation which we may safely assume are among those
concerned in producing the Gulf-stream. One of these is the increased
saltness of its water after the trade-winds have been supplied with
vapour from it, be it much or little; and the other is the diminished
quantum of salt which the Baltic and the Northern Seas contain” (§ 37).
“Now here we have, on one side, the Caribbean Sea and Gulf of Mexico,
with their waters of brine; on the other, the great Polar Basin, the
Baltic, and the North Sea, the two latter with waters that are but
little more than brackish. In one set of these sea-basins the water is
heavy, in the other it is light. Between them the ocean intervenes; but
water is bound to seek and to maintain its level; and here, therefore,
we unmask one of the agents concerned in causing the Gulf-stream” (§
38). To the same effect see §§ 52, 522, 523, 524, 525, 526, 528, 530,
554, 556.
Lieut. Maury’s _two causes neutralize each other_. Here we have two
theories put forth regarding the cause of ocean-currents, the one
in direct opposition to the other. According to the one theory,
ocean-currents exist because the waters of equatorial regions, in
consequence of their higher temperature, are _less dense_ than the
waters of the polar regions; but according to the other theory,
ocean-currents exist because the waters of equatorial regions, in
consequence of their greater saltness, are _more dense_ than the
waters of the polar regions. If the one cause be assigned as a reason
why ocean-currents exist, then the other can be equally assigned as
a reason why they should not exist. According to both theories it is
the difference of density between the equatorial and polar waters that
gives rise to currents; but while the one theory maintains that the
equatorial waters are _lighter_ than the polar, the other holds that
they are _heavier_. Either the one theory or the other may be true,
or neither; but it is logically impossible that both of them can. Let
it be observed that it is not two currents, the one contrary to the
other, with which we have at present to do; it is not temperature
producing currents in one direction, and saltness producing currents
in the contrary direction. We have two theories regarding the origin
of currents, the one diametrically opposed to the other. The tendency
of the one cause assigned is to prevent the action of the other. If
temperature is allowed to act, it will make the inter-tropical waters
lighter than the polar, and then, according to theory, a current will
result. But if we bring saltness into play (the other cause) it will
do the reverse: it will increase the density of the inter-tropical
waters and diminish the density of the polar; and so far as it acts it
will diminish the currents produced by temperature, because it will
diminish the difference of specific gravity between the inter-tropical
and polar regions which had been previously caused by temperature. And
when the effects of saltness are as powerful as those of temperature,
the difference of specific gravity produced by temperature will be
completely effaced, or, in other words, the waters of the equatorial
and polar seas will be of the same density, and consequently no current
will exist. And so long as the two causes continue in action, no
current can arise, unless the energy of the one cause should happen to
exceed that of the other; and even then a current will only exist to
the extent by which the strength of the one exceeds that of the other.
The contrary nature of the two theories will be better seen by
considering the way in which it is supposed that difference in saltness
is produced and acts as a cause.
If there is a constant current resulting from the difference in
saltness between the equatorial and polar waters, then there must be a
cause which maintains this difference. The current is simply the effort
to restore the equilibrium lost by the difference; and the current
would very soon do this, and then all motion would cease, were there
not a constantly operating cause maintaining the disturbance. What,
then, according to Maury, is the cause of this disturbance, or, in
other words, what is it that keeps the equatorial waters salter than
the polar?
The agencies in operation are stated by him to be heat, radiation,
evaporation, precipitation, and secretion of solid matter in the form
of shells, &c. The two most important, however, are evaporation and
precipitation.
The trade-winds enter the equatorial regions as relatively dry winds
thirsting for vapour; consequently they absorb far more moisture than
they give out; and the result is that in inter-tropical regions,
evaporation is much in excess of precipitation; and as fresh water only
is taken up, the salt being left behind, the process, of course, tends
to increase the saltness of the inter-tropical seas. Again, in polar
and extra-tropical regions the reverse is the case; precipitation is in
excess of evaporation. This tends in turn to diminish the saltness of
the waters of those regions. (See on these points §§ 31, 33, 34, 37,
179, 517, 526, and 552.)
In the system of circulation produced by difference of temperature,
as we have already seen, the surface-currents flow from the equator
to the poles, and the under or return currents from the poles to the
equator; but in the system produced by difference of saltness, the
surface currents flow from the poles to the equator, and the return
under currents from the equator to the poles. That the surface currents
produced by difference of saltness flow from the poles to the equator,
Maury thinks is evident for the two following reasons:—
(1) As evaporation is in excess of precipitation in inter-tropical
regions, more water is taken off the surface of the ocean in those
regions than falls upon it in the form of rain. This excess of water
falls in the form of rain on temperate and polar regions, where,
consequently, precipitation is in excess of evaporation. The lifting
of the water off the equatorial regions and its deposit on the polar
tend to lower the level of the ocean in equatorial regions and to raise
the level in polar; consequently, in order to restore the level of
the ocean, the surface water at the polar regions flows towards the
equatorial regions.
(2) As the water taken up at the equator is fresh, and the salt
is left behind, the ocean, in inter-tropical regions, is thus made
saltier and consequently denser. This dense water, therefore, sinks
and passes away as an under current. This water, evaporated from
inter-tropical regions, falls as fresh and lighter water in temperate
and polar regions; and therefore not only is the level of the ocean
raised, but the waters are made lighter. Hence, in order to restore
equilibrium, the waters in temperate and polar regions will flow as
a surface current towards the equator. Under currents will flow from
the equator to the poles, and surface or upper currents from the poles
to the equator. Difference in temperature and difference in saltness,
therefore, in every respect tend to produce opposite effects.
That the above is a fair representation of the way in which Maury
supposes difference in saltness to act as a cause in the production of
ocean-currents will appear from the following quotations:—
“In those regions, as in the trade-wind region, where evaporation is
in excess of precipitation, the general level of this supposed sea
would be altered, and immediately as much water as is carried off by
evaporation would commence to flow in from north and south toward the
trade-wind or evaporation region, to restore the level” (§ 509). “On
the other hand, the winds have taken this vapour, borne it off to the
extra-tropical regions, and precipitated it, we will suppose, where
precipitation is in excess of evaporation. Here is another alteration
of sea-level, by elevation instead of by depression; and hence we
have the motive power for a _surface current from each pole towards
the equator_, the object of which is only to supply the demand for
evaporation in the trade-wind regions” (§ 510).
The above result would follow, supposing the ocean to be fresh. He then
proceeds to consider an additional result that follows in consequence
of the saltness of the ocean.
“Let evaporation now commence in the trade-wind region, as it was
supposed to do in the case of the freshwater seas, and as it actually
goes on in nature—and what takes place? Why a lowering of the sea-level
as before. But as the vapour of salt water is fresh, or nearly so,
fresh water only is taken up from the ocean; that which remains behind
is therefore more salt. Thus, while the level is lowered in the salt
sea, the equilibrium is destroyed because of the saltness of the water;
for the water that remains after evaporation takes place is, on account
of the solid matter held in solution, specifically heavier than it was
before any portion of it was converted into vapour” (§ 517).
“The vapour is taken from the surface-water; the surface-water thereby
becomes more salt, and, under certain conditions, heavier. When it
becomes heavier, it sinks; and hence we have, due to the salts of the
sea, a vertical circulation, namely, a descent of heavier—because
salter and cooler—water from the surface, and an ascent of water that
is lighter—because it is not so salt—from the depths below” (§ 518).
In section 519 he goes on to show that this vapour removed from the
inter-tropical region is precipitated in the polar regions, where
precipitation is in excess of evaporation. “In the precipitating
regions, therefore, the level is destroyed, as before explained, by
elevation, and in the evaporating regions by depression; which, as
already stated, gives rise to a system of _surface_ currents, moved by
gravity alone, from the _poles towards the equator_” (§ 520).
“This fresh water being emptied into the Polar Sea and agitated by the
winds, becomes mixed with the salt; but as the agitation of the sea by
the winds is supposed to extend to no great depth, it is only the upper
layer of salt water, and that to a moderate depth, which becomes mixed
with the fresh. The specific gravity of this upper layer, therefore, is
diminished just as much as the specific gravity of the sea-water in the
evaporating regions was increased. _And thus we have a surface current
of saltish water from the poles towards the equator, and an under
current of water salter and heavier from the equator to the poles_” (§
522).
“This property of saltness imparts to the waters of the ocean another
peculiarity, by which the sea is still better adapted for the
regulation of climates, and it is this: by evaporating fresh water from
the salt in the tropics, the surface water becomes heavier than the
average of sea-water. This heavy water is also warm water; it sinks,
and being a good retainer, but a bad conductor, of heat, this water
is employed in transporting through _under currents_ heat for the
mitigation of climates in far distant regions” (§ 526).
“For instance, let us suppose the waters in a certain part of the
torrid zone to be 90°, but by reason of the fresh water which has been
taken from them in a state of vapour, and consequently, by reason of
the proportionate increase of salts, these waters are heavier than
waters that may be cooler, but not so salt. This being the case, the
tendency would be for this warm but salt and heavy water to flow off as
an _under current towards the polar or some other regions of lighter
water_” (§ 554).
That Maury supposes the warm water at the equator to flow to the polar
regions as an under current is further evident from the fact that he
maintains that the climate of the arctic regions is mitigated by a warm
under current, which comes from the equatorial regions, and passes up
through Davis Straits (see §§ 534−544).
The question now suggests itself: to which of these two antagonistic
causes does Maury really suppose ocean-currents must be referred?
Whether does he suppose, difference in temperature or difference in
saltness, to be the real cause? I have been unable to find anything
from which we can reasonably conclude that he prefers the one cause
to the other. It would seem that he regards both as real causes, and
that he has failed to perceive that the one is destructive of the
other. But it is difficult to conceive how he could believe that the
sea in equatorial regions, by virtue of its higher temperature, is
lighter than the sea in polar regions, while at the same time it _is
not_ lighter but heavier, in consequence of its greater saltness—how
he could believe that the warm water at the equator flows to the poles
as an upper current, and the cold water at the poles to the equator
as an _under_ current, while at the same time the warm water at the
equator does not flow to the poles as a surface current, nor the cold
water at the poles to the equator as an under current, but the reverse.
And yet, unless these absolute impossibilities be possible, how can an
ocean-current be the result of both causes?
The only explanation of the matter appears to be that Maury has failed
to perceive the contradictory nature of his two theories. This fact is
particularly seen when he comes to apply his two theories to the case
of the Gulf-stream. He maintains, as has already been stated, that
the waters of the Gulf-stream are salter than the waters of the sea
through which they flow (see §§ 3, 28, 29, 30, 34, and several other
places). And he states, as we have already seen (see p. 104), that the
existence of the Gulf-stream is due principally to the difference of
density of the water of the Caribbean Sea and the Gulf of Mexico as
compared with that of the great Polar Basin and the North Sea. There
can be no doubt whatever that it is the _density_ of the waters of the
Gulf-stream at its fountain-head, the Gulf of Mexico, resulting from
its superior saltness, and the deficiency of density of the waters in
polar regions and the North Sea, &c., that is here considered to be
unmasked as one of the agents. If this be a cause of the motion of the
Gulf-stream, how then can the difference of temperature between the
waters of inter-tropical and polar regions assist as a cause? This
difference of temperature will simply tend to undo all that has been
done by difference of saltness: for it will tend to make the waters
of the Gulf of Mexico lighter, and the waters of the polar regions
heavier. But Maury maintains, as we have seen, that this difference of
temperature is also a cause, which shows that he does not perceive the
contradiction.
This is still further apparent. He holds, as stated, that “the waters
of the Gulf-stream are salter than the waters of the sea through which
they flow,” and that this excess in saltness, by making the water
heavier, is a cause of the motion of the stream. But he maintains that,
notwithstanding the effect which greater saltness has in increasing
the density of the waters of the Gulf-stream, yet, owing to their
higher temperature, they are actually lighter than the water through
which they flow; and as a proof that this is the case, he adduces the
fact that the surface of the Gulf-stream is roof-shaped (§§ 39−41),
which it could not be were its waters not actually lighter than the
waters through which the stream flows. So it turns out that, in
contradiction to what he had already stated, it is the lesser density
of the waters of the Gulf-stream that is the real cause of their
motion. The greater saltness of the waters, to which he attributes so
much, can in no way be regarded as a cause of motion. Its effect, so
far as it goes, is to stop the motion of the stream rather than to
assist it.
But, again, although he asserts that difference of saltness and
difference of temperature are both causes of ocean-currents, yet he
appears actually to admit that temperature and saltness neutralize each
other so as to prevent change in the specific gravity of the ocean, as
will be seen from the following quotation:—
“It is the trade-winds, then, which prevent the thermal and specific
gravity curves from conforming with each other in inter-tropical seas.
The water they suck up is fresh water; and the salt it contained, being
left behind, is just sufficient to counterbalance, by its weight, the
effect of thermal dilatation upon the specific gravity of sea-water
between the parallels of 34° north and south. As we go from 34° to the
equator, the water grows warmer and expands. It would become lighter;
but the trade-winds, by taking up vapour without salt, make the water
salter, and therefore heavier. The conclusion is, the proportion of
salt in sea-water, its expansibility between 62° and 82°, and the
thirst of the trade-winds for vapour are, where they blow, so balanced
as to produce _perfect compensation_; and a more beautiful compensation
cannot, it appears to me, be found in the mechanism of the universe
than that which we have here stumbled upon. It is a triple adjustment;
the power of the sun to expand, the power of the winds to evaporate,
and the quantity of salts in the sea—these are so proportioned and
adjusted that when both the wind and the sun have each played with
its forces upon the inter-tropical waters of the ocean, _the residuum
of heat and of salt should be just such as to balance each other in
their effects; and so the aqueous equilibrium of the torrid zone is
preserved_” (§ 436, eleventh edition).
“Between 35° or 40° and the equator evaporation is in excess of
precipitation; and though, as we approach the equator on either side
from these parallels, the solar ray warms and expands the surface-water
of the sea, the winds, by the vapour they carry off, and the salt they
leave behind, _prevent it from making that water lighter_” (§ 437,
eleventh edition).
“Philosophers have admired the relations between the size of the earth,
the force of gravity, and the strength of fibre in the flower-stalks of
plants; but how much more exquisite is the system of counterpoises and
adjustments here presented between the sea and its salts, the winds and
the heat of the sun!” (§ 438, eleventh edition).
How can this be reconciled with all that precedes regarding
ocean-currents being the result of difference of specific gravity
caused by a difference of temperature and difference of saltness? Here
is a distinct recognition of the fact that difference in saltness,
instead of producing currents, tends rather to prevent the existence of
currents, by counteracting the effects of difference in temperature.
And so effectually does it do this, that for 40°, or nearly 3,000
miles, on each side of the equator there is absolutely no difference in
the specific gravity of the ocean, and consequently nothing, either as
regards difference of temperature or difference of saltness, that can
possibly give rise to a current.
But it is evident that, if between the equator and latitude 40° the
two effects completely neutralize each other, it is not at all likely
that between latitude 40° and the poles they will not to a large extent
do the same thing. And if so, how can ocean-currents be due either
to difference in temperature or to difference in saltness, far less
to both. If there be any difference of specific gravity of the ocean
between latitude 40° and the poles, it must be only to the extent
by which the one cause has failed to neutralize the other. If, for
example, the waters in latitude 40°, by virtue of higher temperature,
are less dense than the waters in the polar regions, they can be so
only to the extent that difference in saltness has failed to neutralize
the effect of difference in temperature. And if currents result, they
can do so only to the extent that difference in saltness has thus
fallen short of being able to produce complete compensation. Maury,
after stating his views on compensation, seems to become aware of
this; but, strangely, he does not appear to perceive, or, at least, he
does not make any allusion to the fact, that all this is fatal to his
theories about ocean-currents being the combined result of differences
of temperature and of saltness. For, in opposition to all that he
had previously advanced regarding the difficulty of finding a cause
sufficiently powerful to account for such currents as the Gulf-stream,
and the great importance that difference in saltness had in their
production, he now begins to maintain that so great is the influence
of difference in temperature that difference in saltness, and a number
of other compensating causes are actually necessary to prevent the
ocean-currents from becoming too powerful.
“If all the inter-tropical heat of the sun,” he says, “were to pass
into the seas upon which it falls, simply raising the temperature of
their waters, it would create a thermo-dynamical force in the ocean
capable of transporting water scalding hot from the torrid zone, and
spreading it while still in the tepid state around the poles.... Now,
suppose there were no trade-winds to evaporate and to counteract the
dynamical force of the sun, this hot and light water, by becoming
hotter and lighter, would flow off in currents with almost mill-tail
velocity towards the poles, covering the intervening sea with a mantle
of warmth as a garment. The cool and heavy water of the polar basin,
coming out as under currents, would flow equatorially with equal
velocity.”
“Thus two antagonistic forces are unmasked, and, being unmasked, we
discover in them a most exquisite adjustment—a compensation—by which
the dynamical forces that reside in the sunbeam and the trade-wind
are made to counterbalance each other, by which the climates of
inter-tropical seas are regulated, and by which the set, force, and
volume of oceanic currents are measured” (§§ 437 and 438, eleventh
edition).
CHAPTER VII.
EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC
CIRCULATION.—LIEUT. MAURY’S THEORY (_continued_).
Methods of determining the Question.—The Force resulting from
Difference of Specific Gravity.—Sir John Herschel’s Estimate
of the Force.—Maximum Density of Sea-Water.—Rate of Decrease
of Temperature of Ocean at Equator.—-The actual Amount of
Force resulting from Difference of Specific Gravity.—M.
Dubuat’s Experiments.
_How the Question may be Determined._—Whether the circulation of the
ocean is due to difference in specific gravity or not may be determined
in three ways: viz. (1) by direct experiment; (2) by ascertaining the
absolute amount of _force_ acting on the water to produce motion, in
virtue of difference of specific gravity, and thereafter comparing it
with the force which has been shown by experiment to be necessary to
the production of sensible motion; or (3) by determining the greatest
possible amount of _work_ which gravity can perform on the waters in
virtue of difference of specific gravity, and then ascertaining if the
work of gravity does or does not equal the work of the resistances in
the required motion. But Maury has not adopted either of these methods.
_The Force resulting from Difference of Specific Gravity._—I shall
consider first whether the force resulting from difference of specific
gravity be sufficient to account for the motion of ocean-currents.
The inadequacy of this cause has been so clearly shown by Sir John
Herschel, that one might expect that little else would be required than
simply to quote his words on the subject, which are as follows:—
“First, then, if there were no atmosphere, there would be no
Gulf-stream, or any other considerable ocean-current (as distinguished
from a mere surface-drift) whatever. By the action of the sun’s rays,
the _surface_ of the ocean becomes _most_ heated, and the heated water
will, therefore, neither directly tend to _ascend_ (which it could
not do without leaving the sea) nor to _descend_, which it cannot do,
being rendered buoyant, nor to move laterally, no lateral impulse being
given, and which it could only do by reason of a general declivity
of surface, the dilated portion occupying a higher level. Let us see
what this declivity would amount to. The equatorial surface-water
has a temperature of 84°. At 7,200 feet deep the temperature is 39°,
the level of which temperature rises to the surface in latitude 56°.
Taking the dilatability of sea-water to be the same as that of fresh, a
uniformly progressive increase of temperature, from 39° to 84° Fahr.,
would dilate a column of 7,200 feet by 10 feet, to which height,
therefore, above the spheroid of equilibrium (or above the sea-level in
lat. 56°), the equatorial surface is actually raised by dilatation. An
arc of 56° on the earth’s surface measures 3,360 geographical miles;
so that we have a slope of 1/28th of an inch per geographical mile, or
1/32nd of an inch per statute mile for the water so raised to run down.
As the accelerating force corresponding to such a slope (of 1/10th of
a second, 0″·1) is less than one two-millionth part of gravity, we
may dismiss this as a cause capable of creating only a very trifling
surface-drift, and not worth considering, even were it in the proper
direction to form, by concentration, a current from east to west,
_which it could not be, but the very reverse_.”[55]
It is singular how any one, even though he regarded this conclusion as
but a rough approximation to the truth, could entertain the idea that
ocean-currents can be the result of difference in specific gravity.
There are one or two reasons, however, which may be given for the
above not having been generally received as conclusive. Herschel’s
calculations refer to the difference of gravity resulting from
difference of temperature; but this is only one of the causes to which
Maury appeals, and even not the one to which he most frequently refers.
He insists so strongly on the effects of difference of saltness, that
many might think that, although Herschel may have shown that difference
in specific gravity arising from difference of temperature could not
account for the motion of ocean-currents, yet nevertheless that this,
combined with the effects resulting from difference in saltness, might
be a sufficient explanation of the phenomena. Such, of course, would
not be the case with those who perceived the contradictory nature of
Maury’s two causes; but probably many read the “Physical Geography of
the Sea” without being aware that the one cause is destructive of the
other. Again, a few plausible objections, which have never received due
consideration, have been strongly urged by Maury and others against the
theory that ocean-currents can be caused by the impulses of the winds;
and probably these objections appear to militate as strongly against
this theory as Herschel’s arguments against Maury’s.
There is one trifling objection to Herschel’s result: he takes 39° as
the temperature of maximum density. This, however, as we shall see,
does not materially affect his conclusions.
Observations on the temperature of the maximum density of sea-water
have been made by Erman, Despretz, Rossetti, Neumann, Marcet, Hubbard,
Horner, and others. No two of them have arrived at exactly the same
conclusion. This probably arises from the fact that the temperature
of maximum density depends upon the amount of salt held in solution.
No two seas, unless they are equal as to saltness, have the same
temperature of maximum density. The following Table of Despretz will
show how rapidly the temperature of both the freezing-point and of
maximum density is lowered by additional amounts of salt:—
+-----------+-----------------+------------------+
| Amount | Temperature of | Temperature of |
| of salt. | freezing-point. | Maximum density. |
+-----------+-----------------+------------------+
| | ° | ° |
| 0·000123 | −1·21 C. | + 1·19 C. |
| 0·0246 | −2·24 | − 1·69 |
| 0·0371 | −2·77 | − 4·75 |
| 0·0741 | −5·28 | −16·00 |
+-----------+-----------------+------------------+
He found the temperature of maximum density of sea-water, whose density
at 20°C. was 1·0273, to be −3°·67C. (25°·4F.), and the temperature of
freezing-point −2°·55C. (27°·4F.).[56] Somewhere between 25° and 26°
F. may therefore be regarded as the temperature of maximum density
of sea-water of average saltness. We have no reason to believe that
the ocean, from the surface to the bottom, even at the poles, is at
27°·4F., the freezing-point.
The actual slope resulting from difference of specific gravity,
as we shall presently see, does not amount to 10 feet. Herschel’s
estimate was, however, made on insufficient data, both as to the rate
of expansion of sea-water and that at which the temperature of the
ocean at the equator decreases from the surface downwards. We are
happily now in the possession of data for determining with tolerable
accuracy the amount of slope due to difference of temperature between
the equatorial and polar seas. The rate of expansion of sea-water
from 0°C. to 100°C. has been experimentally determined by Professor
Muncke, of Heidelberg.[57] The valuable reports of Captain Nares, of
H.M.S. _Challenger_, lately published by the Admiralty, give the rate
at which the temperature of the Atlantic at the equator decreases
from the surface downwards. These observations show clearly that the
super-heating effect of the sun’s rays does not extend to any great
depth. They also prove that at the equator the temperature decreases
as the depth increases so rapidly that at 60 fathoms from the surface
the temperature is 62°·4, the same as at Madeira at the same depth;
while at the depth of 150 fathoms it is only 51°, about the same as
that in the Bay of Biscay (Reports, p. 11). Here at the very outset
we have broad and important facts hostile to the theory of a flow of
water resulting from difference of temperature between the ocean in
equatorial and temperate and polar regions.
Through the kindness of Staff-Captain Evans, Hydrographer of the
Admiralty, I have been favoured with a most valuable set of serial
temperature soundings made by Captain Nares of the _Challenger_, close
to the equator, between long. 14° 49′ W. and 32° 16′ W. The following
Table represents the mean of the whole of these observations:—
+----------+-------------+
| Fathoms. | Temperature.|
+----------+-------------+
| | ° |
| Surface. | 77·9 |
| 10 | 77·2 |
| 20 | 77·1 |
| 30 | 76·9 |
| 40 | 71·7 |
| 50 | 64·0 |
| 60 | 60·4 |
| 70 | 59·4 |
| 80 | 58·0 |
| 90 | 58·0 |
| 100 | 55·6 |
| 150 | 51·0 |
| 200 | 46·6 |
| 300 | 42·2 |
| 400 | 40·3 |
| 500 | 38·9 |
| 600 | 39·2 |
| 700 | 39·0 |
| 800 | 39·1 |
| 900 | 38·2 |
| 1000 | 36·9 |
| 1100 | 37·6 |
| 1200 | 36·7 |
| 1300 | 35·8 |
| 1400 | 36·4 |
| 1500 | 36·1 |
| Bottom. | 34·7 |
+----------+-------------+
We have in this Table data for determining the height at which the
surface of the ocean at the equator ought to stand above that of the
poles. Assuming 32°F. to be the temperature of the ocean at the poles
from the surface to the bottom and the foregoing to be the rate at
which the temperature of the ocean at the equator decreases from the
surface downwards, and then calculating according to Muncke’s Table of
the expansion of sea-water, we have only 4 feet 6 inches as the height
to which the level of the ocean at the equator ought to stand above
that at the poles in order that the ocean may be in static equilibrium.
In other words, the equatorial column requires to be only 4 feet 6
inches higher than the polar in order that the two may balance each
other.
Taking the distance from the equator to the poles at 6,200 miles, the
force resulting from the slope of 4½ feet in 6,200 will amount to only
1/7,340,000th that of gravity, or about 1/1000th of a grain on a pound
of water. But, as we shall shortly see, there can be no permanent
current resulting from difference of temperature while the two columns
remain in equilibrium, for the current is simply an effort to the
retardation of equilibrium. In order to have permanent circulation
there must be a permanent disturbance of equilibrium. Or, in other
words, the weight of the polar column must be kept in excess of that
of the equatorial. Suppose, then, that the weight of the polar column
exceeds that of the equatorial by 2 feet of water, the difference of
level between the two columns will, in that case, amount to only 2
feet 6 inches. This would give a force of only 1/13,200,000th that of
gravity, or not much over 1/1,900th of a grain on a pound of water,
tending to draw the water down the slope from the equator to the poles,
a force which does not much exceed the weight of a grain on a ton of
water. But it must be observed that this force of a grain per ton would
affect only the water at the surface; a very short distance below the
surface the force, small as it is, would be enormously reduced. If
water were a perfect fluid, and offered no resistance to motion, it
would not only flow down an incline, however small it might be, but
would flow down with an accelerated motion. But water is not a perfect
fluid, and its molecules do offer considerable resistance to motion.
Water flowing down an incline, however steep it may be, soon acquires
a uniform motion. There must therefore be a certain inclination below
which no motion can take place. Experiments were made by M. Dubuat
with the view of determining this limit.[58] He found that when the
inclination was 1 in 500,000, the motion of the water was barely
perceptible; and he came to the conclusion that when the inclination
is reduced to 1 in 1,000,000, all motion ceases. But the inclination
afforded by the difference of temperature between the sea in equatorial
and polar regions does not amount to one-seventh of this, and
consequently it can hardly produce even that “trifling surface-drift”
which Sir John Herschel is willing to attribute to it.
There is an error into which some writers appear to fall to which I
may here refer. Suppose that at the equator we have to descend 10,000
feet before water equal in density to that at the poles is reached. We
have in this case a plain with a slope of 10,000 feet in 6,200 miles,
forming the upper surface of the water of maximum density. Now this
slope exercises no influence in the way of producing a current, as some
seem to think; for it is not a case of disturbed equilibrium, but the
reverse. It is the condition of static equilibrium resulting from a
difference between the temperature of the water at the equator and the
poles. The only slope that has any tendency to produce motion is that
which is formed by the surface of the ocean in the equatorial regions
being higher than the surface at the poles; but this is an inclination
of only 4 feet 6 inches, and is therefore wholly inadequate to produce
such currents as the Gulf-stream.
CHAPTER VIII.
EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC
CIRCULATION.—DR. CARPENTER’S THEORY.
Gulf-stream according to Dr. Carpenter not due to Difference of
Specific Gravity.—Facts to be Explained.—The Explanation of
the Facts.—The Explanation hypothetical.—The Cause assigned
for the hypothetical Mode of Circulation.—Under currents
account for all the Facts better than the Gravitation
Hypothesis.—Known Condition of the Ocean inconsistent with
that Hypothesis.
Dr. Carpenter does not suppose, with Lieut. Maury, that the difference
of temperature between the ocean in equatorial and polar regions can
account for the Gulf-stream and other great currents of the ocean.
He maintains, however, that this difference is quite sufficient to
bring about a slow general interchange of water between the polar and
inter-tropical areas—to induce a general movement of the upper portion
of the ocean from the equator to the poles and a counter-movement of
the under portion in a contrary direction. It is this general movement
which, according to that author, is the great agent by which heat is
distributed over the globe.[59]
In attempting to estimate the adequacy of this hypothesis as an
explanation of the phenomena involved, there are obviously two
questions to be considered: namely, (1) is the difference of
temperature between the sea in inter-tropical and polar regions
sufficiently great to produce the required movement? and (2) assuming
that there is such a movement, does it convey the amount of heat which
Dr. Carpenter supposes? I shall begin with the consideration of the
first of these two points.
But before doing so let us see what the facts are which this
gravitation theory is intended to explain.
_The Facts to be Explained._—Dr. Carpenter considers that the great
mass of warm water proved during recent dredging expeditions to
occupy the depths of the North Atlantic, must be referred, not to the
Gulf-stream, but to a general movement of water from the equator. “The
inference seems inevitable,” he says, “that the bulk of the water in
the warm area must have come thither from the south-west. The influence
of the Gulf-stream proper (meaning by this the body of super-heated
water which issues through the ‘Narrows’ from the Gulf of Mexico), if
it reaches this locality at all (which is very doubtful), could only
affect the _most superficial_ stratum; and the same may be said of
the surface-drift caused by the prevalence of south-westerly winds,
to which some have attributed the phenomena usually accounted for by
the extension of the Gulf-stream to these regions. And the presence
of the body of water which lies between 100 and 600 fathoms deep, and
the range of whose temperature is from 48° to 42°, can scarcely be
accounted for on any other hypothesis than that of a _great general
movement of equatorial water towards the polar area_, of which
movement the Gulf-stream constitutes a peculiar case modified by local
conditions. In like manner the Arctic stream which underlies the warm
superficial stratum in our cold area constitutes a peculiar case,
modified by the local conditions to be presently explained, of _a great
general movement of polar water towards the equatorial area_, which
depresses the temperature of the deepest parts of the great oceanic
basins nearly to the freezing-point.”
It is well-known that, wherever temperature-observations have been
made in the Atlantic, the bottom of that ocean has been found to be
occupied by water of an ice-cold temperature. And this holds true
not merely of the Atlantic, but also of the ocean in inter-tropical
regions—a fact which has been proved by repeated observations, and more
particularly of late by those of Commander Chimmo in the China Sea and
Indian Ocean, where a temperature as low as 32° Fahr. was found at a
depth of 2,656 fathoms. In short, the North Atlantic, and probably the
inter-tropical seas also, may be regarded, Dr. Carpenter considers, as
divided horizontally into two great layers or strata—an upper warm, and
a lower cold stratum. All these facts I, of course, freely admit; nor
am I aware that their truth has been called in question by any one, no
matter what his views may have been as to the mode in which they are to
be explained.
_The Explanation of the Facts._—We have next the explanation of the
facts, which is simply this:—The cold water occupying the bottom of
the Atlantic and of inter-tropical seas is to be accounted for by the
supposition that _it came from the polar regions_. This is obvious,
because the cold possessed by the water could not have been derived
from the crust of the earth beneath: neither could it have come from
the surface; for the temperature of the bottom water is far below the
normal temperature of the latitude in which it is found. Consequently
“the inference seems irresistible that this depression must be produced
and maintained by the convection of cold from the polar towards the
equatorial area.” Of course, if we suppose a flow of water from the
poles towards the equator, we must necessarily infer a counter flow
from the equator towards the poles; and while the water flowing from
equatorial to polar regions will be _warm_, that flowing from polar to
equatorial regions will be _cold_. The doctrine of a mutual interchange
of equatorial and polar water is therefore a _necessary consequence_
from the admission of the foregoing facts. With this _explanation
of the facts_ I need hardly say that I fully agree; nor am I aware
that its correctness has ever been disputed. Dr. Carpenter surely
cannot charge me with overlooking the fact of a mutual interchange of
equatorial and polar water, seeing that my estimate of the thermal
power of the Gulf-stream, from which it is proved that the amount
of heat conveyed from equatorial to temperate and polar regions
is enormously greater than had ever been anticipated, was made a
considerable time before he began to write on the subject of oceanic
circulation.[60] And in my paper “On Ocean-currents in relation to the
Distribution of Heat over the Globe”[61] (the substance of which is
reproduced in Chapters II. and III. of this volume), I have endeavoured
to show that, were it not for the raising of the temperature of polar
and high temperate regions and the lowering of the temperature of
inter-tropical regions by means of this interchange of water, these
portions of the globe would not be habitable by the present existing
orders of beings.
The explanation goes further:—“It is along the surface and upper
portion of the ocean that the equatorial waters flow towards the
poles, and it is along the bottom and under portion of the ocean that
polar waters flow towards the equator; or, in other words, the warm
water keeps the _upper_ portion of the ocean and the cold water the
_under_ portion.” With this explanation I to a great extent agree. It
is evident that, in reference to the northern hemisphere at least, the
most of the water which flows from inter-tropical to polar regions
(as, for example, the Gulf-stream) keeps to the surface and upper
portion of the ocean; but for reasons which I have already stated, a
very large proportion of this water must return in the form of _under_
currents; or, which is the same thing, the return compensating current,
whether it consist of the identical water which originally came from
the equator or not, must flow towards the equator as an under current.
That the cold water which is found at the bottom of the Atlantic and
of inter-tropical seas must have come as under currents is perfectly
obvious, because water which should come along the surface of the ocean
from the polar regions would not be cold when it reached inter-tropical
regions.
_The Explanation hypothetical._—Here the general agreement between
us in a great measure terminates, for Dr. Carpenter is not satisfied
with the explanation generally adopted by the advocates of the
_wind theory_, viz., that the cold water found in temperate and
inter-tropical areas comes from polar regions as compensating under
currents, but advances a _hypothetical_ form of circulation to account
for the phenomenon. He assumes that there is a _general set_ or flow of
the surface and upper portion of the ocean from the equator to polar
regions, and a _general set_ or flow of the bottom and under portion of
the ocean from polar regions to the equator. Mr. Ferrel (_Nature_, June
13, 1872) speaks of that “interchanging motion of the water between the
equator and the pole _discovered_ by Dr. Carpenter.” In this, however,
Mr. Ferrel is mistaken; for Dr. Carpenter not only makes no claim to
any discovery of the kind, but distinctly admits that none such has
yet been made. Although in some of his papers he speaks of a “_set_ of
warm surface-water in the southern oceans toward the Antarctic pole”
as being well known to navigators, yet he nowhere affirms, as far as I
know, that the existence of such a general oceanic circulation as he
advocates has ever been directly determined from observations. This
mode of circulation is _simply inferred_ or _assumed_ in order to
account for the facts referred to above. “At present,” Dr. Carpenter
says, “I claim for it no higher character than that of a good working
_hypothesis_ to be used as a guide in further inquiry” (§ 16); and lest
there should be any misapprehension on this point, he closes his memoir
thus:—“At present, as I have already said, I claim for the doctrine of
a general oceanic circulation no higher a character than that of a good
working _hypothesis_ consistent with our present knowledge of facts,
and therefore entitled to be _provisionally_ adopted for the purpose of
stimulating and directing further inquiry.”
I am unable to agree with him, however, on this latter point. It
seems to me that there is no necessity for adopting any hypothetical
mode of circulation to account for the facts, as they can be quite
well accounted for by means of that mode of circulation which does
_actually exist_. It has been determined from direct observation that
surface-currents flow from equatorial to polar regions, and their
paths have been actually mapped out. But if it is established that
currents flow from equatorial to polar regions, it is equally so that
return currents flow from polar to equatorial regions; for if the one
_actually_ exists, the other of necessity _must_ exist. We know also
on physical grounds, to which I have already referred, and which fall
to be considered more fully in a subsequent chapter, that a very large
portion of the water flowing from polar to equatorial regions must
be in the form of under currents. If there are cold under currents,
therefore, flowing from polar to temperate and equatorial regions,
this is all that we really require to account for the cold water which
is found to occupy the bed of the ocean in those regions. It does not
necessarily follow, because cold water may be found at the bottom of
the ocean all along the equator, that there must be a direct flow
from the polar regions to every point of the equator. Water brought
constantly from the polar regions to various points along the equator
by means of under currents will necessarily accumulate, and in course
of time spread over the bottom of the inter-tropical seas. It must
either do this, or the currents on reaching the equator must bend
upwards and flow to the surface in an unbroken mass. Considerable
portions of some of those currents may no doubt do so and join
surface-currents; but probably the greater portion of the water coming
from polar regions extends itself over the floor of the equatorial
seas. In a letter in _Nature_, January 11, 1872, I endeavoured to show
that the surface-currents of the ocean are not separate and independent
of one another, but form one grand system of circulation, and that
the impelling cause keeping up this system of circulation is not the
_trade-winds_ alone, as is generally supposed, but the _prevailing
winds of the entire globe considered also as one grand system_. The
evidence for this opinion, however, will be considered more fully in
the sequel.
Although the under currents are parts of one general system of oceanic
circulation produced by the impulse of the system of prevailing winds,
yet their direction and position are nevertheless, to a large extent,
determined by different laws. The water at the surface, being moved
by the force of the wind, will follow the path of _greatest pressure
and traction_,—the effects resulting from the general contour of the
land, which to a great extent are common to both sets of currents, not
being taken into account; while, on the other hand, the under currents
from polar regions (which to a great extent are simply “indraughts”
compensating for the water drained from equatorial regions by the
Gulf-stream and other surface currents) will follow, as a general rule,
the path of _least resistance_.
_The Cause assigned for the Hypothetical Mode of Circulation._—Dr.
Carpenter assigns a cause for his mode of circulation; and that cause
he finds in the difference of specific gravity between equatorial
and polar waters, resulting from the difference of temperature
between these two regions. “Two separate questions,” he says, “have
to be considered, which have not, perhaps, been kept sufficiently
distinct, either by Mr. Croll or by myself;—_first_, whether there
is adequate evidence of the existence of a general vertical oceanic
circulation; and _second_, whether, supposing its existence to be
provisionally admitted, a _vera causa_ can be found for it in the
difference of temperature between the oceanic waters of the polar and
equatorial areas” (§ 17). It seems to me that the facts adduced by
Dr. Carpenter do not necessarily require the assumption of any such
mode of circulation as that advanced by him. The phenomena can be
satisfactorily accounted for otherwise; and therefore there does not
appear to be any necessity for considering whether his hypothesis be
sufficient to produce the required effect or not.
_An important Consideration overlooked._—But there is one important
consideration which seems to have been overlooked—namely, the fact
that the sea is salter in inter-tropical than in polar regions, and
that this circumstance, so far as it goes, must tend to neutralize
the effect of difference of temperature. It is probable, indeed, that
the effect produced by difference of temperature is thus entirely
neutralized, and that no difference of density whatever exists between
the sea in inter-tropical and polar regions, and consequently that
there is no difference of level nor anything to produce such a general
motion as Dr. Carpenter supposes. This, I am glad to find, is the
opinion of Professor Wyville Thomson.
“I am greatly mistaken,” says that author, “if the low specific gravity
of the polar sea, the result of the condensation and precipitation
of vapour evaporated from the inter-tropical area, do not fully
counterbalance the contraction of the superficial film by arctic
cold.... Speaking in the total absence of all reliable data, it is my
general impression that if we were to set aside all other agencies, and
to trust for an oceanic circulation to those conditions only which are
relied upon by Dr. Carpenter, if there were any general circulation at
all, which seems very problematical, the odds are rather in favour of
a warm under current travelling northwards by virtue of its excess of
salt, balanced by a surface return current of fresher though colder
arctic water.”[62]
This is what actually takes place on the west and north-west of
Spitzbergen. There the warm water of the Gulf-stream flows underneath
the cold polar current. And it is the opinion of Dr. Scoresby, Mr.
Clements Markham, and Lieut. Maury that this warm water, in virtue
of its greater saltness, is denser than the polar water. Mr. Leigh
Smith found on the north-west of Spitzbergen the temperature at 500
fathoms to be 52°, and once even 64°, while the water on the surface
was only a degree or two above freezing.[63] Mr. Aitken, of Darroch,
in a paper lately read before the Royal Scottish Society of Arts,
showed experimentally that the polar water in regions where the ice is
melting is actually less dense than the warm and more salt tropical
waters. Nor will it help the matter in the least to maintain that
difference of specific gravity is not the reason why the warm water of
the Gulf-stream passes under the polar stream—because if difference
of specific gravity be not the cause of the warm water underlying the
cold water in polar regions, then difference of specific gravity may
likewise not be the cause of the cold water underlying the warm at
the equator; and if so, then there is no necessity for the gravitation
hypothesis of oceanic circulation.
There is little doubt that the super-heated stratum at the surface of
the inter-tropical seas, which stratum, according to Dr. Carpenter,
is of no great thickness, is less dense than the polar water: but if
we take a column extending from the surface down to the bottom of the
ocean, this column at the equator will be found to be as heavy as one
of equal length in the polar area. And if this be the case, then there
can be no difference of level between the equator and the poles, and
no disturbance of static equilibrium nor anything else to produce
circulation.
_Under Currents account for all the Facts better than Dr. Carpenter’s
Hypothesis._—Assuming, for the present, the system of prevailing winds
to be the true cause of oceanic currents, it necessarily follows (as
will be shown hereafter) that a large quantity of Atlantic water must
be propelled into the Arctic Ocean; and such, as we know, is actually
the case. The Arctic Ocean, however, as Professor Wyville Thomson
remarks, is a well-nigh closed basin, not permitting of a free outflow
into the Pacific Ocean of the water impelled into it.
But it is evident that the water which is thus being constantly
carried from the inter-tropical to the arctic regions must somehow
or other find its way back to the equator; in other words, there
must be a return current equal in magnitude to the direct current.
Now the question to be determined is, what path must this return
current take? It appears to me that it will take the _path of least
resistance_, whether that path may happen to be at the surface or under
the surface. But that the path of least resistance will, as a general
rule, lie at a very considerable distance below the surface is, I
think, evident from the following considerations. At the surface the
general direction of the currents is opposite to that of the return
current. The surface motion of the water in the Atlantic is from the
equator to the pole; but the return current must be from the pole to
the equator. Consequently the surface currents will oppose the motion
of any return current unless that current lie at a considerable depth
below the surface currents. Again, the winds, as a general rule, blow
in an opposite direction to the course of the return current, because,
according to supposition, the winds blow in the direction of the
surface currents. From all these causes the path of least resistance to
the return current will, as a general rule, not be at the surface, but
at a very considerable depth below it.
A large portion of the water from the polar regions no doubt leaves
those regions as surface currents; but a surface current of this kind,
on meeting with some resistance to its onward progress along the
surface, will dip down and continue its course as an under current. We
have an example of this in the case of the polar current, which upon
meeting the Gulf-stream on the banks of Newfoundland divides—a portion
of it dipping down and pursuing its course underneath that stream into
the Gulf of Mexico and the Caribbean Sea. And that this under current
is a real and tangible current, in the proper sense of the term, and
not an imperceptible movement of the water, is proved by the fact that
large icebergs deeply immersed in it are often carried southward with
considerable velocity against the united force of the wind and the
Gulf-stream.
Dr. Carpenter refers at considerable length (§ 134) to Mr. Mitchell’s
opinion as to the origin of the polar current, which is the same as
that advanced by Maury, viz., that the impelling cause is difference
of specific gravity. But although Dr. Carpenter quotes Mr. Mitchell’s
opinion, he nevertheless does not appear to adopt it: for in §§ 90−93
and various other places he distinctly states that he does not agree
with Lieut. Maury’s view that the Gulf-stream and polar current
are caused by difference of density. In fact, Dr. Carpenter seems
particularly anxious that it should be clearly understood that he
dissents from the theory maintained by Maury. But he does not merely
deny that the Gulf-stream and polar current can be caused by difference
of density; he even goes so far as to affirm that no sensible current
whatever can be due to that cause, and adduces the authority of Sir
John Herschel in support of that opinion:—“The doctrine of Lieut.
Maury,” he says, “was powerfully and convincingly opposed by Sir
John Herschel; who showed, beyond all reasonable doubt, first, that
the Gulf-stream really has its origin in the propulsive force of the
trade-winds, and secondly, that the greatest disturbance of equilibrium
which can be supposed to result from the agencies invoked by Lieut.
Maury would be utterly inadequate to generate and maintain either the
Gulf-stream or any other sensible current” (§ 92). This being Dr.
Carpenter’s belief, it is somewhat singular that he should advance the
case of the polar current passing under the Gulf-stream as evidence
in favour of his theory; for in reality he could hardly have selected
a case more hostile to that theory. In short, it is evident that, if
a polar current impelled by a force other than that of gravity can
pass from the banks of Newfoundland to the Gulf of Mexico (a distance
of some thousands of miles) under a current flowing in the opposite
direction and, at the same time, so powerful as the Gulf-stream, it
could pass much more easily under comparatively still water, or water
flowing in the same direction as itself. And if this be so, then all
our difficulties disappear, and we satisfactorily explain the presence
of cold polar water at the bottom of inter-tropical seas without having
recourse to the hypothesis advanced by Dr. Carpenter.
But we have an example of an under current more inexplicable on the
gravitation hypothesis than even that of the polar current, viz., the
warm under current of Davis Strait.
There is a strong current flowing north from the Atlantic through Davis
Strait into the Arctic Ocean underneath a surface current passing
southwards in an opposite direction. Large icebergs have been seen to
be carried northwards by this under current at the rate of four knots
an hour against both the wind and the surface current, ripping and
tearing their way with terrific force through surface ice of great
thickness.[64] A current so powerful and rapid as this cannot, as Dr.
Carpenter admits, be referred to difference of specific gravity. But
even supposing that it could, still difference of temperature between
the equatorial and polar seas would not account for it; for the current
in question flows in the _wrong direction_. Nor will it help the matter
the least to adopt Maury’s explanation, viz., that the warm under
current from the south, in consequence of its greater saltness, is
denser than the cold one from the polar regions. For if the water of
the Atlantic, notwithstanding its higher temperature, is in consequence
of its greater saltness so much denser than the polar water on the
west of Greenland as to produce an under current of four knots an hour
in the direction of the pole, then surely the same thing to a certain
extent will hold true in reference to the ocean on the east side of
Greenland. Thus instead of there being, as Dr. Carpenter supposes,
an underflow of polar water south into the Atlantic in virtue of its
_greater_ density, there ought, on the contrary, to be a surface flow
in consequence of its lesser density.
The true explanation no doubt is, that the warm under current from
the south and the cold upper current from the north are both parts
of one grand system of circulation produced by the winds, difference
of specific gravity having no share whatever either in impelling the
currents, or in determining which shall be the upper and which the
lower.
The wind in Baffin’s Bay and Davis Strait blows nearly always in one
direction, viz. from the north. The tendency of this is to produce a
surface or upper current from the north down into the Atlantic, and to
prevent or retard any surface current from the south. The warm current
from the Atlantic, taking the path of least resistance, dips under the
polar current and pursues its course as an under current.
Mr. Clements Markham, in his “Threshold of the Unknown Region,” is
inclined to attribute the motion of the icebergs to tidal action or
to counter under currents. That the motion of the icebergs cannot
reasonably be attributed to the tides is, I think, evident from the
descriptions given both by Midshipman Griffin and by Captain Duncan,
who distinctly saw the icebergs moving at the rate of about four knots
an hour against a surface current flowing southwards. And Captain
Duncan states that the bergs continued their course northwards for
several days, till they ultimately disappeared. The probability is that
this northward current is composed partly of Gulf-stream water and
partly of that portion of polar water which is supposed to flow round
Cape Farewell from the east coast of Greenland. This stream, composed
of both warm and cold water, on reaching to about latitude 65°N., where
it encounters the strong northerly winds, dips down under the polar
current and continues its northward course as an under current.
We have on the west of Spitzbergen, as has already been noticed, a
similar example of a warm current from the south passing under a polar
current. A portion of the Gulf-stream which passes round the west
coast of Spitzbergen flows under an arctic current coming down from
the north; and it does so no doubt because it is here in the region of
prevailing northerly winds, which favour the polar current but oppose
the Gulf-stream. Again, we have a cold and rapid current sweeping
round the east and south of Spitzbergen, a current of which Mr. Lamont
asserts that he is positive he has seen it running at the rate of seven
or eight miles an hour. This current, on meeting the Gulf-stream about
the northern entrance to the German Ocean, dips down under that stream
and pursues its course southwards as an under current.
Several other cases of under currents might be adduced which cannot
be explained on the gravitation theory, and which must be referred to
a system of oceanic circulation produced by the impulse of the wind;
but these will suffice to show that the assumption that the winds can
produce only a mere surface-drift is directly opposed to facts. And
it will not do to affirm that a current which forms part of a general
system of circulation produced by the impulse of the winds cannot
possibly be an under current; for in the case referred to we have
proof that the thing is not only possible but actually exists. This
point, however, will be better understood after we have considered the
evidence in favour of a general system of oceanic currents.
Much of the difficulty experienced in comprehending how under currents
can be produced by the wind, or how an impulse imparted to the surface
of the ocean can ever be transmitted to the bottom, appears to me to
result, to a considerable extent at least, from a slight deception
of the imagination. The thing which impresses us most forcibly in
regard to the ocean is its profound depth. A mean depth of, say, three
miles produces a striking impression; but if we could represent to
the mind the vast area of the ocean as correctly as we can its depth,
_shallowness_ rather than _depth_ would be the impression produced. If
in crossing a meadow we found a sheet of water one hundred yards in
diameter and only an inch in depth, we should not call that a _deep_,
but a very _shallow_ pool. The probability is that we should speak of
it as simply a piece of ground covered with a thin layer of water.
Yet such a thin layer of water would be a correct representation in
miniature of the ocean; for the ocean in relation to its superficial
area is as shallow as the pool of our illustration. In reference to
such a pool or thin film of water, we have no difficulty in conceiving
how a disturbance on its surface would be transmitted to its bottom.
In fact our difficulty is in conceiving how any disturbance extending
over its entire surface should not extend to the bottom. Now if we
could form as accurate a sensuous impression of the vast area of the
ocean as we do of such a pool, all our difficulty in understanding how
the impulses of the wind acting on the vast area of the ocean should
communicate motion down to its bottom would disappear. It is certainly
true that sudden commotions caused by storms do not generally extend to
great depths. Neither will winds of short continuance produce a current
extending far below the surface. But prevailing winds which can produce
such immense surface-flow as that of the great equatorial currents of
the globe and the Gulf-stream, which follow definite directions, must
communicate their motion to great depths, unless water be frictionless,
a thing which it is not. Suppose the upper layer of the ocean to be
forced on by the direct action of the winds with a constant velocity
of, say, four miles an hour, the layer immediately below will be
dragged along with a constant velocity somewhat less than four miles
an hour. The layer immediately below this second layer will in turn be
also dragged along with a constant velocity somewhat less than the one
above it. The same will take place in regard to each succeeding layer,
the constant velocity of each layer being somewhat less than the one
immediately above it, and greater than the one below it. The question
to be determined is, at what depth will all motion cease? I presume
that at present we have not sufficient data for properly determining
this point. The depth will depend, other things being equal, upon the
amount of molecular resistance offered by the water to motion—in other
words, on the amount of the shearing-force of the one layer over the
other. The fact, however, that motion imparted to the surface will
extend to great depths can be easily shown by direct experiment. If a
constant motion be imparted to the surface of water, say, in a vessel,
motion will ultimately be communicated to the bottom, no matter how
wide or how deep the vessel may be. The same effect will take place
whether the vessel be 5 feet deep or 500 feet deep.
_The known Condition of the Ocean inconsistent with Dr. Carpenter’s
Hypothesis._—Dr. Carpenter says that he looks forward with great
satisfaction to the results of the inquiries which are being prosecuted
by the Circumnavigation Expedition, in the hope that the facts brought
to light may establish his theory of a general oceanic circulation; and
he specifies certain of these facts which, if found to be correct, will
establish his theory. It seems to me, however, that the facts to which
he refers are just as explicable on the theory of under currents as on
the theory of a general oceanic circulation. He begins by saying, “If
the views I have propounded be correct, it may be expected that near
the border of the great antarctic ice-barrier a temperature below 30°
will be met with (as it has been by Parry, Martens, and Weyprecht near
Spitzbergen) at no great depth beneath the surface, and that instead of
rising at still greater depths, the thermometer will fall to near the
freezing-point of salt water” (§ 39).
Dr. Carpenter can hardly claim this as evidence in favour of his
theory; for near the borders of the ice-barrier the water, as a matter
of course, could not be expected to have a much higher temperature than
the ice itself. And if the observations be made during summer months,
the temperature of the water at the surface will no doubt be found to
be higher than that of the bottom; but if they be carried on during
winter, the surface-temperature will doubtless be found to be as low as
the bottom-temperature. These are results which do not depend upon any
particular theory of oceanic circulation.
“The bottom temperature of the North Pacific,” he continues, “will
afford a crucial test of the truth of the doctrine. For since the sole
communication of this vast oceanic area with the arctic basin is a
strait so shallow as only to permit an inflow of warm surface water,
its deep cold stratum must be entirely derived from the antarctic area;
and if its bottom temperature is not actually higher than that of the
South Pacific, the glacial stratum ought to be found at a greater depth
north of the equator than south of it” (§ 39).
This may probably show that the water came from the antarctic regions,
but cannot possibly prove that it came in the manner which he supposes.
“In the North Atlantic, again, the comparative limitation of
communication with the arctic area may be expected to prevent its
bottom temperature from being reduced as low as that of the Southern
Atlantic” (§ 39). Supposing the bottom temperature of the South
Atlantic should be found to be lower than the bottom temperature of the
North Atlantic, this fact will be just as consistent with the theory of
under currents as with his theory of a general movement of the ocean.
I am also wholly unable to comprehend how he should imagine, because
the bottom temperature of the South Atlantic happens to be lower, and
the polar water to lie nearer to the surface in this ocean than in the
North Atlantic, that therefore this proves the truth of his theory.
This condition of matters is just as consistent, and even more so, as
will be shown in Chapter XIII., with my theory as with his. When we
consider the immense quantity of warm surface water which, as has been
shown (Chapter V.), is being constantly transferred from the South into
the North Atlantic, we readily understand how the polar water comes
nearer to the surface in the former ocean than in the latter. Every
pound of water, of course, passing from the southern to the northern
hemisphere must be compensated by an equal amount passing from the
northern to the southern hemisphere. But nevertheless the warm water
drained off the South Atlantic is not replaced directly by water from
the north, but by that cold antarctic current, the existence of which
is, unfortunately, too well known to navigators from the immense masses
of icebergs which it brings along with it. In fact, the whole of the
phenomena are just as easily explained upon the principle of under
currents as upon Dr. Carpenter’s theory. But we shall have to return to
this point in Chapter XIII., when we come to discuss a class of facts
which appear to be wholly irreconcilable with the gravitation theory.
Indeed I fear that even although Dr. Carpenter’s expectations should
eventually be realised in the results of the Circumnavigation
Expedition, yet the advocates of the wind theory will still remain
unconverted. In fact the Director of this Expedition has already, on
the wind theory, offered an explanation of nearly all the phenomena
on which Dr. Carpenter relies;[65] and the same has also been done by
Dr. Petermann,[66] who, as is well known, is equally opposed to Dr.
Carpenter’s theory. Dr. Carpenter directs attention to the necessity of
examining the broad and deep channel separating Iceland from Greenland.
The observations which have already been made, however, show that
nearly the entire channel is occupied, on the surface at least, by
water flowing southward from the polar area—a direction the opposite of
what it ought to be according to the gravitation theory. In fact the
surface of one half of the entire area of the ocean, extending from
Greenland to the North Cape, is moving in a direction the opposite of
that which it ought to take according to the theory under review. The
western half of this area is occupied by water which at the surface is
flowing southwards; while the eastern half, which has hitherto been
regarded by almost everybody but Dr. Carpenter himself and Mr. Findlay
as an extension of the Gulf-stream, is moving polewards. The motion of
the western half must be attributed to the winds and not to gravity;
for it is moving in the wrong direction to be accounted for by the
latter cause; but had it been moving in the opposite direction, no
doubt its motion would have been referred to gravitation. To this cause
the motion of the eastern half, which is in the proper direction, is
attributed;[67] but why not assign this motion also to the impulse of
the winds, more especially since the direction of the prevailing winds
blowing over that area coincides with that of the water? If the wind
can produce the motion of the water in the western half, why may not it
do the same in the eastern half?
If there be such a difference of density between equatorial and polar
waters as to produce a general flow of the upper portion of the ocean
poleward, how does it happen that one half of the water in the above
area is moving in opposition to gravity? How is it that in a wide
open sea gravitation should act so powerfully in the one half of it
and with so little effect in the other half? There is probably little
doubt that the ice-cold water of the western half extends from the
surface down to the bottom. And it is also probable that the bottom
water is moving southwards in the same direction as the surface water.
The bottom water in such a case would be moving in harmony with the
gravitation theory; but would Dr. Carpenter on this account attribute
its motion to gravity? Would he attribute the motion of the lower half
to gravity and the upper half to the wind? He could not in consistency
with his theory attribute the motion of the upper half to gravity: for
although the ice-cold water extended to the surface, this could not
explain how gravity should move it southward instead of polewards, as
according to theory it ought to move. He might affirm, if he chose,
that the surface water moves southwards because it is dragged forward
by the bottom water; but if this view be held, he is not entitled to
affirm, as he does, that the winds can only produce a mere surface
drift. If the viscosity and molecular resistance of water be such that,
when the lower strata of the ocean are impelled forward by gravity or
by any other cause, the superincumbent strata extending to the surface
are perforce dragged after them, then, for the same reason, when the
upper strata are impelled forward by the wind or any other cause, the
underlying strata must also be dragged along after them.
If the condition of the ocean between Greenland and the north-western
shore of Europe is irreconcilable with the gravitation theory, we find
the case even worse for that theory when we direct our attention to
the condition of the ocean on the southern hemisphere; for according
to the researches of Captain Duperrey and others on the currents of
the Southern Ocean, a very large portion of the area of that ocean is
occupied by water moving on the surface more in a northward than a
poleward direction. Referring to the deep trough between the Shetland
and the Faroe Islands, called by him the “Lightning Channel,” Dr.
Carpenter says, “If my view be correct, a current-drag suspended in
the _upper_ stratum ought to have a perceptible movement in the N.E.
direction; whilst another, suspended in the _lower_ stratum, should
move S.W.” (§ 40).
Any one believing in the north-eastern extension of the Gulf-stream
and in the Spitsbergen polar under current, to which I have already
referred, would not feel surprised to learn that the surface strata
have a perceptible north-eastward motion, and the bottom strata a
perceptible south-westward motion. North-east and east of Iceland
there is a general flow of cold polar water in a south-east direction
towards the left edge of the Gulf-stream. This water, as Professor Mohn
concludes, “descends beneath the Gulf-stream and partially finds an
outlet in the lower half of the Faroe-Shetland channel.”[68]
_An Objection Considered._—In Nature, vol. ix. p. 423, Dr. Carpenter
has advanced the following objection to the foregoing theory of
under-currents:—“According to Mr. Croll’s doctrine, the whole of that
vast mass of water in the North Atlantic, averaging, say, 1,500 fathoms
in thickness and 3,600 miles in breadth, the temperature of which
(from 40° downwards), as ascertained by the _Challenger_ soundings,
clearly shows it to be mainly derived from a polar source, is nothing
else than _the reflux of the Gulf-stream_. Now, even if we suppose
that the whole of this stream, as it passes Sandy Hook, were to go on
into the closed arctic basin, it would only force out an equivalent
body of water. And as, on comparing the sectional areas of the two,
I find that of the Gulf-stream to be about 1/900th that of the North
Atlantic underflow; and as it is admitted that a large part of the
Gulf-stream returns into the Mid-Atlantic circulation, only a branch of
it going on to the north-east, the extreme improbability (may I not say
impossibility?) that so vast a mass of water can be put in motion by
what is by comparison a mere rivulet (the north-east motion of which,
as a distinct current, has not been traced eastward of 30° W. long.)
seems still more obvious.”
In this objection three things are assumed: (1) that the mass of cold
water 1,500 fathoms deep and 3,600 miles in breadth is in a state of
motion towards the equator; (2) that it cannot be the reflux of the
Gulf-stream, because its sectional area is 900 times as great as that
of the Gulf-stream; (3) that the immense mass of water is, according to
my views, set in motion by the Gulf-stream.
As this objection has an important bearing on the question under
consideration, I shall consider these three assumptions separately
and in their order: (1) That this immense mass of cold water came
originally from the polar regions I, of course, admit, but that the
whole is in a state of motion I certainly do not admit. There is no
warrant whatever for any such assumption. According to Dr. Carpenter
himself, the heating-power of the sun does not extend to any great
depth below the surface; consequently there is nothing whatever to
heat this mass but the heat coming through the earth’s crust. But
the amount of heat derived from this source is so trifling, that an
under current from the arctic regions far less in volume than that
of the Gulf-stream would be quite sufficient to keep the mass at an
ice-cold temperature. Taking the area of the North Atlantic between
the equator and the Tropic of Cancer, including also the Caribbean
Sea and the Gulf of Mexico, to be 7,700,000 square miles, and the
rate at which internal heat passes through the earth’s surface to be
that assigned by Sir William Thomson, we find that the total quantity
of heat derived from the earth’s crust by the above area is equal to
about 88 × 10^{15} foot-pounds per day. But this amount is equal to
only 1/894th that conveyed by the Gulf-stream, on the supposition that
each pound of water carries 19,300 foot-pounds of heat. Consequently
an under current from the polar regions of not more than 1/35th the
volume of the Gulf-stream would suffice to keep the entire mass of
water of that area within 1° of what it would be were there no heat
derived from the crust of the earth; that is to say, were the water
conveyed by the under current at 32°, internal heat would not maintain
the mass of the ocean in the above area at more than 33°. The entire
area of the North Atlantic from the equator to the arctic circle is
somewhere about 16,000,000 square miles. An under current of less than
1/17th that of the Gulf-stream coming from the arctic regions would
therefore suffice to keep the entire North Atlantic basin filled with
ice-cold water. In short, whatever theory we adopt regarding oceanic
circulation, it follows equally as a necessary consequence that the
entire mass of the ocean below the stratum heated by the sun’s rays
must consist of cold water. For if cold water be continually coming
from the polar regions either in the form of under currents, or in the
form of a general underflow as Dr. Carpenter supposes, the entire under
portion of the ocean must ultimately become occupied by cold water; for
there is no source from which this influx of water can derive heat,
save from the earth’s crust. But the amount thus derived is so trifling
as to produce no sensible effect. For example, a polar under current
one half the size of the Gulf-stream would be sufficient to keep the
entire water of the globe (below the stratum heated by the sun’s rays)
at an ice-cold temperature. Internal heat would not be sufficient under
such circumstances to maintain the mass 1° Fahr. above the temperature
it possessed when it left the polar regions.
It follows therefore that the presence of the immense mass of ice-cold
water in the great depths of the ocean is completely accounted for by
under currents, and there is no necessity for supposing it to be all
in a state of motion towards the equator. In fact, this very state of
things, which the general oceanic circulation hypothesis was devised to
explain, results as a necessary consequence of polar under currents.
Unless these were entirely stopped it is physically impossible that the
ocean could be in any other condition.
But suppose that this immense mass of cold water occupying the great
depths of the ocean were, as Dr. Carpenter assumes it to be, in a
state of constant motion towards the equator, and that its sectional
area were 900 times that of the Gulf-stream, it would not therefore
follow that the quantity of water passing through this large sectional
area must be greater than that flowing through a sectional area of
the Gulf-stream; for the quantity of water flowing through this large
sectional area depends entirely on the rate of motion.
I am wholly unable to understand how it could be supposed that this
underflow, according to my view, is set in motion by the Gulf-stream,
seeing that I have shown that the return under current is as much due
to the impulse of the wind as the Gulf-stream itself.
Dr. Carpenter lays considerable stress on the important fact
established by the _Challenger_ expedition, that the great depths of
the sea in equatorial regions are occupied by ice-cold water, while
the portion heated by the sun’s rays is simply a thin stratum at the
surface. It seems to me that it would be difficult to find a fact more
hostile to his theory than this. Were it not for this upper stratum
of heated water there would be no difference between the equatorial
and polar columns, and consequently nothing to produce motion. But the
thinner this stratum is the less is the difference, and the less there
is to produce motion.
CHAPTER IX.
EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC
CIRCULATION.—THE MECHANICS OF DR. CARPENTER’S THEORY.
Experimental Illustration of the Theory.—The Force exerted by
Gravity.—Work performed by Gravity.—Circulation not by
Convection.—Circulation depends on Difference in Density
of the Equatorial and Polar Columns.—Absolute Amount of
Work which can be performed by Gravity.—How Underflow is
produced.—How Vertical Descent at the Poles and Ascent at
the Equator is produced.—The Gibraltar Current.—Mistake in
Mechanics concerning it.—The Baltic Current.
_Experiment to illustrate Theory._—In support of the theory of a
general movement of water between equatorial and polar regions, Dr.
Carpenter adduces the authority of Humboldt and of Prof. Buff.[69]
I have been unable to find anything in the writings of either from
which it can be inferred that they have given this matter special
consideration. Humboldt merely alludes to the theory, and that in the
most casual manner; and that Prof. Buff has not carefully investigated
the subject is apparent from the very illustration quoted by Dr.
Carpenter from the “Physics of the Earth.” “The water of the ocean at
great depths,” says Prof. Buff, “has a temperature, even under the
equator, nearly approaching to the freezing-point. This low temperature
cannot depend on any influence of the sea-bottom.... The fact, however,
is explained by a continual current of cold water flowing from the
polar regions towards the equator. The following well-known experiment
clearly illustrates the manner of this movement. A glass vessel is to
be filled with water with which some powder has been mixed, and is then
to be heated _at bottom_. It will soon be seen, from the motion of the
particles of powder, that currents are set up in opposite directions
through the water. Warm water rises from the bottom up through the
middle of the vessel, and spreads over the surface, while the colder
and therefore heavier liquid falls down at the sides of the glass.”
This illustration is evidently intended to show not merely the form
and direction of the great system of oceanic circulation, but also the
mode in which the circulation is induced by heat. It is no doubt true
that if we apply heat (say that of a spirit-lamp) to the bottom of a
vessel filled with water, the water at the bottom of the vessel will
become heated and rise to the surface; and if the heat be continued
an ascending current of warm water will be generated; and this, of
course, will give rise to a compensating under current of colder water
from all sides. In like manner it is also true that, if heat were
applied to the bottom of the ocean in equatorial regions, an ascending
current of hot water would be also generated, giving rise to an under
current of cold water from the polar regions. But all this is the
diametrically opposite of what actually takes place in nature. The heat
is not applied to the bottom of the ocean, so as to make the water
there lighter than the water at the surface, and thus to generate an
ascending current; but the heat is applied to the surface of the ocean,
and the effect of this is to prevent an ascending current rather than
to produce one, for it tends to keep the water at the surface lighter
than the water at the bottom. In order to show how the heat of the sun
produces currents in the ocean, Prof. Buff should have applied the
heat, not to the bottom of his vessel, but to the upper surface of the
water. But this is not all, the form of the vessel has something to
do with the matter. The wider we make the vessel in proportion to its
depth, the more difficult it is to produce currents by means of heat.
But in order to represent what takes place in nature, we ought to have
the same proportion between the depth and the superficial area of the
water in our vessel as there is between the depth and the superficial
area of the sea. The mean depth of the sea may be taken roughly to be
about three miles.[70] The distance between pole and pole we shall take
in round numbers to be 12,000 miles. The sun may therefore be regarded
as shining upon a circular sea 12,000 miles in diameter and three miles
deep. The depth of the sea to its diameter is therefore as 1 to 4,000.
Suppose, now, that in our experiment we make the depth of our vessel
one inch, we shall require to make its diameter 4,000 inches, or 333
feet, say, in round numbers, 100 yards in diameter. Let us, then, take
a pool of water 100 yards in diameter, and one inch deep. Suppose the
water to be at 32°. Apply heat to the upper surface of the pool, so
as to raise the temperature of the surface of the water to 80° at the
centre of the pool, the temperature diminishing towards the edge, where
it is at 32°. It is found that at a depth of two miles the temperature
of the water at the equator is about as low as that of the poles. We
must therefore suppose the water at the centre of our pool to diminish
in temperature from the surface downwards, so that at a depth of half
an inch the water is at 32°. We have in this case a thin layer of warm
water half an inch thick at the centre, and gradually thinning off to
nothing at the edge of the pool. The lightest water, be it observed,
is at the surface, so that an ascending or a descending current
is impossible. The only way whereby the heat applied can have any
tendency to produce motion is this:—The heating of the water expands
it, consequently the surface of the pool must stand at a little higher
level at its centre than at its edge, where no expansion takes place;
and therefore, in order to restore the level of the pool, the water at
the centre will tend to flow towards the sides. But what is the amount
of this tendency? Its amount will depend upon the amount of slope,
but the slope in the case under consideration amounts to only 1 in
7,340,000.
_Dr. Carpenter’s Experiment._—In order to obviate the objection to
Professor Buff’s experiment Dr. Carpenter has devised another mode.
But I presume his experiment was intended rather to illustrate the way
in which the circulation of the ocean, according to his theory, takes
place, than to prove that it actually does take place. At any rate, all
that can be claimed for the experiment is the proof that water will
circulate in consequence of difference of specific gravity resulting
from difference of temperature. But this does not require proof, for no
physicist denies it. The point which requires to be proved is this. Is
the difference of specific gravity which exists in the ocean sufficient
to produce the supposed circulation? Now his mode of experimenting
will not prove this, unless he makes his experiment agree with the
conditions already stated.
But I decidedly object to the water being heated in the way in which it
has been done by him in his experiment before the Royal Geographical
Society; for I feel somewhat confident that in this experiment the
circulation resulted not from difference of specific gravity, as was
supposed, but rather from the way in which the heat was applied. In
that experiment the one half of a thick metallic plate was placed in
contact with the upper surface of the water at one end of the trough;
the other half, projecting over the end of the trough, was heated
by means of a spirit-lamp. It is perfectly obvious that though the
temperature of the great mass of the water under the plate might not
be raised over 80° or so, yet the molecules in contact with the metal
would have a very high temperature. These molecules, in consequence of
their expansion, would be unable to sink into the cooler and denser
water underneath, and thus escape the heat which was being constantly
communicated to them from the heated plate. But escape they must, or
their temperature would continue to rise until they would ultimately
burst into vapour. They cannot ascend, neither can they descend: they
therefore must be expelled by the heat from the plate in a horizontal
direction. The next layer of molecules from beneath would take their
place and would be expelled in a similar manner, and this process would
continue so long as the heat was applied to the plate. A circulation
would thus be established by the direct expansive force of vapour, and
not in any way due to difference of specific gravity, as Dr. Carpenter
supposes.
But supposing the heated bar to be replaced by a piece of ice,
circulation would no doubt take place; but this proves nothing more
than that difference of density will produce circulation, which is what
no one calls in question.
The case referred to by Dr. Carpenter of the heating apparatus in
London University is also unsatisfactory. The water leaves the boiler
at 120° and returns to it at 80°. The difference of specific gravity
between the water leaving the boiler and the water returning to it
is supposed to produce the circulation. It seems to me that this
difference of specific gravity has nothing whatever to do with the
matter. The cause of the circulation must be sought for in the boiler
itself, and not in the pipes. The heat is applied to the bottom of
the boiler, not to the top. What is the temperature of the molecules
in contact with the bottom of the boiler directly over the fire, is
a question which must be considered before we can arrive at a just
determination of the causes which produce circulation in the pipes
of a heating apparatus such as that to which Dr. Carpenter refers.
But, in addition to this, as the heat is applied to the bottom of the
boiler and not to the top, convection comes into play, a cause which,
as we shall find, does not come into play in the theory of oceanic
circulation at present under our consideration.
_The Force exerted by Gravity._—Dr. Carpenter speaks of his doctrine of
a general oceanic circulation sustained by difference of temperature
alone, “as one of which physical geographers could not recognise the
importance, so long as they remained under the dominant idea that
the temperature of the deep sea is everywhere 39°.” And he affirms
that “until it is clearly apprehended that sea-water becomes more and
more dense as its temperature is reduced, the immense motive power of
polar cold cannot be understood.” But in chap. vii. and also in the
Phil. Mag. for October, 1870 and 1871, I proved that if we take 39°
as the temperature of maximum density the force exerted by gravity
tending to produce circulation is just as great as when we take 32°.
The reason for this is that when we take 32° as the temperature of
maximum density, although we have, it is true, a greater elevation of
the ocean above the place of maximum density, yet this latter occurs at
the poles; while on the other hand, when we take 39°, the difference
of level is less—the place not being at the poles but in about lat.
56°. Now the shorter slope from the equator to lat. 56° is as steep as
the larger one from the equator to the poles, and consequently gravity
exerts as much force in the production of motion in the one case as in
the other. Sir John Herschel, taking 39° as the temperature of maximum
density, estimated the slope at 1/32nd of an inch per mile, whereas
we, taking 32° as the actual temperature of maximum density of the
polar seas and calculating from modern data, find that the slope is not
one-half that amount, and that the force of gravity tending to produce
circulation is much less than Herschel concluded it to be. The reason,
therefore, why physical geographers did not adopt the theory that
oceanic circulation is the result of difference of temperature could
not possibly be the one assigned by Dr. Carpenter, viz., that they had
under-estimated the force of gravity by taking 39° instead of 32° as
the temperature of maximum density.
_The Work performed by Gravity._—But in order clearly to understand
this point, it will be better to treat the matter according to the
third method, and consider not the mere _force_ of gravity impelling
the waters, but the amount of _work_ which gravitation is capable of
performing.
Let us then assume the correctness of my estimate, that the height of
the surface of the ocean at the equator above that at the poles is 4
feet 6 inches, for in representing the mode in which difference of
specific gravity produces circulation it is of no importance what we
may fix upon as the amount of the slope. In order, therefore, to avoid
fractions of a foot, I shall take the slope at 4 feet instead of 4½
feet, which it actually is. A pound of water in flowing down this slope
from the equator to either of the poles will perform 4 foot-pounds of
work; or, more properly speaking, gravitation will. Now it is evident
that when this pound of water has reached the pole, it is at the
bottom of the slope, and consequently cannot descend further. Gravity,
therefore, cannot perform any more work upon it; as it can only do so
while the thing acted upon continues to descend—that is, moves under
the force exerted. But the water will not move under the influence
of gravity unless it move downward; it being in this direction only
that gravity acts on the water. “But,” says Dr. Carpenter, “the effect
of surface-cold upon the water of the polar basin will be to reduce
the temperature of its whole mass below the freezing-point of fresh
water, the surface-stratum _sinking_ as it is cooled in virtue of its
diminished bulk and increased density, and being replaced by water not
yet cooled to the same degree.”[71] By the cooling of the whole mass
of polar water by cold and the heating of the water at the equator by
the sun’s rays the polar column of water, as we have seen, is rendered
denser than the equatorial one, and in order that the two may balance
each other, the polar column is necessarily shorter than the equatorial
by 4 feet; and thus it is that the slope of 4 feet is formed. It is
perfectly true that the water which leaves the equator warm and light,
becomes by the time it reaches the pole cold and dense. But unless
it be denser than the underlying polar water it will not sink down
_through_ it.[72] We are not told, however, why it should be colder
than the whole mass underneath, which, according to Dr. Carpenter,
is cooled by polar cold. But that he does suppose it to sink to the
bottom in consequence of its contraction by cold would appear from the
following quotation:—
“Until it is clearly apprehended that sea-water becomes more and more
dense as its temperature is reduced, and that it consequently continues
to sink until it freezes, the immense motor power of polar cold cannot
be apprehended. But when this has been clearly recognised, it is seen
that the application of _cold at the surface_ is precisely equivalent
as a moving power to that application of _heat at the bottom_ by which
the circulation of water is sustained in every heating apparatus that
makes use of it” (§ 25).
The application of cold at the surface is thus held to be equivalent
as a motor power to the application of heat at the bottom. But heat
applied to the bottom of a vessel produces circulation by _convection_.
It makes the molecules at the bottom expand, and they, in consequence
of buoyancy, rise _through_ the water in the vessel. Consequently if
the action of cold at the surface in polar regions is equivalent to
that of heat, the cold must contract the molecules at the surface and
make them sink _through_ the mass of polar water beneath. But assuming
this to be the meaning in the passage just quoted, how much colder is
the surface water than the water beneath? Let us suppose the difference
to be one degree. How much work, then, will gravity perform upon this
one pound of water which is one degree colder than the mass beneath
supposed to be at 32°? The force with which the pound of water will
sink will not be proportional to its weight, but to the difference
of weight between it and a similar bulk of the water through which
it sinks. The difference between the weight of a pound of water at
31° and an equal volume of water at 32° is 1/29,000th of a pound. Now
this pound of water in sinking to a depth of 10,000 feet, which is
about the depth at which a polar temperature is found at the equator,
would perform only one-third of a foot-pound of work. And supposing
it were three degrees colder than the water beneath, it would in
sinking perform only one foot-pound. This would give us only 4 + 1 = 5
foot-pounds as the total amount that could be performed by gravitation
on the pound of water from the time that it left the equator till
it returned to the point from which it started. The amount of work
performed in descending the slope from the equator to the pole and in
sinking to a depth of 10,000 feet or so through the polar water assumed
to be warmer than the surface water, comprehends the total amount of
work that gravitation can possibly perform; so that the amount of force
gained by such a supposition over and above that derived from the slope
is trifling.
It would appear, however, that this is not what is meant after all.
What Dr. Carpenter apparently means is this: when a quantity of water,
say a layer one foot thick, flows down from the equator to the pole,
the polar column becomes then heavier than the equatorial by the
weight of this additional layer. A layer of water equal in quantity
is therefore pressed away from the bottom of the column and flows off
in the direction of the equator as an under current, the polar column
at the same time sinking down one foot until equilibrium of the polar
and equatorial columns is restored. Another foot of water now flows
down upon the polar column and another foot of water is displaced
from below, causing, of course, the column to descend an additional
foot. The same process being continually repeated, a constant downward
motion of the polar column is the result. Or, perhaps, to express the
matter more accurately, owing to the constant flow of water from the
equatorial regions down the slope, the weight of the polar column is
kept always in excess of that of the equatorial; therefore the polar
column in the effort to restore equilibrium is kept in a constant state
of descent. Hence he terms it a “vertical” circulation. The following
will show Dr. Carpenter’s theory in his own words:—
“The action of cold on the surface water of each polar area will be
exerted as follows:—
“(_a_) In diminishing the height of the polar column as compared with
that of the equatorial, so that a lowering of its _level_ is produced,
which can only be made good by a surface-flow from the latter towards
the former.
“(_b_) In producing an excess in the downward _pressure_ of the column
when this inflow has restored its level, in virtue of the increase of
specific gravity it has gained by its reduction in volume; whereby a
portion of its heavy bottom-water is displaced laterally, causing a
further reduction of level, which draws in a further supply of the
warmer and lighter water flowing towards its surface.
“(_c_) In imparting a downward _movement_ to each new surface-stratum
as its temperature undergoes reduction; so that the _entire column_ may
be said to be in a state of constant descent, like that which exists in
the water of a tall jar when an opening is made at its bottom, and the
water which flows away through it is replaced by an equivalent supply
poured into the top of the jar” (§ 23).
But if this be his theory, as it evidently is, then the 4 foot-pounds
(the amount of work performed by the descent of the water down the
slope) comprehends all the work that gravitation can perform on a pound
of water in making a complete circuit from the equator to the pole and
from the pole back to the equator.
This, I trust, will be evident from the following considerations. When
a pound of water has flowed down from the equator to the pole, it has
descended 4 feet, and is then at the foot of the slope. Gravity has
therefore no more power to pull it down to a lower level. It will not
sink through the polar water, for it is not denser than the water
beneath on which it rests. But it may be replied that although it will
not sink through the polar water, it has nevertheless made the polar
column heavier than the equatorial, and this excess of pressure forces
a pound of water out from beneath and allows the column to descend.
Suppose it may be argued that a quantity of water flows down from the
equator, so as to raise the level of the polar water by, say, one foot.
The polar column will now be rendered heavier than the equatorial by
the weight of one foot of water. The pressure of the one foot will
thus force a quantity of water laterally from the bottom and cause the
entire column to descend till the level of equilibrium is restored. In
other words, the polar column will sink one foot. Now in the sinking of
this column work is performed by gravity. A certain amount of work is
performed by gravity in causing the water to flow down the slope from
the equator to the pole, and, in addition to this, a certain amount is
performed by gravity in the vertical descent of the column.
I freely admit this to be sound reasoning, and admit that so much is
due to the slope and so much to the vertical descent of the water. But
here we come to the most important point, viz., is there the full slope
of 4 feet and an additional vertical movement? Dr. Carpenter seems
to conclude that there is, and that this vertical force is something
in addition to the force which I derive from the slope. And here, I
venture to think, is a radical error into which he has fallen in regard
to the whole matter. Let it be observed that, when water circulates
from difference of specific gravity, this vertical movement is just as
real a part of the process as the flow down the slope; but the point
which I maintain is that _there is no additional power derived from
this vertical movement over and above what is derived from the full
slope_—or, in other words, that this _primum mobile_, which he says I
have overlooked, has in reality no existence.
Perhaps the following diagram will help to make the point still
clearer:—
[Illustration: Fig. 1.]
Let P (fig. 1) be the surface of the ocean at the pole, and E the
surface at the equator; P O a column of water at the pole, and E Q a
column at the equator. The two columns are of equal weight, and balance
each other; but as the polar water is colder, and consequently denser
than the equatorial, the polar column is shorter than the equatorial,
the difference in the length of the two columns being 4 feet. The
surface of the ocean at the equator E is 4 feet higher than the surface
of the ocean at the pole P; there is therefore a slope of 4 feet from E
to P. The molecules of water at E tend to flow down this slope towards
P. The amount of work performed by gravity in the descent of a pound of
water down this slope from E to P is therefore 4 foot-pounds.
But of course there can be no permanent circulation while the full
slope remains. In order to have circulation the polar column must be
heavier than the equatorial. But any addition to the weight of the
polar column is at the expense of the slope. In proportion as the
weight of the polar column increases the less becomes the slope. This,
however, makes no difference in the amount of work performed by gravity.
Suppose now that water has flowed down till an addition of one foot
of water is made to the polar column, and the difference of level,
of course, diminished by one foot. The surface of the ocean in this
case will now be represented by the dotted line P′ E, and the slope
reduced from 4 feet to 3 feet. Let us then suppose a pound of water to
leave E and flow down to P′; 3 foot-pounds will be the amount of work
performed. The polar column being now too heavy by the extent of the
mass of water P′ P one foot thick, its extra pressure causes a mass of
water equal to P′ P to flow off laterally from the bottom of the column.
The column therefore sinks down one foot till P′ reaches P. Now the
pound of water in this vertical descent from P′ to P has one foot-pound
of work performed on it by gravity; this added to the 3 foot-pounds
derived from the slope, gives a total of 4 foot-pounds in passing from
E to P′ and then from P′ to P. This is the same amount of work that
would have been performed had it descended directly from E to P. In
like manner it can be proved that 4 foot-pounds is the amount of work
performed in the descent of every pound of water of the mass P′ P. The
first pound which left E flowed down the slope directly to P, and
performed 4 foot-pounds of work. The last pound flowed down the slope E
P′, and performed only 3 foot-pounds; but in descending from P′ to P it
performed the other one foot-pound. A pound leaving at a period exactly
intermediate between the two flowed down 3½ feet of slope and descended
vertically half a foot. Whatever path a pound of water might take, by
the time that it reached P, 4 foot-pounds of work would be performed.
But no further work can be performed after it reaches P.
But some will ask, in regard to the vertical movement, is it only in
the descent of the water from P′ to P that work is performed? Water
cannot descend from P′ to P, it will be urged, unless the entire column
P O underneath descend also. But the column P O descends by means of
gravity. Why, then, it will be asked, is not the descent of the column
a motive power as real as the descent of the mass of water P′ P?
That neither force nor energy can be derived from the mere descent of
the polar column P O is demonstrable thus:—The reason why the column P
O descends is because, in consequence of the mass of water P′ P resting
on it, its weight is in excess of the equatorial column E Q. But the
force with which the column descends is equal, not to the weight of
the column, but to the weight of the mass P′ P; consequently as much
work would be performed by gravity in the descent of the mass P′ P (the
one foot of water) alone as in the descent of the entire column P′ O,
10,000 feet in height. Suppose a ton weight is placed in each scale of
a balance: the two scales balance each other. Place a pound weight in
one of the scales along with the ton weight and the scale will descend.
But it descends, not with the pressure of a ton and a pound, but with
the pressure of the pound weight only. In the descent of the scale,
say, one foot, gravity can perform only one foot-pound of work. In like
manner, in the descent of the polar column, the only work available is
the work of the mass P′ P laid on the top of the column. But it must be
observed that in the descent of the column from P′ to P, a distance of
one foot, each pound of water of the mass P′ P does not perform one
foot-pound of work; for the moment that a molecule of water reaches P,
it then ceases to perform further work. The molecules at the surface P′
descend one foot before reaching P; the molecules midway between P′ and
P descend only half a foot before reaching P, and the molecules at the
bottom of the mass are already at P, and therefore cannot perform any
work. The mean distance through which the entire mass performs work is
therefore half a foot. One foot-pound per pound of water represents in
this case the amount of work derived from the vertical movement.
That such is the case is further evident from the following
considerations. Before the polar column begins to descend, it is
heavier than the equatorial by the weight of one foot of water; but
when the column has descended half a foot, the polar column is heavier
than the equatorial by the weight of only half a foot of water; and,
as the column continues to descend, the force with which it descends
continues to diminish, and when it has sunk to P the force is zero.
Consequently the mean pressure or weight with which the one foot of
water P′ P descended was equal to that of a layer of half a foot of
water; in other words, each pound of water, taking the mass as a whole,
descended with the pressure or weight of half a pound. But a half
pound descending one foot performs half a foot-pound; so that whether
we consider the _full pressure acting through the mean distance, or
the mean pressure acting through the full distance, we get the same
result_, viz. a half foot-pound as the work of vertical descent.
Now it will be found, as we shall presently see, that if we calculate
the mean amount of work performed in descending the slope from the
equator to the pole, 3½ foot-pounds per pound of water is the amount.
The water at the bottom of the mass P P′ moved, of course, down the
full slope E P 4 feet. The water at the top of the mass which descended
from E to P′ descended a slope of only 3 feet. The mean descent of the
whole mass is therefore 3½ feet. And this gives 3½ foot-pounds as the
mean amount of work per pound of water in descending the slope; this,
added to the half foot-pound derived from vertical descent, gives 4
foot-pounds as the total amount of work per pound of the mass.
I have in the above reasoning supposed one foot of water accumulated
on the polar column before any vertical descent takes place. It is
needless to remark that the same conclusion would have been arrived
at, viz., that the total amount of work performed is 4 foot-pounds per
pound of water, supposing we had considered 2 feet, or 3 feet, or even
4 feet of water to have accumulated on the polar column before vertical
motion took place.
I have also, in agreement with Dr. Carpenter’s mode of representing
the operation, been considering the two effects, viz., the flowing of
the water down the slope and the vertical descent of the polar column
as taking place alternately. In nature, however, the two effects take
place simultaneously; but it is needless to add that the amount of work
performed would be the same whether the effects took place alternately
or simultaneously.
I have also represented the level of the ocean at the equator as
remaining permanent while the alterations of level were taking place at
the pole. But in representing the operation as it would actually take
place in nature, we should consider the equatorial column to be lowered
as the polar one is being raised. We should, for example, consider the
one foot of water P′ P put upon the polar column as so much taken off
the equatorial column. But in viewing the problem thus we arrive at
exactly the same results as before.
Let P (Fig. 2), as in Fig. 1, be the surface of the ocean at the pole,
and E the surface at the equator, there being a slope of 4 feet from E
to P. Suppose now a quantity of water, E E′, say, one foot thick, to
flow from off the equatorial regions down upon the polar. It will thus
lower the level of the equatorial column by one foot, and raise the
level of the polar column by the same amount. I may, however, observe
that the one foot of water in passing from E to P would have its
temperature reduced from 80° to 32°, and this would produce a slight
contraction. But as the weight of the mass would not be affected, in
order to simplify our reasoning we may leave this contraction out of
consideration. Any one can easily satisfy himself that the assumption
that E E′ is equal to P′ P does not in any way affect the question at
issue—the only effect of the contraction being to _increase_ by an
infinitesimal amount the work done in descending the slope, and to
_diminish_ by an equally infinitesimal amount the work done in the
vertical descent. If, for example, 3 foot-pounds represent the amount
of work performed in descending the slope, and one foot-pound the
amount performed in the vertical descent, on the supposition that E′ E
does not contract in passing to the pole, then 3·0024 foot-pounds will
represent the work of the slope, and 0·9976 foot-pounds the work of
vertical descent when allowance is made for the contraction. But the
total amount of work performed is the same in both cases. Consequently,
to simplify our reasoning, we may be allowed to assume P′ P to be equal
to E E′.
[Illustration: Fig. 2.]
The slope E P being 4 feet, the slope E′ P′ is consequently 2 feet;
the mean slope for the entire mass is therefore 3 feet. The mean
amount of work performed by the descent of the mass will of course
be 3 foot-pounds per pound of water. The amount of work performed by
the vertical descent of P′ P ought therefore to be one foot-pound per
pound. That this is the amount will be evident thus:—The transference
of the one foot of water from the equatorial column to the polar
disturbs the equilibrium by making the equatorial column too light by
one foot of water and the polar column too heavy by the same amount of
water. The polar column will therefore tend to sink, and the equatorial
to rise till equilibrium is restored. The difference of weight of
the two columns being equal to 2 feet of water, the polar column
will begin to descend with a pressure of 2 feet of water; and the
equatorial column will begin to rise with an equal amount of pressure.
When the polar column has descended half a foot the equatorial column
will have risen half a foot. The pressure of the descending polar
column will now be reduced to one foot of water. And when the polar
column has descended another foot, P′ will have reached P, and E′
will have reached E; the two columns will then be in equilibrium. It
therefore follows that the mean pressure with which the polar column
descended the one foot was equal to the pressure of one foot of water.
Consequently the mean amount of work performed by the descent of the
mass was equal to one foot-pound per pound of water; this, added to the
3 foot-pounds derived from the slope, gives a total of 4 foot-pounds.
In whatever way we view the question, we are led to the conclusion that
if 4 feet represent the amount of slope between the equatorial and
polar columns when the two are in equilibrium, then 4 foot-pounds is
the total amount of work that gravity can perform upon a pound of water
in overcoming the resistance to motion in its passage from the equator
to the pole down the slope, and then in its vertical descent to the
bottom of the ocean.
But it will be replied, not only does the one foot of water P′ P
descend, but the entire column P O, 10,000 feet in length, descends
also. What, then, it will be asked, becomes of the force which gravity
exerts in the descent of this column? We shall shortly see that this
force is entirely applied in work against gravity in other parts of
the circuit; so that not a single foot-pound of this force goes to
overcome cohesion, friction, and other resistances; it is all spent in
counteracting the efforts which gravity exerts to stop the current in
another part of the circuit.
I shall now consider the next part of the movement, viz., the under
or return current from the bottom of the polar to the bottom of the
equatorial column. What produces this current? It is needless to say
that it cannot be caused directly by gravity. Gravitation cannot
directly draw any body horizontally along the earth’s surface. The
water that forms this current is pressed out laterally by the weight
of the polar column, and flows, or rather is pushed, towards the
equator to supply the vacancy caused by the ascent of the equatorial
column. There is a constant flow of water from the equator to the poles
along the surface, and this draining of the water from the equator is
supplied by the under or return current from the poles. But the only
power which can impel the water from the bottom of the polar column
to the bottom of the equatorial column is the pressure of the polar
column. But whence does the polar column derive its pressure? It can
only press to the extent that its weight exceeds that of the equatorial
column. That which exerts the pressure is therefore the mass of water
which has flowed down the slope from the equator upon the polar column.
It is in this case the vertical movement that causes this under
current. The energy which produces this current must consequently be
derived from the 4 foot-pounds resulting from the slope; for the energy
of the vertical movement, as has already been proved, is derived from
this source; or, in other words, whatever power this vertical movement
may exert is so much deducted from the 4 foot-pounds derived from the
full slope.
Let us now consider the fourth and last movement, viz., the ascent of
the under current to the surface of the ocean at the equator. When
this cold under current reaches the equatorial regions, it ascends
to the surface to the point whence it originally started on its
circuit. What, then, lifts the water from the bottom of the equatorial
column to its top? This cannot be done directly, either by heat or
by gravity. When heat, for example, is applied to the bottom of a
vessel, the heated water at the bottom expands and, becoming lighter
than the water above, rises through it to the surface; but if the
heat be applied to the surface of the water instead of to the bottom,
the heat will not produce an ascending current. It will tend rather
to prevent such a current than to produce one—the reason being that
each successive layer of water will, on account of the heat applied,
become hotter and consequently lighter than the layer below it, and
colder and consequently heavier than the layer above it. It therefore
cannot ascend, because it is too heavy; nor can it descend, because
it is too light. But the sea in equatorial regions is heated from
above, and not from below; consequently the water at the bottom does
not rise to the surface at the equator in virtue of any heat which it
receives. A layer of water can never raise the temperature of a layer
below it to a higher temperature than itself; and since it cannot do
this, it cannot make the layer under it lighter than itself. That which
raises the water at the equator, according to Dr. Carpenter’s theory,
must be the downward pressure of the polar column. When water flows
down the slope from the equator to the pole, the polar column, as we
have seen, becomes too heavy and the equatorial column too light;
the former then sinks and the latter rises. It is the sinking of the
polar column which raises the equatorial one. When the polar column
descends, as much water is pressed in underneath the equatorial column
as is pressed from underneath the polar column. If one foot of water
is pressed from under the polar column, a foot of water is pressed in
under the equatorial column. Thus, when the polar column sinks a foot,
the equatorial column rises to the same extent. The equatorial water
continuing to flow down the slope, the polar column descends: a foot
of water is again pressed from underneath the polar column and a foot
pressed in under the equatorial. As foot after foot is thus removed
from the bottom of the polar column while it sinks, foot after foot is
pushed in under the equatorial column while it rises; so by this means
the water at the surface of the ocean in polar regions descends to
the bottom, and the water at the bottom in equatorial regions ascends
to the surface—the effect of solar heat and polar cold continuing, of
course, to maintain the surface of the ocean in equatorial regions at a
higher level than at the poles, and thus keeping up a constant state of
disturbed equilibrium. Or, to state the matter in Dr. Carpenter’s own
words, “The cold and dense polar water, as it flows in at the bottom of
the equatorial column, will not directly take the place of that which
has been drafted off from the surface; but this place will be filled
by the rising of the whole superincumbent column, which, being warmer,
is also lighter than the cold stratum beneath. Every new arrival from
the poles will take its place below that which precedes it, since its
temperature will have been less affected by contact with the warmer
water above it. In this way an ascending movement will be imparted to
the whole equatorial column, and in due course every portion of it will
come under the influence of the surface-heat of the sun.”[73]
But the agency which raises up the water of the under current to the
surface is the pressure of the polar column. The equatorial column
cannot rise directly by means of gravity. Gravity, instead of raising
the column, exerts all its powers to prevent its rising. Gravity
here is a force acting against the current. It is the descent of
the polar column, as has been stated, that raises the equatorial
column. Consequently the entire amount of work performed by gravity
in pulling down the polar column is spent in raising the equatorial
column. Gravity performs exactly as much work in preventing motion
in the equatorial column as it performs in producing motion in the
polar column; so that, so far as the vertical parts of Dr. Carpenter’s
circulation are concerned, gravity may be said neither to produce
motion nor to prevent it. And this remark, be it observed, applies not
only to P O and E Q, but also to the parts P′ P and E E′ of the two
columns. When a mass of water E E′, say one foot deep, is removed off
the equatorial column and placed upon the polar column, the latter
column is then heavier than the former by the weight of two feet of
water. Gravity then exerts more force in pulling the polar column down
than it does in preventing the equatorial column from rising; and
the consequence is that the polar column begins to descend and the
equatorial column to rise. But as the polar column continues to descend
and the equatorial to rise, the power of gravity to produce motion in
the polar column diminishes, and the power of gravity to prevent motion
in the equatorial column increases; and when P′ descends to P and E′
rises to E, the power of gravity to prevent motion in the equatorial
column is exactly equal to the power of gravity to produce motion in
the polar column, and consequently motion ceases. It therefore follows
that the entire amount of work performed by the descent of P′ P is
spent in raising E′ E against gravity.
It follows also that inequalities in the sea-bottom cannot in any
way aid the circulation; for although the cold under current should
in its progress come to a deep trough filled with water less dense
than itself, it would no doubt sink to the bottom of the hollow; yet
before it could get out again as much work would have to be performed
against gravity as was performed by gravity in sinking it. But whilst
inequalities in the bed of the ocean would not aid the current, they
would nevertheless very considerably retard it by the obstructions
which they would offer to the motion of the water.
We have been assuming that the weight of P′ P is equal to that of E E′;
but the mass P′ P must be greater than E E′ because P′ P has not only
to raise E E′, but to impel the under current—to push the water along
the sea-bottom from the pole to the equator. So we must have a mass of
water, in addition to P′ P, placed on the polar column to enable it to
produce the under current in addition to the raising of the equatorial
column.
It follows also that the amount of work which can be performed by
gravity depends entirely on the _difference_ of temperature between
the equatorial and the polar waters, and is wholly independent of the
way in which the temperature may decrease from the equator to the
poles. Suppose, in agreement with Dr. Carpenter’s idea,[74] that the
equatorial heat and polar cold should be confined to limited areas, and
that through the intermediate space no great difference of temperature
should prevail. Such an arrangement as this would not increase the
amount of work which gravity could perform; it would simply make the
slope steeper at the two extremes and flatter in the intervening space.
It would no doubt aid the surface-flow of the water near the equator
and the poles, but it would retard in a corresponding degree the flow
of the water in the intermediate regions. In short, it would merely
destroy the uniformity of the slope without aiding in the least degree
the general motion of the water.
It is therefore demonstrable that _the energy derived from the full
slope, whatever that slope may be, comprehends all that can possibly be
obtained from gravity_.
It cannot be urged as an objection to what has been advanced that I
have determined simply the amount of the force acting on the water at
the surface of the ocean and not that on the water at all depths—that I
have estimated the amount of work which gravity can perform on a given
quantity of water at the surface, but not the total amount of work
which gravity can perform on the entire ocean. This objection will not
stand, because it is at the surface of the ocean where the greatest
difference of temperature, and consequently of density, exists between
the equatorial and polar waters, and therefore there that gravity
exerts its greatest force. And if gravity be unable to move the water
at the surface, it is much less able to do so under the surface. So
far as the question at issue is concerned, any calculations as to the
amount of force exerted by gravity at various depths are needless.
It is maintained also that the winds cannot produce a vertical current
except under some very peculiar conditions. We have already seen that,
according to Dr. Carpenter’s theory, the vertical motion is caused
by the water flowing off the equatorial column, down the slope, upon
the polar column, thus destroying the equilibrium between the two by
diminishing the weight of the equatorial column and increasing that of
the polar column. In order that equilibrium may be restored, the polar
column sinks and the equatorial one rises. Now must not the same effect
occur, supposing the water to be transferred from the one column to
the other, by the influence of the winds instead of by the influence
of gravity? The vertical descent and ascent of these columns depend
entirely upon the difference in their weights, and not upon the nature
of the agency which makes this difference. So far as difference of
weight is concerned, 2 feet of water, propelled down the slope from the
equatorial column to the polar by the winds, will produce just the same
effect as though it had been propelled by gravity. If vertical motion
follows as a necessary consequence from a transference of water from
the equator to the poles by gravity, it follows equally as a necessary
consequence from the same transference by the winds; so that one is not
at liberty to advocate a vertical circulation in the one case and to
deny it in the other.
_Gravitation Theory of the Gibraltar Current._—If difference of
specific gravity fails to account for the currents of the ocean in
general, it certainly fails in a still more decided manner to account
for the Gibraltar current. The existence of the submarine ridge
between Capes Trafalgar and Spartel, as was shown in the Phil. Mag.
for October, 1871, p. 269, affects currents resulting from difference
of specific gravity in a manner which does not seem to have suggested
itself to Dr. Carpenter. The pressure of water and other fluids is
not like that of a solid—not like that of the weight in the scale of
a balance, simply a downward pressure. Fluids press downwards like
the solids, but they also press laterally. The pressure of water is
hydrostatic. If we fill a basin with water or any other fluid, the
fluid remains in perfect equilibrium, provided the sides of the basin
be sufficiently strong to resist the pressure. The Mediterranean and
Atlantic, up to the level of the submarine ridge referred to, may be
regarded as huge basins, the sides of which are sufficiently strong to
resist all pressure. It follows that, however much denser the water
of the Mediterranean may be than that of the Atlantic, it is only the
water above the level of the ridge that can possibly exercise any
influence in the way of disturbing equilibrium, so as to cause the
level of the Mediterranean to stand lower than that of the Atlantic.
The water of the Atlantic below the level of this ridge might be as
light as air, and that of the Mediterranean as heavy as molten lead,
but this could produce no disturbance of equilibrium; and if there be
no difference of density between the Atlantic and the Mediterranean
waters from the surface down to the level of the top of the ridge, then
there can be nothing to produce the circulation which Dr. Carpenter
infers. Suppose both basins empty, and dense water to be poured into
the Mediterranean, and water less dense into the Atlantic, until they
are both filled up to the level of the ridge, it is evident that the
heavier water in the one basin can exercise no influence in raising
the level of the lighter water in the other basin, the entire pressure
being borne by the sides of the basins. But if we continue to pour in
water till the surface is raised, say one foot, above the level of the
ridge, then there is nothing to resist the lateral pressure of this one
foot of water in the Mediterranean but the counter pressure of the one
foot in the Atlantic. But as the Mediterranean water is denser than the
Atlantic, this one foot of water will consequently exert more pressure
than the one foot of water of the Atlantic. We must therefore continue
to pour more water into the Atlantic until its lateral pressure equals
that of the Mediterranean. The two seas will then be in equilibrium,
but the surface of the Atlantic will of course be at a higher level
than the surface of the Mediterranean. The difference of level will be
proportionate to the difference in density of the waters of the two
seas. But here we come to the point of importance. In determining the
difference of level between the two seas, or, which is the same thing,
the difference of level between a column of the Atlantic and a column
of the Mediterranean, we must take into consideration _only the water
which lies above the level of the ridge_. If there be one foot of water
above the ridge, then there is a difference of level proportionate to
the difference of pressure between the one foot of water of the two
seas. If there be 2 feet, 3 feet, or any number of feet of water above
the level of the ridge, the difference of level is proportionate to
the 2 feet, 3 feet, or whatever number of feet there may be of water
above the ridge. If, for example, 13 should represent the density of
the Mediterranean water and 12 the density of the Atlantic water, then
if there were one foot of water in the Mediterranean above the level of
the ridge, there would require to be one foot one inch of water in the
Atlantic above the ridge in order that the two might be in equilibrium.
The difference of level would therefore be one inch. If there were 2
feet of water, the difference of level would be 2 inches; if 3 feet,
the difference would be 3 inches, and so on. And this would follow,
no matter what the actual depth of the two basins might be; the water
below the level of the ridge exercising no influence whatever on the
level of the surface.
Taking Dr. Carpenter’s own data as to the density of the Mediterranean
and Atlantic waters, what, then, is the difference of density? The
submarine ridge comes to within 167 fathoms of the surface; say, in
round numbers, to within 1,000 feet. What are the densities of the two
basins down to the depth of 1,000 feet? According to Dr. Carpenter
there is little, if any, difference. His own words on this point are
these:—“A comparison of these results leaves no doubt that there is
an excess of salinity in the water of the Mediterranean above that of
the Atlantic; but that this excess _is_ slight in the surface-water,
whilst somewhat greater in the deeper water” (§ 7). “Again, it was
found by examining samples of water taken from the surface, from 100
fathoms, from 250 fathoms, and from 400 fathoms respectively, that
whilst the _first two_ had the _characteristic temperature and density
of Atlantic water_, the last two had the characteristics and density of
Mediterranean water” (§ 13). Here, at least to the depth of 100 fathoms
or 600 feet, there is little difference of density between the waters
of the two basins. Consequently down to the depth of 600 feet, there is
nothing to produce any sensible disturbance of equilibrium. If there
be any sensible disturbance of equilibrium, it must be in consequence
of difference of density which may exist between the depths of 600
feet and the surface of the ridge. We have nothing to do with any
difference which may exist between the water of the Mediterranean and
the Atlantic below the ridge; the water in the Mediterranean basin may
be as heavy as mercury below 1,000 feet: but this can have no effect
in disturbing equilibrium. The water to the depth of 600 feet being of
the same density in both seas, the length of the two columns acting on
each other is therefore reduced to 400 feet—that is, to that stratum of
water lying at a depth of from 600 to the surface of the ridge 1,000
feet below the surface. But, to give the theory full justice, we shall
take the Mediterranean stratum at the density of the deep water of
the Mediterranean, which he found to be about 1·029, and the density
of the Atlantic stratum at 1·026. The difference of density between
the two columns is therefore ·003. Consequently, if the height of the
Mediterranean column be 400 feet, it will be balanced by the Atlantic
column of 401·2 feet; the difference of level between the Mediterranean
and the Atlantic cannot therefore be more than 1·2 foot. The amount
of work that can be performed by gravity in the case of the Gibraltar
current is little more than one foot-pound per pound of water, an
amount of energy evidently inadequate to produce the current.
It is true that in his last expedition Dr. Carpenter found the
bottom-water on the ridge somewhat denser than Atlantic water at the
same depth, the former being 1·0292 and the latter 1·0265; but it
also proved to be denser than Mediterranean water at the same depth.
He found, for example, that “the dense Mediterranean water lies about
100 fathoms nearer the surface over a 300-fathoms bottom, than it
does where the bottom sinks to more than 500 fathoms” (§ 51). But any
excess of density which might exist at the ridge could have no tendency
whatever to make the Mediterranean column preponderate over the
Atlantic column, any more than could a weight placed over the fulcrum
of a balance have a tendency to make the one scale weigh down the other.
If the objection referred to be sound, it shows the mechanical
impossibility of the theory. It proves that whether there be an under
current or not, or whether the dense water lying in the deep trough of
the Mediterranean be carried over the submarine ridge into the Atlantic
or not, the explanation offered by Dr. Carpenter is one which cannot be
admitted. It is incumbent on him to explain either (1) how the almost
infinitesimal difference of density which exists between the Atlantic
and Mediterranean columns down to the level of the ridge can produce
the upper and under currents carrying the deep and dense water of
the Mediterranean over the ridge, or (2) how all this can be done by
means of the difference of density which exists below the level of the
ridge.[75] What the true cause of the Gibraltar current really is will
be considered in Chap. XIII.
_The Baltic Current._—The entrance to the Baltic Sea is in some
places not over 50 or 60 feet deep. It follows, therefore, from what
has already been proved in regard to the Gibraltar current, that the
influence of gravity must be even still less in causing a current in
the Baltic strait than in the Gibraltar strait.
CHAPTER X.
EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCULATION.—DR.
CARPENTER’S THEORY.—OBJECTIONS CONSIDERED.
_Modus Operandi_ of the Matter.—Polar Cold considered by Dr.
Carpenter the _Primum Mobile_.—Supposed Influence of
Heat derived from the Earth’s Crust.—Circulation without
Difference of Level.—A Confusion of Ideas in Reference to the
supposed Agency of Polar Cold.—M. Dubuat’s Experiments.—A
Begging of the Question at Issue.—Pressure as a Cause of
Circulation.
In the foregoing chapter, the substance of which appeared in the
Phil. Mag. for October, 1871, I have represented the manner in which
difference of specific gravity produces circulation. But Dr. Carpenter
appears to think that there are some important points which I have
overlooked. These I shall now proceed to consider in detail.
“Mr. Croll’s whole manner of treating the subject,” he says, “is so
different from that which it appears to me to require, and he has so
completely misapprehended my own view of the question, that I feel it
requisite to present this in fuller detail in order that physicists and
mathematicians, having both sides fully before them, may judge between
us” (§ 26).[76]
He then refers to a point so obvious as hardly to require
consideration, viz., the effect which results when the surface of
the entire area of a lake or pond of water is cooled. The whole of
the surface-film, being chilled at the same time, sinks through the
subjacent water, and a new film from the warmer layer immediately
beneath the surface rises into its place. This being cooled in its
turn, sinks, and so on. He next considers what takes place when only
a portion of the surface of the pond is cooled, and shows that in this
case the surface-film which descends is replaced not from beneath, but
by an inflow from the neighbouring area.
“That such must be the case,” says Dr. Carpenter, “appears to me so
self-evident that I am surprised that any person conversant with the
principles of physical science should hesitate in admitting it, still
more that he should explicitly deny it. But since others may feel the
same difficulty as Mr. Croll, it may be worth while for me to present
the case in a form of yet more elementary simplicity” (§ 29).
Then, in order to show the mode in which the general oceanic
circulation takes place, he supposes two cylindrical vessels, W and
C, of equal size, to be filled with sea-water. Cylinder W represents
the equatorial column, and the water contained in it has its
temperature maintained at 60°; whilst the water in the other cylinder
C, representing the polar column, has its temperature maintained at
30° by means of the constant application of cold at the top. Free
communication is maintained between the two cylinders at top and
bottom; and the water in the cold cylinder being, in virtue of its
low temperature, denser than the water in the warm cylinder, the two
columns are therefore not in static equilibrium. The cold, and hence
heavier column tends to produce an outflow of water from its bottom to
the bottom of the warm column, which outflow is replaced by an inflow
from the top of the warm column to the top of the cold column. In fact,
we have just a simple repetition of what he has given over and over
again in his various memoirs on the subject. But why so repeatedly
enter into the _modus operandi_ of the matter? Who feels any difficulty
in understanding how the circulation is produced?
_Polar Cold considered by Dr. Carpenter the Primum Mobile._—It is
evident that Dr. Carpenter believes that he has found in polar _cold_
an agency the potency of which, in producing a general oceanic
circulation, has been overlooked by physicists; and it is with the view
of developing his ideas on this subject that he has entered so fully
and so frequently into the exposition of his theory. “If I have myself
done anything,” he says, “to strengthen the doctrine, it has been by
showing that polar cold, rather than equatorial heat, is the _primum
mobile_ of this circulation.”[77]
The influence of the sun in heating the waters of the inter-tropical
seas is, in Dr. Carpenter’s manner of viewing the problem, of no
great importance. The efficient cause of motion he considers resides
in _cold_ rather than in _heat_. In fact, he even goes the length
of maintaining that, as a power in the production of the general
interchange of equatorial and polar water, the effect of polar cold is
so much superior to that of inter-tropical heat, that the influence of
the latter may be _practically disregarded_.
“Suppose two basins of ocean-water,” he says, “connected by a strait to
be placed under such different climatic conditions that the surface of
one is exposed to the heating influence of tropical sunshine, whilst
the surface of the other is subjected to the extreme cold of the
sunless polar winter. The effect of the surface-heat upon the water
of the tropical basin will be for the most part limited (as I shall
presently show) to its uppermost stratum, and may here be _practically
disregarded_.”[78]
Dr. Carpenter’s idea regarding the efficiency of cold in producing
motion seems to me to be not only opposed to the generally received
views on the subject, but wholly irreconcilable with the ordinary
principles of mechanics. In fact, there are so many points on which
Dr. Carpenter’s theory of a “General _Vertical_ Oceanic Circulation”
differs from the generally received views on the subject of circulation
by means of difference of specific gravity, that I have thought it
advisable to enter somewhat minutely into the consideration of the
mechanics of that theory, the more so as he has so repeatedly asserted
that eminent physicists agree with what he has advanced on the subject.
According to the generally received theory, the circulation is due to
the _difference of density_ between the sea in equatorial and polar
regions. The real efficient cause is gravity; but gravity cannot act
when there is no difference of specific gravity. If the sea were of
equal density from the poles to the equator, gravity could exercise no
influence in the production of circulation; and the influence which it
does possess is in proportion to the difference of density. But the
difference of density between equatorial and polar waters is in turn
due not absolutely either to polar cold or to tropical heat, but to
both—or, in other words, to the _difference_ of temperature between
the polar and equatorial seas. This difference, in the very nature of
things, must be as much the result of equatorial heat as of polar cold.
If the sea in equatorial regions were not being heated by the sun as
rapidly as the sea in polar regions is being cooled, the difference of
temperature between them, and consequently the difference of density,
would be diminishing, and in course of time would disappear altogether.
As has already been shown, it is a necessary consequence that the
water flowing from equatorial to polar regions must be compensated by
an equal amount flowing from polar to equatorial regions. Now, if the
water flowing from polar to equatorial regions were not being heated
as rapidly as the water flowing from equatorial to polar regions is
being cooled, the equatorial seas would gradually become colder and
colder until no sensible difference of temperature existed between
them and the polar oceans. In fact, _equality of the two rates_ is
necessary to the very existence of such a general circulation as that
advocated by Dr. Carpenter. If he admits that the general interchange
of equatorial and polar water advocated by him is caused by the
difference of density between the water at the equator and the poles,
resulting from difference of temperature, then he must admit also that
this difference of density is just as much due to the heating of the
equatorial water by the sun as it is to the cooling of the polar water
by radiation and other means—or, in other words, that it is as much due
to equatorial heat as to polar cold. And if so, it cannot be true that
polar cold rather than equatorial heat is the “_primum mobile_” of
this circulation; and far less can it be true that the heating of the
equatorial water by the sun is of so little importance that it may be
“practically disregarded.”
_Supposed Influence of Heat derived from the Earth’s Crust._—There is,
according to Dr. Carpenter, another agent concerned in the production
of the general oceanic circulation, viz., the heat derived by the
bottom of the ocean from the crust of the earth.[79] We have no reason
to believe that the quantity of internal heat coming through the
earth’s crust is greater in one part of the globe than in another; nor
have we any grounds for concluding that the bottom of inter-tropical
seas receives more heat from the earth’s crust than the bottom of those
in polar regions. But if the polar seas receive as much heat from this
source as the seas within the tropics, then the difference of density
between the two cannot possibly be due to heat received from the
earth’s crust; and this being so, it is mechanically impossible that
internal heat can be a cause in the production of the general oceanic
circulation.
_Circulation without Difference of Level._—There is another part of
the theory which appears to me irreconcilable with mechanics. It is
maintained that this general circulation takes place without any
difference of level between the equator and the poles. Referring to the
case of the two cylinders W and C, which represent the equatorial and
polar columns respectively, Dr. Carpenter says:—
“The force which will thus lift up the entire column of water in W
is that which causes the descent of the entire column in C, namely,
the excess of gravity constantly acting in C,—the levels of the
two columns, and consequently their heights, being maintained at a
_constant equality_ by the free passage of surface-water from W to C.”
“The whole of Mr. Croll’s discussion of this question, however,” he
continues, “proceeds upon the assumption that the levels of the polar
and equatorial columns are _not kept at an_ _equality_, &c.” (§ 30.)
And again, “Now, so far from asserting (as Captain Maury has done) that
the trifling difference of level arising from inequality of temperature
is adequate to the production of ocean-currents, I simply affirm that
as fast as the level is disturbed by change of temperature it will be
restored by gravity.” (§ 23.)[80]
[Illustration: Fig. 3.]
In order to understand more clearly how the circulation under
consideration cannot take place without a difference of level, let W E
(Fig. 3) represent the equatorial column, and C P the polar column. The
equatorial column is warmer than the polar column because it receives
_more_ heat from the sun than the latter; and the polar is colder
than the equatorial column because it receives _less_. The difference
in the density of the two columns results from their difference of
temperature; and the difference of temperature results in turn from the
difference in the quantity of heat received from the sun by each. Or,
to express the matter in other words, the difference of density (and
consequently the circulation under consideration) is due to the excess
of heat received from the sun by the equatorial over that received by
the polar column; so that to leave out of account the super-heating of
the inter-tropical waters by the sun is to leave out of account the
very thing of all others that is absolutely essential to the existence
of the circulation. The water being assumed to be the same in both
columns and differing only as regards temperature, and the equatorial
column possessing more heat than the polar, and being therefore less
dense than the latter, it follows, in order that the two columns may
be in static equilibrium, that the surface of the equatorial column
must stand at a higher level than that of the polar. This produces the
slope W C from the equator to the pole. The extent of the slope will of
course depend upon the extent of the difference of their temperatures.
But, as was shown on a former occasion,[81] it is impossible that
static equilibrium can ever be fully obtained, because the slope
occasioned by the elevation of the equatorial column above the polar
produces what we may be allowed to call a _molecular_ disturbance of
equilibrium. The surface of the ocean, or the molecules of water lying
on the slope, are not in a position of equilibrium, but tend, in virtue
of gravity, to roll down the slope in the direction of the polar column
C. It will be observed that the more we gain of static equilibrium
of the entire ocean the greater is the slope, and consequently the
greater is the disturbance of molecular equilibrium; and, _vice versâ_,
the more molecular equilibrium is restored by the reduction of the
slope, the greater is the disturbance of static equilibrium. _It is
therefore absolutely impossible that both conditions of equilibrium can
be fulfilled at the same time so long as a difference of temperature
exists between the two columns._ And this conclusion holds true even
though we should assume water to be a perfect fluid absolutely devoid
of viscosity. It follows, therefore, that a general oceanic circulation
without a difference of level is a _mechanical impossibility_.
In a case of actual circulation due to difference of gravity, there
is always a constant disturbance of both _static_ and molecular
equilibrium. Column C is always higher and column W always lower than
it ought to be were the two in equilibrium; but they never can be at
the same level.
It is quite conceivable, of course, that the two conditions of
equilibrium may be fulfilled alternately. We can conceive column C
remaining stationary till the water flowing from column W has restored
the level. And after the level is restored we can conceive the polar
column C sinking and the equatorial column W rising till the two
perfectly balance each other. Such a mode of circulation, consisting
of an alternate surface-flow and vertical descent and ascent of the
columns, though conceivable, is in reality impossible in nature; for
there are no means by which the polar column C could be supported
from sinking till the level had been restored. But Dr. Carpenter does
not assume that the general oceanic circulation takes place in this
intermitting manner; according to him, the circulation is _constant_.
He asserts that there is a “_continual_ transference of water from the
bottom of C to the bottom of W, and from the top of W to the top of C,
with a _constant_ descending movement in C and a _constant_ ascending
movement in W” (§ 29). But such a condition of things is irreconcilable
with the idea of “the levels of the two columns, and consequently their
heights, being maintained at a _constant_ equality” (§ 29).
Although Dr. Carpenter does not admit the existence of a permanent
difference of level between the equator and the pole, he nevertheless
speaks of a depression of level in the polar basin resulting from the
contraction by cooling of the water flowing into it. This reduction of
level induces an inflow of water from the surrounding area; “and since
what is drawn away,” to quote his own words, “is supplied from a yet
greater distance, the continued cooling of the surface-stratum in the
polar basin will cause a ‘set’ of waters towards it, to be propagated
backwards through the whole intervening ocean in communication with
it until it reaches the tropical area.” The slope produced between
the polar basin and the surrounding area, if sufficiently great, will
enable the water in the surrounding area to flow polewards; but unless
this slope extend to the equator, it will not enable the tropical
waters also to flow polewards. One of two things necessarily follows:
either the slope extends from the equator to the pole, or water can
flow from the equator to the pole without a slope. If Dr. Carpenter
maintains the former, he contradicts himself; and if he adopts the
latter, he contradicts an obvious principle of mechanics.
_A Confusion of Ideas in Reference to the supposed Agency of Polar
Cold._—It seems to me that Dr. Carpenter has been somewhat misled by a
slight confusion of ideas in reference to the supposed agency of polar
cold. This is brought out forcibly in the following passage from his
memoir in the Proceedings of the Royal Geographical Society, vol. xv.
“Mr. Croll, in arguing against the doctrine of a general oceanic
circulation sustained by difference of temperature, and _justly
maintaining_ that such a circulation cannot be produced by the
application of heat at the surface, has entirely ignored the agency of
cold.”
It is here supposed that there are two agents at work in the production
of the general oceanic circulation. The one agent is _heat_, acting
at the equatorial regions; and the other agent is _cold_, acting at
the polar regions. It is supposed that the agency of cold is far more
powerful than that of heat. In fact so trifling is the agency of
equatorial heat in comparison with that of polar cold that it may be
“practically disregarded”—left out of account altogether,—polar cold
being the _primum mobile_ of the circulation. It is supposed also that
I have considered the efficiency of one of the agents, viz., heat, and
found it totally inadequate to produce the circulation in question; and
it is admitted also that my conclusions are perfectly correct. But then
I am supposed to have left out of account the other agent, viz., polar
cold, the only agent possessing real potency. Had I taken into account
polar cold, it is supposed that I should have found at once a cause
perfectly adequate to produce the required effect.
This is a fair statement of Dr. Carpenter’s views on the subject; I am
unable, at least, to attach any other meaning to his words. And I have
no doubt they are also the views which have been adopted by those who
have accepted his theory.
It must be sufficiently evident from what has already been stated,
that the notion of there being two separate agents at work producing
circulation, namely heat and cold, the one of which is assumed to have
much more potency than the other, is not only opposed to the views
entertained by physicists, but is also wholly irreconcilable with the
ordinary principles of mechanics. But more than this, if we analyze the
subject a little so as to remove some of the confusion of ideas which
besets it, we shall find that these views are irreconcilable with even
Dr. Carpenter’s own explanation of the cause of the general oceanic
circulation.
_Cold_ is not a something positive imparted to the polar waters giving
them motion, and of which the tropical waters are deprived. If, dipping
one hand into a basin filled with tropical water at 80° and the other
into one filled with polar water at 32°, we refer to our _sensations_,
we call the water in the one _hot_ and that in the other _cold_; but
so far as the water itself is concerned heat and cold simply mean
difference in the amounts of heat possessed. Both the polar and the
tropical water possess a certain amount of energy in the form of heat,
only the polar water does not possess so much of it as the tropical.
How, then, according to Dr. Carpenter, does polar cold impart motion
to the water? The warm water flowing in upon the polar column becomes
chilled by cold, but it is not cooled below that of the water
underneath; for, according to Dr. Carpenter, the ocean in polar regions
is as cold and as dense underneath as at the surface. The cooled
surface-water does not sink through the water underneath, like the
surface-water of a pond chilled during a frosty night. “The descending
motion in column C will not consist,” he says, “in a successional
descent of surface-films from above downwards, but it will be a
downward movement of the _entire mass_, as if water in a tall jar
were being drawn off through an orifice at the bottom” (§ 29). There
is a downward motion of the entire column, producing an outflow of
water at the bottom towards the equatorial column W, which outflow is
compensated by an inflow from the top of the equatorial column to the
top of the polar column C. But what causes column C to descend? The
cause of the descent is its excess of weight over that of column W.
Column C descends and column W ascends, for the same reason that in
a balance the heavy scale descends and the light scale rises. Column
C descends not simply because it is cold, but because it is _colder_
than column W. Column C descends not simply because in consequence of
being cold it is dense and therefore heavy, but because in consequence
of being cold it is _denser_ and therefore _heavier_ than column W.
It might be as cold as frozen mercury and as heavy as lead; but it
would not on that account descend unless it were heavier than column
W. The descent of column C and ascent of column W, and consequently
the general oceanic circulation, results, therefore, according to Dr.
Carpenter’s explanation, from the _difference_ in the weights of the
two columns; and the difference in the weights of the two columns
results from their _difference_ of density; and the difference of
density of the two columns in turn results from their _difference_ of
temperature. But it has already been proved that the difference of
temperature between the polar and equatorial columns depends wholly on
the difference in the amount of heat received by each from the sun. The
equatorial column W possesses more heat than the polar column C, solely
because it receives more heat from the sun than column C. Consequently
Dr. Carpenter’s statement that the circulation is produced by polar
cold rather than by equatorial heat, is just as much in contradiction
to his own theory as it is to the principles of mechanics. Again, his
admission that the general oceanic circulation “cannot be produced by
the application of heat to the surface,” is virtually a giving up the
whole point in debate; for according to his gravitation theory, and
every form of that theory, the circulation results from _difference_ of
temperature between equatorial and polar seas; but this difference, as
we have seen, is entirely owing to the difference in the amount of heat
received from the sun at these two places. The heat received, however,
is “surface-heat;” for it is at the surface that the ocean receives all
its heat from the sun; and consequently if surface-heat cannot produce
the effect required, nothing else can.
_M. Dubuat’s Experiments._—Referring to the experiments of M. Dubuat
adduced by me to show that water would not run down a slope of 1
in 1,820,000,[82] he says, “Now the experiments of M. Dubuat had
reference, not to the slow restoration of level produced by the motion
of water on itself, but to the sensible movement of water flowing over
solid surfaces and retarded by its friction against them” (§ 22).
Dr. Carpenter’s meaning, I presume, is that if the incline consist
of any solid substance, water will not flow down it; but if it be
made of _water_ itself, _water_ will flow down it. But in M. Dubuat’s
experiments it was only the molecules in actual _contact_ with the
solid incline that could possibly be retarded by friction against it.
The molecules not in contact with the solid incline evidently rested
upon an _incline of water_, and were at perfect liberty to roll down
that incline if they chose; but they did not do so; and consequently M.
Dubuat’s experiment proved that water will not flow over itself on an
incline of 1 in 1,000,000.
_A Begging of the Question at Issue._—“It is to be remembered,” says
Dr. Carpenter, “that, however small the original amount of movement
may be, a _momentum_ tending to its continuance _must_ be generated
from the instant of its commencement; so that if the initiating force
be in constant action, there will be a _progressive acceleration_ of
its rate, until the increase of resistance equalises the tendency to
further acceleration. Now, if it be admitted that the propagation of
the disturbance of equilibrium from one column to another is simply
_retarded_, _not_ prevented, by the viscosity of the liquid, I cannot
see how the conclusion can be resisted, that the constantly maintained
difference of gravity between the polar and equatorial columns really
acts as a _vis viva_ in maintaining a circulation between them” (§ 35).
If it be true, as Dr. Carpenter asserts, that in the case of the
general oceanic circulation advocated by him “viscosity” simply
_retards_ motion, but does not _prevent_ it, I certainly agree with him
“that the constantly maintained difference of gravity between the polar
and equatorial columns really acts as a _vis viva_ in maintaining a
circulation between them.” But to assert that it merely retards, but
does not prevent, motion, is simply _begging the question at issue_.
It is an established principle that if the _force_ resisting motion be
greater than the force tending to produce it, then no motion can take
place and no work can be performed. The experiments of M. Dubuat prove
that the _force_ of the molecular resistance of water to motion is
_greater_ than the _force_ derived from a slope of 1 in 1,000,000; and
therefore it is simply begging the question at issue to assert that it
is _less_. The experiments of MM. Barlow, Rainey, and others, to which
he alludes, are scarcely worthy of consideration in relation to the
present question, because we know nothing whatever regarding the actual
amount of force producing motion of the water in these experiments,
further than that it must have been enormously greater than that
derived from a slope of 1 in 1,000,000.
_Supposed Argument from the Tides._—Dr. Carpenter advances Mr.
Ferrel’s argument in regard to the tides. The power of the moon to
disturb the earth’s water, he asserts, is, according to Herschel,
only 1/11,400,000th part of gravity, and that of the sun not over
1/25,736,400th part of gravity; yet the moon’s attractive force, even
when counteracted by the sun, will produce a rise of the ocean. But as
the disturbance of gravity produced by difference of temperature is far
greater than the above, it ought to produce circulation.
It is here supposed that the force exerted by gravity on the ocean,
resulting from difference of temperature, tending to produce the
general oceanic circulation, is much greater than the force exerted
on the ocean by the moon in the production of the tides. But if we
examine the subject we shall find that the opposite is the case. The
attraction of the moon tending to lift the waters of the ocean acts
directly on every molecule from the surface to the bottom; but the
force of gravity tending to produce the circulation in question acts
directly on only a portion of the ocean. Gravity can exercise no direct
force in impelling the underflow from the polar to the equatorial
regions, nor in raising the water to the surface when it reaches the
equatorial regions. Gravity can exercise no direct influence in pulling
the water horizontally along the earth’s surface, nor in raising it
up to the surface. The pull of gravity is always _downwards_, never
_horizontally_ nor upwards. Gravity will tend to pull the surface-water
from the equator to the poles because here we have _descent_. Gravity
will tend to sink the polar column because here also we have _descent_.
But these are the only parts of the circuit where gravity has any
tendency to produce motion. Motion in the other parts of the circuit,
viz., along the bottom of the ocean from the poles to the equator and
in raising the equatorial column, is produced by the _pressure_ of the
polar column; and consequently it is only _indirectly_ that gravity may
be said to produce motion in those parts. It is true that on certain
portions of the ocean the force of gravity tending to produce motion is
greater than the force of the moon’s attraction, tending to produce the
tides; but this portion of the ocean is of inconsiderable extent. The
total force of gravity acting on the entire ocean tending to produce
circulation is in reality prodigiously less than the total force of the
moon tending to produce the tides.
It is no doubt a somewhat difficult problem to determine accurately
the total amount of force exercised by gravity on the ocean; but for
our present purpose this is not necessary. All that we require at
present is a very rough estimate indeed. And this can be attained by
very simple considerations. Suppose we assume the mean depth of the
sea to be, say, three miles. The mean depth may yet be found to be
somewhat less than this, or it may be found to be somewhat greater;
a slight mistake, however, in regard to the mass of the ocean will
not materially affect our conclusions. Taking the depth at 3 miles,
the force or direct pull of gravity on the entire waters of the ocean
tending to the production of the general circulation will not amount to
more than 1/24,000,000,000th that of gravity, or only about 1/2,100th
that of the attraction of the moon in the production of the tides. Let
it be observed that I am referring to the force or pull of gravity,
and not to hydrostatic pressure.
The moon, by raising the waters of the ocean, will produce a slope of 2
feet in a quadrant; and because the raised water sinks and the level is
restored, Mr. Ferrel concludes that a similar slope of 2 feet produced
by difference of temperature will therefore be sufficient to produce
motion and restore level. But it is overlooked that the restoration of
level in the case of the tides is as truly the work of the moon as the
disturbance of that level is. For the water raised by the attraction of
the moon at one time is again, six hours afterwards, pulled down by the
moon when the earth has turned round a quadrant.
No doubt the earth’s gravity alone would in course of time restore
the level; but this does not follow as a logical consequence from Mr.
Ferrel’s premises. If we suppose a slope to be produced in the ocean by
the moon and the moon’s attraction withdrawn so as to allow the water
to sink to its original level, the raised side will be the heaviest and
the depressed side the lightest; consequently the raised side will tend
to sink and the depressed side will tend to rise, in order that the
ocean may regain its static equilibrium. But when a difference of level
is produced by difference of temperature, the raised side is always the
lightest and the depressed side is always the heaviest; consequently
the very effort which the ocean makes to maintain its equilibrium
tends to prevent the level being restored. The moon produces the tides
chiefly by means of a simple yielding of the entire ocean considered as
a mass; whereas in the case of a general oceanic circulation the level
is restored by a _flow_ of water at or near the surface. Consequently
the amount of friction and molecular resistance to be overcome in the
restoration of level in the latter case is much greater than in the
former. The moon, as the researches of Sir William Thomson show, will
produce a tide in a globe composed of a substance where no currents or
general flow of the materials could possibly take place.
_Pressure as a Cause of Circulation._—We shall now briefly refer to
the influence of pressure (the indirect effects of gravity) in the
production of the circulation under consideration. That which causes
the polar column C to descend and the equatorial column W to ascend,
as has repeatedly been remarked, is the difference in the weight of
the two columns. The efficient cause in the production of the movement
is, properly speaking, gravity; _cold_ at the poles and _heat_ at the
equator, or, what is the same thing, the _excess_ of heat received
by the equator over that received by the poles is what maintains the
difference of temperature between the two columns, and consequently is
that also which maintains the difference of weight between them. In
other words, difference of temperature is the cause which maintains
the _state of disturbed equilibrium_. But the efficient cause of
the circulation in question is gravity. Gravity, however, could not
act without this state of disturbed equilibrium; and difference of
temperature may therefore be called, in relation to the circulation,
a necessary _condition_, while gravity may be termed the _cause_.
Gravity sinks column C _directly_, but it raises column W _indirectly_
by means of pressure. The same holds true in regard to the motion of
the bottom-waters from C to W, which is likewise due to pressure. The
pressure of the excess of the weight of column C over that of column W
impels the bottom-water equatorwards and lifts the equatorial column.
But on this point I need not dwell, as I have in the preceding chapter
entered into a full discussion as to how this takes place.
We come now to the most important part of the inquiry, viz., how is
the surface-water impelled from the equator to the poles? Is pressure
from behind the impelling force here as in the case of the bottom-water
of the ocean? It seems to me that, in attempting to account for the
surface-flow from the equator to the poles, Dr. Carpenter’s theory
signally fails. The force to which he appeals appears to be wholly
inadequate to produce the required effect.
The experiments of M. Dubuat, as already noticed, prove that, any slope
which can possibly result from the difference of temperature between
the equator and the poles is wholly insufficient to enable gravity to
move the waters; but it does not necessarily prove that the _pressure_
resulting from the raised water at the equator may not be sufficient to
produce motion. This point will be better understood from the following
figure, where, as before, P C represents the polar column and E W the
equatorial column.
[Illustration: Fig. 4.]
It will be observed that the water in that wedge-shaped portion W C
W′ forming the incline cannot be in a state of static equilibrium.
A molecule of water at O, for example, will be pressed more in the
direction of C than in the direction of W′, and the amount of this
excess of pressure towards C will depend upon the height of W above
the line C W′. It is evident that the pressure tending to move the
molecule at O towards C will be far greater than the direct pull of
gravity tending to draw a molecule at O′ lying on the surface of the
incline towards C. The experiments of M. Dubuat prove that the direct
force of gravity will not move the molecule at O′—that is, cause it to
roll down the incline W C; but they do not prove that it may not yield
to pressure from above, or that the pressure of the column W W′ will
not move the molecule at O. The pressure is caused by gravity, and
cannot, of course, enable gravity to perform more work than what is
derived from the energy of gravity; it will enable gravity, however,
to overcome resistance, which it could not do by direct action. But
whether the pressure resulting from the greater height of the water
at the equator due to its higher temperature be actually sufficient
to produce displacement of the water is a question which I am wholly
unable to answer.
If we suppose 4 feet 6 inches to be the height of the equatorial
surface above the polar required to make the two columns balance
each other, the actual difference of level between the two columns
will certainly not be more than one-half that amount, because, if a
circulation exist, the weight of the polar column must always be in
excess of that of the equatorial. But this excess can only be obtained
at the expense of the surface-slope, as has already been shown at
length. The surface-slope probably will not be more than 2 feet or 2
feet 6 inches. Suppose the ocean to be of equal density from the poles
to the equator, and that by some means or other the surface of the
ocean at the equator is raised, say, 2 feet above that of the poles,
then there can be little doubt that in such a case the water would
soon regain its level; for the ocean at the equator being heavier than
at the poles by the weight of a layer 2 feet in thickness, it would
sink at the former place and rise at the latter until equilibrium was
restored, producing, of course, a very slight displacement of the
bottom-waters towards the poles. It will be observed, however, that
restoration of level in this case takes place by a simple yielding, as
it were, of the entire mass of the ocean without displacement of the
molecules of the water over each other to any great extent. In the case
of a slope produced by difference of temperature, however, the raised
portion of the ocean is not heavier but lighter than the depressed
portion, and consequently has no tendency to sink. Any movement which
the ocean as a mass makes in order to regain equilibrium tends, as we
have seen, rather to increase the difference of level than to reduce
it. Restoration of level can only be produced by the forces which are
in operation in the wedge-shaped mass W C W′, constituting the slope
itself. But it will be observed by a glance at the Figure that, in
order to the restoration of level, a large portion of the water W W′ at
the equator will require to flow to C, the pole.
According to the general _vertical_ oceanic circulation theory,
pressure from behind is not one of the forces employed in the
production of the flow from the equator to the poles. This is evident;
for there can be no pressure from behind acting on the water if there
be no slope existing between the equator and the poles. Dr. Carpenter
not only denies the actual existence of a slope, but denies the
necessity for its existence. But to deny the existence of a slope is to
deny the existence of pressure, and to deny the necessity for a slope
is to deny the necessity for pressure. That in Dr. Carpenter’s theory
the surface-water is supposed to be _drawn_ from the equator to the
poles, and not _pressed_ forward by a force from behind, is further
evident from the fact that he maintains that the force employed is not
_vis a tergo_ but _vis a fronte_.[83]
CHAPTER XI.
THE INADEQUACY OF THE GRAVITATION THEORY PROVED BY ANOTHER
METHOD.
Quantity of Heat which can be conveyed by the General Oceanic
Circulation trifling.—Tendency in the Advocates of the
Gravitation Theory to under-estimate the Volume of the
Gulf-stream.—Volume of the Stream as determined by the
_Challenger_.—Immense Volume of Warm Water discovered by
Captain Nares.—Condition of North Atlantic inconsistent with
the Gravitation Theory.—Dr. Carpenter’s Estimate of the
Thermal Work of the Gulf-stream.
I shall now proceed by another method to prove the inadequacy of such
a general oceanic circulation as that which Dr. Carpenter advocates.
By contrasting the quantity of heat carried by the Gulf-stream from
inter-tropical to temperate and polar regions with such amount as
can possibly be conveyed in the same direction by means of a general
oceanic circulation, it will become evident that the latter sinks into
utter insignificance before the former.
In my earlier papers on the amount of heat conveyed by the
Gulf-stream,[84] I estimated the volume of that stream as _equal
to that_ of a current 50 miles broad and 1,000 feet deep, flowing
(from the surface to the bottom) at 4 miles an hour. Of course I did
not mean, as Dr. Carpenter seems to suppose, that the stream at any
particular place is 50 miles broad and 1,000 feet deep, or that it
actually flows at the uniform rate of 4 miles an hour at surface and
bottom. All I meant was, that the Gulf-stream is _equal to that_ of
a current of the above size and velocity. But in my recent papers on
Ocean-currents, the substance of which appears in the present volume,
to obviate any objections on the grounds of having over-estimated the
volume, I have taken that at one half this estimate, viz., equal to
a current 50 miles broad and 1,000 feet deep flowing at the rate of
2 miles an hour. I have estimated the mean temperature of the stream
as it passes the Straits of Florida to be 65°, and have supposed that
the water in its course becomes ultimately cooled down on an average
to 40°. In this case each pound of water conveys 19,300 foot-pounds of
heat from the Gulf of Mexico, to be employed in warming temperate and
polar regions. Assuming these data to be correct, it follows that the
amount of heat transferred from the Gulf of Mexico by this stream per
day amounts to 77,479,650,000,000,000,000 foot-pounds. This enormous
quantity of heat is equal to one-fourth of all that is received from
the sun by the whole of the Atlantic Ocean from the Tropic of Cancer up
to the Arctic Circle.
This is the amount of heat conveyed from inter-tropical to temperate
and polar regions by the Gulf-stream. What now is the amount conveyed
by means of the General Oceanic Circulation?
According to this theory there ought to be as much warm water flowing
from inter-tropical regions towards the Antarctic as towards the Arctic
Circle. We may, therefore, in our calculations, consider that the heat
which is received in tropical regions to the south of the equator goes
to warm the southern hemisphere, and that received on the north side
of the equator to warm the northern hemisphere. The warm currents
found in the North Atlantic in temperate regions we may conclude came
from the regions lying to the north of the equator,—or, in other
words, from that part of the Atlantic lying between the equator and
the Tropic of Cancer. At least, according to the gravitation theory,
we have no reason to believe that the quantity of warm water flowing
from tropical to temperate and polar regions in the Atlantic is
greater than the area between the equator and the Tropic of Cancer
can supply—because it is affirmed that a very large proportion of the
cold water found in the North Atlantic comes, not from the arctic, but
from the antarctic regions. But if the North Atlantic is cooled by a
cold stream from the southern hemisphere, the southern hemisphere in
turn must be heated by a warm current from the North Atlantic—unless
we assume that the compensating current flowing from the Atlantic into
the southern hemisphere is as cold as the antarctic current, which is
very improbable. But Dr. Carpenter admits that the quantity of warm
water flowing from the Atlantic in equatorial regions towards the
south is even greater than that flowing northwards. “The unrestricted
communication,” he says, “which exists between the antarctic area and
the great Southern Ocean-basins would involve, if the doctrine of a
general oceanic circulation be admitted, a much more considerable
interchange of waters between the antarctic and the equatorial areas
than is possible in the northern hemisphere.”[85]
We have already seen that, were it not for the great mass of warm water
which finds its way to the polar regions, the temperature of these
regions would be enormously lower than they really are. It has been
shown likewise that the comparatively high temperature of north-western
Europe is due to the same cause. But if it be doubtful whether the
Gulf-stream reaches our shores, and if it be true that, even supposing
it did, it “could only affect the _most superficial_ stratum,” and that
the great mass of warm water found by Dr. Carpenter in his dredging
expeditions came directly from the equatorial regions, and not from
the Gulf-stream, then the principal part of the heating-effect must be
attributed, not to the Gulf-stream, but to the general flow of water
from the equatorial regions. It surely would not, then, be too much to
assume that the quantity of heat conveyed from equatorial regions by
this general flow of water into the North Atlantic is at least equal to
that conveyed by the Gulf-stream. If we assume this to be the amount of
heat conveyed by the two agencies into the Atlantic from inter-tropical
regions, it will, of course, be equal to twice that conveyed by the
Gulf-stream alone.
We shall now consider whether the area of the Atlantic to the north of
the equator is sufficient to supply the amount of heat demanded by Dr.
Carpenter’s theory.
The entire area of the Atlantic, extending from the equator to the
Tropic of Cancer, including the Caribbean Sea and the Gulf of Mexico,
is about 7,700,000 square miles.
The quantity of heat conveyed by the Gulf-stream through the Straits of
Florida is, as we have already endeavoured to show, equal to all the
heat received from the sun by 1,560,935 square miles at the equator.
The annual quantity of heat received from the sun by the torrid zone
per unit surface, taking the mean of the whole zone, is to that
received by the equator as 39 to 40, consequently the quantity of
heat conveyed by the Gulf-stream is equal to all the heat received by
1,600,960 square miles of the Atlantic in the torrid zone.
But if, according to Dr. Carpenter’s views, the quantity of heat
conveyed from the tropical regions is double that conveyed by the
Gulf-stream, the amount of heat in this case conveyed into the Atlantic
in temperate regions will be equal to all the heat received from the
sun by 3,201,920 square miles of the Atlantic between the equator and
the Tropic of Cancer. This is 32/77ths of all the heat received from
the sun by that area.
Taking the annual quantity received per unit surface at the equator at
1,000, the quantities received by the three zones would be respectively
as follows:—
Equator 1000
Torrid zone 975
Temperate zone 757
Frigid zone 454
Now, if we remove from the Atlantic in tropical regions 32/77ths of the
heat received from the sun, we remove 405 parts from every 975 received
from the sun, and consequently only 570 parts per unit surface remain.
It has been shown[86] that the quantity of heat conveyed by the
Gulf-stream from the equatorial regions into the temperate regions
is equal to 100/412ths of all the heat received by the Atlantic in
temperate regions. But according to the theory under consideration the
quantity removed is double this, or equal to 100/206ths of all the heat
received from the sun. But the amount received from the sun is equal
to 757 parts per unit surface; add then to this 100/206ths of 757, or
367, and we have 1,124 parts of heat per unit surface as the amount
possessed by the Atlantic in temperate regions. The Atlantic should in
this case be much warmer in temperate than in tropical regions; for
in temperate regions it would possess 1,124 parts of heat per unit
surface, whereas in tropical regions it would possess only 570 parts
per unit surface. Of course the heat conveyed from tropical regions
does not all remain in temperate regions; a very considerable portion
of it must pass into the arctic regions. Let us, then, assume that
one half goes to warm the Arctic Ocean, and the other half remains
in the temperate regions. In this case 183·5 parts would remain, and
consequently 757 + 183·5 = 940·5 parts would be the quantity possessed
by the Atlantic in temperate regions, a quantity which still exceeds by
no less than 370·5 parts the heat possessed by the Atlantic in tropical
regions.
As one half of the amount of heat conveyed from the tropical regions
is assumed to go into the Arctic Ocean, the quantity passing into
that ocean would therefore be equal to that which passes through the
Straits of Florida, an amount which, as we have found, is equal to all
the heat received from the sun by 3,436,900 square miles of the Arctic
Ocean.[87] The entire area covered by sea beyond the Arctic Circle is
under 5,000,000 square miles; but taking the Arctic Ocean in round
numbers at 5,000,000 square miles, the quantity of heat conveyed into
it by currents to that received from the sun would therefore be as
3,436,900 to 5,000,000.
The amount received on the unit surface of the arctic regions we have
seen to be 454 parts. The amount received from the currents would
therefore be 312 parts. This gives 766 parts of heat per unit surface
as the quantity possessed by the Arctic Ocean. Thus the Arctic Ocean
also would contain more heat than the Atlantic in tropical regions; for
the Atlantic in these regions would, in the case under consideration,
possess only 570 parts, while the Arctic Ocean would possess 766
parts. It is true that more rays are cut off in arctic regions than in
tropical; but still, after making due allowance for this, the Arctic
Ocean, if the theory we are considering were true, ought to be as warm
as, if not warmer than, the Atlantic in tropical regions. The relative
quantities of heat possessed by the three zones would therefore be as
follows:—
Atlantic, in torrid zone 570
〃 in temperate zone 940
〃 in frigid zone 766
It is here assumed, however, that none of the heat possessed by the
Gulf-stream is derived from the southern hemisphere, which, we know,
is not the case. But supposing that as much as one half of the heat
possessed by the stream came from the southern hemisphere, and that the
other half was obtained from the seas lying between the equator and the
Tropic of Cancer, the relative proportions of heat possessed by the
three zones per given area would be as follows:—
Atlantic, in torrid zone 671
〃 in temperate zone 940
〃 in frigid zone 766
This proves incontestably that, supposing there is such a general
oceanic circulation as is maintained, the quantity of heat conveyed by
means of it into the North Atlantic and Arctic Oceans must be trifling
in comparison with that conveyed by the Gulf-stream; for if it nearly
equalled that conveyed by the Gulf-stream, then not only the North
Atlantic in temperate regions, but even the Arctic Ocean itself would
be much warmer than the inter-tropical seas. In fact, so far as the
distribution of heat over the globe is concerned, it is a matter of
indifference whether there really is or is not such a thing as this
general oceanic circulation. The enormous amount of heat conveyed by
the Gulf-stream alone puts it beyond all doubt that ocean-currents are
the great agents employed in distributing over the globe the excess of
heat received by the sea in inter-tropical regions.
It is therefore, so far as concerns the theory of a General Oceanic
Circulation, of the utmost importance that the advocates of that
theory should prove that I have over-estimated the thermal power of
the Gulf-stream. This, however, can only be done by detecting some
error either in my computation or in the data on which it is based;
yet neither Dr. Carpenter nor any one else, as far as I know, has
challenged the accuracy of my figures. The question at issue is the
correctness of the data; but the only part of the data which can
possibly admit of being questioned is my estimate of the _volume_
and _temperature_ of the stream. Dr. Carpenter, however, does not
maintain that I have over-estimated the temperature of the stream; on
the contrary, he affirms that I have really under-estimated it. “If we
assume,” he remarks, “the limit of the stratum above 60° as that of
the real Gulf-stream current, we shall find its average temperature to
be somewhat higher than it has been stated by Mr. Croll, who seems to
have taken 65° as the average of the water flowing through the entire
channel. The average surface temperature of the Florida channel for
the whole year is 80°; and we may fairly set the average of the entire
outgoing stream, down to the plane of 60°, at 70°, instead of 65° as
estimated by Mr. Croll” (§ 141). It follows, then, that every pound of
water of the Gulf-stream actually conveys 5 units of heat more than
I have estimated it to do—the amount conveyed being 30 units instead
of 25 units as estimated by me. Consequently, if the Gulf-stream be
equal to that of a current of merely 41½ miles broad and 1,000 feet
deep, flowing at the rate of 2 miles an hour, it will still convey the
estimated quantity of heat. But this estimate of the volume of the
stream, let it be observed, barely exceeds _one-third_ of that given
by Herschel, Maury, and Colding,[88] and is little more than one-half
that assigned to it by Mr. Laughton, while it very little exceeds that
given by Mr. Findlay,[89] an author whom few will consider likely to
overrate either the volume or heating-power of the stream.
The important results obtained during the _Challenger_ expedition have
clearly proved that I have neither over-estimated the temperature nor
the volume of the Gulf-stream. Between Bermuda and Sandy Hook the
stream is 60 miles broad and 600 feet deep, with a maximum velocity of
from 3½ to 4 miles an hour. If the mean velocity of the entire section
amounts to 2¼ miles an hour, which it probably does, the volume of the
stream must equal that given in my estimate. But we have no evidence
that all the water flowing through the Straits of Florida passes
through the section examined by the officers of the _Challenger_. Be
this, however, as it may, the observations made between St. Thomas
and Sandy Hook reveal the existence of an immense flow of warm water,
2,300 feet deep, entirely distinct from the water included in the above
section of the Gulf-stream proper. As the thickest portion of this
immense body of water joins the warm water of the Gulf-stream, Captain
Nares considers that “it is evidently connected with it, and probably
as an offshoot.” At Sandy Hook, according to him, it extends 1,200
feet deeper than the Gulf-stream itself, but off Charleston, 600 miles
nearer the source, the same temperature is found at the same depth.
But whether it be an offshoot of the Gulf-stream or not, one thing is
certain, it can only come from the Gulf of Mexico or from the Caribbean
Sea. This mass of water, after flowing northwards for about 1,000
miles, turns to the right and crosses the Atlantic in the direction of
the Azores, where it appears to thin out.
If, therefore, we take into account the combined heat conveyed by
both streams, my estimate of the heat transferred from inter-tropical
regions into the North Atlantic will be found rather under than above
the truth.
_Dr. Carpenter’s Estimate of the Thermal Work of the Gulf-stream._—In
the appendix to an elaborate memoir on Oceanic Circulation lately
read before the Geographical Society, Dr. Carpenter endeavours to
show that I have over-estimated the thermal work of the Gulf-stream.
In that memoir[90] he has also favoured us with his own estimate of
the sectional area, rate of flow, and temperature of the stream. Even
adopting his data, however, I find myself unable to arrive at his
conclusions.
Let us consider first his estimate of the sectional area of the
stream. He admits that “it is impossible, in the present state of our
knowledge, to arrive at any exact estimate of the sectional area of the
stream; since it is for the most part only from the temperatures of
its different strata that we can judge whether they are, or are not,
in movement, and what is the direction of their movement.” Now it is
perfectly evident that our estimate of the sectional area of the stream
will depend upon what we assume to be its bottom temperature. If, for
example, we assume 70° to be the bottom temperature, we shall have a
small sectional area. Taking the temperature at 60°, the sectional
area will be larger, and if 50° be assumed to be the temperature, the
sectional area will be larger still, and so on. Now the small sectional
area obtained by Dr. Carpenter arises from the fact of his having
assumed the high temperature of 60° to be that of the bottom of the
stream. He concludes that all the water below 60° has an inward flow,
and that it is only that portion from 60° and upwards which constitutes
the Gulf-stream. I have been unable to find any satisfactory evidence
for assuming so high a temperature for the bottom of the stream. It
must be observed that the water underlying the Gulf-stream is not
the ordinary water of the Atlantic, but the cold current from the
arctic regions. In fact, it is the same water which reaches the
equator at almost every point with a temperature not much above the
freezing-point. It is therefore highly improbable that the under
surface of the Gulf-stream has a temperature so high as 60°.
Dr. Carpenter’s method of measuring the mean velocity of the
Gulf-stream is equally objectionable. He takes the mean annual rate at
the surface in the “Narrows” to be two miles an hour and the rate at
the bottom to be zero, and he concludes from this that the average rate
of the whole is one mile an hour—the arithmetical mean between these
two extremes. Now it will be observed that this conclusion only holds
true on the supposition that the breadth of the stream is as great at
the bottom as at the surface, which of course it is not. All admit that
the sides of the Gulf-stream are not perpendicular, but slope somewhat
in the manner of the banks of a river. The stream is broad at the
surface and narrows towards the bottom. It is therefore evident that
the upper half of the section has a much larger area than the lower;
the quantity of water flowing through the upper half with a greater
velocity than one mile an hour must be much larger than the quantity
flowing through the lower half with a less velocity than one mile an
hour.
His method of estimating the mean temperature of the stream is even
more objectionable. He says, “The average surface temperature of the
Florida Channel for the whole year is 80°, and we may set the average
of the entire outgoing stream down to the plane of 60° at 70°, instead
of 65°, as estimated by Mr. Croll.” If 80° be the surface and 60° be
the bottom temperature, temperature and rate of velocity being assumed
of course to decrease uniformly from the surface downwards, how is it
possible that 70° can be the average temperature? The amount of water
flowing through the upper half of the section, with a temperature above
70°, is far more than the amount flowing through the under half of the
section, with a temperature below 70°. Supposing the lower half of the
section to be as large as the upper half, which it is not, still the
quantity of water flowing through it would only equal one-third of
that flowing through the upper half, because the mean velocity of the
water in the lower half would be only half a mile per hour, whereas
the mean velocity of that in the upper half would be a mile and a half
an hour. But the area of the lower half is much less than that of the
upper half, consequently the amount of water whose temperature is under
70° must be even much under one-third of that, the temperature of which
is above 70°.
Had Dr. Carpenter taken the proper method of estimating the mean
temperature, he would have found that 75°, even according to his own
data, was much nearer the truth than 70°. I pointed out, several years
ago,[91] the fallacy of estimating the mean temperature of a stream in
this way.
So high a mean temperature as 75° for the Gulf-stream, even in the
Florida Channel, is manifestly absurd, but if 60° be the bottom
temperature of the stream, the mean temperature cannot possibly be much
under that amount. It is, of course, by under-estimating the sectional
area of the stream that its mean temperature is over-estimated. We
cannot reduce the mean temperature without increasing the sectional
area. If my estimate of 65° be taken as the mean temperature, which I
have little doubt will yet be found to be not far from the truth, Dr.
Carpenter’s estimate of the sectional area must be abandoned. For if
65° be the mean temperature of the stream, its bottom temperature must
be far under 60°, and if the bottom temperature be much under 60°, then
the sectional area must be greater than he estimates it to be.
Be this, however, as it may; even if we suppose that 60° will
eventually be found to be the actual bottom temperature of the
Gulf-stream, nevertheless, if the total quantity of heat conveyed by
the stream from inter-tropical regions be estimated in the proper way,
we shall still find that amount to be so enormous, that there is not
sufficient heat remaining in those regions to supply Dr. Carpenter’s
oceanic circulation with a quantity as great for distribution in the
North Atlantic.
It therefore follows (and so far as regards the theory of Secular
changes of climate, this is all that is worth contending for) that
Ocean-currents and not a General Oceanic Circulation resulting from
gravity, are the great agents employed in the distribution of heat over
the globe.
CHAPTER XII.
MR. A. G. FINDLAY’S OBJECTIONS CONSIDERED.
Mr. Findlay’s Estimate of the Volume of the Gulf-stream.—Mean
Temperature of a Cross Section less than Mean Temperature
of Stream.—Reason of such Diversity of Opinion regarding
Ocean-currents.—More rigid Method of Investigation necessary.
At the conclusion of the reading of Dr. Carpenter’s paper before the
Royal Geographical Society, on January 9th, 1871, Mr. Findlay made the
following remarks:—
“When, by the direction of the United States Government, ten or eleven
years ago, the narrowest part of the Gulf-stream was examined, figures
were obtained which shut out all idea of its ever reaching our shores
as a heat-bearing current. In the narrowest part, certainly not more
than from 250 to 300 cubic miles of water pass per diem. Six months
afterwards that water reaches the banks of Newfoundland, and nine or
twelve months afterwards the coast of England, by which time it is
popularly supposed to cover an area of 1,500,000 square miles. The
proportion of the water that passes through the Gulf of Florida will
not make a layer of water more than 6 inches thick per diem over such
a space. Every one knows how soon a cup of tea cools; and yet it is
commonly imagined that a film of only a few inches in depth, after the
lapse of so long a time, has an effect upon our climate. There is no
need for calculations; the thing is self-evident.”[92]
About five years ago, Mr. Findlay objected to the conclusions which I
had arrived at regarding the enormous heating-power of the Gulf-stream
on the ground that I had over-estimated the volume of the stream. He
stated that its volume was only about the half of what I had estimated
it to be. To obviate this objection, I subsequently reduced the volume
to one-half of my former estimate.[93] But taking the volume at this
low estimate, it was nevertheless found that the quantity of heat
conveyed into the Atlantic through the Straits of Florida by means of
the stream was equal to about _one-fourth_ of all the heat received
from the sun by the Atlantic from the latitude of the Strait of Florida
up to the Arctic Circle.
Mr. Findlay, in his paper read before the British Association, affirmed
that the volume of the stream is somewhere from 294 to 333 cubic miles
per day; but in his remarks at the close of Dr. Carpenter’s address, he
stated it to be not greater than from 250 to 300 cubic miles per day. I
am unable to reconcile any of those figures with the data from which he
appears to have derived them. In his paper to the British Association,
he remarks that “the Gulf-stream at its outset is not more than 39½
miles wide, and 1,200 feet deep.” From all attainable data, he computes
the mean annual rate of motion to be 65·4 miles per day; but as the
rate decreases with the depth, the mean velocity of the whole mass does
not exceed 49·4 miles per day. When he speaks of the mean velocity of
the Gulf-stream being so and so, he must refer to the mean velocity at
some particular place. This is evident; for the mean velocity entirely
depends upon the sectional area of the stream. The place where the
mean velocity is 49·4 miles per day must be the place where it is 39½
miles broad and 1,200 feet deep; for he is here endeavouring to show us
how small the volume of the stream actually is. Now, unless the mean
velocity refers to the place where he gives us the breadth and depth
of the stream, his figures have no bearing on the point in question.
But a stream 39½ miles broad and 1,200 feet deep has a sectional area
of 8·97 square miles, and this, with a mean velocity of 49·4 miles
per day, will give 443 cubic miles of water. The amount, according to
my estimate, is 459 cubic miles per day; it therefore exceeds Mr.
Findlay’s estimate by only 16 cubic miles.
Mr. Findlay does not, as far as I know, consider that I have
over-estimated the mean temperature of the stream. He states[94] that
between Sand Key and Havana the Gulf-stream is about 1,200 feet deep,
and that it does not reach the summit of a submarine ridge, which he
states has a temperature of 60°. It is evident, then, that the bottom
of the stream has a temperature of at least 60°, which is within 5° of
what I regard as the mean temperature of the mass. But the surface of
the stream is at least 17° above this mean. Now, when we consider that
it is at the upper parts of the stream, the place where the temperature
is so much above 65°, that the motion is greatest, it is evident that
the mean temperature of the entire moving mass must, according to Mr.
Findlay, be considerably over 65°. It therefore follows, according
to his own data, that the Gulf-stream conveys into the Atlantic an
amount of heat equal to one-fourth of all the heat which the Atlantic,
from the latitude of the Straits of Florida up to the arctic regions,
derives from the sun.
But it must be borne in mind that although the mean temperature of the
cross section should be below 65°, it does not therefore follow that
the mean temperature of the _water flowing through this cross section_
must be below that temperature, for it is perfectly obvious that the
mean temperature of the mass of water flowing through the cross section
in a given time must be much higher than that of the cross section
itself. The reason is very simple. It is in the upper half of the
section where the high temperature exists; but as the velocity of the
stream is much greater in its upper than in its lower half, the greater
portion of the water passing through this cross section is water of
high temperature.
But even supposing we were to halve Mr. Findlay’s own estimate, and
assume that the volume of the stream is equal to only 222 cubic miles
of water per day instead of 443, still the amount of heat conveyed
would be equal to one-eighth part of the heat received from the sun by
the Atlantic. But would not the withdrawal of an amount of heat equal
to one-eighth of that received from the sun greatly affect the climate
of the Atlantic? Supposing we take the mean temperature of the Atlantic
at, say, 56°; this will make its temperature 295° above that of space.
Extinguish the sun and stop the Gulf-stream, and the temperature ought
to sink 295°. How far, then, ought the temperature to sink, supposing
the sun to remain and the Gulf-stream to stop? Would not the withdrawal
of the stream cause the temperature to sink some 30°? Of course, if
the Gulf-stream were withdrawn and everything else were to remain the
same, the temperature of the Atlantic would not actually remain 30°
lower than at present; for heat would flow in from all sides and partly
make up for the loss of the stream. But nevertheless 30° represents the
amount of temperature maintained by means of the heat from the stream.
And this, be it observed, is taking the volume of the stream at a lower
estimate than even Mr. Findlay himself would be willing to admit. Mr.
Findlay says that, by the time the Gulf-stream reaches the shores of
England, it is supposed to cover a space of 1,500,000 square miles.
“The proportion of water that passes through the Straits of Florida
will not make,” according to him, “a layer of water more than 6 inches
thick per diem over such a space.” But a layer of water 6 inches thick
cooling 25° will give out 579,000 foot-pounds of heat per square foot.
If, therefore, the Gulf-stream, as he asserts, supplies 6 inches per
day to that area, then every square foot of the area gives off per
day 579,000 foot-pounds of heat. The amount of heat received from the
sun per square foot in latitude 55°, which is not much above the mean
latitude of Great Britain, is 1,047,730 foot-pounds per day, taking, of
course, the mean of the whole year; _consequently this layer of water
gives out an amount of heat equal to more than_ one-half _of all that
is received from the sun_. But assuming that the stream should leave
the half of its heat on the American shores and carry to the shores of
Britain only 12½° of heat, still we should have 289,500 foot-pounds per
square foot, which notwithstanding _is more than equal to_ one-fourth
_of that received from the sun_. If an amount of heat so enormous
cannot affect climate, what can?
I shall just allude to one other erroneous notion which prevails in
regard to the Gulf-stream; but it is an error which I by no means
attribute either to Mr. Findlay or to Dr. Carpenter. The error to which
I refer is that of supposing that when the Gulf-stream widens out to
hundreds of miles, as it does before it reaches our shores, its depth
must on this account be much less than when it issues from the Gulf of
Mexico. Although the stream may be hundreds of miles in breadth, there
is no necessity why it should be only 6 inches, or 6 feet, or 60 feet,
or even 600 feet in depth. It may just as likely be 6,000 feet deep as
6 inches.
_The Reason why such Diversity of Opinion prevails in Regard to
Ocean-currents._—In conclusion I venture to remark that more than
nine-tenths of all the error and uncertainty which prevail, both
in regard to the cause of ocean-currents and to their influence on
climate, is due, not, as is generally supposed, to the intrinsic
difficulties of the subject, but rather to the defective methods
which have hitherto been employed in its investigation—that is, in
not treating the subject according to the rigid methods adopted in
other departments of physics. What I most particularly allude to is
the disregard paid to the modern method of determining the amount of
effects in _absolute measure_.
But let me not be misunderstood on this point. I by no means suppose
that the _absolute quantity_ is the thing always required for its
own sake. It is in most cases required simply as a means to an end;
and very often that end is the knowledge of the _relative_ quantity.
Take, for example, the Gulf-stream. Suppose the question is asked,
to what extent does the heat conveyed by that stream influence the
climate of the North Atlantic? In order to the proper answering of this
question, the principal thing required is to know what proportion the
amount of heat conveyed by the stream into the Atlantic bears to that
received from the sun by that area. We want the _relative proportions_
of these two quantities. But how are we to obtain them? We can only
do so by determining first the _absolute_ quantity of each. We must
first measure each before we can know how much the one is greater
than the other, or, in other words, before we can know their relative
proportions. We have the means of determining the absolute amount
of heat received from the sun by a given area at any latitude with
tolerable accuracy; but the same cannot be done with equal accuracy in
regard to the amount of heat conveyed by the Gulf-stream, because the
volume and mean temperature of the stream are not known with certainty.
Nevertheless we have sufficient data to enable us to fix upon such a
maximum and minimum value to these quantities as will induce us to
admit that the truth must lie somewhere between them. In order to give
full justice to those who maintain that the Gulf-stream exercises
but little influence on climate, and to put an end to all further
objections as to the uncertainty of my data, I shall take a minimum
to which none of them surely can reasonably object, viz. that the
volume of the stream is not over 230 cubic miles per day, and the heat
conveyed per pound of water not over 12½ units. Calculating from these
data, we find that the amount of heat carried into the North Atlantic
is equal to one-sixteenth of all the heat received from the sun by that
area. There are, I presume, few who will not admit that the actual
proportion is much higher than this, probably as high as 1 to 3, or 1
to 4. But, who, without adopting the method I have pursued, could ever
have come to the conclusion that the proportion was even 1 to 16? He
might have guessed it to be 1 to 100 or 1 to 1000, but he never would
have guessed it to be 1 to 16. Hence the reason why the great influence
of the Gulf-stream as a heating agent has been so much under-estimated.
The same remarks apply to the gravitation theory of the cause of
currents. Viewed simply as a theory it looks very reasonable. There is
no one acquainted with physics but will admit that the tendency of the
difference of temperature between the equator and the poles is to cause
a surface current from the equator towards the poles, and an under
current from the poles to the equator. But before we can prove that
this tendency does actually produce such currents, another question
must be settled, viz. is this force sufficiently great to produce
the required motion? Now when we apply the method to which I refer,
and determine the absolute amount of the force resulting from the
difference of specific gravity, we discover that not to be the powerful
agent which the advocates of the gravitation theory suppose, but a
force so infinitesimal as not to be worthy of being taken into account
when considering the causes by which currents are produced.
CHAPTER XIII.
THE WIND THEORY OF OCEANIC CIRCULATION.
Ocean-currents not due alone to the Trade-winds.—An Objection
by Maury.—Trade-winds do not explain the Great Antarctic
Current.—Ocean-currents due to the System of Winds.—The
System of Currents agrees with the System of the
Winds.—Chart showing the Agreement between the System
of Currents and System of Winds.—Cause of the Gibraltar
Current.—North Atlantic an immense Whirlpool.—Theory of Under
Currents.—Difficulty regarding Under Currents obviated.—Work
performed by the Wind in impelling the Water forward.—The
_Challenger’s_ crucial Test of the Wind and Gravitation
Theories.—North Atlantic above the Level of Equator.—Thermal
Condition of the Southern Ocean irreconcilable with the
Gravitation Theory.
_Ocean-currents not due alone to the Trade-winds._—The generally
received opinion amongst the advocates of the wind theory of oceanic
circulation is that the Gulf-stream and other currents of the ocean are
due to the impulse of the trade-winds. The tendency of the trade-winds
is to impel the inter-tropical waters along the line of the equator
from east to west; and were those regions not occupied in some places
by land, this equatorial current would flow directly round the
globe. Its westward progress, however, is arrested by the two great
continents, the old and the new. On approaching the land the current
bifurcates, one portion trending northwards and the other southwards.
The northern branch of the equatorial current of the Atlantic passes
into the Caribbean Sea, and after making a circuit of the Gulf of
Mexico, flows northward and continues its course into the Arctic
Ocean. The southern branch, on the other hand, is deflected along the
South-American coast, constituting what is known as the Brazilian
current. In the Pacific a similar deflection occurs against the
Asiatic coast, forming a current somewhat resembling the Gulf-stream,
a portion of which (Kamtschatka current) in like manner passes into
the arctic regions. In reference to all these various currents, the
impelling cause is supposed to be the force of the trade-winds.
It is, however, urged as an objection by Maury and other advocates of
the gravitation theory, that a current like the Gulf-stream, extending
as far as the arctic regions, could not possibly be impelled and
maintained by a force acting at the equatorial regions. But this is
a somewhat weak objection. It seems to be based upon a misconception
of the magnitude of the force in operation. It does not take into
account that this force acts on nearly the whole area of the ocean in
inter-tropical regions. If, in a basin of water, say three feet in
diameter, a force is applied sufficient to produce a surface-flow one
foot broad across the centre of the basin, the water impelled against
the side will be deflected to the extremes of the vessel. And this
result does not in any way depend upon the size of the basin. The
same effect which occurs in a small basin will occur in a large one,
provided the proportion between the breadth of the belt of water put in
motion and the size of the vessel be the same in both cases. It does
not matter, therefore, whether the diameter of the basin be supposed to
be three feet, or three thousand miles, or ten thousand miles.
There is a more formidable objection, however, to the theory.
The trade-winds will account for the Gulf-stream, Brazil, Japan,
Mozambique, and many other currents; but there are currents, such as
some of the polar currents, which cannot be so accounted for. Take,
for example, the great antarctic current flowing northward into the
Pacific. This current does not bend to the left under the influence
of the earth’s rotation and continue its course in a north-westerly
direction, but actually bends round to the right and flows eastward
against the South-American coast, in direct opposition both to the
influence of rotation and to the trade-winds. The trade-wind theory,
therefore, is insufficient to account for all the facts. But there is
yet another explanation, which satisfactorily solves our difficulties.
The currents of the ocean owe their origin, not to the trade-winds
alone, but to the _prevailing_ winds of the globe (including, of
course, the trade-winds).
_Ocean-currents due to the System of Winds._—If we leave out of account
a few small inland sheets of water, the globe may be said to have but
one sea, just as it possesses only one atmosphere. We have accustomed
ourselves, however, to speak of parts or geographical divisions of
the one great ocean, such as the Atlantic and the Pacific, as if they
were so many separate oceans. And we have likewise come to regard the
currents of the ocean as separate and independent of one another. This
notion has no doubt to a considerable extent militated against the
acceptance of the theory that the currents are caused by the winds, and
not by difference of specific gravity; for it leads to the conclusion
that currents in a sea must flow in the direction of the prevailing
winds blowing over that particular sea. The proper view of the matter,
as I hope to be able to show, is that which regards the various
currents merely as members of one grand system of circulation produced,
not by the trade-winds alone, nor by the prevailing winds proper alone,
but by the combined action of all the prevailing winds of the globe,
regarded as one system of circulation.
If the winds be the impelling cause of currents, the _direction_ of the
currents will depend upon two circumstances, viz.:—(1) the direction
of the prevailing winds of the globe, including, of course, under this
term the prevailing winds proper and the trade-winds; and (2) the
conformation of land and sea. It follows, therefore, that as a current
in any given sea is but a member of a general system of circulation,
its direction is determined, not alone by the prevailing winds blowing
over the sea in question, but by the general system of prevailing
winds. It may consequently sometimes happen that the general system
of winds may produce a current directly opposite to the prevailing
wind blowing over the current. The accompanying Chart (Plate I.) shows
how exactly the system of ocean-currents agrees with the system of
the prevailing winds. The fine lines indicate the paths of the
prevailing winds, and the fine arrows the direction in which the wind
blows along those paths. The large arrows show the direction of the
principal ocean-currents.
[Illustration: PLATE I.
CHART SHOWING THE GENERAL AGREEMENT BETWEEN THE SYSTEM OF OCEAN
CURRENTS AND WINDS.
W. & A. K. Johnston, Edinb^r. and London.]
The directions and paths of the prevailing winds have been taken from
Messrs. Johnston’s small physical Atlas, which, I find, agrees exactly
with the direction of the prevailing winds as deduced from the four
quarterly wind charts lately published by the Hydrographic Department
of the Admiralty. The direction of the ocean-currents has been taken
from the Current-chart published by the Admiralty.
In every case, without exception, the direction of the main currents of
the globe agrees exactly with the direction of the prevailing winds.
There could not possibly be a more convincing proof that those winds
are the cause of the ocean-currents than this general agreement of the
two systems as indicated by the chart. Take, for example, the North
Atlantic. The Gulf-stream follows exactly the path of the prevailing
winds. The Gulf-stream bifurcates in mid-Atlantic; so does the wind.
The left branch of the stream passes north-eastwards into the arctic
regions, and the right branch south-eastwards by the Azores; so does
the wind. The south-eastern branch of the stream, after passing the
Canaries, re-enters the equatorial current and flows into the Gulf
of Mexico; the same, it will be observed, holds true of the wind. A
like remarkable agreement exists in reference to all the other leading
currents of the ocean. This is particularly seen in the case of the
great antarctic current between long. 140° W. and 160° W. This current,
flowing northwards from the antarctic regions, instead of bending to
the left under the influence of rotation, turns to the right when it
enters the regions of the westerly winds, and flows eastwards towards
the South-American shores. In fact, all the currents in this region of
strong westerly winds flow in an easterly or north-easterly direction.
Taking into account the effects resulting from the conformation of
sea and land, the system of ocean-currents agrees precisely with
the system of the winds. All the principal currents of the globe are
in fact moving in the exact direction in which they ought to move,
assuming the winds to be the sole impelling cause. In short, so perfect
is the agreement between the two systems, that, given the system of
winds and the conformation of sea and land, and the direction of all
the currents of the ocean, or more properly the system of oceanic
circulation, might be determined _à priori_. Or given the system of the
ocean-currents together with the conformation of sea and land, and the
direction of the prevailing winds could also be determined _à priori_.
Or, thirdly, given the system of winds and the system of currents,
and the conformation of sea and land might be roughly determined. For
example, it can be shown by this means that the antarctic regions
are probably occupied by a continent and not by a number of separate
islands, nor by sea.
While holding that the currents of the ocean form one system of
circulation, we must not be supposed to mean that the various currents
are connected end to end, having the same water flowing through them
all in succession like that in a heating apparatus. All that is
maintained is simply this, that the currents are so mutually related
that any great change in one would modify the conditions of all the
others. For example, a great increase or decrease in the easterly flow
of antarctic water in the Southern Ocean would decrease or increase,
as the case might be, the strength of the West Australian current;
and this change would modify the equatorial current of the Indian
Ocean, a modification which in like manner would affect the Agulhas
current and the Southern Atlantic current—this last leading in turn
to a modification of the equatorial current of the Atlantic, and
consequently of the Brazilian current and the Gulf-stream. Furthermore,
since a current impelled by the winds, as Mr. Laughton in his excellent
paper on Ocean-currents justly remarks, tends to leave a vacancy
behind, it follows that a decrease or increase in the Gulf-stream would
affect the equatorial current, the Agulhas current, and all the other
currents back to the antarctic currents. Again, a large modification
in the great antarctic drift-current would in like manner affect all
the currents of the Pacific. On the other hand, any great change in
the currents of the Pacific would ultimately affect the currents of
the Atlantic and Indian Oceans, through its influence on the Cape Horn
current, the South Australian current, and the current passing through
the Asiatic archipelago; and _vice versâ_, any changes in the currents
of the Atlantic or Indian Oceans would modify the currents of the
Pacific.
_Cause of Gibraltar Current._—I may now consider the cause of the
Gibraltar current. There can be little doubt that this current owes its
origin (as Mr. Laughton points out) to the Gulf-stream. “I conceive,”
that author remarks, “that the Gibraltar current is distinctly a stream
formed by easterly drift of the North Atlantic, which, although it
forms a southerly current on the coast of Portugal, is still strongly
pressed to the eastward and seeks the first escape it can find. So
great indeed does this pressure seem to be, that more water is forced
through the Straits than the Mediterranean can receive, and a part
of it is ejected in reverse currents, some as lateral currents on
the surface, some, it appears, as an under current at a considerable
depth.”[95] The funnel-shaped nature of the strait through which the
water is impelled helps to explain the existence of the under current.
The water being pressed into the narrow neck of the channel tends to
produce a slight banking up; and as the pressure urging the water
forward is greatest at the surface and diminishes rapidly downwards,
the tendency to the restoration of level will cause an underflow
towards the Atlantic, because below the surface the water will find the
path of least resistance. It is evident indeed that this underflow will
not take place toward the Mediterranean, from the fact that that sea is
already filled to overflowing by the current received from the outside
ocean.
If we examine the Current-chart published by the Hydrographic
Department of the Admiralty, we shall find the Gibraltar current
represented as merely a continuation of the S.E. flow of Gulf-stream
water. Now, if the arrows shown upon this chart indicate correctly the
direction of the flow, we must become convinced that the Gulf-stream
water cannot possibly avoid passing through the Gibraltar Strait. Of
course the excess of evaporation over that of precipitation within
the Mediterranean area would alone suffice to produce a considerable
current through the Strait; but this of itself would not fill that
inland sea to overflowing.[96]
The Atlantic may, in fact, be regarded as an immense whirlpool with the
Saragossa Sea as its vortex; and although it is true, as will be seen
from an inspection of the Chart, that the wind blows round the Atlantic
along the very path taken by the water, impelling the water forward
along every inch of its course, yet nevertheless it must hold equally
true that the water has a tendency to flow off in a straight line at
a tangent to the circular course in which it is moving. But the water
is so hemmed in on all sides that it cannot leave this circular path
except only at two points; and at these two points it actually does
flow outwards. On the east and west sides the land prevents any such
outflow. Similarly, in the south the escape of the water is frustrated
by the pressure of the opposing currents flowing from that quarter;
while in the north it is prevented by the pressure exerted by polar
currents from Davis Strait and the Arctic Ocean. But in the Strait of
Gibraltar and in the north-eastern portion of the Atlantic between
Iceland and the north-eastern shores of Europe there is no resistance
offered: and at these two points an outflow does actually take place.
In both cases, however, especially the latter, the outflow is greatly
aided by the impulse of the prevailing winds.
No one, who will glance at the accompanying chart (Plate I.) showing
how the north-eastern branch of the Gulf-stream bends round and, of
course, necessarily presses against the coast, can fail to understand
how the Atlantic water should be impelled into the Gibraltar Strait,
even although the loss sustained by the Mediterranean from evaporation
did not exceed the gain from rain and rivers.
_Theory of Under Currents._—The consideration that ocean-currents are
simply parts of a system of circulation produced by the system of
prevailing winds, and not by the impulse of the trade-winds alone,
helps to remove the difficulty which some have in accounting for the
existence of under currents without referring them to difference of
specific gravity. Take the case of the Gulf-stream, which passes
under the polar stream on the west of Spitzbergen, this latter stream
passing in turn under the Gulf-stream a little beyond Bear Island. The
polar streams have their origin in the region of prevailing northerly
winds, which no doubt extends to the pole. The current flowing past
the western shores of Spitzbergen, throughout its entire course up
to near the point where it disappears under the warm waters of the
Gulf-stream, lies in the region of these same northerly winds. Now why
should this current cease to be a surface current as soon as it passes
out of the region of northerly into that of south-westerly winds? The
explanation seems to be this: when the stream enters the region of
prevailing south-westerly winds, its progress southwards along the
surface of the ocean is retarded both by the wind and by the surface
water moving in opposition to its course; but being continually pressed
forward by the impulse of the northerly winds acting along its whole
course back almost to the pole, perhaps, or as far north at least as
the sea is not wholly covered with ice, the polar current cannot stop
when it enters the region of opposing winds and currents; it must move
forward. But the water thus pressed from behind will naturally take
the _path of least resistance_. Now in the present case this path will
necessarily lie at a considerable distance below the surface. Had the
polar stream simply to contend with the Gulf-stream flowing in the
opposite direction, it would probably keep the surface and continue its
course along the side of that stream; but it is opposed by the winds,
from which it cannot escape except by dipping down under the surface;
and the depth to which it will descend will depend upon the depth of
the surface current flowing in the opposite direction. There is no
necessity for supposing a heaping up of the water in order to produce
by pressure a force sufficient to impel the under current. The pressure
of the water from behind is of itself enough. The same explanation, of
course, applies to the case of the Gulf-stream passing under the polar
stream. And if we reflect that these under currents are but parts of
the general system of circulation, and that in most cases they are
currents compensating for water drained off at some other quarter, we
need not wonder at the distance which they may in some cases flow, as,
for example, from the banks of Newfoundland to the Gulf of Mexico.
The under currents of the Gulf-stream are necessary to compensate for
the water impelled southwards by the northerly winds; and again, the
polar under currents are necessary to compensate for the water impelled
northward by the south and south-westerly winds.
But it may be asked, how do the opposing currents succeed in crossing
each other? It is evident that the Gulf-stream must plunge through
the whole thickness of the polar stream before it can become an
under current, and so likewise must the cold water of the polar-flow
pass through the genial water of the Gulf-stream in order to get
underneath it and continue on its course towards the south. The
accompanying diagram (Plate II., Fig. 1) will render this sufficiently
intelligible.
[Illustration: _Fig. 3_ PLATE II.
_Map shewing meeting of the Gulf-stream and Polar Current (from
D^r. Petermann’s Geographische Mittheilungen._) _The curved lines
are Isotherms; temperatures are in Fahrenheit._]
[Illustration: _Fig. 1_
_Diagram to shew how two opposing currents intersect each other_]
[Illustration: _Surface Plan to shew how two opposing currents meet
each other_
W. & A. K. Johnston, Edinb^r. and London.
_Fig. 2_]
Now these two great ocean-currents are so compelled to intersect each
other for the simple reason that they cannot turn aside, the one to the
left and the other to the right. When two broad streams like those in
question are pressed up against each other, they succeed in mutually
intersecting each other’s path by breaking up into bands or belts—the
cold water being invaded and pierced as it were by long tongues of
warm water, while at the same time the latter is similarly intersected
by corresponding protrusions of cold water. The two streams become
in a manner interlocked, and the one passes through the other very
much as we pass the fingers of one hand between the fingers of the
other. The diagram (Plate II., Fig. 2), representing the surface of
the ocean at the place of meeting of two opposing currents, will show
this better than description. At the surface the bands necessarily
assume the tongue-shaped appearance represented in the diagram, but
when they have succeeded in mutually passing down through the whole
thickness of the opposing currents, they then unite and form two
definite under currents, flowing in opposite directions. The polar
bands, after penetrating the Gulf-stream, unite below to form a
southward-flowing under current, and in the same way the Gulf-stream
bands, uniting underneath the polar current, continue in their
northerly course as a broad under current of warm water. That this is
a correct representation of what actually occurs in nature becomes
evident from an inspection of the current charts. Thus in the chart
of the North Atlantic which accompanies Dr. Petermann’s Memoir on the
Gulf-stream, we observe that south of Spitzbergen the polar current and
the Gulf-stream are mutually interpenetrated—long tongues invading and
dipping down underneath the Gulf-stream, while in like manner the polar
current becomes similarly intersected by well-marked protrusions of
warm water flowing from the south. (See Plate II., Fig. 3.)
No accurate observations, as far as I know, have been made regarding
the amount of work performed by the wind in impelling the water
forward; but when we consider the great retarding effect of objects
on the earth’s surface, it is quite apparent that the amount of work
performed on the surface of the ocean must be far greater than is
generally supposed. For example, Mr. Buchan, Secretary to the Scottish
Meteorological Society, has shown[97] that a fence made of slabs of
wood three inches in width and three inches apart from each other is a
protection even during high winds to objects on the lee side of it, and
that a wire screen with meshes about an inch apart affords protection
during a gale to flower-pots. The same writer was informed by Mr. Addie
that such a screen put up at Rockville was torn to pieces by a storm of
wind, the wire screen giving way much in the same way as sails during a
hurricane at sea.
_The “Challenger’s” Crucial Test of the Wind and Gravitation Theories
of Oceanic Circulation._—It has been shown in former chapters that all
the facts which have been adduced in support of the gravitation theory
are equally well explained by the wind theory. We may now consider a
class of facts which do not appear to harmonize with either theory. The
recent investigations of the _Challenger_ Expedition into the thermal
state of the ocean reveal a condition of things which appears to me
utterly irreconcilable with the gravitation theory.
It is a condition absolutely essential to the gravitation theory that
the surface of the ocean should be highest in equatorial regions and
slope downwards to either pole. Were water absolutely frictionless, an
incline, however small, would be sufficient to produce a surface-flow
from the equator to the poles, but to induce such an effect some slope
there must be, or gravitation could exercise no power in drawing the
surface-water polewards.
The researches of the _Challenger_ Expedition bring to light the
striking and important fact that the general surface of the North
Atlantic in order to produce equilibrium must stand at a higher level
than at the equator. In other words the surface of the Atlantic is
lowest at the equator, and rises with a gentle slope to well-nigh the
latitude of England. If this be the case, then it is mechanically
impossible that, as far as the North Atlantic is concerned, there can
be any such general movement as Dr. Carpenter believes. Gravitation can
no more cause the surface-water of the Atlantic to flow towards the
arctic regions than it can compel the waters of the Gulf of Mexico up
the Mississippi into the Missouri. The impossibility is equally great
in both cases.
In order to prove what has been stated, let us take a section of the
mid-Atlantic, north and south, across the equator; and, to give the
gravitation theory every advantage, let us select that particular
section adopted by Dr. Carpenter as the one of all others most
favourable to his theory, viz., Section marked No. VIII. in his memoir
lately read before the Royal Geographical Society.[98]
The fact that the polar cold water comes so near the surface at the
equator is regarded by Dr. Carpenter as evidence in favour of the
gravitation theory. On first looking at Dr. Carpenter’s section it
forcibly struck me that if it was accurately drawn, the ocean to be
in equilibrium would require to stand at a higher level in the North
Atlantic than at the equator. In order, therefore, to determine
whether this is the case or not I asked the hydrographer of the
Admiralty to favour me with the temperature soundings indicated in the
section, a favour which was most obligingly granted. The following
are the temperature soundings at the three stations A, B, and C. The
temperature of C are the mean of six soundings taken along near the
equator:—
+--------+----------------+----------------+----------------------+
| | A | B | C |
| | | | |
| Depth |Lat. 37° 54′ N. |Lat. 23° 10′ N. | Mean of six |
| in |Long. 41° 44′ W.|Long. 38° 42′ W.|temperature soundings |
|Fathoms.| | | near equator. |
| | | +---------+------------+
| | Temperature. | Temperature. | Depth in|Temperature.|
| | | | Fathoms.| |
+--------+----------------+----------------+---------+------------+
| | ° | ° | | ° |
|Surface.| 70·0 | 72·0 |Surface. | 77·9 |
| 100 | 63·5 | 67·0 | 10 | 77·2 |
| 200 | 60·6 | 57·6 | 20 | 77·1 |
| 300 | 60·0 | 52·5 | 30 | 76·9 |
| 400 | 54·8 | 47·7 | 40 | 71·7 |
| 500 | 46·7 | 43·7 | 50 | 64·0 |
| 600 | 41·6 | 41·7 | 60 | 60·4 |
| 700 | 40·6 | 40·6 | 70 | 59·4 |
| 800 | 38·1 | 39·4 | 80 | 58·0 |
| 900 | 37·8 | 39·2 | 90 | 58·0 |
| 1000 | 37·9 | 38·3 | 100 | 55·6 |
| 1100 | 37·1 | 38·0 | 150 | 51·0 |
| 1200 | 37·1 | 37·6 | 200 | 46·6 |
| 1300 | 37·2 | 36·7 | 300 | 42·2 |
| 1400 | 37·1 | 36·9 | 400 | 40·3 |
| 1500 | .. | 36·7 | 500 | 38·9 |
| 2700 | 35·2 | .. | 600 | 39·2 |
| 2720 | .. | 35·4 | 700 | 39·0 |
| | | | 800 | 39·1 |
| | | | 900 | 38·2 |
| | | | 1000 | 36·9 |
| | | | 1100 | 37·6 |
| | | | 1200 | 36·7 |
| | | | 1300 | 35·8 |
| | | | 1400 | 36·4 |
| | | | 1500 | 36·1 |
| | | | Bottom. | 34·7 |
+--------+----------------+----------------+---------+------------+
On computing the extent to which the three columns A, B, and C are each
expanded by heat according to Muncke’s table of the expansion of sea
water for every degree Fahrenheit, I found that column B, in order to
be in equilibrium with C (the equatorial column), would require to have
its surface standing fully 2 feet 6 inches above the level of column C,
and column A fully 3 feet 6 inches above that column. In short, it is
evident that there must be a gradual rise from the equator to latitude
38° N. of 3½ feet. Any one can verify the accuracy of these results by
making the necessary computations for himself.[99]
[Illustration: PLATE III.
W. & A. K. Johnston, Edinb^r. and London.
SECTION OF THE ATLANTIC nearly North and South, between LAT. 38° N.
& LAT. 38° S.]
I may observe that, had column C extended to the same depth as columns
A and B, the difference of level would be considerably greater, for
column C requires to balance only that portion of columns A and B
which lies above the level of its base. Suppose a depth of ocean equal
to that of column C to extend to the north pole, and the polar water
to have a uniform temperature of 32° from the surface to the bottom,
then, in order to produce equilibrium, the surface of the ocean at
the equator would require to be 4 feet 6 inches above that at the
pole. But the surface of the ocean at B would be 7 feet, and at A 8
feet, above the poles. Gravitation never could have caused the ocean
to assume this form. It is impossible that this immense mass of warm
water, extending to such a depth in the North Atlantic, could have been
brought from equatorial regions by means of gravitation. And, even
if we suppose this accumulation of warm water can be accounted for
by some other means, still its presence precludes the possibility of
any such surface-flow as that advocated by Dr. Carpenter. For so long
as the North Atlantic stands 3½ feet above the level of the equator,
gravitation can never move the equatorial waters polewards.
There is another feature of this section irreconcilable with the
gravitation theory. It will be observed that the accumulation of warm
water is all in the North Atlantic, and that there is little or none
in the south. But according to the gravitation theory it ought to
have been the reverse. For owing to the unrestricted communication
between the equatorial and antarctic regions, the general flow of
water towards the south pole is, according to that theory, supposed to
be greater than towards the north, and consequently the quantity of
warm equatorial water in the South Atlantic ought also to be greater.
Dr. Carpenter himself seems to be aware of this difficulty besetting
the theory, and meets it by stating that “the upper stratum of the
North Atlantic is not nearly as much cooled down by its limited polar
underflow, as that of the South Atlantic is by the vast movement of
antarctic water which is constantly taking place towards the equator.”
But this “vast movement of antarctic water” necessarily implies a vast
counter-movement of warm surface-water. So that if there is more polar
water in the South Atlantic to produce the cooling effect, there should
likewise be more warm water to be cooled.
According to the wind theory of oceanic circulation the explanation of
the whole phenomena is simple and obvious. It has already been shown
that owing to the fact that the S. E. trades are stronger than the N.
E., and blow constantly over upon the northern hemisphere, the warm
surface-water of the South Atlantic is drifted across the equator. It
is then carried by the equatorial current into the Gulf of Mexico, and
afterwards of course forms a part of the Gulf-stream.
The North Atlantic, on the other hand, not only does not lose its
surface heat like the equatorial and South Atlantic, but it receives
from the Gulf-stream in the form of warm water an amount of heat, as we
have seen, equal to one-fourth of all the heat which it receives from
the sun. The reason why the warm surface strata are so much thicker
on the North Atlantic than on the equatorial regions is perfectly
obvious. The surface-water at the equator is swept into the Gulf of
Mexico by the trade-winds and the equatorial current, as rapidly as it
is heated by the sun, so that it has not time to gather to any great
depth. But all this warm water is carried by the Gulf-stream into the
North Atlantic, where it accumulates. That this great depth of warm
water in the North Atlantic, represented in the section, is derived
from the Gulf-stream, and not from a direct flow from the equator due
to gravitation, is further evident from the fact that temperature
sounding A in latitude 38° N. is made through that immense body of warm
water, upwards of 300 fathoms thick, extending from Bermuda to near the
Azores, discovered by the _Challenger_ Expedition, and justly regarded
by Captain Nares as an offshoot of the Gulf-stream. This, in Captain
Nares’s Report, is No. 8 “temperature sounding,” between Bermuda and
the Azores; sounding B is No. 6 “temperature curve,” between Teneriffe
and St. Thomas.
There is an additional reason to the one already stated why the
surface temperature of the South Atlantic should be so much below
that of the North. It is perfectly true that whatever amount of water
is transferred from the southern hemisphere to the northern must be
compensated by an equal amount from the northern to the southern
hemisphere, nevertheless the warm water which is carried off the South
Atlantic by the winds is not directly compensated by water from the
north, but by that cold antarctic current whose existence is so well
known to mariners from the immense masses of ice which it brings from
the Southern Ocean.
_Thermal Condition of Southern Ocean._——The thermal condition of the
Southern Ocean, as ascertained by the _Challenger_ Expedition, appears
to me to be also irreconcilable with the gravitation theory. Between
the parallels of latitude 65° 42′ S. and 50° 1′ S., the ocean, with
the exception of a thin stratum at the surface heated by the sun’s
rays, was found, down to the depth of about 200 fathoms, to be several
degrees colder than the water underneath.[100] The cold upper stratum
is evidently an antarctic current, and the warm underlying water an
equatorial under current. But, according to the gravitation theory, the
colder water should be underneath.
The very fact of a mass of water, 200 fathoms deep and extending over
fifteen degrees of latitude, remaining above water of three or four
degrees higher temperature shows how little influence difference of
temperature has in producing motion. If it had the potency which some
attribute to it, one would suppose that this cold stratum should sink
down and displace the warm water underneath. If difference of density
is sufficient to move the water horizontally, surely it must be more
than sufficient to cause it to sink vertically.
CHAPTER XIV.
THE WIND THEORY OF OCEANIC CIRCULATION IN RELATION TO CHANGE
OF CLIMATE.
Direction of Currents depends on Direction of the Winds.—Causes
which affect the Direction of Currents will affect
Climate.—How Change of Eccentricity affects the Mode
of Distribution of the Winds.—Mutual Reaction of Cause
and Effect.—Displacement of the Great Equatorial
Current.—Displacement of the Median Line between the Trades,
and its Effect on Currents.—Ocean-currents in Relation to the
Distribution of Plants and Animals.—Alternate Cold and Warm
Periods in North and South.—Mr. Darwin’s Views quoted.—How
Glaciers at the Equator may be accounted for.—Migration
across the Equator.
_Ocean-currents in Relation to Change of Climate._—In my attempts to
prove that oceanic circulation is produced by the winds and not by
difference of specific gravity, and that ocean-currents are the great
distributors of heat over the globe, my chief aim has been to show
the bearing which these points have on the grand question of secular
changes of climate during geological epochs, more particularly in
reference to that mystery the cause of the glacial epoch.
In concluding this discussion regarding oceanic circulation, I may
therefore be allowed briefly to recapitulate those points connected
with the subject which seem to shed most light on the question of
changes of climate.
The complete agreement between the systems of ocean-currents and
winds not only shows that the winds are the impelling cause of the
currents, but it also indicates to what an extent the _directions_ of
the currents are determined by the winds, or, more properly, to what an
extent their directions are determined by the _direction_ of the winds.
We have seen in Chapter II. to what an enormous extent the climatic
conditions of the globe are dependent on the distribution of heat
effected by means of ocean-currents. It has been there pointed out
that, if the heat conveyed from inter-tropical to temperate and polar
regions by oceanic circulation were restored to the former, the
equatorial regions would then have a temperature about 55° warmer,
and the high polar regions a climate 83° colder than at present. It
follows, therefore, that any cause which will greatly affect the
currents or greatly change their paths and mode of distribution, will
of necessity seriously affect the climatic condition of the globe. But
as the existence of these currents depends on the winds, and their
direction and form of distribution depend upon the direction and form
of distribution of the winds, any cause which will greatly affect the
winds will also greatly affect the currents, and consequently will
influence the climatic condition of the globe. Again, as the existence
of the winds depends mainly on the difference of temperature between
equatorial and polar regions, any cause which will greatly affect this
difference of temperature will likewise greatly affect the winds; and
these will just as surely react on the currents and climatic conditions
of the globe. A simple increase or decrease in the difference of
temperature between equatorial and polar regions, though it would
certainly produce an increase or a decrease, as the case might be, in
the strength of the winds, and consequently in the strength of the
currents, would not, however, greatly affect the mode of _distribution_
of the winds, nor, as a consequence, the mode of _distribution_ of
the currents. But although a simple change in the difference of
temperature between the equator and the poles would not produce a
different _distribution_ of aërial, and consequently of ocean-currents,
nevertheless a _difference in the difference_ of temperature between
the equator and the two poles would do so; that is to say, any cause
that should increase the difference of temperature between the equator
and the pole on the one hemisphere, and decrease that difference on
the other, would effect a change in the distribution of the aërial
currents, which change would in turn produce a corresponding change in
the distribution of ocean-currents.
It has been shown[101] that an increase in the eccentricity of the
earth’s orbit tends to lower the temperature of the one hemisphere and
to raise the temperature of the other. It is true that an increase of
eccentricity does not afford more heat to the one hemisphere than to
the other; nevertheless it brings about a condition of things which
tends to lower the temperature of the one hemisphere and to raise the
temperature of the other. Let us imagine the eccentricity to be at its
superior limit, 0·07775, and the winter solstice in the aphelion. The
midwinter temperature, owing to the increased distance of the sun,
would be lowered enormously; and the effect of this would be to cause
all the moisture which now falls as rain during winter in temperate
regions to fall as snow. Nor is this all; the winters would not merely
be colder than now, but they would also be much longer. At present the
summer half-year exceeds the winter half year by nearly eight days; but
at the period in question the winters would be longer than the summers
by upwards of thirty-six days. The heat of the sun during the short
summer, for reasons which have already been explained, would not be
sufficient to melt the snow of winter; so that gradually, year by year,
the snow would continue to accumulate on the ground.
On the southern hemisphere the opposite condition of things would
obtain. Owing to the nearness of the sun during the winter of that
hemisphere, the moisture of the air would be precipitated as rain in
regions where at present it falls as snow. This and the shortness of
the winter would tend to produce a decrease in the quantity of snow.
The difference of temperature between the equatorial and the temperate
and polar regions would therefore be greater on the northern than on
the southern hemisphere; and, as a consequence, the aërial currents
of the former hemisphere would be stronger than those of the latter.
This would be more especially the case with the trade-winds. The
N.E. trades being stronger than the S.E. trades would blow across the
equator, and the median line between them would therefore be at some
distance to the south of the equator. Thus the equatorial waters would
be impelled more to the southern than to the northern hemisphere; and
the warm water carried over in this manner to the southern hemisphere
would tend to increase the difference of temperature between the two
hemispheres. This change, again, would in turn tend to strengthen the
N.E. and to weaken the S.E. trades, and would thus induce a still
greater flow of equatorial waters into the southern hemisphere—a
result which would still more increase the difference of temperature
between the northern and southern hemisphere, and so on—the one cause
so reacting on the other as to increase its effects, as was shown at
length in Chapter IV.
It was this mutual reaction of those physical agents which led, as was
pointed out in Chapter IV., to that extraordinary condition of climate
which prevailed during the glacial epoch.
There is another circumstance to be considered which perhaps more
than any thing else would tend to lower the temperature of the one
hemisphere and to raise the temperature of the other; and this is
the _displacement of the great equatorial current_. During a glacial
period in the northern hemisphere the median line between the trades
would be shifted very considerably south of the equator; and the same
would necessarily be the case with the great equatorial currents, the
only difference being that the equatorial currents, other things being
equal, would be deflected farther south than the median line. For the
water impelled by the strong N.E. trades would be moving with greater
velocity than the waters impelled by the weaker S.E. trades, and, of
course, would cross the median line of the trades before its progress
southwards could be arrested by the counteracting influence of the S.E.
trades. Let us glance briefly at the results which would follow from
such a condition of things. In the first place, as was shown on former
occasions,[102] were the equatorial current of the Atlantic (the feeder
of the Gulf-stream) shifted considerably south of its present position,
it would not bifurcate, as it now does, off Cape St. Roque, owing to
the fact that the whole of the waters would strike obliquely against
the Brazilian coast and thus be deflected into the Southern Ocean. The
effect produced on the climate of the North Atlantic and North-Western
Europe by the withdrawal of the water forming the Gulf-stream, may be
conceived from what has already been stated concerning the amount of
heat conveyed by that stream. The heat thus withdrawn from the North
Atlantic would go to raise the temperature of the Southern Ocean and
antarctic regions. A similar result would take place in the Pacific
Ocean. Were the equatorial current of that ocean removed greatly to
the south of its present position, it would not then impinge and be
deflected upon the Asiatic coast, but upon the continent of Australia;
and the greater portion of its waters would then pass southward into
the Southern Ocean, while that portion passing round the north of
Australia (owing to the great strength of the N.E. trades) would rather
flow into the Indian Ocean than turn round, as now, along the east
coast of Asia by the Japan Islands. The stoppage of the Japan current,
combined with the displacement of the equatorial current to the south
of the equator, would greatly lower the temperature of the whole of the
North Pacific and adjoining continents, and raise to a corresponding
degree the temperature of the South Pacific and Southern Ocean. Again,
the waters of the equatorial current of the Indian Ocean (owing to the
opposing N.E. trades), would not, as at present, find their way round
the Cape of Good Hope into the North Atlantic, but would be deflected
southwards into the Antarctic Sea.
We have in the present state of things a striking example of the extent
to which the median line between the two trades may be shifted, and the
position of the great equatorial currents of the ocean may be affected,
by a slight difference in the relative strength of the two aërial
currents. The S.E. trades are at present a little stronger than the
N.E.; and the consequence is that they blow across the equator into the
northern hemisphere to a distance sometimes of 10 or 15°, so that the
mean position of the median line lies at least 6 or 7 degrees north of
the equator.
And it is doubtless owing to the superior strength of the S.E. trades
that so much warm water crosses the equator from the South to the North
Atlantic, and that the main portion of the equatorial current flows
into the Caribbean Sea rather than along the Brazilian coast. Were the
two trades of equal strength, the transference of heat into the North
Atlantic from the southern hemisphere by means of the Southern Atlantic
and equatorial currents would be much less than at present. The same
would also hold true in regard to the Pacific.
_Ocean-currents in Relation to the Distribution of Plants and
Animals._—In the fifth and last editions of the “Origin of Species,”
Mr. Darwin has done me the honour to express his belief that the
foregoing view regarding alternate cold and warm periods in north
and south during the glacial epoch explains a great many facts in
connection with the distribution of plants and animals which have
always been regarded as exceedingly puzzling.
There are certain species of plants which occur alike in the temperate
regions of the southern and northern hemispheres. At the equator these
same temperate forms are found on elevated mountains, but not on the
lowlands. How, then, did these temperate forms manage to cross the
equator from the northern temperate regions to the southern, and _vice
versâ_? Mr. Darwin’s solution of the problem is (in his own words) as
follows:—
“As the cold became more and more intense, we know that arctic forms
invaded the temperate regions; and from the facts just given, there
can hardly be a doubt that some of the more vigorous, dominant, and
widest-spreading temperate forms invaded the equatorial lowlands.
The inhabitants of these hot lowlands would at the same time have
migrated to the tropical and subtropical regions of the south; for the
southern hemisphere was at this period warmer. On the decline of the
glacial period, as both hemispheres gradually recovered their former
temperatures, the northern temperate forms living on the lowlands under
the equator would have been driven to their former homes or have been
destroyed, being replaced by the equatorial forms returning from the
south. Some, however, of the northern temperate forms would almost
certainly have ascended any adjoining high land, where, if sufficiently
lofty, they would have long survived like the arctic forms on the
mountains of Europe.”
“In the regular course of events the southern hemisphere would in
its turn be subjected to a severe glacial period, with the northern
hemisphere rendered warmer; and then the southern temperate forms
would invade the equatorial lowlands. The northern forms which had
before been left on the mountains would now descend and mingle with the
southern forms. These latter, when the warmth returned, would return
to their former homes, leaving some few species on the mountains, and
carrying southward with them some of the northern temperate forms which
had descended from their mountain fastnesses. Thus we should have some
few species identically the same in the northern and southern temperate
zones and on the mountains of the intermediate tropical regions” (p.
339, sixth edition).
Additional light is cast on this subject by the results already stated
in regard to the enormous extent to which the temperature of the
equator is affected by ocean-currents. Were there no transferrence of
heat from equatorial to temperate and polar regions, the temperature
of the equator, as has been remarked, would probably be about 55°
warmer than at present. In such a case no plant existing on the face of
the globe could live at the equator unless on some elevated mountain
region. On the other hand, were the quantity of warm water which is
being transferred from the equator to be very much increased, the
temperature of inter-tropical latitudes might be so lowered as easily
to admit of temperate species of plants growing at the equator. A
lowering of the temperature at the equator some 20° or 30° is all that
would be required; and only a moderate increase in the volume of the
currents proceeding from the equator, taken in connection with the
effects flowing from the following considerations, might suffice to
produce that result. During the glacial epoch, when the one hemisphere
was under ice and the other enjoying a warm and equable climate, the
median line between the trades may have been shifted to almost the
tropical line of the warm hemisphere. Under such a condition of things
the warmest part would probably be somewhere about the tropic of the
warm hemisphere, and not, as now, at the equator; for since all, or
nearly all, the surface-water of the equator would then be impelled
over to the warm hemisphere, the tropical regions of that hemisphere
would be receiving nearly double their present amount of warm water.
Again, as the equatorial current at this time would be shifted towards
the tropic of the warm hemisphere, the surface-water would not, as at
present, be flowing in equatorial regions parallel to the equator,
but obliquely across it from the cold to the warm hemisphere. This of
itself would tend greatly to lower the temperature of the equator.
It follows, therefore, as a necessary consequence, that during the
glacial epoch, when the one hemisphere was under snow and ice and
the other enjoying a warm and equable climate, the temperature of
the equator would be lower than at present. But when the glaciated
hemisphere (which we may assume to be the northern) began to grow
warmer and the climate of the southern or warm hemisphere to get
colder, the median line of the trades and the equatorial currents
of the ocean also would begin to move back from the southern tropic
towards the equator. This would cause the temperature of the equator
to rise and to continue rising until the equatorial currents reached
their normal position. When the snow began to accumulate on the
southern hemisphere and to disappear on the northern, the median line
of the trades and the equatorial currents of the ocean would then
begin to move towards the northern tropic as they had formerly towards
the southern. The temperature of the equator would then again begin
to sink, and continue to do so until the glaciation of the southern
hemisphere reached its maximum. This oscillation of the thermal equator
to and fro across the geographical equator would continue so long as
the alternate glaciation of the two hemispheres continued.
This lowering of the temperature of the equator during the severest
part of the glacial epoch will help to explain the former existence of
glaciers in inter-tropical regions at no very great elevation above the
sea-level, evidence of which appears recently to have been found by
Professor Agassiz, Mr. Belt, and others.
The glacial _epoch_ may be considered as contemporaneous in both
hemispheres. But the epoch consisted of a succession of cold and warm
_periods_, the cold periods of one hemisphere coinciding with the warm
periods of the other, and _vice versâ_.
_Migration across the Equator._—Mr. Belt[103] and others have felt
some difficulty in understanding how, according to theory, the plants
and animals of temperate regions could manage to migrate from one
hemisphere to the other, seeing that in their passage they would have
to cross the thermal equator. The oscillation to and fro of the thermal
equator across the geographical, removes every difficulty in regard to
how the migration takes place. When, for example, a cold period on the
northern hemisphere and the corresponding warm one on the southern were
at their maximum, the thermal equator would by this time have probably
passed beyond the Tropic of Capricorn. The geographical equator would
then be enjoying a subtropical, if not a temperate condition of
climate, and the plants and animals of the northern hemisphere would
manage then to reach the equator. When the cold began to abate on
the northern and to increase on the southern hemisphere, the thermal
equator would commence its retreat towards the geographical. The plants
and animals from the north, in order to escape the increasing heat as
the thermal equator approached them, would begin to ascend the mountain
heights; and when that equator had passed to its northern limit, and
the geographical equator was again enjoying a subtropical condition of
climate, the plants and animals would begin to descend and pursue their
journey southwards as the cold abated on the southern hemisphere.
CHAPTER XV.
WARM INTER-GLACIAL PERIODS.
Alternate Cold and Warm Periods.—Warm Inter-glacial Periods
a Test of Theories.—Reason why their Occurrence has not
been hitherto recognised.—Instances of Warm Inter-glacial
Periods.—Dranse, Dürnten, Hoxne, Chapelhall, Craiglockhart,
Leith Walk, Redhall Quarry, Beith, Crofthead, Kilmaurs,
Sweden, Ohio, Cromer, Mundesley, &c., &c.—Cave and River
Deposits.—Occurrence of Arctic and Warm Animals in some Beds
accounted for.—Mr. Boyd Dawkins’s Objections.—Occurrence
of Southern Shells in Glacial Deposits.—Evidence of Warm
Inter-glacial Periods from Mineral Borings.—Striated
Pavements.—Reason why Inter-glacial Land-surfaces are so rare.
_Alternate Cold and Warm Periods._—If the theory developed in the
foregoing chapters in reference to the cause of secular changes of
climate be correct, it follows that that long age known as the glacial
epoch did not, as has hitherto been generally supposed, consist of one
long unbroken period of cold and ice. Neither did it consist, as some
have concluded, of two long periods of ice with an intervening mild
period, but it must have consisted of a long succession of cold and
warm periods; the warm periods of the one hemisphere corresponding in
time with the cold periods of the other and _vice versâ_. It follows
also from theory that as the cold periods became more and more severe,
the warm intervening periods would become more and more warm and
equable. As the ice began to accumulate during the cold periods in
subarctic and temperate regions in places where it previously did not
exist, so in like manner during the corresponding warm periods it would
begin to disappear in arctic regions where it had held enduring sway
throughout the now closing cycle. As the cold periods in the southern
hemisphere became more and more severe, the ice would continue to
advance northwards in the temperate regions; but at that very same
time the intervening warm periods in the northern hemisphere would
become warmer and warmer and more equable, and the ice of the arctic
regions would continue to disappear farther and farther to the north,
till by the time that the ice had reached a maximum during the cold
antarctic periods, Greenland and the arctic regions would, during the
warm intervening periods, be probably free of ice and enjoying a mild
and equable climate. Or we may say that as the one hemisphere became
cold the other became warm, and when the cold reached a maximum in the
one hemisphere, the warmth would reach a maximum in the other. The time
when the ice had reached its greatest extension on the one hemisphere
would be the time when it had disappeared from the other.
_Inter-glacial Periods a Test of Theories._—Here we have the grand
crucial test of the truth of the foregoing theory of the cause of
the glacial epoch. That the glacial epoch should have consisted of a
succession of cold and warm periods is utterly inconsistent with all
previous theories which have been advanced to account for it. What,
then, is the evidence of geology on this subject? If the glacial epoch
can be proved from geological evidence to have consisted of such a
succession of cold and warm periods, then I have little doubt but the
theory will soon be generally accepted. But at the very outset an
objection meets us, viz., why call an epoch, which consisted as much of
warm periods as of cold, a glacial epoch, or an “Ice Age,” as Mr. James
Geikie tersely expresses it? Why not as well call it a warm epoch as a
cold one, seeing that, according to theory, it was just as much a warm
as a cold epoch? The answer to this objection will be fully discussed
in the chapter on the Reason of the Imperfection of Geological Records.
But in the meantime, I may remark that it will be shown that the epoch
known as the glacial has been justly called the glacial epoch or “Ice
Age,” because the geological evidences of the cold periods remain in
a remarkably perfect state, whilst the evidences of the warm periods
have to a great extent disappeared. The reason of this difference
in the two cases will be discussed in the chapter to which I have
referred. Besides, the condition of things during the cold periods was
so extraordinary, so exceptional, so totally different from those now
prevailing, that even supposing the geological records of the warm
periods had been as well preserved as those of the cold, nevertheless
we should have termed the epoch in question a glacial epoch. There
is yet another reason, however, for our limited knowledge of warm
inter-glacial periods. Till very lately, little or no attention was
paid by geologists to this part of the subject in the way of keeping
records of cases of inter-glacial deposits which, from time to time,
have been observed. Few geologists ever dreamt of such a thing as
warm periods during the age of ice, so that when intercalated beds of
sand and gravel, beds of peat, roots, branches, trunks, leaves, and
fruits of trees were found in the boulder clay, no physical importance
was attached to them, and consequently no description or record of
them ever kept. In fact, all such examples were regarded as purely
accidental and exceptional, and were considered not worthy of any
special attention. A case which came under my own observation will
illustrate my meaning. An intelligent geologist, some years ago, read a
paper before one of our local geological societies, giving an account
of a fossiliferous bed of clay found intercalated between two distinct
beds of till. In this intercalated bed were found rootlets and stems of
trees, nuts, and other remains, showing that it had evidently been an
old inter-glacial land surface. In the transactions of the society a
description of the two beds of till was given, but no mention whatever
was made of the intercalated bed containing the organic remains,
although this was the only point of any real importance.
Since the theory that the glacial epoch resulted from a high state
of eccentricity of the earth’s orbit began to receive some little
acceptance, geologists have paid a good deal of attention to cases of
intercalated beds in the till containing organic remains, and the
result is that we have already a great body of evidence of a geological
nature in favour of warm inter-glacial periods, and I have little
doubt that in the course of a few years the former occurrence of warm
inter-glacial periods will be universally admitted.
I shall now proceed to give a very brief outline of the evidence
bearing on the subject. But the cases to which I shall have to refer
are much too numerous to allow me to enter into details.
_Inter-glacial Beds of Switzerland._—The first geologist, so far as I
am aware, who directed attention to evidence of a break in the cold of
the glacial epoch was M. Morlot. It is now twenty years ago since he
announced the existence of a warm period during the glacial epoch from
geological evidence connected with the glacial drift of the Alps.[104]
The rivers of Switzerland, he found, show on their banks three
well-marked terraces of regularly stratified and well-rounded shingle,
identical with the modern deposits of the rivers. They stand at 50,
100, and 150 feet above the present level of the rivers. These terraces
were evidently formed by the present system of rivers when these flowed
at a higher level, and extend up the Alps to a height of from 3,000 to
4,000 feet above the level of the sea. There is a terrace bordering the
Rhine at Camischollas, above Disentis, 4,400 feet above the level of
the sea, proving that during the period of its formation the Alps were
free of ice up to the height of 4,400 feet above the sea-level. It is
well known that a glacial period must have succeeded the formation of
these drifts, for they are in many places covered with erratics. At
Geneva, for example, an erratic drift nearly 50 feet thick is seen to
rest on the drift of the middle terrace, which rises 100 feet above the
level of the lake. But it is also evident that a glacial period must
have preceded the formation of the drift beds, for they are found to
lie in many places upon the unstratified boulder clay or _till_. M.
Morlot observed in the neighbourhood of Clareus, from 7 to 9 feet of
drift resting upon a bed of true till 40 feet thick; the latter was
composed of a compact blue clay, containing worn and scratched alpine
boulders and without any trace of stratification. In the gorge of
Dranse, near Thoron, M. Morlot found the whole three formations in a
direct superimposed series. At the bottom was a mass of compact till or
boulder clay, 12 feet thick, containing boulders of alpine limestone.
Over this mass came regularly stratified beds 150 feet thick, made
up of rounded pebbles in horizontal beds. Above this again lay a
second formation of unstratified boulder clay, with erratic blocks and
striated pebbles, which constituted the left lateral moraine of the
great glacier of the Rhone, when it advanced for the second time to the
Lake of Geneva. A condition of things somewhat similar was observed by
M. Ischer in the neighbourhood of Berne.
These facts, M. Morlot justly considers, prove the existence of two
glacial periods separated by an intermediate one, during which the
ice, which had not only covered Switzerland, but the greater part of
Europe, disappeared even in the principal valleys of the Alps to a
height of more than 4,400 feet above the present level of the sea. This
warm period, after continuing for long ages, was succeeded by a second
glacial period, during which the country was again covered with ice as
before. M. Morlot even suggests the possibility of these alternations
of cold and warm periods depending upon a cosmical cause. “Wild as it
may have appeared,” he says, “when first started, the idea of general
and periodical eras of refrigeration for our planet, connected perhaps
with some cosmic agency, may eventually prove correct.”[105]
Shortly afterwards, evidence of a far more remarkable character was
found in the glacial drift of Switzerland, namely, the famous lignite
beds of Dürnten. In the vicinity of Utznach and Dürnten, on the Lake of
Zurich, and near Mörschwyl, on the Lake of Constance, there are beds of
coal or lignite, nearly 12 feet thick, lying directly on the boulder
clay. Overlying these beds is another mass of drift and clay 30 feet
in thickness, with rounded blocks, and on the top of this upper drift
lie long angular erratics, which evidently have been transported on
the back of glaciers.[106] Professor Vogt attributes their transport
to floating ice; but he evidently does so to avoid the hypothesis of a
warm period during the glacial epoch.
Here we have proof not merely of the disappearance of the ice during
the glacial epoch, but of its absence during a period of sufficient
length to allow of the growth of 10 or 12 feet of coal. Professor Heer
thinks that this coal-bed, when in the condition of peat, must have
been 60 feet thick; and assuming that one foot of peat would be formed
in a century, he concludes that 6,000 years must have been required
for the growth of the coal plants. According to Liebig, 9,600 years
would be required. This, as we have already seen, is about the average
duration of a warm period.
In these beds have been found the bones of the elephant (_E. Merkii_),
stag, cave-bear, and other animals. Numerous insects have also been met
with, which further prove the warm, mild condition of climate which
must have prevailed at the time of the formation of the lignite.
At Hoxne, near Diss, in Suffolk, a black peaty mass several feet thick,
containing fragments of wood of the oak, yew, and fir, was found,
overlying the boulder clay.[107] Professor Vogt believes that this peat
bed is of the same age as the lignite beds of Switzerland.
In the glacial drift of North America, particularly about Lake
Champlain and the valley of the St. Lawrence, there is similar evidence
of two glacial periods with an intervening non-glacial or warm
period.[108]
_Glacial and Inter-glacial Periods of the Southern Hemisphere_—(_South
Africa_).—Mr. G. W. Stow, in a paper on the “Geology of South
Africa,”[109] describes a recent glaciation extending over a large
portion of Natal, British Kaffraria, the Kaga and Krome mountains,
which he attributes to the action of land-ice. He sums up the phenomena
as follows:—“The rounding off of the hills in the interiors of the
ancient basins; the numerous dome-shaped (_roches moutonnée_) rocks;
the enormous erratic boulders in positions where water could not have
carried them; the frequency of unstratified clays—clays with imbedded
angular boulders; drift and lofty mounds of boulders; large tracts of
country thickly spread over with unstratified clays and superimposed
fragments of rock; the Oliphant’s-Hoek clay, and the vast piles of Enon
conglomerate.” In addition to these results of ice-action, he records
the discovery by himself of distinct ice-scratches or groovings on the
surface of the rocks at Reit-Poort in the Tarka, and subsequently[110]
the discovery by Mr. G. Gilfillan of a large boulder at Pniel with
_striæ_ distinctly marked upon it, and also that the same observer
found that almost every boulder in the gravel at “Moonlight Rush” had
unmistakable striæ on one or more sides.
In South Africa there is evidence not only of a glacial condition
during the Pliocene period, but also of a warmer climate than now
prevails in that region. “The evidence,” says Mr. Stow, “of the
Pliocene shells of the superficial limestone of the Zwartkops heights,
and elsewhere, leads us to believe that the climate of South Africa
must have been of a far more tropical character than at present.
“Take, for instance, the characteristic _Venericardia_ of that
limestone. This has migrated along the coast some 29° or 30° and is now
found within a few degrees of the equator, near Zanzibar, gradually
driven, as I presume it must have been, further and further north by a
gradual lowering of the temperature of the more southern parts of this
coast since the limestone was deposited.”
“During the formation of the shell-banks in the Zwartkops estuary,
younger than the Pliocene limestone, the immense number of certain
species of shells, which have as yet been found living only in
latitudes nearer the equator, point to a somewhat similar though a more
modified change of temperature.”
_Inter-glacial Beds of Scotland._—Upwards of a dozen years ago,
Professor Geikie arrived, from his own observations of the glacial
drift of Scotland, at a similar conclusion to that of M. Morlot
regarding the intercalation of warm periods during the glacial epoch;
and the facts on which Professor Geikie’s conclusions were based are
briefly as follows. In a cliff of boulder clay on the banks of the
Slitrig Water, near the town of Hawick, he observed a bed of stones
or shingle. Over the lower stratum of stones lay a few inches of
well-stratified sand, silt, and clay, some of the layers being black
and peaty, _with enclosed vegetable fibres_ in a crumbling state.[111]
There were some 30 or 40 feet of boulder clay above these stratified
beds, and 15 or 20 feet under them. The stones in the shingle band
were identical with those of the boulder clay, but they showed no
striations, and were more rounded and water-worn, and resembled in
every respect the stones now lying in the bed of the Slitrig. The
section of the cliff stood as under:—
1. Vegetable soil.
2. Boulder clay, thirty to forty feet.
{ 3. Yellowish gravelly sand.
Stratified beds { 4. Peaty silt and clay.
{ 5. Fine ferruginous sand.
{ 6. Coarse shingle, two to three feet.
7. Coarse, stiff boulder clay, fifteen to twenty feet.
A few more cases of intercalation of stratified materials in the true
till were also found in the same valley.
In a cliff of stiff brown boulder clay, about 20 feet high, on the
banks of the Carmichael Water, Lanarkshire, Professor Geikie observed
a stratified bed of clay about 3 or 4 inches in thickness. About a mile
higher up the stream, he found a series of beds of gravel, sand, and
clay in the true _till_. “A thin seam of _peaty matter_,” he says, “was
observed to run for a few inches along the bottom of a bed of clay and
then disappear, while in a band of fine laminated clay with thin sandy
partings occasional _fragments of mouldering wood_ were found.”[112]
At Chapelhall, near Airdrie, a sand-bed has been extensively mined
under about 114 feet of till. This bed of finely stratified sand
is about 20 feet thick. In it were found lenticular beds of fine
pale-coloured clay containing layers of peat and decaying twigs and
branches. Professor Geikie found the vegetable fibres, though much
decayed, still distinct, and the substance when put into the fire
burned with a dull lambent flame. Underlying these stratified beds, and
forming the floor of the mine, is a deposit of _the true till_ about
24 feet in thickness. In another pit adjoining, the till forming the
floor is 30 feet thick, but it is sometimes absent altogether, so as to
leave the sand beds resting directly on the sandstone and shale of the
coal-measures. At some distance from this sand-pit an old buried river
channel was met with in one of the pit workings. This channel was found
to contain a coating of boulder clay, on which the laminated sands and
clays reposed, showing, as Professor Geikie has pointed out, that this
old channel had been filled with boulder clay, and then re-excavated
to allow of the deposition of the stratified deposits. Over all lay a
thick mantle of boulder clay which buried the whole.
A case somewhat similar was found by Professor Nicol in a cutting on
the Edinburgh and Leith Railway. In many places the till had been
worn into hollows as if part of it had been removed by the action of
running water.[113] One of these hollows, about 5 or 6 feet wide by 3
or 4 feet deep, closely resembled the channel of a small stream. It
was also filled with gravel and sand, in all respects like that found
in such a stream at the present day. It was seen to exhibit the same
characters on both sides of the cutting, but Professor Nicol was unable
to determine how far it may have extended beyond; but he had no doubt
whatever that it had been formed by a stream of water. Over this old
watercourse was a thick deposit of true till.
In reference to the foregoing cases, Professor Geikie makes the
following pertinent remarks:—“Here it is evident that the scooping out
of this channel belongs to the era of the boulder clay. It must have
been effected during a pause in the deposition of the clay, when a run
of water could find its way along the inequalities of the surface of
the clay. This pause must have been of sufficient duration to enable
the runnel to excavate a capacious channel for itself, and leave in it
a quantity of sand and shingle. We can scarcely doubt that when this
process was going on the ground must have been a land surface, and
could not have been under the sea. And lastly, we see from the upper
boulder clay that the old conditions returned, the watercourse was
choked up, and another mass of chaotic boulder clay was tumbled down
upon the face of the country. This indicates that the boulder clay is
not the result of one great catastrophe, but of slow and silent, yet
mighty, forces acting sometimes with long pauses throughout a vast
cycle of time.”[114]
At Craiglockhart Hill, about a mile south of Edinburgh, an extensive
bed of fine sand of from one to three feet in thickness was found
between two distinct masses of true boulder clay or till. The sand was
extensively used for building purposes during the erection of the city
poorhouse a few years ago. In this sand-bed I found a great many tree
roots in the position in which they had grown. During the time of the
excavations I visited the place almost daily, and had every opportunity
of satisfying myself that this sand-bed, prior to the time of the
formation of the upper boulder clay, must have been a land surface
on which the roots had grown. In no case did I find them penetrating
into the upper boulder clay, and in several places I found stones of
the upper clay resting directly on the broken ends of the roots. These
roots were examined by Professor Balfour, but they were so decayed that
he was unable to determine their character.
In digging a foundation for a building in Leith Walk, Edinburgh, a few
years ago, two distinct beds of sand were passed through, the upper,
about 10 feet in thickness, rested upon what appeared to be a denuded
surface of the lower bed. In this lower bed, which evidently had been
a land surface, numbers of tree roots were found. I had the pleasure
of examining them along with my friend Mr. C. W. Peach, who first
directed my attention to them. In no instance were the roots found
in the upper bed. That these roots did not belong to trees which had
grown on the present surface and penetrated to that depth, was further
evident from the fact that in one or two cases we found the roots
broken off at the place where they had been joined to the trunk, and
there the upper sand-bed over them was more than 10 feet in thickness.
If we assume that the roots belonged to trees which had grown on the
present surface, then we must also assume, what no one would be willing
to admit, that the trunks of the trees had grown downwards into the
earth to a depth of upwards of ten feet. I have shown these roots to
several botanists, but none of them could determine to what trees they
belonged. The surface of the ground at the spot in question is 45 feet
above sea-level. Mr. Peach and I have found similar roots in the under
sand-bed at several other places in the same neighbourhood. That they
belong to an inter-glacial period appears probable for the following
reasons:—(1.) This upper sand-bed is overlaid by a tough clay, which
in all respects appears to be the same as the Portobello clay, which
we know belongs to the glacial series. In company with Mr. Bennie,
I found the clay in some places to be contorted in a similar manner
to the Portobello clays. (2.) In a sand-pit about one or two hundred
yards to the west of where the roots were found, the sand-bed was
found contorted in the most extraordinary manner to a depth of about 15
feet. In fact, for a space of more than 30 feet, the bedding had been
completely turned up on end without the fine layers being in the least
degree broken or disarranged, showing that they had been upturned by
some enormous powers acting on a large mass of the sand.
One of the best examples of true till to be met with in the
neighbourhood of Edinburgh is at Redhall Quarry, about three miles to
the south-west of the city. In recently opening up a new quarry near
the old one a bed of peat was found intercalated in the thick mass of
till overlying the rock. The clay overlying and underlying the peat-bed
was carefully examined by Mr. John Henderson,[115] and found to be true
till.
In a quarry at Overtown, near Beith, Ayrshire, a sedimentary bed of
clay, intercalated between two boulder clays, was some years ago
observed by Mr. Robert Craig, of the Glasgow Geological Society. This
bed filled an elliptical basin about 130 yards long, and about 30 yards
broad. Its thickness averaged from one to two feet. This sedimentary
bed rested on the till on the north-east end of the basin, and was
itself overlaid on the south-west end by the upper bed of till. The
clay bed was found to be full of roots and stems of the common hazel.
That these roots had grown in the position in which they were found
was evident from the fact that they were in many places found to pass
into the “cutters” or fissures of the limestone, and were here found
in a flattened form, having in growing accommodated themselves to the
size and shape of the fissures. Nuts of the hazel were plentifully
found.[116]
At Hillhead, some distance from Overtown, there is a similar
intercalated bed full of hazel remains, and a species of freshwater
_Ostracoda_ was detected by Mr. David Robertson.
In a railway cutting a short distance from Beith, Mr. Craig pointed out
to my colleague, Mr. Jack, and myself, a thin layer of peaty matter,
extending for a considerable distance between an upper and lower mass
of till; and at one place we found a piece of oak about four feet in
length and about seven or eight inches in thickness. This oak boulder
was well polished and striated.
Not far from this place is the famous Crofthead inter-glacial bed, so
well known from the description given by Mr. James Geikie and others
that I need not here describe it. I had the pleasure of visiting the
section twice while it was well exposed, once, in company with Mr.
James Geikie, and I do not entertain the shadow of a doubt as to its
true inter-glacial character.
In the silt, evidently the mud of an inter-glacial lake, were found the
upper portion of the skull of the great extinct ox (_Bos primigenius_),
horns of the Irish elk or deer, and bones of the horse. In the detailed
list of the lesser organic remains found in the intercalated peat-bed
by Mr. J. A. Mahony,[117] are the following, viz., three species of
_Desmidaceæ_, thirty-one species of _Diatomaceæ_, eleven species of
mosses, nine species of phanerogamous plants, and several species of
annelids, crustacea, and insects. This list clearly shows that the
inter-glacial period, represented by these remains, was not only mild
and warm, but of considerable duration. Mr. David Robertson found in
the clay under the peat several species of _Ostracoda_.
The well-known Kilmaurs bed of peaty matter in which the remains of
the mammoth and reindeer were found, has now by the researches of the
Geological Survey been proved to be of inter-glacial age.[118]
In Ireland, as shown by Professors Hull and Harkness, the inter-glacial
beds, called by them the “manure gravels,” contain numerous fragments
of shells indicating a more genial climate than prevailed when the
boulder clays lying above and below them were formed.[119]
In Sweden inter-glacial beds of freshwater origin, containing plants,
have been met with by Herr Nathorst and also by Herr Holmström.[120]
In North America Mr. Whittlesey describes inter-glacial beds of blue
clay enclosing pieces of wood, intercalated with beds of hard pan
(till). Professor Newberry found at Germantown, Ohio, an immense bed of
peat, from 12 to 20 feet in thickness, underlying, in some places 30
feet, and in other places as much as 80 feet, of till, and overlying
drift beds. The uppermost layers of the peat contain undecomposed
sphagnous mosses, grasses, and sedges, but in the other portions of
the bed abundant fragments of coniferous wood, identified as red cedar
(_Juniperus virginiana_), have been found. Ash, hickory, sycamore,
together with grape-vines and beech-leaves, were also met with, and
with these the remains of the mastodon and great extinct beaver.[121]
_Inter-glacial Beds of England._—Scotland has been so much denuded by
the ice sheet with which it was covered during the period of maximum
glaciation that little can be learned in this part of the island
regarding the early history of the glacial epoch. But in England,
and more especially in the south-eastern portion of it, matters are
somewhat different. We have, in the Norwich Crag and Chillesford beds,
a formation pretty well developed, which is now generally regarded as
lying at the base of the Glacial Series. That this formation is of a
glacial character is evident from the fact of its containing shells of
a northern type, such as _Leda lanceolata_, _Cardium Groènlandicum_,
_Lucina borealis_, _Cyprina Islandica_, _Panopæa Norvegica_, and
_Mya truncata_. But the glacial character of the formation is
more strikingly brought out, as Sir Charles Lyell remarks, by the
predominance of such species as _Rhynchonella psittacea_, _Tellina
calcarea_, _Astarte borealis_, _Scalaria Groènlandica_, and _Fusus
carinatus_.
_The “Forest Beds.”_—Immediately following this in the order of
time comes the famous “Forest Bed” of Cromer. This buried forest has
been traced for more than forty miles along the coast from Cromer to
near Kessengland, and consists of stumps of trees standing erect,
attached to their roots, penetrating the original soil in which they
grew. Here and in the overlying fluvio-marine beds we have the first
evidence of at least a temperate, if not a warm, inter-glacial period.
This is evident from the character of the flora and fauna belonging
to these beds. Among the trees we have, for example, the Scotch and
spruce fir, the yew, the oak, birch, the alder, and the common sloe.
There have also been found the white and yellow water-lilies, the
pond-weed, and others. Amongst the mammalia have been met with the
_Elephas meridionalis_, also found in the Lower Pliocene beds of the
Val d’Arno, near Florence; _Elephas antiquus_, _Hippopotamus major_,
_Rhinoceros Etruscus_, the two latter Val d’Arno species, the roebuck,
the horse, the stag, the Irish elk, the _Cervus Polignacus_, found
also at Mont Perrier, France, _C. verticornis_, and _C. carnutorum_,
the latter also found in Pliocene strata of St. Prest, France. In
the fluvio-marine series have been found the _Cyclas omnica_ and the
_Paludina marginata_, a species of mollusc still found in the South of
France, but no longer inhabiting the British Isles.
Above the forest bed and fluvio-marine series comes the well-known
unstratified Norwich boulder till, containing immense blocks 6 or 8
feet in diameter, many of which must have come from Scandinavia, and
above the unstratified till are a series of contorted beds of sand and
gravel. This series may be considered to represent a period of intense
glaciation. Above this again comes the middle drift of Mr. Searles
Wood, junior, yielding shells which indicate, as is now generally
admitted, a comparatively mild condition of climate. Upon this middle
drift lies the upper boulder clay, which is well developed in South
Norfolk and Suffolk, and which is of unmistakable glacial origin. Newer
than all these are the Mundesley freshwater beds, which lie in a hollow
denuded out of the foregoing series. In this formation a black peaty
deposit containing seeds of plants, insects, shells, and scales and
bones of fishes, has been found, all indicating a mild and temperate
condition of climate. Among the shells there is, as in the forest bed,
the _Paludina marginata_. And that an arctic condition of things in
England followed is believed by Mr. Fisher and others, on the evidence
of the “Trail” described by the former observer.
_Cave and River Deposits._—Evidence of the existence of warm periods
during the glacial epoch is derived from a class of facts which
have long been regarded by geologists as very puzzling, namely, the
occurrence of mollusca and mammalia of a southern type associated
in England and on the continent with those of an extremely arctic
character. For example, _Cyrena fluminalis_ is a shell which does not
live at present in any European river, but inhabits the Nile and parts
of Asia, especially Cashmere. _Unio littoralis_, extinct in Britain,
is still abundant in the Loire; _Paludina marginata_ does not exist
in this country. These shells of a southern type have been found in
post-tertiary deposits at Gray’s Thurrock, in Essex; in the valley
of the Ouse, near Bedford; and at Hoxne, in Suffolk, associated with
a _Hippopotamus_ closely allied to that now inhabiting the Nile, and
_Elephas antiquus_, an animal remarkable for its southern range.
Amongst other forms of a southern type which have been met with in
the cave and river deposits, are the spotted hyæna from Africa,
an animal, says Mr. Dawkins, identical, except in size, with the
cave hyæna, the African elephant (_E. Africanus_), and the _Elephas
meridionalis_, the great beaver (_Trogontherium_), the cave hyæna
(_Hyæna spelæa_), the cave lion (_Felis leo_, var. _spelæa_), the lynx
(_Felis lynx_), the sabre-toothed tiger (_Machairodus latidens_), the
rhinoceros (_Rhinoceros megarhinus_ and _R. leptorhinus_). But the
most extraordinary thing is that along with these, associated in the
same beds, have been found the remains of such animals of an arctic
type as the glutton (_Gulo luscus_), the ermine (_Mustela erminea_),
the reindeer (_Cervus tarandus_), the musk-ox or musk-sheep (_Ovibos
moschatus_), the aurochs (_Bison priscus_), the woolly rhinoceros
(_Rhinoceros tichorhinus_), the mammoth (_Elephas primigenius_), and
others of a like character. According to Mr. Boyd Dawkins, these
southern animals extended as far north as Yorkshire in England, and
the northern animals as far south as the latitude of the Alps and
Pyrenees.[122]
_The Explanation of the Difficulty._—As an explanation of these
puzzling phenomena, I suggested, in the Philosophical Magazine for
November, 1868, that these southern animals lived in our island during
the warm periods of the glacial epoch, while the northern animals
lived during the cold periods. This view I am happy to find has lately
been supported by Sir John Lubbock; further, Mr. James Geikie, in his
“Great Ice Age,” and also in the Geological Magazine, has entered so
fully into the subject and brought forward such a body of evidence
in support of it, that, in all probability, it will, ere long, be
generally accepted. The only objection which has been advanced, so far
as I am aware, deserving of serious consideration, is that by Mr. Boyd
Dawkins, who holds that if these migrations had been _secular_ instead
of seasonal, as is supposed by Sir Charles Lyell and himself, the
arctic and southern animals would now be found in separate deposits.
It is perfectly true that if there had been only one cold and one warm
period, each of geologically immense duration, the remains might, of
course, be expected to have been found in separate beds; but when
we consider that the glacial epoch consisted of a long _succession
of alternate cold and warm periods_, of not more than ten or twelve
thousand years each, we can hardly expect that in the river deposits
belonging to this long cycle we should be able to distinguish the
deposits of the cold periods from those of the warm.
_Shell Beds._—Evidence of warm inter-glacial periods may be justly
inferred from the presence of shells of a southern type which have been
found in glacial beds, of which some illustrations follow.
In the southern parts of Norway, from the present sea-level up to 500
feet, are found glacial shell beds, similar to those of Scotland. In
these beds _Trochus magus_, _Tapes decussata_, and _Pholas candida_
have been found, shells which are distributed between the Mediterranean
and the shores of England, but no longer live round the coasts of
Norway.
At Capellbacken, near Udevalla, in Sweden, there is an extensive bed of
shells 20 to 30 feet in thickness. This formation has been described
by Mr. Gwyn Jeffreys.[123] It consists of several distinct layers,
apparently representing many epochs and conditions. Its shells are of a
highly arctic character, and several of the species have not been found
living south of the arctic circle. But the remarkable circumstance
is that it contains _Cypræa lurida_, a Mediterranean shell, which
Mr. Jeffreys, after some hesitation, believed to belong to the bed.
Again, at Lilleherstehagen, a short distance from Capellbacken,
another extensive deposit is exposed. “Here the upper layer,” says Mr.
Jeffreys, “gives a singular result. Mixed with the universal _Trophon
clathratus_ (which is a high northern species, and found living only
within the arctic circle) are many shells of a southern type, such are
_Ostrea edulis_, _Tapes pullastra_, _Corbula gibba_, and _Aporrhais
pes-pelicani_.”
At Kempsey, near Worcester, a shell bed is described by Sir R.
Murchison in his “Silurian System” (p. 533), in which _Bulla ampulla_
and a species of _Oliva_, shells of a southern type, have been found.
A case somewhat similar to the above is recorded by the Rev. Mr.
Crosskey as having been met with in Scotland at the Kyles of Bute.
“Among the Clyde beds, I have found,” he says, “a layer containing
shells, in which those of a more southern type appear to exist in
greater profusion and perfection than even in our present seas. It is
an open question,” he continues, “whether our climate was not slightly
warmer than it is now between the glacial epoch and the present
day.”[124]
In a glacial bed near Greenock, Mr. A. Bell found the fry of living
Mediterranean forms, viz., _Conus Mediterraneus_ and _Cardita trapezia_.
Although deposits containing shells of a temperate or of a southern
type in glacial beds have not been often recorded, it by no means
follows that such deposits are actually of rare occurrence. That
glacial beds should contain deposits indicating a temperate or a
warm condition of climate is a thing so contrary to all preconceived
opinions regarding the sequence of events during the glacial epoch,
that most geologists, were they to meet with a shell of a southern
type in one of those beds, would instantly come to the conclusion that
its occurrence there was purely accidental, and would pay no special
attention to the matter.
_Evidence derived from “Borings.”_—With the view of ascertaining if
additional light would be cast on the sequence of events, during the
formation of the boulder clay, by an examination of the journals of
bores made through a great depth of surface deposits, I collected,
during the summer of 1867, about two hundred and fifty such records,
put down in all parts of the mining districts of Scotland. An
examination of these bores shows most conclusively that the opinion
that the boulder clay, or lower till, is one great undivided formation,
is wholly erroneous.
These two hundred and fifty bores represent a total thickness of 21,348
feet, giving 86 feet as the mean thickness of the deposits passed
through. Twenty of these have one boulder clay, with beds of stratified
sand or gravel beneath the clay; twenty-five have _two_ boulder clays,
with stratified beds of sand and gravel between; ten have _three_
boulder clays; one has _four_ boulder clays; two have _five_ boulder
clays; and no one has fewer than _six_ separate masses of boulder
clay, with stratified beds of sand and gravel between; sixteen have
two or three separate boulder clays, differing altogether in colour
and hardness, without any stratified beds between. We have, therefore,
out of two hundred and fifty bores, seventy-five of them representing
a condition of things wholly different from that exhibited to the
geologist in ordinary sections.
The full details of the character of the deposits passed through by
these bores, and their bearing on the history of the glacial epoch,
have been given by Mr. James Bennie, in an interesting paper read
before the Glasgow Geological Society,[125] to which I would refer all
those interested in the subject of surface geology.
The evidence afforded by these bores of the existence of warm
inter-glacial periods will, however, fall to be considered in a
subsequent chapter.[126]
Another important and unexpected result obtained from these bores to
which we shall have occasion to refer, was the evidence which they
afforded of a Continental Period.
_Striated Pavements._—It has been sometimes observed that in horizontal
sections of the boulder clay, the stones and boulders are all striated
in one uniform direction, and this has been effected over the original
markings on the boulders. It has been inferred from this that a pause
of long duration must have taken place in the formation of the boulder
clay, during which the ice disappeared and the clay became hardened
into a solid mass. After which the old condition of things returned,
glaciers again appeared, passed over the surface of the hardened clay
with its imbedded boulders, and ground it down in the same way as they
had formerly done the solid rocks underneath the clay.
An instance of striated pavements in the boulder clay was observed by
Mr. Robert Chambers in a cliff between Portobello and Fisherrow. At
several places a narrow train of blocks was observed crossing the line
of the beach, somewhat like a quay or mole, but not more than a foot
above the general level. All the blocks _had flat sides uppermost,
and all the flat sides were striated in the same direction_ as that
of the rocky surface throughout the country. A similar instance was
also observed between Leith and Portobello. “There is, in short,” says
Mr. Chambers, “a surface of the boulder clay, deep down in the entire
bed, which, to appearance, has been in precisely the same circumstances
as the fast rock surface below had previously been. It has had in its
turn to sustain the weight and abrading force of the glacial agent,
in whatever form it was applied; and the additional deposits of the
boulder clay left over this surface may be presumed to have been formed
by the agent on that occasion.”[127]
Several cases of a similar character were observed by Mr. James
Smith, of Jordanhill, on the beach at Row, and on the shore of the
Gareloch.[128] Between Dunbar and Cockburnspath, Professor Geikie found
along the beach, for a space of 30 or 40 square yards, numbers of large
blocks of limestone with flattened upper sides, imbedded in a stiff red
clay, and all striated in one direction. On the shores of the Solway he
found another example.[129]
The cases of striated pavements recorded are, however, not very
numerous. But this by no means shows that they are of rare occurrence
in the boulder clay. These pavements, of course, are to be found only
in the interior of the mass, and even there they can only be seen
along a horizontal section. But sections of this kind are rarely to be
met with, for river channels, quarries, railway cuttings, and other
excavations of a similar character which usually lay open the boulder
clay, exhibit vertical sections only. It is therefore only along the
sea-shore, as Professor Geikie remarks, where the surface of the clay
has been worn away by the action of the waves, that opportunities have
hitherto been presented to the geologist for observing them.
There can be little doubt that during the warm periods of the glacial
epoch our island would be clothed with a luxuriant flora. At the end
of a cold period, when the ice had disappeared, the whole face of the
country would be covered over to a considerable depth with a confused
mass of stones and boulder clay. A surface thus wholly destitute of
every seed and germ would probably remain for years without vegetation.
But through course of time life would begin to appear, and during
the thousands of years of perpetual summer which would follow, the
soil, uncongenial as it no doubt must have been, would be forced to
sustain a luxuriant vegetation. But although this was the case, we
need not wonder that now scarcely a single vestige of it remains; for
when the ice sheet again crept over the island everything animate and
inanimate would be ground down to powder. We are certain that prior
to the glacial epoch our island must have been covered with life and
vegetation. But not a single vestige of these are now to be found;
no, not even of the very soil on which the vegetation grew. The solid
rock itself upon which the soil lay has been ground down to mud by the
ice sheet, and, to a large extent, as Professor Geikie remarks, swept
away into the adjoining seas.[130] It is now even more difficult to
find a trace of the ancient soil _under_ the boulder clay than it is
to find remains of the soil of the warm periods _in_ that clay. As
regards Scotland, cases of old land surfaces under the boulder clay are
as seldom recorded as cases of old land surfaces in it. In so far as
geology is concerned, there is as much evidence to show that our island
was clothed with vegetation during the glacial epoch as there is that
it was so clothed prior to that epoch.
CHAPTER XVI.
WARM INTER-GLACIAL PERIODS IN ARCTIC REGIONS.
Cold Periods best marked in Temperate, and Warm Periods
in Arctic, Regions.—State of Arctic Regions during
Glacial Period.—Effects of Removal of Ice from Arctic
Regions.—Ocean-Currents; Influence on Arctic Climate.—Reason
why Remains of Inter-glacial Period are rare in Arctic
Regions.—Remains of Ancient Forests in Banks’s Land, Prince
Patrick’s Island, &c.—Opinions of Sir R. Murchison, Captain
Osborn, and Professor Haughton.—Tree dug up by Sir E. Belcher
in lat. 75° N.
In the temperate regions the cold periods of the glacial epoch would be
far more marked than the warm inter-glacial periods. The condition of
things which prevailed during the cold periods would differ far more
widely from that which now prevails than would the condition of things
during the warm periods. But as regards the polar regions the reverse
would be the case; there the warm inter-glacial periods would be far
more marked than the cold periods. The condition of things prevailing
in those regions during the warm periods would be in strongest contrast
to what now obtains, but this would not hold true in reference to the
cold periods; for during the latter, matters there would be pretty
much the same as at present, only a good deal more severe. The reason
of this may be seen from what has already been stated in Chapter IV.;
but as it is a point of considerable importance in order to a proper
understanding of the physical state of things prevailing in polar
regions during the glacial epoch, I shall consider this part of the
subject more fully.
During the cold periods, our island, and nearly all places in the
northern temperate regions down to about the same latitude, would be
covered with snow and ice, and all animal and vegetable life within
the glaciated area would to a great extent be destroyed. The presence
of the ice would of itself, for reasons already explained, lower the
mean annual temperature to near the freezing-point. The summers,
notwithstanding the proximity of the sun, would not be warm, on the
contrary their temperature would rise little above the freezing-point.
An excess of evaporation would no doubt take place, owing to the
increase in the intensity of the sun’s rays, but this result would only
tend to increase the snowfall.[131]
During the warm periods our country and the regions under consideration
would experience conditions not differing much from those of the
present, but the climate would probably be somewhat warmer and more
equable. The proximity of the sun during winter would prevent snow
from falling. The summers, owing to the greater distance of the sun,
would probably be somewhat colder than they are now. But the loss of
heat during summer would be to a large extent compensated for by two
causes to which we must here refer. (1.) The much greater amount of
heat conveyed by ocean-currents than at present. (2.) Our summers are
now cooled to a considerable extent by cold aërial currents from the
ice-covered regions of the north. But during the period in question
there would be little or no ice in arctic regions, consequently the
winds would be comparatively warm, whatever direction they came from.
Let us next direct our attention to the state of things in the arctic
regions during the glacial epoch. At present Greenland and other parts
of the arctic regions occupied by land are almost wholly covered
with ice, and as a consequence nearly destitute of vegetable life.
During the cold periods of the glacial epoch the quantity of snow
falling would doubtless be greater and the ice thicker, but as regards
organic life, matters would not probably be much worse than they are
at present. In fact, so far as Greenland and the antarctic continent
are concerned, they are about as destitute of plant life as they can
be. Although an increase in the thickness of the arctic ice would not
greatly alter the present state of matters in those regions, yet what
a transformation would ensue upon the disappearance of the ice! This
would not only raise the summer temperature some twenty degrees or so,
but would afford the necessary conditions for the existence of abundant
animal and plant life. The severity of the climate of Greenland is
due to a very considerable extent, as we have already seen, to the
presence of ice. Get rid of the permanent ice, and the temperature of
the country, _cæteris paribus_, would instantly rise. That Greenland
should ever have enjoyed a temperate climate, capable of supporting
abundant vegetation, has often been matter of astonishment, but this
wonder diminishes when we reflect that during the warm periods it would
be in the arctic regions that the greatest heating effect would take
place, this being due mainly to the transference of nearly all the warm
inter-tropical waters to one hemisphere.
It has been shown in Chapter II. that the heating effects at present
resulting from the transference of heat by ocean-currents increase as
we approach the poles. As a consequence of this it follows that during
the warm periods, when the quantity of warm water transferred would be
nearly doubled, the _increase of heat resulting from this cause would
itself increase_ as the warm pole was approached. This effect, combined
with the shortness of the winter in perihelion and the nearness of the
sun during that season, would prevent the accumulation of snow. During
summer, the sun, it is true, would be at a much greater distance from
the earth than at present, but it must be borne in mind that for a
period of three months the quantity of heat received from the sun at
the north pole would be greater than that received at the equator.
Consequently, after the winter’s snow was melted, this great amount of
heat would go to raise the temperature, and the arctic summer could
not be otherwise than hot. It is not hot at present, but this, be it
observed, is because of the presence of the ice. When we take all these
facts into consideration we need not be surprised that Greenland once
enjoyed a condition of climate totally different from that which now
obtains in that region.
It is, therefore, in the arctic and antarctic regions where we ought
to find the most marked and decided evidence of warm inter-glacial
periods. And doubtless such evidence would be abundantly forthcoming
had these regions not been subjected to such intense denudation since
the glacial epoch, and were so large a portion of the land not still
buried beneath an icy covering, and therefore beyond the geologist’s
reach. Only on islands and such outlying places as are not shrouded in
snow and ice can we hope to meet with any trace of the warm periods of
the glacial epoch: and we may now proceed to consider what relics of
these warm periods have actually been discovered in arctic regions.
_Evidence of Warm Periods in Arctic Regions._—The fact that stumps,
&c., of full-grown trees have been found in places where at present
nothing is to be met with but fields of snow and ice, and where the
mean annual temperature scarcely rises above the zero of the Fahrenheit
thermometer, is good evidence to show that the climate of the arctic
regions was once much warmer than now. The remains of an ancient forest
were discovered by Captain McClure, in Banks’s Land, in latitude 74°
48′. He found a great accumulation of trees, from the sea-level to an
elevation of upwards of 300 feet. “I entered a ravine,” says Captain
McClure, “some miles inland, and found the north side of it, for a
depth of 40 feet from the surface, composed of one mass of wood similar
to what I had before seen.”[132] In the ravine he observed a tree
protruding about 8 feet, and 3 feet in circumference. And he further
states that, “_From the perfect state of the bark_, and the position of
the trees so far from the sea, there can be but little doubt that they
grew originally in the country.” A cone of one of these fir-trees was
brought home, and was found to belong apparently to the genus _Abies_,
resembling _A. (Pinus) alba_.
In Prince Patrick’s Island, in latitude 76° 12′ N., longitude 122°
W., near the head of Walker Inlet, and a considerable distance in the
interior in one of the ravines, a tree protruding about 10 feet from
a bank was discovered by Lieutenant Mecham. It proved to be 4 feet
in circumference. In its neighbourhood several others were seen, all
of them similar to some he had found at Cape Manning; each of them
measured 4 feet round and 30 feet in length. The carpenter stated that
the trees resembled larch. Lieutenant Mecham, from their appearance and
position, concluded that they must have grown in the country.[133]
Trees under similar conditions were also found by Lieutenant Pim on
Prince Patrick’s Island, and by Captain Parry on Melville Island, all
considerably above the present sea-level and at a distance from the
shore. On the coast of New Siberia, Lieutenant Anjou found a cliff of
clay containing stems of trees still capable of being used for fuel.
“This remarkable phenomenon,” says Captain Osborn, “opens a vast field
for conjecture, and the imagination becomes bewildered in trying to
realise that period of the world’s history when the absence of ice and
a milder climate allowed forest trees to grow in a region where now the
ground-willow and dwarf-birch have to struggle for existence.”
Sir Roderick Murchison came to the conclusion that all those trees
were drifted to their present position when the islands of the arctic
archipelago were submerged. But it was the difficulty of accounting
for the growth of trees in such a region which led him to adopt this
hypothesis. His argument is this: “If we imagine,” he says, “that the
timber found in those latitudes grew on the spot we should be driven
to adopt the anomalous hypothesis that, notwithstanding physical
relations of land and water similar to those which now prevail, trees
of large size grew on such _terra firma_ within a few degrees of the
north pole!—a supposition which I consider to be wholly incompatible
with the data in our possession, and at variance with the laws of the
isothermal lines.”[134] This reasoning of Sir Roderick’s may be quite
correct, on the supposition that changes of climate are due to changes
in the distribution of sea and land, as advocated by Sir Charles Lyell.
But these difficulties disappear if we adopt the views advocated in
the foregoing chapters. As Captain Osborn has pointed out, however,
Sir Roderick’s hypothesis leaves the real difficulty untouched. “A
very different climate,” he says, “must then have existed in those
regions to allow driftwood so perfect as to retain its bark to reach
such great distances; and perhaps it may be argued that if that sea was
sufficiently clear of ice to allow such timber to drift unscathed to
Prince Patrick’s Land, that that _very absence of a frozen sea would
allow fir-trees to grow in a soil naturally fertile_.”[135]
As has been already stated, all who have seen those trees in arctic
regions agree in thinking that they grew _in situ_. And Professor
Haughton, in his excellent account of the arctic archipelago appended
to McClintock’s “Narrative of Arctic Discoveries,” after a careful
examination of the entire evidence on the subject, is distinctly of
the same opinion; while the recent researches of Professor Heer put it
beyond doubt that the drift theory must be abandoned.
Undoubtedly the arctic archipelago was submerged to an extent that
could have admitted of those trees being floated to their present
positions. This, as we shall see, follows from theory; but submergence,
without a warmer condition of climate, would not enable trees to reach
those regions with their bark entire.
But in reality we are not left to theorise on the subject, for we
have a well-authenticated case of one of those trees being got by
Captain Belcher standing erect in the position in which it grew. It was
found immediately to the northward of the narrow strait opening into
Wellington Sound, in lat. 75° 32′ N. long. 92° W., and about a mile and
a half inland. The tree was dug up out of the frozen ground, and along
with it a portion of the soil which was immediately in contact with the
roots. The whole was packed in canvas and brought to England. Near to
the spot several knolls of peat mosses about nine inches in depth were
found, containing the bones of the lemming in great numbers. The tree
in question was examined by Sir William Hooker, who gave the following
report concerning it, which bears out strongly the fact of its having
grown _in situ_.
“The piece of wood brought by Sir Edward Belcher from the shores of
Wellington Channel belongs to a species of pine, probably to the _Pinus
(Abies) alba_, the most northern conifer. The structure of the wood
of the specimen brought home differs remarkably in its anatomical
character from that of any other conifer with which I am acquainted.
Each concentric ring (or annual growth) consists of two zones of
tissue; one, the outer, that towards the circumference, is broader, of
a pale colour, and consists of ordinary tubes of fibres of wood, marked
with discs common to all coniferæ. These discs are usually opposite
one another when more than one row of them occur in the direction of
the length of the fibre; and, what is very unusual, present radiating
lines from the central depression to the circumference. Secondly,
the inner zone of each annual ring of wood is narrower, of a dark
colour, and formed of more slender woody fibres, with thicker walls in
proportion to their diameter. These tubes have few or no discs upon
them, but are covered with spiral striæ, giving the appearance of each
tube being formed of a twisted band. The above characters prevail in
all parts of the wood, but are slightly modified in different rings.
Thus the outer zone is broader in some than in others, the disc-bearing
fibres of the outer zone are sometimes faintly marked with spiral
striæ, and the spirally marked fibres of the inner zone sometimes bear
discs. These appearances suggest the annual recurrence of some special
cause that shall thus modify the first and last formed fibres of each
year’s deposit, so that that first formed may differ in amount as
well as in kind from that last formed; and the peculiar conditions of
an arctic climate appear to afford an adequate solution. The inner,
or first-formed zone, must be regarded as imperfectly developed,
being deposited at a season when the functions of the plant are very
intermittently exercised, and when a few short hours of sunshine are
daily succeeded by many of extreme cold. As the season advances the
sun’s heat and light are continuous during the greater part of the
twenty-four hours, and the newly formed wood fibres are hence more
perfectly developed, they are much longer, present no signs of striæ,
but are studded with discs of a more highly organized structure than
are usual in the natural order to which this tree belongs.”[136]
Another circumstance which shows that the tree had grown where it was
found is the fact that in digging up the roots portions of the leaves
were obtained. It may also be mentioned that near this place was found
an old river channel cut deeply into the rock, which, at some remote
period, when the climate must have been less rigorous than at present,
had been occupied by a river of considerable size.
Now, it is evident that if a tree could have grown at Wellington Sound,
there is no reason why one might not have grown at Banks’s Land, or
at Prince Patrick’s Island. And, if the climatic condition of the
country would allow one tree to grow, it would equally as well allow
a hundred, a thousand, or a whole forest. If this, then, be the case,
Sir Roderick’s objection to the theory of growth _in situ_ falls to the
ground.
Another circumstance which favours the idea that those trees grew
during the glacial epoch is the fact that although they are recent,
geologically speaking, and belong to the drift series, yet they are,
historically speaking, very old. The wood, though not fossilized, is so
hardened and changed by age that it will scarcely burn.
CHAPTER XVII.
FORMER GLACIAL EPOCHS.—REASON OF THE IMPERFECTION OF
GEOLOGICAL RECORDS IN REFERENCE TO THEM.
Two Reasons why so little is known of Glacial Epochs.—Evidence
of Glaciation to be found on Land-surfaces.—Where are all
our ancient Land-surfaces?—The stratified Rocks consist
of a Series of old Sea-bottoms.—Transformation of a
Land-surface into a Sea-bottom obliterates all Traces of
Glaciation.—Why so little remains of the Boulder Clays of
former Glacial Epochs.—Records of the Glacial Epoch are fast
disappearing.—Icebergs do not striate the Sea-bottom.—Mr.
Campbell’s Observations on the Coast of Labrador.—Amount
of Material transported by Icebergs much exaggerated.—Mr.
Packard on the Glacial Phenomena of Labrador.—Boulder Clay
the Product of Land-ice.—Palæontological Evidence.—Paucity of
Life characteristic of a Glacial Period.—Warm Periods better
represented by Organic Remains than cold.—Why the Climate
of the Tertiary Period was supposed to be warmer than the
present.—Mr. James Geikie on the Defects of Palæontological
Evidence.—Conclusion.
_Two Reasons why so little is known of former Glacial Epochs._—If the
glacial epoch resulted from the causes discussed in the foregoing
chapters, then such epochs must have frequently supervened. We may,
therefore, now proceed to consider what evidence there is for the
former occurrence of excessive conditions of climate during previous
geological ages. When we begin our inquiry, however, we soon find
that the facts which have been recorded as evidence in favour of the
action of ice in former geological epochs are very scanty indeed. Two
obvious reasons for this may be given, namely, (1) The imperfection
of the geological records themselves, and (2) the little attention
hitherto paid toward researches of this kind. The notion, once so
prevalent, that the climate of our earth was much warmer in the earlier
geological ages than it is now, and that it has ever since been
gradually becoming cooler, was wholly at variance with the idea of
former ice-periods. And this conviction of the _à priori_ improbability
of cold periods having obtained during Palæozoic and Mesozoic ages
tended to prevent due attention being paid to such facts as seemed to
bear upon the subject. But our limited knowledge of former glacial
epochs must no doubt be attributed chiefly to the actual imperfection
of the geological records. So great is this imperfection that the mere
absence of direct geological evidence cannot reasonably be regarded as
sufficient proof that the conclusions derived from astronomical and
physical considerations regarding former ice-periods are improbable.
Nor is this all. The geological records of ancient glacial conditions
are not only imperfect, but, as I shall endeavour to show, this
imperfection _follows as a natural consequence from the principles of
geology itself_. There are not merely so many blanks or gaps in the
records, but a reason exists in the very nature of geological evidence
why such breaks in the record might reasonably be expected to occur.
_Evidence of Glaciation to be found chiefly on Land-surfaces._—It is on
a land-surface that the principal traces of the action of ice during
a glacial epoch are left, for it is there that the stones are chiefly
striated, the rocks ground down, and the boulder clay formed. But where
are all our ancient land-surfaces? They are not to be found. The total
thickness of the stratified rocks of Great Britain is, according to
Professor Ramsay, nearly fourteen miles. But from the top to the bottom
of this enormous pile of deposits there is hardly a single land-surface
to be detected. True patches of old land-surfaces of a local character
exist, such, for example, as the dirt-beds of Portland; but, with the
exception of coal-seams, every general formation from top to bottom
has been accumulated under water, and none but the under-clays _ever
existed as a land_-surface. And it is here, in such a formation,
that the geologist has to collect all his information regarding the
existence of former glacial epochs. The entire stratified rocks of the
globe, with the exception of the coal-beds and under-clays (in neither
of which would one expect to find traces of ice-action), consist almost
entirely of a _series of old sea-bottoms_, with here and there an
occasional freshwater deposit. Bearing this in mind, what is the sort
of evidence which we can now hope to find in these old sea-bottoms of
the existence of former ice-periods?
Every geologist of course admits that the stratified rocks are not
old land-surfaces, but a series of old sea-bottoms formed out of
the accumulated material derived from the degradation of primeval
land-surfaces. And it is true that all land-surfaces once existed
as sea-bottoms; but the stratified rocks consist of a series of old
sea-bottoms which never were land-surfaces. Many of them no doubt
have been repeatedly above the sea-level, and may once have possessed
land-surfaces; but these, with the exception of the under-clays of the
various coal measures, the dirt-beds of Portland, and one or two more
patches, have all been denuded away. The important bearing which this
consideration has on the nature of the evidence which we can now expect
to find of the existence of former glacial epochs has certainly been
very much overlooked.
If we examine the matter fully we shall be led to conclude that the
_transformation of a land-surface into a sea-bottom_ will probably
completely obliterate every trace of glaciation which that land-surface
may once have presented. We cannot, for example, expect to meet with
polished and striated stones belonging to a former land glaciation; for
such stones are not carried down bodily and unchanged by our rivers
and deposited in the sea. They become broken up by subaërial agencies
into gravel, sand, and clay, and in this condition are transported
seawards. Nor even if we supposed it possible that the stones and
boulders derived from a mass of till could be carried down to sea by
river-action, could we at the same time fail to admit that such stones
would be deprived of all their ice-markings, and become water-worn and
rounded on the way.[137]
Nor can we expect to find boulder clay among the stratified rocks, for
boulder clay is not carried down as such and deposited in the sea, but
under the influence of the denuding agents becomes broken up into soft
mud, clay, sand, and gravel, as it is gradually peeled off the land and
swept seawards. Patches of boulder clay may have been now and again
forced into the sea by ice and eventually become covered up; but such
cases are wholly exceptional, and their absence in any formation cannot
fairly be adduced as a proof that that formation does not belong to a
glacial period.
The only evidence of the existence of land-ice during former periods
which we can reasonably expect to meet with in the stratified rocks,
consists of erratic blocks which may have been transported by icebergs
and dropped into the sea. But unless the glaciers of such epochs
reached the sea, we could not possibly possess even this evidence.
Traces in the stratified rocks of the effects of land-ice during former
epochs must, in the very nature of things, be rare indeed. The only
sort of evidence which, as a general rule, we may expect to detect,
is the presence of large erratic blocks imbedded in strata which from
their constitution have evidently been formed in still water. But this
is quite enough; for it proves the existence of ice at the time the
strata were being deposited as conclusively as though we saw the ice
floating with the blocks upon it. This sort of evidence, when found in
low latitudes, ought to be received as conclusive of the existence of
former glacial epochs; and, no doubt, would have been so received had
it not been for the idea that, if these blocks had been transported
by ice, there ought in addition to have been found striated stones,
boulder clay, and other indications of the agency of land-ice.
Of course all erratics are not necessarily transported by masses of
ice broken from the terminal front of glaciers. The “ice foot,” formed
by the freezing of the sea along the coasts of the higher latitudes of
Greenland, carries seawards immense quantities of blocks and _débris_.
And again stones and boulders are frequently frozen into river-ice,
and when the ice breaks up in spring are swept out to sea, and may be
carried some little distance before they are dropped. But both these
cases can occur only in regions where the winters are excessive; nor
is it at all likely that such ice-rafts will succeed in making a long
voyage. If, therefore, we could assure ourselves that the erratics
occasionally met with in certain old geological formations in low
latitudes were really transported from the land by an ice-foot or a
raft of river-ice, we should be forced to conclude that very severe
climatic conditions must have obtained in such latitudes at the time
the erratics were dispersed.
The reason why we now have, comparatively speaking, so little direct
evidence of the existence of former glacial periods will be more
forcibly impressed upon the mind, if we reflect on how difficult it
would be in a million or so of years hence to find any trace of what
we now call the glacial epoch. The striated stones would by that time
be all, or nearly all, disintegrated, and the till washed away and
deposited in the bottom of the sea as stratified sands and clays. And
when these became consolidated into rock and were raised into dry land,
the only evidence that we should probably then have that there ever
had been a glacial epoch would be the presence of large blocks of the
older rocks, which would be found imbedded in the upraised formation.
We could only infer that there had been ice at work from the fact that
by no other known agency could we conceive such blocks to have been
transported and dropped in a still sea.
Probably few geologists believe that during the Middle Eocene and
the Upper Miocene periods our country passed through a condition of
glaciation as severe as it has done during the Post-pliocene period;
yet when we examine the subject carefully, we find that there is
actually no just ground to conclude that it has not. For, in all
probability, throughout the strata to be eventually formed out of the
destruction of the now existing land-surfaces, evidence of ice-action
will be as scarce as in Eocene or Miocene strata.
If the stratified rocks forming the earth’s crust consisted of a series
of old land-surfaces instead (as they actually do) of a series of old
sea-bottoms, then probably traces of many glacial periods might be
detected.
Nearly all the evidence which we have regarding the glacial epoch
has been derived from what we find on the now existing land-surfaces
of the globe. But probably not a vestige of this will exist in the
stratified beds of future ages, formed out of the destruction of the
present land-surfaces. Even the very arctic shell-beds themselves,
which have afforded to the geologist such clear proofs of a frozen sea
during the glacial epoch, will not be found in those stratified rocks;
for they must suffer destruction along with everything else which now
exists above the sea-level. There is probably not a single relic of
the glacial epoch which has ever been seen by the eye of man that will
be treasured up in the stratified rocks of future ages. Nothing that
does not lie buried in the deeper recesses of the ocean will escape
complete disintegration and appear imbedded in those formations. It
is only those objects which lie in our existing sea-bottoms that will
remain as monuments of the glacial epoch of the Post-tertiary period.
And, moreover, it will only be those portions of the sea-bottoms that
may happen to be upraised into dry land that will be available to the
geologist of future ages. The point to be determined now is this:—_Is
it probable that the geologist of the future will find in the rocks
formed out of the now existing sea-bottoms more evidence of a glacial
epoch during Post-tertiary times than we now do of one during, say, the
Miocene, the Eocene, or the Permian period?_ Unless this can be proved
to be the case, we have no ground whatever to conclude that the cold
periods of the Miocene, Eocene, and Permian periods were not as severe
as that of the glacial epoch. This is evident, for the only relics
which now remain of the glacial epochs of those periods are simply
what happened to be protected in the then existing sea-bottoms. Every
vestige that lay on the land would in all probability be destroyed by
subaërial agency and carried into the sea in a sedimentary form. But
before we can determine whether or not there is more evidence of the
glacial epoch in our now existing sea-bottoms than there is of former
glacial epochs in the stratified rocks (which are in reality the
sea-bottoms belonging to ancient epochs), we must first ascertain what
is the nature of those marks of glaciation which are to be found in a
sea-bottom.
_Icebergs do not striate the Sea-bottom._—We know that the rocky face
of the country was ground down and striated during the glacial epoch;
and this is now generally believed to have been done by land-ice. But
we have no direct evidence that the floor of the ocean, beyond where it
may have been covered with land-ice, was striated. Beyond the limits
of the land-ice it could be striated only by means of icebergs. But
do icebergs striate the rocky bed of the ocean? Are they adapted for
such work? It seems to be often assumed that they are. But I have been
totally unable to find any rational grounds for such a belief. Clean
ice can have but little or no erosive power, and never could scratch a
rock. To do this it must have grinding materials in the form of sand,
mud, or stones. But the bottoms of icebergs are devoid of all such
materials. Icebergs carry the grinding materials on their backs, not on
their bottoms. No doubt, when the iceberg is launched into the deep,
great masses of sand, mud, and stones will be adhering to its bottom.
But no sooner is the berg immersed, than a melting process commences
at its sides and lower surface in contact with the water; and the
consequence is, the materials adhering to the lower surface soon drop
off and sink to the bottom of the sea. The iceberg, divested of these
materials, can now do very little harm to the rocky sea-bottom over
which it floats. It is true that an iceberg moving with a velocity
of a few miles an hour, if it came in contact with the sea-bottom,
would, by the mere force of concussion, tear up loose and disjointed
rocks, and hurl some of the loose materials to a distance; but it would
do but little in the way of grinding down the rock against which it
struck. But even supposing the bottom of the iceberg were properly
shod with the necessary grinding materials, still it would be but a
very inefficient grinding agent; for a _floating_ iceberg would not
be in contact with the sea-bottom. And if it were in contact with the
sea-bottom, it would soon become stranded and, of course, motionless,
and under such conditions could produce no effect.
It is perfectly true that although the bottom of the berg may be devoid
of grinding materials, yet these may be found lying on the surface
of the submarine rock over which the ice moves. But it must be borne
in mind that the same current which will move the icebergs over the
surface of the rock will move the sand, mud, and other materials
over it also; so that the markings effected by the ice would in all
probability be erased by the current. In the deep recesses of the
ocean the water has been found to have but little or no motion. But
icebergs always follow the path of currents; and it is very evident
that at the comparatively small depth of a thousand feet or so reached
by icebergs the motion of the water will be considerable; and the
continual shifting of the small particles of the mud and sand will in
all probability efface the markings which may be made now and again by
a passing berg.
Much has been said regarding the superiority of icebergs as grinding
and striating agents in consequence of the great velocity of their
motion in comparison with that of land-ice. But it must be remembered
that it is while the iceberg is floating, and before it touches the
rock, that it possesses high velocity. When the iceberg runs aground,
its motion is suddenly arrested or greatly reduced. But if the iceberg
advancing upon a sloping sea-bottom is raised up so as to exert great
pressure, it will on this account be the more suddenly arrested,
the motion will be slow, and the distance passed over short, before
the berg becomes stranded. If it exerts but little pressure on the
sea-bottom, it may retain a considerable amount of motion and advance
to a considerable distance before it is brought to a stand; but,
exerting little pressure, it can perform but little work. Land-ice
moves slowly, but then it exerts enormous pressure. A glacier 1,000
feet in thickness has a pressure on its rocky bed equal to about 25
tons on the square foot; but an iceberg a mile in thickness, forced up
on a sloping sea-bottom to an elevation of 20 feet (and this is perhaps
more than any ocean-current could effect), would only exert a pressure
of about half a ton on the square foot, or about 1/50th part of the
pressure of the glacier 1,000 feet in thickness. A great deal has been
said about the erosive and crushing power of icebergs of enormous
thickness, as if their thickness gave them any additional pressure. An
iceberg 100 feet in thickness will exert just as much pressure as one
a mile in thickness. The pressure of an iceberg is not like that of a
glacier, in proportion to its thickness, but to the height to which it
is raised out of the water. An iceberg 100 feet in thickness raised 10
feet will exert exactly the same pressure as one a mile in thickness
raised to an equal height.
To be an efficient grinding agent, steadiness of motion, as well as
pressure, is essential. A rolling or rocking motion is ill-adapted
for grinding down and striating a rock. A steady rubbing motion under
pressure is the thing required. But an iceberg is not only deficient in
pressure, but also deficient in steadiness of motion. When an iceberg
moving with considerable velocity comes on an elevated portion of the
sea-bottom, it does not move steadily onwards over the rock, unless
the pressure of the berg on the rock be trifling. The resistance being
entirely at the bottom of the iceberg, its momentum, combined with the
pressure of the current, applied wholly above the point of resistance,
tends to make the berg bend forward, and in some cases upset (when
it is of a cubical form). The momentum of the moving berg, instead
of being applied in forcing it over the rock against which it comes
in contact, is probably all consumed in work against gravitation in
raising the berg upon its front edge. After the momentum is consumed,
unless the berg be completely upset, it will fall back under the force
of gravitation to its original position. But the momentum which it
acquires from gravitation in falling backwards carries it beyond its
position of repose in an opposite direction. It will thus continue to
rock backwards and forwards until the friction of the water brings it
to rest. The momentum of the berg, instead of being applied to the work
of grinding and striating the sea-bottom, will chiefly be consumed in
heat in the agitation of the water. But if the berg does advance, it
will do so with a rocking unsteady motion, which, as Mr. Couthouy[138]
and Professor Dana[139] observe, will tend rather to obliterate
striations than produce them.
A floating berg moves with great steadiness; but a berg that has run
aground cannot advance with a steady motion. If the rock over which the
berg moves offers little resistance, it may do so; but in such a case
the berg could produce but little effect on the rock.
Dr. Sutherland, who has had good opportunities to witness the effects
of icebergs, makes some most judicious remarks on the subject. “It
will be well” he says, “to bear in mind that when an iceberg _touches
the ground, if that ground be hard and resisting, it must come to a
stand_, and the propelling power continuing, a slight leaning over in
the water, or yielding motion of the whole mass, may compensate readily
for being so suddenly arrested. If, however, the ground be soft, so
as not to arrest the motion of the iceberg at once, a moraine will be
the result; but the moraine thus raised will tend to bring it to a
stand.”[140]
There is another cause referred to by Professor Dana, which, to a
great extent, must prevent the iceberg from having an opportunity of
striating the sea-bottom, even though it were otherwise well adapted
for so doing. It is this: the bed of the ocean in the track of
icebergs must be pretty much covered with stones and rubbish dropped
from the melting bergs. And this mass of rubbish will tend to protect
the rock.[141]
If icebergs cannot be shown _à priori_, from mechanical considerations,
to be well adapted for striating the sea-bottom, one would naturally
expect, from the confident way in which it is asserted that they are
so adapted, that the fact has been at least established by actual
observation. But, strange as it may appear, we seem to have little or
no proof that icebergs actually striate the bed of the ocean. This can
be proved from the direct testimony of the advocates of the iceberg
theory themselves.
We shall take the testimony of Mr. Campbell, the author of two
well-known works in defence of the iceberg theory, viz., “Frost and
Fire,” and “A Short American Tramp.” Mr. Campbell went in the fall of
the year 1864 to the coast of Labrador, the Straits of Belle Isle, and
the Gulf of St. Lawrence, for the express purpose of witnessing the
effects of icebergs, and testing the theory which he had formed, that
the ice-markings of the glacial epoch were caused by floating ice and
not by land-ice, as is now generally believed.
The following is the result of his observations on the coast of
Labrador.
Hanly Harbour, Strait of Belle Isle:—“The water is 37° F. in July....
As fast as one island of ice grounds and bursts, another takes its
place; and in winter the whole strait is blocked up by a mass which
swings bodily up and down, grating along the bottom at all depths....
Examined the beaches and rocks at the water-line, especially in sounds.
Found the rocks ground smooth, _but not striated_, in the sounds”
(_Short American Tramp_, pp. 68, 107).
Cape Charles and Battle Harbour:—“But though these harbours are all
frozen every winter, the _rocks at the water-line are not striated_”
(p. 68).
At St. Francis Harbour:—“The water-line is much rubbed, smooth, _but
not striated_” (p. 72).
Cape Bluff:—“Watched the rocks with a telescope, and _failed to make
out striæ anywhere_; but the water-line is everywhere rubbed smooth”
(p. 75).
Seal Islands:—“_No striæ are to be seen at the land-wash in these
sounds or on open sea-coasts near the present water-line_” (p. 76).
He only mentions having here found striations in the three following
places along the entire coast of Labrador visited by him; and in regard
to two of these, it seems very doubtful that the markings were made by
modern icebergs.
Murray’s Harbour:—“This harbour was blocked up with ice on the 20th of
July. The water-line is rubbed, and in _some places_ striated” (p. 69).
Pack Island:—“The water-line in a narrow sound was polished and
striated in the direction of the sound, about N.N.W. This seems to be
fresh work done by heavy ice drifting from Sandwich Bay; _but, on the
other hand, stages with their legs in the sea, and resting on these
very rocks, are not swept away by the ice_” (p. 96). If these markings
were modern, why did not the “heavy ice” remove the small fir poles
supporting the fishing-stages?
Red Bay:—“Landed half-dressed, and found some striæ perfectly fresh at
the water-level, but weathered out a short distance _inland_” (p. 107).
The striations “inland” could not have been made by modern icebergs;
and it does not follow that because the markings at the water-level
were not weathered they were produced by modern ice.
These are the evidences which he found that icebergs striate rocks,
on a coast of which he says that, during the year he visited it, “the
winter-drift was one vast solid raft of floes and bergs more than 150
miles wide, and perhaps 3,000 feet thick at spots, driven by a whole
current bodily over one definite course, year after year, since this
land was found” (p. 85).
But Mr. Campbell himself freely admits that the floating ice which
comes aground along the shores does not produce striæ. “It is
sufficiently evident,” he says, “_that glacial striæ are not produced
by thin bay-ice_” (p. 76). And in “Frost and Fire,” vol. ii., p. 237,
he states that, “from a careful examination of the water-line at many
spots, it appears that bay-ice grinds rocks, _but does not produce
striation_.”
“It is impossible,” he continues, “to get at rocks over which heavy
icebergs now move; but a mass 150 miles wide, perhaps 3,000 feet thick
in some parts, and moving at the rate of a mile an hour, or more,
_appears to be an engine amply sufficient_ to account for striæ on
rising rocks.” And in “American Tramp,” p. 76, he says, “_striæ must be
made_ in deep water by the large masses which seem to pursue the even
tenor of their way in the steady current which flows down the coast.”
Mr. Campbell, from a careful examination of the sea-bottom along the
coast, finds that the small icebergs do not produce striæ, but the
large ones, which move over rocks impossible to be got at, “must”
produce them. They “appear” to be amply sufficient to do so. If the
smaller bergs cannot striate the sea-bottom, why must the larger ones
do so? There is no reason why the smaller bergs should not move as
swiftly and exert as much pressure on the sea-bottom as the larger
ones. And even supposing that they did not, one would expect that the
light bergs would effect on a smaller scale what the heavy ones would
do on a larger.
I have no doubt that when Mr. Campbell visited Labrador he expected to
find the sea-coast under the water-line striated by means of icebergs,
and was probably not a little surprised to find that it actually was
not. And I have no doubt that were the sea-bottom in the tracks of the
large icebergs elevated into view, he would find to his surprise that
it was free from striations also.
So far as observation is concerned, we have no grounds from what Mr.
Campbell witnessed to conclude that icebergs striate the sea-bottom.
The testimony of Dr. Sutherland, who has had opportunities of seeing
the effects of icebergs in arctic regions, leads us to the same
conclusion. “Except,” he says, “from the evidence afforded by plants
and animals at the bottom, we have _no means whatever_ to ascertain
the effect produced by icebergs upon the rocks.[142] In the Malegat
and Waigat I have seen whole clusters of these floating islands,
drawing from 100 to 250 fathoms, moving to and fro with every return
and recession of the tides. I looked very earnestly for grooves and
scratches left by icebergs and glaciers in the rocks, but always failed
to discover any.”[143]
We shall now see whether river-ice actually produces striations or not.
If floating ice under any form can striate rocks, one would expect that
it ought to be done by river-ice, seeing that such ice is obliged to
follow one narrow definite track.
St. John’s River, New Brunswick:—“This river,” says Mr. Campbell,
“is obstructed by ice during five months of the year. When the ice
goes, there is wild work on the bank. Arrived at St. John, drove
to the suspension-bridge.... At this spot, if _anywhere in the
world_, river-ice ought to produce striation. The whole drainage of
a wide basin and one of the strongest tides in the world, here work
continually in one rock-groove; and in winter this water-power is armed
with heavy ice. _There are no striæ_ about the water-line.”[144]
River St. Lawrence:—“In winter the power of ice-floats driven by
water-power is tremendous. The river freezes and packs ice till the
flow of water is obstructed. The rock-pass at Quebec is like the
Narrows at St. John’s, Newfoundland. The whole pass, about a mile
wide, was paved with great broken slabs and round boulders of worn ice
as big as small shacks, piled and tossed, and heaped and scattered
upon the level water below and frozen solid.... This kind of ice does
NOT _produce striation_ at the water-margin at Quebec. At Montreal,
when the river ‘goes,’ the ice goes with it with a vengeance.... The
_piers are not yet striated_ by river-ice at Montreal.... The rocks
at the high-water level have _no trace_ of glacial striæ.... The rock
at Ottawa is rubbed by river-ice every spring, and _always in one
direction, but it is not striated_.... The surfaces are all rubbed
smooth, and the edges of broken beds are rounded where exposed to the
ice; _but there are no striæ_.”[145]
When Sir Charles Lyell visited the St. Lawrence in 1842, at Quebec he
went along with Colonel Codrington “and searched carefully below the
city in the channel of the St. Lawrence, at low water, near the shore,
for the signs of glacial action at the precise point where the chief
pressure and friction of packed ice are exerted every year,” but found
none.
“At the bridge above the Falls of Montmorenci, over which a large
quantity of ice passes every year, the gneiss is polished, and kept
perfectly free from lichens, but not more so than rocks similarly
situated at waterfalls in Scotland. In none of these places were any
long straight grooves observable.”[146]
The only thing in the shape of modern ice-markings which he seems to
have met with in North America was a few straight furrows half an inch
broad in soft sandstone, at the base of a cliff at Cape Blomidon in the
Bay of Fundy, at a place where during the preceding winter “packed”
ice 15 feet thick had been pushed along when the tide rose over the
sandstone ledges.[147]
The very fact that a geologist so eminent as Sir Charles Lyell, after
having twice visited North America, and searched specially for modern
ice-markings, was able to find only two or three scratches, upon a soft
sandstone rock, which he could reasonably attribute to floating ice,
ought to have aroused the suspicion of the advocates of the iceberg
theory that they had really formed too extravagant notions regarding
the potency of floating ice as a striating agent.
There is no reason to believe that the grooves and markings noticed
by M. Weibye and others on the Scandinavian coast and other parts of
northern Europe were made by icebergs.
Professor Geikie has clearly shown, from the character and direction
of the markings, that they are the production of land-ice.[148] If
the floating ice of the St. Lawrence and the icebergs of Labrador are
unable to striate and groove the rocks, it is not likely that those of
northern Europe will be able to do so.
It will not do for the advocates of the iceberg theory to assume, as
they have hitherto done, that, as a matter of course, the sea-bottom is
being striated and grooved by means of icebergs. They must prove that.
They must either show that, as a matter of fact, icebergs are actually
efficient agents in striating the sea-bottom, or prove from mechanical
principles that they must be so. The question must be settled either by
observation or by reason; mere opinion will not do.
_The Amount of Material transported by Icebergs much exaggerated._—The
transporting of boulders and rubbish, and not the grinding and
striating of rocks, is evidently the proper function of the iceberg.
But even in this respect I fear too much has been attributed to it.
In reading the details of voyages in the arctic regions one cannot help
feeling surprised how seldom reference is made to stones and rubbish
being seen on icebergs. Arctic voyagers, like other people, when they
are alluding to the geological effects of icebergs, speak of enormous
quantities of stones being transported by them; but in reading the
details of their voyages, the impression conveyed is that icebergs with
stones and blocks of rock upon them are the exceptions. The greater
portion of the narratives of voyages in arctic regions consists of
interesting and detailed accounts of the voyager’s adventures among the
ice. The general appearance of the icebergs, their shape, their size,
their height, their colour, are all noticed; but rarely is mention
made of stones being seen. That the greater number of icebergs have
no stones or rubbish on them is borne out by the positive evidence of
geologists who have had opportunities of seeing icebergs.
Mr. Campbell says:—“It is remarkable that up to this time we have only
seen a few doubtful stones on bergs which we have passed.... Though
no bergs with stones _on them or in them_ have been approached during
this voyage, many on board the _Ariel_ have been close to bergs heavily
laden.... A man who has had some experience of ice has _never seen a
stone on a berg_ in these latitudes. Captain Anderson, of the _Europa_,
who is a geologist, has _never seen a stone on a berg_ in crossing the
Atlantic. _No stones were clearly seen on this trip._”[149] Captain Sir
James Anderson (who has long been familiar with geology, has spent a
considerable part of his life on the Atlantic, and has been accustomed
to view the iceberg as a geologist as well as a seaman) has never seen
a stone on an iceberg in the Atlantic. This is rather a significant
fact.
Sir Charles Lyell states that, when passing icebergs on the Atlantic,
he “was most anxious to ascertain whether there was any mud, stones,
or fragments of rocks on any one of these floating masses; but after
examining about forty of them without perceiving any signs of frozen
matter, I left the deck when it was growing dusk.”[150] After he had
gone below, one was said to be seen with something like stones upon it.
The captain and officers of the ship assured him that they had _never
seen a stone upon a berg_.
The following extract from Mr. Packard’s “Memoir on the Glacial
Phenomena of Labrador and Maine,” will show how little is effected by
the great masses of floating ice on the Labrador coast either in the
way of grinding and striating the rocks, or of transporting stones,
clay, and other materials.
“Upon this coast, which during the summer of 1864 was lined with a
belt of floe-ice and bergs probably two hundred miles broad, and which
extended from the Gulf of St. Lawrence at Belles Amours to the arctic
seas, this immense body of floating ice seemed _directly_ to produce
but little alteration in its physical features. If we were to ascribe
the grooving and polishing of rocks to the action of floating ice-floes
and bergs, how is it that the present shores far above (500), and at
least 250 feet below, the water-line are often jagged and angular,
though constantly stopping the course of masses of ice impelled four to
six miles an hour by the joint action of tides, currents, and winds? No
boulders, or gravel, or mud were seen upon any of the bergs or masses
of shore-ice. They had dropped all burdens of this nature nearer their
points of detachment in the high arctic regions.” ...
“This huge area of floating ice, embracing so many thousands of square
miles, was of greater extent, and remained longer upon the coast, in
1864, than for forty years previous. It was not only pressed upon the
coast by the normal action of the Labrador and Greenland currents,
which, in consequence of the rotatory motion of the earth, tended to
force the ice in a south-westerly direction, but the presence of the
ice caused the constant passage of cooler currents of air from the
sea over the ice upon the heated land, giving rise during the present
season to a constant succession of north-easterly winds from March
until early in August, which further served to crowd the ice into every
harbour and recess upon the coast. It was the universal complaint
of the inhabitants that the easterly winds were more prevalent, and
the ice ‘held’ later in the harbours this year than for many seasons
previous. Thus the fisheries were nearly a failure, and vegetation
greatly retarded in its development. But so far as polishing and
striating the rocks, depositing drift material, and thus modifying
the contour of the surface of the present coast, this modern mass of
bergs and floating ice effected comparatively little. Single icebergs,
when small enough, entered the harbours, and there stranding, soon
pounded to pieces upon the rocks, melted, and disappeared. From Cape
Harrison, in lat. 55°, to Caribo Island, was an interrupted line of
bergs stranded in 80 to 100 or more fathoms, often miles apart, while
others passed to the seaward down by the eastern coast of Newfoundland,
or through the Straits of Belle Isle.”[151]
_Boulder Clay the Product of Land-ice._—There is still another point
connected with icebergs to which we must allude, viz., the opinion
that great masses of the boulder clay of the glacial epoch were formed
from the droppings of icebergs. If boulder clay is at present being
accumulated in this manner, then traces of the boulder clay deposits of
former epochs might be expected to occur. It is perfectly obvious that
_unstratified_ boulder clay could not have been formed in this way.
Stones, gravel, sand, clay, and mud, the ingredients of boulder clay,
tumbled all together from the back of an iceberg, could not sink to the
bottom of the sea without separating. The stones would reach the bottom
first, then the gravel, then the sand, then the clay, and last of all
the mud, and the whole would settle down in a stratified form. But,
besides, how could the _clay_ be derived from icebergs? Icebergs derive
their materials from the land before they are launched into the deep,
and while they are in the form of land-ice. The materials which are
found on the backs of icebergs are what fell upon the ice from mountain
tops and crags projecting above the ice. Icebergs are chiefly derived
from continental ice, such as that of Greenland, where the whole
country is buried under one continuous mass, with only a lofty mountain
peak here and there rising above the surface. And this is no doubt
the chief reason why so few icebergs have stones upon their backs.
The continental ice of Greenland is not, like the glaciers of the
Alps, covered with loose stones. Dr. Robert Brown informs me that no
moraine matter has ever been seen on the inland ice of Greenland. It is
perfectly plain that clay does not fall upon the ice. What falls upon
the ice is stones, blocks of rocks, and the loose _débris_. Clay and
mud we know, from the accounts given by arctic voyagers, are sometimes
washed down upon the coast-ice; but certainly very little of either can
possibly get upon an iceberg. Arctic voyagers sometimes speak of seeing
clay and mud upon bergs; but it is probable that if they had been near
enough they would have found that what they took for clay and mud were
merely dust and rubbish.
Undoubtedly the boulder clay of many places bears unmistakable evidence
of having been formed under water; but it does not on that account
follow that it was formed from the droppings of icebergs. The fact
that the boulder clay in every case _is chiefly composed of materials
derived from the country on which the clay lies_, proves that it was
not formed from matter transported by icebergs. The clay, no doubt,
contains stones and boulders belonging to other countries, which in
some cases may have been transported by icebergs; but the clay itself
has not come from another country. But if the clay itself has been
derived from the country on which it lies, then it is absurd to suppose
that it was deposited from icebergs. The clay and materials which are
found on icebergs are derived from the land on which the iceberg is
formed; but to suppose that icebergs, after floating about upon the
ocean, should always return to the country which gave them birth, and
there deposit their loads, is rather an extravagant supposition.
From the facts and considerations adduced we are, I would venture to
presume, warranted to conclude that, with the exception of what may
have been produced by land-ice, very little in the shape of boulder
clay or striated rocks belonging to the glacial epoch lies buried
under the ocean—and that when the now existing land-surfaces are all
denuded, probably scarcely a trace of the glacial epoch will then be
found, except the huge blocks that were transported by icebergs and
dropped into the sea. It is therefore probable that we have as much
evidence of the existence of a glacial epoch during former periods as
the geologists of future ages will have of the existence of a glacial
epoch during the Post-tertiary period, and that consequently we are not
warranted in concluding that the glacial epoch was something unique in
the geological history of our globe.
_Palæontological Evidence._—It might be thought that if glacial epochs
have been numerous, we ought to have abundance of palæontological
evidence of their existence. I do not know if this necessarily follows.
Let us take the glacial epoch itself for example, which is quite a
modern affair. Here we do not require to go and search in the bottom
of the sea for the evidence of its existence; for we have the surface
of the land in almost identically the same state in which it was when
the ice left it, with the boulder clay and all the wreck of the ice
lying upon it. But what geologist, with all these materials before him,
would be able to find out from palæontological evidence alone that
there had been such an epoch? He might search the whole, but would not
be able to find fossil evidence from which he could warrantably infer
that the country had ever been covered with ice. We have evidence
in the fossils of the Crag and other deposits of the existence of a
colder condition of climate prior to the true glacial period, and in
the shell-beds of the Clyde and other places of a similar state of
matters after the great ice-sheets had vanished away. But in regard
to the period of the true boulder clay or till, when the country was
enveloped in ice, palæontology has almost nothing whatever to tell
us. “Whatever may be the cause,” says Sir Charles Lyell, “the fact is
certain that over large areas in Scotland, Ireland, and Wales, I might
add throughout the northern hemisphere on both sides of the Atlantic,
the stratified drift of the glacial period is very commonly devoid of
fossils.”[152]
In the “flysch” of the Eocene of the Alps, to which we shall have
occasion to refer in the next chapter, in which the huge blocks are
found which prove the existence of ice-action during that period, few
or no fossils have been found. So devoid of organic remains is that
formation, that it is only from its position, says Sir Charles, that
it is known to belong to the middle or “nummulitic” portion of the
great Eocene series. Again, in the conglomerates at Turin, belonging
to the Upper Miocene period, in which the angular blocks of limestone
are found which prove that during that period Alpine glaciers reached
the sea-level in the latitude of Italy, not a single organic remain has
been found. It would seem that an extreme paucity of organic life is a
characteristic of a glacial period, which warrants us in concluding
that the absence of organic remains in any formation otherwise
indicative of a cold climate cannot be regarded as sufficient evidence
that that formation does not belong to a cold period.
In the last chapter it was shown why so little evidence of the warm
periods of the glacial epoch is now forthcoming. The remains of the
_faunas_ and _floras_ of those periods were nearly wholly destroyed and
swept into the adjoining seas by the ice-sheet that covered the land.
It is upon the present land-surface that we find the chief evidence
of the last glacial epoch, but the traces of the warm periods of that
epoch are hardly now to be met with in that position since they have
nearly all been obliterated or carried into the sea.
In regard to former glacial epochs, however, ice-marked rocks,
scratched stones, moraines, till, &c., no longer exist; the
land-surfaces of those old times have been utterly swept away. The only
evidence, therefore, of such ancient glacial epochs, that we can hope
to detect, must be sought for in the deposits that were laid down upon
the sea-bottom; where also we may expect to find traces of the warm
periods that alternated during such epochs with glacial conditions. It
is plain, moreover, that the palæontological evidence in favour of warm
periods will always be the most abundant and satisfactory.
Judging from geological evidence alone, we naturally conclude that, as
a general rule, the climate of former periods was somewhat warmer than
it is at the present day. It is from fossil remains that the geologist
principally forms his estimate of the character of the climate during
any period. Now, in regard to fossil remains, the warm periods will
always be far better represented than the cold; for we find that, as
a _general rule, those formations which geologists are inclined to
believe indicate a cold condition of climate are remarkably devoid of
fossil remains_. If a geologist does not keep this principle in view,
he will be very apt to form a wrong estimate of the general character
of the climate of a period of such enormous length as say the Tertiary.
Suppose that the presently existing sea-bottoms, which have been
forming since the commencement of the glacial epoch, were to become
consolidated into rock and thereafter to be elevated into dry land, we
should then have a formation which might be properly designated the
Post-pliocene. It would represent the time which has elapsed from the
beginning of the glacial epoch to the present day. Suppose one to be
called upon as a geologist to determine from that formation what was
the general character of the climate during the period in question,
what would probably be the conclusion at which he would arrive? He
would probably find here and there patches of boulder clay containing
striated and ice-worn stones. Now and again he would meet with bones
of the mammoth and the reindeer, and shells of an arctic type. He
would likewise stumble upon huge blocks of the older rocks imbedded
in the formation, from which he would infer the existence of icebergs
and glaciers reaching the sea-level. But, on the whole, he would
perceive that the greater portion of the fossil remains met with in
this formation implied a warm and temperate condition of climate. At
the lower part of the formation, corresponding to the time of the true
boulder clay, there would be such a scarcity of organic remains that
he would probably feel at a loss to say whether the climate at that
time was cold or hot. But if the intense cold of the glacial epoch
was not continuous, but broken up by intervening warm periods during
which the ice, to a considerable extent at least, disappeared for a
long period of time (and there are few geologists who have properly
studied the subject who will positively deny that such was the case),
then the country would no doubt during those warm periods possess an
abundance of plant and animal life. It is quite true that we may almost
search in vain on the present land-surface for the organic remains
which belonged to those inter-glacial periods; for they were nearly
all swept away by the ice which followed. But no doubt in the deep
recesses of the ocean, buried under hundreds of feet of sand, mud,
clay, and gravel, lie multitudes of the plants and animals which then
flourished on the land, and were carried down by rivers into the sea.
And along with these lie the skeletons, shells, and other exuviæ of
the creatures which flourished in the warm seas of those periods. Now
looking at the great abundance of fossils indicative of warm and genial
conditions which the lower portions of this formation would contain,
the geologist might be in danger of inferring that the earlier part
of the Post-pliocene period was a warmer period, whereas we, at the
present day, looking at the matter from a different standpoint, declare
that part to have been characterized by cold or glacial conditions. No
doubt, if the beds formed during the cold periods of the glacial epoch
could be distinguished from those formed during the warm periods, the
fossil remains of the one would indicate a cold condition of climate,
and those of the other a warm condition; but still, taking the entire
epoch as a whole, the percentage of fossil remains indicative of a
warm condition would probably so much exceed that indicative of a cold
condition, that we should come to the conclusion that the character
of the climate, as a whole, during the epoch in question was warm and
equable.
As geologists we have, as a rule, no means of arriving at a knowledge
of the character of the climate of any given period but through an
examination of the sea-bottoms belonging to that period; for these
contain all the evidence upon the subject. But unless we exercise
caution, we shall be very apt, in judging of the climate of such
a period, to fall into the same error that we have just now seen
one might naturally fall into were he called upon to determine the
character of the climate during the glacial epoch from the nature of
the organic remains which lie buried in our adjoining seas. On this
point Mr. J. Geikie’s observations are so appropriate, that I cannot
do better than introduce them here. “When we are dealing,” says this
writer, “with formations so far removed from us in time, and in which
the animal and plant remains depart so widely from existing forms of
life, we can hardly expect to derive much aid from the fossils in our
attempts to detect traces of cold climatic conditions. The arctic
shells in our Post-tertiary clays are convincing proofs of the former
existence in our latitude of a severe climate; but when we go so far
back as Palæozoic ages, we have no such clear evidence to guide us.
All that palæontologists can say regarding the fossils belonging to
these old times is simply this, that they seem to indicate, generally
speaking, mild, temperate, or genial, and even sometimes tropical,
conditions of climate. Many of the fossils, indeed, if we are to reason
from analogy at all, could not possibly have lived in cold seas. But,
for aught that we know, there may have been alternations of climate
during the deposition of each particular formation; and these changes
may be marked by the presence or absence, or by the greater or less
abundant development, of certain organisms at various horizons in
the strata. Notwithstanding all that has been done, our knowledge of
the natural history of these ancient seas is still very imperfect;
and therefore, in the present state of our information, we are not
entitled to argue, from the general aspect of the fossils in our older
formations, that the temperature of the ancient seas was never other
than mild and genial.”[153]
_Conclusion._—From what has already been stated it will, I trust, be
apparent that, assuming glacial epochs during past geological ages to
have been as numerous and as severe as the Secular theory demands,
still it would be unreasonable to expect to meet with abundant traces
of them. The imperfection of the geological record is such that we
ought not to be astonished that so few relics of former ice ages have
come down to us. It will also be apparent that the palæontological
evidence of a warm condition of climate having obtained during any
particular age, is no proof that a glacial epoch did not also supervene
during the same cycle of time. Indeed it is quite the reverse; for
the warm conditions of which we have proof may indicate merely the
existence of an inter-glacial period. Furthermore, if the Secular
theory of changes of climate be admitted, then evidence of a warm
condition of climate having prevailed in arctic regions during any
past geological age may be regarded as presumptive proof of the
existence of a glacial epoch; that is to say, of an epoch during
which cold and warm conditions of climate alternated. Keeping these
considerations in view, we shall now proceed to examine briefly what
evidence we at present have of the former existence of glacial epochs.
CHAPTER XVIII.
FORMER GLACIAL EPOCHS; GEOLOGICAL EVIDENCE OF.
Cambrian Conglomerate of Islay and North-west of
Scotland.—Ice-action in Ayrshire and Wigtownshire
during Silurian Period.—Silurian Limestones in Arctic
Regions.—Professor Ramsay on Ice-action during Old
Red Sandstone Period.—Warm Climate in Arctic Regions
during Old Red Sandstone Period.—Professor Geikie and
Mr. James Geikie on a Glacial Conglomerate of Lower
Carboniferous Age.—Professor Haughton and Professor Dawson
on Evidence of Ice-action during Coal Period.—Mr. W. T.
Blanford on Glaciation in India during Carboniferous
Period.—Carboniferous Formations of Arctic Regions.—Professor
Ramsay on Permian Glaciers.—Permian Conglomerate in
Arran.—Professor Hull on Boulder Clay of Permian Age.—Permian
Boulder Clay of Natal.—Oolitic Boulder Conglomerate in
Sutherlandshire.—Warm Climate in North Greenland during
Oolitic Period.—Mr. Godwin-Austen on Ice-action during
Cretaceous Period.—Glacial Conglomerates of Eocene Age in the
Alps.—M. Gastaldi on the Ice-transported Limestone Blocks of
the Superga.—Professor Heer on the Climate of North Greenland
during Miocene Period.
CAMBRIAN PERIOD.
_Island of Islay._—Good evidence of ice-action has been observed by
Mr. James Thomson, F.G.S.,[154] in strata which he believes to be of
Cambrian age. At Port Askaig, Island of Islay, below a precipitous
cliff of quartzite 70 feet in height, there is a mass of arenaceous
talcose schist containing fragments of granite, some angular, but
most of them rounded, and of all sizes, from mere particles to
large boulders. As there is no granite in the island from which
these boulders could have been derived, he justly infers that they
must have been transported by the agency of ice. The probability
of his conclusion is strengthened by the almost total absence of
stratification in the deposit in question.
_North-west of Scotland._—Mr. J. Geikie tells me that much of the
Cambrian conglomerate in the north-west of Scotland strongly reminds
him of the coarse shingle beds (Alpine diluvium) which so often crowd
the old glacial valleys of Switzerland and Northern Italy. In many
places the stones of the Cambrian conglomerate have a subangular,
blunted shape, like those of the re-arranged moraine débris of Alpine
countries.
SILURIAN PERIOD.
_Wigtownshire._—The possibility of glacial action so far back as
the Silurian age has been suggested. In beds of slate and shales in
Wigtownshire of Lower Silurian age Mr. J. Carrick Moore found beds of
conglomerate of a remarkable character. The fragments generally vary
from the size of one inch to a foot in diameter, but in some of the
beds, boulders of 3, 4, and even 5 feet in diameter occur. There are
no rocks in the neighbourhood from which any of these fragments could
have been derived. The matrix of this conglomerate is sometimes a green
trappean-looking sandstone of exceeding toughness, and sometimes an
indurated sandstone indistinguishable from many common varieties of
greywacke.[155]
_Ayrshire._—Mr. James Geikie states that in Glenapp, and near
Dalmellington, he found embedded in Lower Silurian strata blocks
and boulders from one foot to 5 feet in diameter of gneiss,
syenite, granite, &c., none of which belong to rocks of those
neighbourhoods.[156] Similar cases have been found in Galway, Ireland,
and at Lisbellaw, south of Enniskillen.[157] In America, Professor
Dawson describes Silurian conglomerates with boulders 2 feet in
diameter.
_Arctic Regions._—The existence of warm inter-glacial periods
during that age may be inferred from the fact that in the arctic
regions we find widespread masses of Silurian limestones containing
encrinites, corals, and mollusca, and other fossil remains, for an
account of which see Professor Haughton’s geological account of the
Arctic Archipelago appended to McClintock’s “Narrative of Arctic
Discoveries.”[158]
OLD RED SANDSTONE.
_North of England._—According to Professor Ramsay and some other
geologists the brecciated, subangular conglomerates and boulder beds
of the Old Red Sandstone of Scotland and the North of England are of
glacial origin. When these conglomerates and the recent boulder clay
come together it is difficult to draw the line of demarcation between
them.
Professor Ramsay observed some very remarkable facts in connection
with the Old Red Sandstone conglomerates of Kirkby Lonsdale, and
Sedburgh, in Westmoreland and Yorkshire. I shall give the results of
his observations in his own words.
“The result is, that we have found many stones and blocks distinctly
scratched, and on others the ghosts of scratches nearly obliterated
by age and chemical action, probably aided by pressure at a time when
these rocks were buried under thousands of feet of carboniferous
strata. In some cases, however, the markings were probably produced
within the body of the rock itself by pressure, accompanied by
disturbance of the strata; but in others the longitudinal and cross
striations convey the idea of glacial action. The shapes of the stones
of these conglomerates, many of which are from 2 to 3 feet long, their
flattened sides and subangular edges, together with the confused manner
in which they are often arranged (like stones in the drift), have
long been enough to convince me of their ice-borne character; and the
scratched specimens, when properly investigated, may possibly convince
others.”[159]
_Isle of Man._—The conglomerate of the Old Red Sandstone in the Isle of
Man has been compared by Mr. Cumming to “a consolidated ancient boulder
clay.” And he remarks, “Was it so that those strange trilobitic-looking
fishes of that era had to endure the buffeting of ice-waves, and to
struggle amidst the wreck of ice-floes and the crush of bergs?”[160]
_Australia._—A conglomerate similar to that of Scotland has been found
in Victoria, Australia, by Mr. Selwyn, at several localities. Along
the Wild Duck Creek, near Heathcote, and also near the Mia-Mia, Spring
Plains, Redesdale, localities in the Colony of Victoria, where it was
examined by Messrs. Taylor and Etheridge, Junior, this conglomerate
consists of a mixture of granite pebbles and boulders of various
colours and textures, porphyries, indurated sandstone, quartz, and
a peculiar flint-coloured rock in a matrix of bluish-grey very hard
mud-cement.[161] Rocks similar to the pebbles and blocks composing the
conglomerate do not occur in the immediate neighbourhood; and from the
curious mixture of large and small angular and water-worn fragments
it was conjectured that it might possibly be of glacial origin.
Scratched stones were not observed, although a careful examination was
made. From similar mud-pebble beds on the Lerderderg River, Victoria,
Mr. P. Daintree obtained a few pebbles grooved after the manner of
ice-scratched blocks.[162]
And the existence of a warm condition of climate during the Old Red
Sandstone period is evidenced by the fossiliferous limestones of
England, Russia, and America. On the banks of the Athabasca River,
Rupert-Land, Sir John Richardson found beds of limestone containing
_Producti_, _Spiriferi_, an _Orthis_ resembling _O. resupinata_,
_Terebratula reticularis_,[163] and a _Pleurotomaria_, which, in the
opinion of the late Dr. Woodward, who examined the specimens, are
characteristic of Devonian rocks of Devonshire.
CARBONIFEROUS PERIOD.
_France._—It is now a good many years since Mr. Godwin-Austen directed
attention to what he considered evidence of ice-action during the coal
period. This geologist found in the carboniferous strata of France
large angular blocks which he could not account for without inferring
the former action of ice. “Whether from local elevation,” he says,
“or from climatic conditions, there are certain appearances over the
whole which imply that at one time the temperature must have been very
low, as glacier-action can alone account for the presence of the large
angular blocks which occur in the lowest detrital beds of many of the
southern coal-basins.”[164]
_Scotland._—In Scotland great beds of conglomerate are met with in
various parts, which are now considered by Professor Geikie, Mr.
James Geikie, and other officers of the Geological Survey who have
had opportunities of examining them, to be of glacial origin. “They
are,” says Mr. James Geikie, “quite unstratified, and the stones often
show that peculiar blunted form which is so characteristic of glacial
work.”[165] Many of the stones found by Professor Geikie, several of
which I have had an opportunity of seeing, are well striated.
In 1851 Professor Haughton brought forward at the Geological Society
of Dublin, a case of angular fragments of granite occurring in the
carboniferous limestone of the county of Dublin; and he explained the
phenomena by the supposition of the transporting power of ice.
_North America._—In one of the North American coal-fields Professor
Newberry found a boulder of quartzite 17 inches by 12 inches, imbedded
in a seam of coal. Similar facts have also been recorded both in the
United States, and in Nova Scotia. Professor Dawson describes what he
calls a gigantic esker of Carboniferous age, on the outside of which
large travelled boulders were deposited, probably by drift-ice; while
in the swamps within, the coal flora flourished.[166]
_India._—Mr. W. T. Blanford, of the Geological Survey of India, states
that in beds considered to be of Carboniferous age are found large
boulders, some of them as much as 15 feet in diameter. The bed in
which these occur is a fine silt, and he refers the deposition of the
boulders to ice-action. Within the last three years his views have
received singular confirmation in another part of India, where beds
of limestone were found striated below certain overlying strata. The
probability that these appearances are due, as Mr. Blanford says, to
the action of ice, is strengthened by the consideration that about five
degrees farther to the north of the district in question rises the
cold and high table-land of Thibet, which during a glacial epoch would
undoubtedly be covered with ice that might well descend over the plains
of India.[167]
_Arctic Regions._—A glacial epoch during the Carboniferous age may be
indirectly inferred from the probable existence of warm inter-glacial
periods, as indicated by the limestones with fossil remains found in
arctic regions.
That an equable condition of climate extended to near the north pole
is proved by the fact that in the arctic regions vast masses of
carboniferous limestone, having all the characters of the mountain
limestone of England, have been found. “These limestones,” says
Mr. Isbister, “are most extensively developed in the north-east
extremity of the continent, where they occupy the greater part of
the coast-line, from the north side of the Kotzebue Sound to within
a few miles of Point Barrow, and form the chief constituent of the
lofty and conspicuous headlands of Cape Thomson, Cape Lisburn, and
Cape Sabine.”[168] Limestone of the same age occurs extensively
along the Mackenzie River. The following fossils have been found
in these limestones:—_Terebratula resupinata_,[169] _Lithostrotion
basaltiforme_, _Cyathophyllum dianthum_, _C. flexuosum_, _Turbinolia
mitrata_, _Productus Martini_,[170] _Dentalium Sarcinula_,
_Spiriferi_, _Orthidæ_, and encrinital fragments in the greatest
abundance.
Among the fossils brought home from Depôt Point, Albert Land, by
Sir E. Belcher, Mr. Salter found the following, belonging to the
Carboniferous period:—_Fusulina hyperborea_, _Stylastrea inconferta_,
_Zaphrentis ovibos_, _Clisiophyllum tumulus_, _Syringopora (Aulopora)_,
_Fenestella Arctica_, _Spirifera Keilhavii_, _Productus cora_, _P.
semireticulatus_.[171]
Coal-beds of Carboniferous age are extensively developed in arctic
regions. The fuel is of a highly bituminous character, resembling, says
Professor Haughton, the gas coals of Scotland. The occurrence of coal
in such high latitudes indicates beyond doubt that a mild and temperate
condition of climate must, during some part of the Carboniferous age,
have prevailed up to the very pole.
“In the coal of Jameson’s Land, on the east side of Greenland, lying
in latitude 71°, and in that of Melville Island, in latitude 75° N.,
Professor Jameson found plants resembling fossils of the coal-fields of
Britain.”[172]
PERMIAN PERIOD.
_England._—From the researches of Professor Ramsay in the Permian
breccias, we have every reason to believe that during a part of the
Permian age our country was probably covered with glaciers reaching
to the sea. These brecciated stones, he states, are mostly angular
or subangular, with flattened sides and but very slightly rounded at
the edges, and are imbedded in a deep red marly paste. At Abberley
Hill some of the masses are from 2 to 3 feet in diameter, and in one
of the quarries, near the base of Woodbury Hill, Professor Ramsay
saw one 2 feet in diameter. Another was observed at Woodbury Rock, 4
feet long, 3 feet broad, and 1½ feet thick. The boulders were found
in South Staffordshire, Enville, in Abberley and Malvern Hills,
and other places. “They seem,” he says, “to have been derived from
the conglomerate and green, grey, and purple Cambrian grits of the
Longmynd, and from the Silurian quartz-rocks, slates, felstones,
felspathic ashes, greenstones, and Upper Caradoc rocks of the country
between the Longmynd and Chirbury. But then,” he continues, “the south
end of the Malvern Hills is from forty to fifty miles, the Abberleys
from twenty-five to thirty-five miles, Enville from twenty to thirty
miles, and South Staffordshire from thirty-five to forty miles distant
from that country.”[173]
It is physically impossible, Professor Ramsay remarks, that these
blocks could have been transported to such distances by any other
agency than that of ice. Had they been transported by water, supposing
such a thing possible, they would have been rounded and water-worn,
whereas many of these stones are flat slabs, and most of them have
their edges but little rounded. And besides many of them are highly
polished, and others grooved and finely striated, exactly like those of
the ancient glaciers of Scotland and Wales. Some of these specimens are
to be seen in the Museum of Practical Geology, Jermyn Street.
_Scotland._—In the Island of Arran, Mr. E. A. Wunsch and Mr. James
Thomson found a bed of conglomerate which they considered of Permian
age, and probably of glacial origin. This conglomerate enclosed angular
fragments of various schistose, volcanic, and limestone rocks, and
contained carboniferous fossils.
_Ireland._—At Armagh, Ireland, Professor Hull found boulder beds of
Permian age, containing pebbles and boulders, sometimes 2 feet in
diameter. Some of the boulders must have been transported from a
region lying about 30 miles to the north-west of the locality in which
they now occur. It is difficult to conceive, says Professor Hull,
how rock fragments of such a size could have been carried to their
present position by any other agency than that of floating ice. This
boulder-bed is overlaid by a recent bed of boulder clay. Professor
Ramsay, who also examined the section, agrees with Professor Hull that
the bed is of Permian age, and unquestionably of ice-formation.[174]
Professor Ramsay feels convinced that the same conclusions which he has
drawn in regard to the Permian breccia of England will probably yet be
found to hold good in regard to much of that of North Germany.[175] And
there appears to be some ground for concluding that the cold of that
period even reached to India.[176]
_South Africa._—An ancient boulder clay, supposed to be either
of Permian or Jurassic age, has been extensively found in Natal,
South Africa. This deposit, discovered by Dr. Sutherland, the
Surveyor-General of the colony, is thus described by Dr. Mann:—
“The deposit itself consists of a greyish-blue argillaceous matrix,
containing fragments of granite, gneiss, graphite, quartzite,
greenstone, and clay-slate. These imbedded fragments are of various
size, from the minute dimensions of sand-grains up to vast blocks
measuring 6 feet across, and weighing from 5 to 10 tons. They are
smoothed, as if they had been subject to a certain amount of attrition
in a muddy sediment; but they are not rounded like boulders that
have been subjected to sea-breakers. The fracture of the rock is not
conchoidal, and there is manifest, in its substance, a rude disposition
towards wavy stratification.”
“Dr. Sutherland inclines to think that the transport of vast massive
blocks of several tons’ weight, the scoring of the subjacent surfaces
of sandstone, and the simultaneous deposition of minute sand-grains
and large boulders in the same matrix, all point to one agency as the
only one which can be rationally admitted to account satisfactorily
for the presence of this remarkable formation in the situations in
which it is found. He believes that the boulder-bearing clay of Natal
is of analogous nature to the great Scandinavian drift, to which it
is certainly intimately allied in intrinsic mineralogical character;
that it is virtually a vast moraine of olden time; and that ice, in
some form or other, has had to do with its formation, at least so far
as the deposition of the imbedded fragments in the amorphous matrix are
concerned.”[177]
In the discussion which followed the reading of Dr. Sutherland’s paper,
Professor Ramsay pointed out that in the Natal beds enormous blocks of
rock occurred, which were 60 or 80 miles from their original home, and
still remained angular; and there was a difficulty in accounting for
the phenomena on any other hypothesis than that suggested.
Mr. Stow, in his paper on the Karoo beds, has expressed a similar
opinion regarding the glacial character of the formation.[178]
But we have in the Karoo beds evidence not only of glaciation, but of a
much warmer condition of things than presently exists in that latitude.
This is shown from the fact that the shells of the _Trigona_-beds
indicate a tropical or subtropical condition of climate.
_Arctic Regions._—The evidence which we have of the existence of a
warm climate during the Permian period is equally conclusive. The
close resemblance of the _flora_ of the Permian period to that of
Carboniferous times evidently points to the former prevalence of a warm
and equable climate. And the existence of the magnesian limestone in
high latitudes seems to indicate that during at least a part of the
Permian period, just as during the accumulation of the carboniferous
limestone, a warm sea must have obtained in those latitudes.
OOLITIC PERIOD.
_North of Scotland._—There is not wanting evidence of something like
the action of ice during the Oolitic period.[179]
In the North of Scotland Mr. James Geikie says there is a coarse
boulder conglomerate associated with the Jurassic strata in the east
of Sutherland, the possibly glacial origin of which long ago suggested
itself to Professor Ramsay and other observers. Mr. Judd believes the
boulders to have been floated down by ice from the Highland mountains
at the time the Jurassic strata were being accumulated.
_North Greenland._—During the Oolitic period a warm condition of
climate extended to North Greenland. For example, in Prince Patrick’s
Island, at Wilkie Point, in lat. 76° 20′ N., and long. 117° 20′
W., Oolitic rocks containing an ammonite (_Ammonites McClintocki_,
Haughton), like _A. concavus_ and other shells of Oolitic species,
were found by Captain McClintock.[180] In Katmai Bay, near Behring’s
Straits, the following Oolitic fossils were discovered—_Ammonites
Wasnessenskii_, _A. biplex_, _Belemnites paxillosus_, and _Unio
liassinus_.[181] Captain McClintock found at Point Wilkie, in Prince
Patrick’s Island, lat. 76° 20′, a bone of _Ichthyosaurus_, and Sir E.
Belcher found in Exmouth Island, lat. 76° 16′ N., and long. 96° W., at
an elevation of 570 feet above the level of the sea, bones which were
examined by Professor Owen, and pronounced to be those of the same
animal.[182] Mr. Salter remarks that at the time that these fossils
were deposited, “a condition of climate something like that of our own
shores was prevailing in latitudes not far short of 80° N.”[183] And
Mr. Jukes says that during the Oolitic period, “in latitudes where
now sea and land are bound in ice and snow throughout the year, there
formerly flourished animals and plants similar to those living in our
own province at that time. The questions thus raised,” continues Mr.
Jukes, “as to the climate of the globe when cephalopods and reptiles
such as we should expect to find only in warm or temperate seas,
could live in such high latitudes, are not easy to answer.”[184] And
Professor Haughton remarks, that he thinks it highly improbable that
any change in the position of land and water could ever have produced a
temperature in the sea at 76° north latitude which would allow of the
existence of ammonites, especially species so like those that lived
at the same time in the tropical warm seas of the South of England
and France at the close of the Liassic, and commencement of the Lower
Oolitic period.[185]
The great abundance of the limestone and coal of the Oolitic system
shows also the warm and equable condition of the climate which must
have then prevailed.
CRETACEOUS PERIOD.
_Croydon._—A large block of crystalline rock resembling granite was
found imbedded in a pit, on the side of the old London and Brighton
road near Purley, about two miles south of Croydon. Mr. Godwin-Austen
has shown conclusively that it must have been transported there by
means of floating ice. This boulder was associated with loose sea-sand,
coarse shingle, and a smaller boulder weighing twenty or twenty-five
pounds, and all water-worn. These had all sunk together without
separating. Hence they must have been firmly held together, both during
the time that they were being floated away, and also whilst sinking to
the bottom of the cretaceous sea. Mr. Godwin-Austen supposes the whole
to have been carried away frozen to the bottom of a mass of ground-ice.
When the ice from melting became unable to float the mass attached to
it, the whole would then sink to the bottom together.[186]
_Dover._—While the workmen were employed in cutting the tunnel on
the London, Chatham, and Dover Railway, between Lydden Hill and
Shepherdswell, a few miles from Dover, they came upon a mass of coal
imbedded in chalk, at a depth of 180 feet. It was about 4 feet square,
and from 4 to 10 inches thick. The coal was friable and highly
bituminous. It resembled some of the Wealden or Jurassic coal, and
was unlike the true coal of the coal-measures. The specific gravity
of the coal precluded the supposition that it could have floated away
of itself into the cretaceous sea. “Considering its friability,” says
Mr. Godwin-Austen, “I do not think that the agency of a floating tree
could have been engaged in its transport; but, looking at its flat,
angular form, it seems to me that its history may agree with what I
have already suggested with reference to the boulder in the chalk
at Croydon. We may suppose that during the Cretaceous period some
bituminous beds of the preceding Oolitic period lay so as to be covered
with water near the sea-margin, or along some river-bank, and from
which portions could be carried off by ice, and so drifted away, until
the ice was no longer able to support its load.”[187]
Mr. Godwin-Austen then mentions a number of other cases of blocks
being found in the chalk. In regard to those cases he appropriately
remarks that, as the cases where the occurrence of such blocks has
been observed are likely to be far less numerous than those which have
escaped observation, or failed to have been recorded, and as the chalk
exposed in pits and quarries bears only a most trifling proportion to
the whole horizontal extent of the formation, we have no grounds to
conclude that the above are exceptional cases.
Boulders have also been found in the cretaceous strata of the Alps by
Escher von der Linth.[188]
The existence of warm periods during the Cretaceous age is plainly
shown by the character of the flora and fauna of that age. The fact
that chalk is of organic origin implies that the climate must have
been warm and genial, and otherwise favourable to animal life. This is
further manifested by such plants as _Cycas_ and _Zamia_, which betoken
a warm climate, and by the corals and huge sauroid reptiles which then
inhabited our waters.
It is, in fact, the tropical character of the fauna of that period
which induced Sir Charles Lyell to reject Mr. Godwin-Austen’s idea that
the boulders found in the chalk had been transported by floating ice.
Such a supposition, implying a cold climate, “is,” Sir Charles says,
“inconsistent with the luxuriant growth of large chambered univalves,
numerous corals, and many fish, and other fossils of tropical forms.”
The recent discovery of the Cretaceous formation in Greenland shows
that during that period a mild and temperate condition of climate
must have prevailed in that continent up to high latitudes. “This
formation in Greenland,” says Dr. Robert Brown, “has only been recently
separated from the Miocene formation, with which it is associated and
was supposed to be a part of. It is, as far as we yet know, only found
in the vicinity of Kome or Koke, near the shores of Omenak Fjord, in
about 70° north latitude, though traces have been found elsewhere
on Disco, &c. The fossils hitherto brought to Europe have been very
few, and consist of plants which are now preserved in the Stockholm
and Copenhagen Museums. From these there seems little doubt that the
age assigned to this limited deposit (so far as we yet know) by the
celebrated palæontologist, Professor Oswald Heer, of Zurich, is the
correct one.”[189] Dr. Brown gives a list of the Cretaceous flora found
in Greenland.
EOCENE PERIOD.
_Switzerland._—In a coarse conglomerate belonging to the “_flysch_”
of Switzerland, an Eocene formation, there are found certain immense
blocks, some of which consist of a variety of granite which is not
known to occur _in situ_ in any part of the Alps. Some of the blocks
are 10 feet and upwards in length, and one at Halekeren, at the Lake of
Thun, is 105 feet in length, 90 feet in breadth, and 45 feet in height.
Similar blocks are found in the Apennines. These unmistakably indicate
the presence of glaciers or floating ice. This conclusion is further
borne out by the fact that the “_flysch_” is destitute of organic
remains. But the hypothesis that these huge masses were transported
to their present sites by glaciers or floating ice has been always
objected to, says Sir Charles Lyell, “on the ground that the Eocene
strata of Nummulitic age in Switzerland, as well as in other parts of
Europe, contain genera of fossil plants and animals characteristic of a
warm climate. And it has been particularly remarked,” he continues, “by
M. Desor that the strata most nearly associated with the ‘_flysch_’ in
the Alps are rich in echinoderms of the _Spatangus_ family which have a
decided tropical aspect.”[190]
But according to the theory of Secular Changes of Climate, the very
fact that the “_flysch_” is immediately associated with beds indicating
a warm or even tropical condition of climate, is one of the strongest
proofs which could be adduced in favour of its glacial character, for
the more severe a cold period of a glacial epoch is, the warmer will be
the periods which immediately precede and succeed. These crocodiles,
tortoises, and tropical flora probably belong to a warm Eocene
inter-glacial period.
MIOCENE PERIOD.
_Italy._—We have strong evidence in favour of the opinion that a
glacial epoch existed during the Miocene period. It has been shown
by M. Gastaldi, that during that age Alpine glaciers extended to the
sea-level.
Near Turin there is a series of hills, rising about 500 or 600 feet
above the valleys, composed of beds of Miocene sandstone, marl, and
gravel, and loose conglomerate. These beds have been carefully examined
and described by M. Gastaldi.[191] The hill of the Luperga has been
particularly noticed by him. Many of the stones in these beds are
striated in a manner similar to those found in the true till or boulder
clay of this country. But what is most remarkable is the fact that
large erratic blocks of limestone, many of them from 10 to 15 feet in
diameter, are found in abundance in these beds. It has been shown by
Gastaldi that these blocks have all been derived from the outer ridge
of the Alps on the Italian side, namely, from the range extending from
Ivrea to the Lago Maggiore, and consequently they must have travelled
from twenty to eighty miles. So abundant are these large blocks, that
extensive quarries have been opened in the hills for the sake of
procuring them. These facts prove not only the existence of glaciers
on the Alps during the Miocene period, but of glaciers extending to
the sea and breaking up into icebergs; the stratification of the beds
amongst which the blocks occur sufficiently indicating aqueous action
and the former presence of the sea.
That the glaciers of the Southern Alps actually reached to the sea,
and sent their icebergs adrift over what are now the sunny plains of
Northern Italy, is sufficient proof that during the cold period of
Miocene times the climate must have been very severe. Indeed, it may
well have been as severe as, if not even more excessive than, the
intensest severity of climate experienced during the last great glacial
epoch.
_Greenland._—Of the existence of warm conditions during Miocene times,
geology affords us abundant evidence. I shall quote the opinion of Sir
Charles Lyell on this point:—
“We know,” says Sir Charles, “that Greenland was not always covered
with snow and ice; for when we examine the tertiary strata of Disco
Island (of the Upper Miocene period), we discover there a multitude
of fossil plants which demonstrate that, like many other parts of the
arctic regions, it formerly enjoyed a mild and genial climate. Among
the fossils brought from that island, lat. 70° N., Professor Heer has
recognised _Sequoia Landsdorfii_, a coniferous species which flourished
throughout a great part of Europe in the Miocene period. The same
plant has been found fossil by Sir John Richardson within the Arctic
Circle, far to the west on the Mackenzie River, near the entrance of
Bear River; also by some Danish naturalists in Iceland, to the east.
The Icelandic surturband or lignite, of this age, has also yielded a
rich harvest of plants, more than thirty-one of them, according to
Steenstrup and Heer, in a good state of preservation, and no less than
fifteen specifically identical with Miocene plants of Europe. Thirteen
of the number are arborescent; and amongst others is a tulip-tree
(_Liriodendron_), with its fruit and characteristic leaves, a plane
(_Platanus_), a walnut, and a vine, affording unmistakable evidence
of a climate in the parallel of the Arctic Circle which precludes the
supposition of glaciers then existing in the neighbourhood, still less
any general crust of continental ice like that of Greenland.”[192]
At a meeting of the British Association, held at Nottingham in August
1866, Professor Heer read a valuable paper on the “Miocene Flora of
North Greenland.” In this paper some remarkable conclusions as to the
probable temperature of Greenland during the Miocene period were given.
Upwards of sixty different species brought from Atanekerdluk, a place
on the Waigat opposite Disco, in lat. 70° N., have been examined by him.
A steep hill rises on the coast to a height of 1,080 feet, and at
this level the fossil plants are found. Large quantities of wood in
a fossilized or carbonized condition lie about. Captain Inglefield
observed one trunk thicker than a man’s body standing upright. The
leaves, however, are the most important portion of the deposit. The
rock in which they are found is a sparry iron ore, which turns reddish
brown on exposure to the weather. In this rock the leaves are found, in
places packed closely together, and many of them are in a very perfect
condition. They give us a most valuable insight into the nature of the
vegetation which formed this primeval forest.
He arrives at the following conclusions:—
1. _The fossilized plants of Atanekerdluk cannot have been drifted from
any great distance. They must have grown on the spot where they were
found._
This is shown—
(_a_) By the fact that Captain Inglefield and Dr. Ruik observed trunks
of trees standing upright.
(_b_) By the great abundance of the leaves, and the perfect state of
preservation in which they are found.
(_c_) By the fact that we find in the stone both fruits and seeds of
the trees whose leaves are also found there.
(_d_) By the occurrence of insect remains along with the leaves.
2. _The flora of Atanekerdluk is Miocene._
3. _The flora is rich in species._
4. _The flora proves without a doubt that North Greenland, in the
Miocene epoch, had a climate much warmer than its present one. The
difference must be at least_ 29° F.
Professor Heer discusses at considerable length this proposition. He
says that the evidence from Greenland gives a final answer to those
who objected to the conclusions as to the Miocene climate of Europe
drawn by him on a former occasion. It is quite impossible that the
trees found at Atanekerdluk could ever have flourished there if
the temperature were not far higher than it is at present. This is
clear from many of the species, of which we find the nearest living
representative 10° or even 20° of latitude to the south of the locality
in question.
The trees of Atanekerdluk were not, he says, all at the extreme
northern limit of their range, for in the Miocene flora of Spitzbergen,
lat. 78° N., we find the beech, plane, hazelnut, and some other species
identical with those from Greenland, and we may conclude, he thinks,
that the firs and poplars which we meet at Atanekerdluk and Bell Sound,
Spitzbergen, must have reached up to the North Pole if land existed
there in the tertiary period.
“The hills of fossilized wood,” he adds, “found by McClure and his
companions in Banks’s Land (lat. 74° 27′ N.), are therefore discoveries
which should not astonish us, they only confirm the evidence as to the
original vegetation of the polar regions which we have derived from
other sources.”
The _Sequoia landsdorfii_ is the most abundant of the trees of
Atanekerdluk. The _Sequoia sempervirens_ is its present representative.
This tree has its extreme northern limit about lat. 53° N. For its
existence it requires a summer temperature of 59° or 61° F. Its fruit
requires a temperature of 64° for ripening. The winter temperature must
not fall below 34°, and that of the whole year must be at least 49°.
The temperature of Atanekerdluk during the time that the Miocene flora
grew could not have been under the above.[193]
Professor Heer concludes his paper as follows:—
“I think these facts are convincing, and the more so that they are not
insulated, but confirmed by the evidence derivable from the Miocene
flora of Iceland, Spitzbergen, and Northern Canada. These conclusions,
too, are only links in the grand chain of evidence obtained from the
examination of the Miocene flora of the whole of Europe. They prove to
us that we could not by any re-arrangement of the relative positions
of land and water produce for the northern hemisphere a climate which
would explain the phenomena in a satisfactory manner. We must only
admit that we are face to face with a problem, whose solution in all
probability must be attempted, and, we doubt not, completed by the
astronomer.”
CHAPTER XIX.
GEOLOGICAL TIME.—PROBABLE DATE OF THE GLACIAL EPOCH.
Geological Time measurable from Astronomical Data.—M. Leverrier’s
Formulæ.—Tables of Eccentricity for 3,000,000 Years in the
Past and 1,000,000 Years in the Future.—How the Tables have
been computed.—Why the Glacial Epoch is more recent than had
been supposed.—Figures convey a very inadequate Conception
of immense Duration.—Mode of representing a Million of
Years.—Probable Date of the Glacial Epoch.
If those great Secular variations of climate which we have been
considering be indirectly the result of changes in the eccentricity
of the earth’s orbit, then we have a means of determining, at least
so far as regards recent epochs, when these variations took place.
If the glacial epoch be due to the causes assigned, we have a means
of ascertaining, with tolerable accuracy, not merely the date of its
commencement, but the length of its duration. M. Leverrier has not
only determined the superior limit of the eccentricity of the earth’s
orbit, but has also given formulæ by means of which the extent of the
eccentricity for any period, past or future, may be computed.
A well-known astronomer and mathematician, who has specially
investigated the subject, is of opinion that these formulæ give results
which may be depended upon as approximately correct for _four millions
of years_ past and future. An eminent physicist has, however, expressed
to me his doubts as to whether the results can be depended on for a
period so enormous. M. Leverrier in his Memoir has given a table of the
eccentricity for 100,000 years before and after 1800 A.D., computed
for intervals of 10,000 years. This table, no doubt, embraces a period
sufficiently great for ordinary astronomical purposes, but it is by far
too limited to afford information in regard to geological epochs.
With the view of ascertaining the probable date of the glacial epoch,
as well as the character of the climate for a long course of ages,
Table I. was computed from M. Leverrier’s formulæ.[194] It shows the
eccentricity of the earth’s orbit and longitude of the perihelion for
3,000,000 of years back, and 1,000,000 of years to come, at periods
50,000 years apart.
On looking over the table it will be seen that there are three
principal periods when the eccentricity rose to a very high value,
with a few subordinate maxima between. It will be perceived also that
during each of those periods the eccentricity does not remain at the
same uniform value, but rises and falls, in one case twice, and in the
other two cases three times. About 2,650,000 years back we have the
eccentricity almost at its inferior limit. It then begins to increase,
and fifty thousand years afterwards, namely at 2,600,000 years ago, it
reaches ·0660; fifty thousand years after this period it has diminished
to ·0167, which is about its present value. It then begins to increase,
and in another fifty thousand years, namely at 2,500,000 years ago, it
approaches to almost the superior limit, its value being then ·0721. It
then begins to diminish, and at 2,450,000 years ago it has diminished
to ·0252. These two maxima, separated by a minimum and extending over a
period of 200,000 years, constitute the first great period of high
eccentricity. We then pass onwards for upwards of a million and a half
years, and we come to the second great period. It consists of three
maxima separated by two minima. The first maximum occurred at 950,000
years ago, the second or middle one at 850,000 years ago, and the
third and last at 750,000 years ago—the whole extending over a period
of nearly 300,000 years. Passing onwards for another million and half
years, or to about 800,000 years in the future, we come to the third
great period. It also consists of three maxima one hundred thousand
years apart. Those occur at the periods 800,000, 900,000, and 1,000,000
years to come, respectively, separated also by two minima. Those three
great periods, two of them in the past and one of them in the future,
included in the Table, are therefore separated from each other by an
interval of upwards of 1,700,000 years.
[Illustration: PLATE IV
W. & A. K. Johnston, Edinb^r. and London.
DIAGRAM REPRESENTING THE VARIATIONS IN THE ECCENTRICITY OF THE
EARTH’S ORBIT FOR THREE MILLION OF YEARS BEFORE 1800 A.D. ONE
MILLION OF YEARS AFTER IT.
_The Ordinates are joined by straight lines where the values, at
intervals of 10,000 years, between them have not been determined._]
In this Table there are seven periods when the earth’s orbit becomes
nearly circular, four in the past and three in the future.
The Table shows also four or five subordinate periods of high
eccentricity, the principal one occurring 200,000 years ago.
The variations of eccentricity during the four millions of years, are
represented to the eye diagrammatically in Plate IV.
In order to determine with more accuracy the condition of the earth’s
orbit during the three periods of great eccentricity included in Table
I., I computed the values for periods of ten thousand years apart, and
the results are embodied in Tables II., III., and IV.
There are still eminent astronomers and physicists who are of opinion
that the climate of the globe never could have been seriously affected
by changes in the eccentricity of its orbit. This opinion results, no
doubt, from viewing the question as a purely astronomical one. Viewed
from an astronomical standpoint, as has been already remarked, there
is actually nothing from which any one could reasonably conclude with
certainty whether a change of eccentricity would seriously affect
climate or not. By means of astronomy we ascertain the extent of the
eccentricity at any given period, how much the winter may exceed
the summer in length (or the reverse), how much the sun’s heat is
increased or decreased by a decrease or an increase of distance,
and so forth; but we obtain no information whatever regarding how
these will actually affect climate. This, as we have already seen,
must be determined wholly from physical considerations, and it is
an exceedingly complicated problem. An astronomer, unless he has
given special attention to the physics of the question, is just as
apt to come to a wrong conclusion as any one else. The question
involves certain astronomical elements; but when these are determined
everything then connected with the matter is purely physical. Nearly
all the astronomical elements of the question are comprehended in the
accompanying Tables.
TABLE I.
THE ECCENTRICITY AND LONGITUDE OF THE PERIHELION OF THE EARTH’S
ORBIT FOR 3,000,000 YEARS IN THE PAST AND 1,000,000 YEARS IN
THE FUTURE, COMPUTED FOR INTERVALS OF 50,000 YEARS.
+---------------------------------------------+
| PAST TIME. |
+------------------+-------------+------------+
| Number of years |Eccentricity.|Longitude of|
|before epoch 1800.| |perihelion. |
+------------------+-------------+------------+
| | | ° ′ |
| −3,000,000 | 0·0365 | 39 30 |
| −2,950,000 | 0·0170 | 210 39 |
| −2,900,000 | 0·0442 | 200 52 |
| −2,850,000 | 0·0416 | 0 18 |
| −2,800,000 | 0·0352 | 339 14 |
| −2,750,000 | 0·0326 | 161 22 |
| −2,700,000 | 0·0330 | 65 37 |
| −2,650,000 | 0·0053 | 318 40 |
| −2,600,000 | 0·0660 | 190 4 |
| −2,550,000 | 0·0167 | 298 34 |
| −2,500,000 | 0·0721 | 338 36 |
| −2,450,000 | 0·0252 | 109 33 |
| −2,400,000 | 0·0415 | 116 40 |
| −2,350,000 | 0·0281 | 308 23 |
| −2,300,000 | 0·0238 | 195 25 |
| −2,250,000 | 0·0328 | 141 18 |
| −2,200,000 | 0·0352 | 307 6 |
| −2,150,000 | 0·0183 | 307 5 |
| −2,100,000 | 0·0304 | 98 40 |
| −2,050,000 | 0·0170 | 334 46 |
| −2,000,000 | 0·0138 | 324 4 |
| −1,950,000 | 0·0427 | 120 32 |
| −1,900,000 | 0·0336 | 188 31 |
| −1,850,000 | 0·0503 | 272 14 |
| −1,800,000 | 0·0334 | 354 52 |
| −1,750,000 | 0·0350 | 65 25 |
| −1,700,000 | 0·0085 | 95 13 |
| −1,650,000 | 0·0035 | 168 23 |
| −1,600,000 | 0·0305 | 158 42 |
| −1,550,000 | 0·0239 | 225 57 |
| −1,500,000 | 0·0430 | 303 29 |
| −1,450,000 | 0·0195 | 57 11 |
| −1,400,000 | 0·0315 | 97 35 |
| −1,350,000 | 0·0322 | 293 38 |
| −1,300,000 | 0·0022 | 0 48 |
| −1,250,000 | 0·0475 | 105 50 |
| −1,200,000 | 0·0289 | 239 34 |
| −1,150,000 | 0·0473 | 250 27 |
| −1,100,000 | 0·0311 | 55 24 |
| −1,050,000 | 0·0326 | 4 8 |
| −1,000,000 | 0·0151 | 248 22 |
| −950,000 | 0·0517 | 97 51 |
| −900,000 | 0·0102 | 135 2 |
| −850,000 | 0·0747 | 239 28 |
| −800,000 | 0·0132 | 343 49 |
| −750,000 | 0·0575 | 27 18 |
| −700,000 | 0·0220 | 208 13 |
| −650,000 | 0·0226 | 141 29 |
| −600,000 | 0·0417 | 32 34 |
| −550,000 | 0·0166 | 251 50 |
| −500,000 | 0·0388 | 193 56 |
| −450,000 | 0·0308 | 356 52 |
| −400,000 | 0·0170 | 290 7 |
| −350,000 | 0·0195 | 182 50 |
| −300,000 | 0·0424 | 23 29 |
| −250,000 | 0·0258 | 59 39 |
| −200,000 | 0·0569 | 168 18 |
| −150,000 | 0·0332 | 242 56 |
| −100,000 | 0·0473 | 316 18 |
| −50,000 | 0·0131 | 50 14 |
+------------------+-------------+------------+
+---------------------------------------------+
| FUTURE TIME. |
+------------------+-------------+------------+
| Number of years |Eccentricity.|Longitude of|
|before epoch 1800.| |perihelion. |
+------------------+-------------+------------+
| | | ° ′ |
| A.D. 1800 | 0·0168 | 99 30 |
| +50,000 | 0·0173 | 38 12 |
| +100,000 | 0·0191 | 114 50 |
| +150,000 | 0·0353 | 201 57 |
| +200,000 | 0·0246 | 279 41 |
| +250,000 | 0·0286 | 350 54 |
| +300,000 | 0·0158 | 172 29 |
| +350,000 | 0·0098 | 201 40 |
| +400,000 | 0·0429 | 6 9 |
| +450,000 | 0·0231 | 98 37 |
| +500,000 | 0·0534 | 157 26 |
| +550,000 | 0·0259 | 287 31 |
| +600,000 | 0·0395 | 285 43 |
| +650,000 | 0·0169 | 144 3 |
| +700,000 | 0·0357 | 17 12 |
| +750,000 | 0·0195 | 0 53 |
| +800,000 | 0·0639 | 140 38 |
| +850,000 | 0·0144 | 176 41 |
| +900,000 | 0·0659 | 291 16 |
| +950,000 | 0·0086 | 115 13 |
| +1,000,000 | 0·0528 | 57 31 |
+------------------+-------------+------------+
TABLE II.
ECCENTRICITY, LONGITUDE OF THE PERIHELION, &C., &C., FOR INTERVALS OF
10,000 YEARS, FROM 2,650,000 TO 2,450,000 YEARS AGO.
THE GLACIAL EPOCH OF THE _Eocene period_ IS PROBABLY COMPREHENDED
WITHIN THIS TABLE.
+---------+------------+-----------+-----------+-------------------------------------------+
| I. | II. | III. | IV. | Winter occurring in aphelion. |
| | | | +---------+---------+-----------+-----------+
| | | | | V. | VI. | VII. | VIII. |
|Number of|Eccentricity|Longitude | Number of |Excess of|Midwinter|Number of | Midwinter |
| years | of orbit. | of | degrees | winter |intensity|degrees by |temperature|
| before | |perihelion.| passed | over | of the | which the | of Great |
| A.D. | | |over by the| summer, | sun’s | midwinter | Britain. |
| 1800. | | |perihelion.|in days. | heat. |temperature| |
| | | | Motion | | Present |is lowered | |
| | | |retrograde | |intensity| | |
| | | |at periods | | =1000. | | |
| | | | marked R. | | | | |
+---------+------------+-----------+-----------+---------+---------+-----------+-----------+
| | | ° | | | | | |
|2,650,000| 0·0053 | 318 40 | ° ′ | | | F. | F. |
|2,640,000| 0·0173 | 54 25 | 95 45 | | | ° | ° |
|2,630,000| 0·0331 | 93 37 | 39 12 | 15·4 | 906 | 26·2 | 12·8 |
|2,620,000| 0·0479 | 127 12 | 33 35 | 22·2 | 884 | 33·3 | 5·7 |
|2,610,000| 0·0591 | 158 36 | 31 24 | 27·4 | 862 | 38·3 | 0·7 |
|2,600,000| 0·0660 | 190 4 | 31 28 | 30·6 | 851 | 41·5 | −2·5 |
|2,590,000| 0·0666 | 220 28 | 30 24 | 30·9 | 850 | 41·8 | −2·8 |
|2,580,000| 0·0609 | 249 56 | 29 28 | 28·3 | 859 | 39·2 | −0·2 |
+---------+------------+-----------+-----------+---------+---------+-----------+-----------+
|2,570,000| 0·0492 | 277 24 | 27 28 | 22·9 | 878 | 33·9 | 5·1 |
|2,560,000| 0·0350 | 305 2 | 27 38 | 16·2 | 902 | 27·1 | 11·9 |
|2,550,000| 0·0167 | 298 34 | R 6 28 | | | | |
|2,540,000| 0·0192 | 253 58 | R 44 36 | | | | |
|2,530,000| 0·0369 | 259 19 | 5 21 | 17·1 | 899 | 28·0 | 11·0 |
|2,520,000| 0·0537 | 283 7 | 23 48 | 25·0 | 871 | 35·9 | 3·1 |
|2,510,000| 0·0660 | 310 4 | 26 57 | 30·6 | 851 | 41·5 | −2·5 |
|2,500,000| 0·0721 | 338 36 | 28 32 | 33·5 | 841 | 44·2 | −5·2 |
|2,490,000| 0·0722 | 7 36 | 29 0 | 33·6 | 841 | 44·3 | −5·3 |
|2,480,000| 0·0662 | 35 46 | 28 10 | 30·8 | 850 | 41·7 | −2·7 |
|2,470,000| 0·0553 | 63 26 | 27 40 | 25·7 | 868 | 36·6 | 2·4 |
|2,460,000| 0·0410 | 89 13 | 25 47 | 19·1 | 892 | 30·0 | 9·0 |
|2,450,000| 0·0252 | 109 33 | 20 20 | 11·7 | | | |
+---------+------------+-----------+-----------+---------+---------+-----------+-----------+
TABLE III.
ECCENTRICITY, LONGITUDE OF THE PERIHELION, &C., &C., FOR INTERVALS OF
10,000 YEARS, FROM 1,000,000 TO 750,000 YEARS AGO.
THE GLACIAL EPOCH OF THE _Miocene period_ IS PROBABLY COMPREHENDED
WITHIN THIS TABLE.
+---------+------------+-----------+-----------+-------------------------------------------+
| I. | II. | III. | IV. | Winter occurring in aphelion. |
| | | | +---------+---------+-----------+-----------+
| | | | | V. | VI. | VII. | VIII. |
|Number of|Eccentricity|Longitude | Number of |Excess of|Midwinter|Number of | Midwinter |
| years | of orbit. | of | degrees | winter |intensity|degrees by |temperature|
| before | |perihelion.| passed | over | of the | which the | of Great |
| A.D. | | |over by the| summer, | sun’s | midwinter | Britain. |
| 1800. | | |perihelion.|in days. | heat. |temperature| |
| | | | Motion | | Present |is lowered | |
| | | |retrograde | |intensity| | |
| | | |at periods | | =1000. | | |
| | | | marked R. | | | | |
+---------+------------+-----------+-----------+---------+---------+-----------+-----------+
| | | ° ′ | | | | | |
|1,000,000| 0·0151 | 248 22 | ° ′ | | | F. | F. |
| 990,000| 0·0224 | 313 50 | 65 28 | | | ° | ° |
| 980,000| 0·0329 | 358 2 | 44 12 | 15·3 | 906 | 26·1 | 12·9 |
| 970,000| 0·0441 | 32 40 | 34 38 | 20·5 | 887 | 31·5 | 7·5 |
| 960,000| 0·0491 | 66 49 | 34 9 | 22·8 | 878 | 33·8 | 5·2 |
| 950,000| 0·0517 | 97 51 | 31 2 | 24·0 | 874 | 35·0 | 4·0 |
| 940,000| 0·0495 | 127 42 | 29 51 | 23·0 | 878 | 34·0 | 5·0 |
| 930,000| 0·0423 | 156 11 | 28 29 | 19·7 | 890 | 30·6 | 8·4 |
| 920,000| 0·0305 | 181 40 | 25 29 | 14·2 | 910 | 25·0 | 14·0 |
| 910,000| 0·0156 | 194 15 | 12 35 | | | | |
| 900,000| 0·0102 | 135 2 | R 59 13 | | | | |
| 890,000| 0·0285 | 127 1 | R 8 1 | | | | |
| 880,000| 0·0456 | 152 33 | 25 32 | 21·2 | 884 | 32·2 | 6·8 |
| 870,000| 0·0607 | 180 23 | 27 50 | 28·2 | 859 | 39·0 | 0·0 |
| 860,000| 0·0708 | 209 41 | 29 18 | 32·9 | 843 | 43·6 | −4·6 |
| 850,000| 0·0747 | 239 28 | 29 47 | 34·7 | 837 | 45·3 | −6·3 |
| 840,000| 0·0698 | 269 14 | 29 46 | 32·4 | 845 | 43·2 | −4·2 |
| 830,000| 0·0623 | 298 28 | 29 14 | 29·0 | 857 | 40·0 | −1·0 |
| 820,000| 0·0476 | 326 4 | 27 36 | 22·1 | 881 | 33·1 | 5·9 |
| 810,000| 0·0296 | 348 30 | 22 26 | | | | |
| 800,000| 0·0132 | 343 49 | R 4 41 | | | | |
| 790,000| 0·0171 | 293 19 | R 50 30 | | | | |
| 780,000| 0·0325 | 303 37 | 10 18 | 15·2 | 907 | 26·0 | 13·0 |
| 770,000| 0·0455 | 328 38 | 25 1 | 21·2 | 884 | 32·2 | 6·8 |
| 760,000| 0·0540 | 357 12 | 28 34 | 25·1 | 870 | 36·0 | 3·0 |
| 750,000| 0·0575 | 27 18 | 30 6 | 26·7 | 864 | 37·7 | 1·3 |
| 740,000| 0·0561 | 58 30 | 31 12 | 26·1 | 867 | 37·0 | 2·0 |
| 730,000| 0·0507 | 90 55 | 32 25 | 23·6 | 876 | 34·6 | 4·4 |
| 720,000| 0·0422 | 125 14 | 34 19 | 19·6 | 890 | 30·6 | 8·4 |
| 710,000| 0·0307 | 177 26 | 52 12 | 14·3 | 910 | 25·0 | 14·0 |
| 700,000| 0·0220 | 208 13 | 30 47 | | | | |
+---------+------------+-----------+-----------+---------+---------+-----------+-----------+
TABLE IV.
ECCENTRICITY, LONGITUDE OF THE PERIHELION, &C., &C., FOR INTERVALS OF
10,000 YEARS, FROM 250,000 YEARS AGO TO THE PRESENT DATE.
THE _Glacial epoch_ IS PROBABLY COMPREHENDED WITHIN THIS TABLE.
+---------+------------+-----------+-----------+-------------------------------------------+
| I. | II. | III. | IV. | Winter occurring in aphelion. |
| | | | +---------+---------+-----------+-----------+
| | | | | V. | VI. | VII. | VIII. |
|Number of|Eccentricity|Longitude | Number of |Excess of|Midwinter|Number of | Midwinter |
| years | of orbit. | of | degrees | winter |intensity|degrees by |temperature|
| before | |perihelion.| passed | over | of the | which the | of Great |
| A.D. | | |over by the| summer, | sun’s | midwinter | Britain. |
| 1800. | | |perihelion.|in days. | heat. |temperature| |
| | | | Motion | | Present |is lowered | |
| | | |retrograde | |intensity| | |
| | | |at periods | | =1000. | | |
| | | | marked R. | | | | |
+---------+------------+-----------+-----------+---------+---------+-----------+-----------+
| | | ° ′ | | | | F. | F. |
| 250,000| 0·0258 | 59 39 | ° ′ | | | ° | ° |
| 240,000| 0·0374 | 74 58 | 15 19 | 17·4 | 898 | 28·3 | 10·7 |
|S 230,000| 0·0477 | 102 49 | 27 51 | 22·2 | 885 | 33·2 | 5·8 |
|S 220,000| 0·0497 | 124 33 | 21 44 | 23·2 | 877 | 34·1 | 4·9 |
|S 210,000| 0·0575 | 144 55 | 20 22 | 26·7 | 864 | 37·7 | 1·3 |
| 200,000| 0·0569 | 168 18 | 23 23 | 26·5 | 865 | 37·4 | 1·6 |
|S 190,000| 0·0532 | 190 4 | 21 46 | 24·7 | 871 | 35·7 | 3·3 |
|S 180,000| 0·0476 | 209 22 | 19 18 | 22·1 | 881 | 33·1 | 5·9 |
|S 170,000| 0·0437 | 228 7 | 18 45 | 20·3 | 887 | 31·3 | 7·7 |
| 160,000| 0·0364 | 236 38 | 8 31 | 16·9 | 900 | 27·8 | 11·2 |
| 150,000| 0·0332 | 242 56 | 6 18 | 15·4 | 905 | 26·2 | 12·8 |
| 140,000| 0·0346 | 246 29 | 3 33 | 16·1 | 903 | 26·9 | 12·1 |
| 130,000| 0·0384 | 259 34 | 13 5 | 17·8 | 896 | 28·8 | 10·2 |
| 120,000| 0·0431 | 274 47 | 15 13 | 20·1 | 888 | 31·0 | 8·0 |
| 110,000| 0·0460 | 293 48 | 19 1 | 21·4 | 883 | 32·4 | 6·6 |
| 100,000| 0·0473 | 316 18 | 22 30 | 22·0 | 881 | 33·0 | 6·0 |
|L 90,000| 0·0452 | 340 2 | 23 44 | 21·0 | 885 | 32·0 | 7·0 |
|L 80,000| 0·0398 | 4 13 | 24 11 | 18·5 | 894 | 29·4 | 9·6 |
|L 70,000| 0·0316 | 27 22 | 23 9 | 14·7 | 908 | 25·5 | 13·5 |
|L 60,000| 0·0218 | 46 8 | 18 46 | | | | |
| 50,000| 0·0131 | 50 14 | 4 6 | | | | |
|L 40,000| 0·0109 | 28 36 | R 21 38 | | | | |
|L 30,000| 0·0151 | 5 50 | R 22 46 | | | | |
|L 20,000| 0·0188 | 44 0 | 38 10 | | | | |
|L 10,000| 0·0187 | 78 28 | 34 28 | | | | |
|A.D. 1800| 0·0168 | 99 30 | 21 2 | | | | |
+---------+------------+-----------+-----------+---------+---------+-----------+-----------+
In Tables II., III., and IV., column I. represents the dates of the
periods, column II. the eccentricity, column III. the longitude of
the perihelion. In Table IV. the eccentricity and the longitude of
the perihelion of the six periods marked with an S are copied from a
letter of Mr. Stone to Sir Charles Lyell, published in the Supplement
of the Phil. Mag. for June, 1865; the eight periods marked L are copied
from M. Leverrier’s Table, to which reference has been made. For the
correctness of everything else, both in this Table and in the other
three, I alone am responsible.
Column IV. gives the number of degrees passed over by the perihelion
during each 10,000 years. From this column it will be seen how
irregular is the motion of the perihelion. At four different periods
it had a retrograde motion for 20,000 years. Column V. shows the
number of days by which the winter exceeds the summer when the winter
occurs in aphelion. Column VI. shows the intensity of the sun’s heat
during midwinter, when the winter occurs in aphelion, the present
midwinter intensity being taken at 1,000. These six columns comprehend
all the astronomical part of the Tables. Regarding the correctness of
the principles upon which these columns are constructed, there is no
diversity of opinion. But these columns afford no direct information
as to the character of the climate, or how much the temperature is
increased or diminished. To find this we pass on to columns VII. and
VIII., calculated on physical principles. Now, unless the physical
principles upon which these three columns are calculated be wholly
erroneous, change of eccentricity must undoubtedly very seriously
affect climate. Column VII. shows how many degrees Fahrenheit the
temperature is lowered by a decrease in the intensity of the sun’s heat
corresponding to column VI. For example, 850,000 years ago, if the
winters occurred then in aphelion, the direct heat of the sun during
midwinter would be only 837/1000 of what it is at present at the same
season of the year, and column VII. shows that this decrease in the
intensity of the sun’s heat would lower the temperature 45°·3 F.
The principle upon which this result is arrived at is this:--The
temperature of space, as determined by Sir John Herschel, is −239°
F. M. Pouillet, by a different method, arrived at almost the same
result. If we take the midwinter temperature of Great Britain at
39°, then 239° + 39° = 278° will represent the number of degrees of
rise due to the sun’s heat at midwinter; in other words, it takes a
quantity of sun-heat which we have represented by 1000 to maintain the
temperature of the earth’s surface in Great Britain 278° above the
temperature of space. Were the sun extinguished, the temperature of
our island would sink 278° below its present midwinter temperature,
or to the temperature of space. But 850,000 years ago, as will be
seen from Table III., if the winters occurred in aphelion, the heat
of the sun at midwinter would only equal 837 instead of 1000 as at
present. Consequently, if it takes 1,000 parts of heat to maintain the
temperature 278° above the temperature of space, 837 parts of heat will
only be able to maintain the temperature 232°·7 above the temperature
of space; for 232°·7 is to 278 as 837 is to 1,000. Therefore, if the
temperature was then only 232°·7 above that of space, it would be
45°·3 below what it is at present. This is what the temperature would
be on the supposition, of course, that it depended wholly on the
sun’s intensity and was not modified by other causes. This method has
already been discussed at some length in Chapter II. But whether these
values be too high or too low, one thing is certain, that a very slight
increase or a very slight decrease in the quantity of heat received
from the sun must affect temperature to a considerable extent. The
direct heat of the moon, for example, cannot be detected by the finest
instruments which we possess; yet from 238,000 observations made at
Prague during 1840−66, it would seem that the temperature is sensibly
affected by the mere change in the lunar perigee and inclination of the
moon’s orbit.[195]
Column VIII. gives the midwinter temperature. It is found by
subtracting the numbers in column VII. from 39°, the present midwinter
temperature.
I have not given a Table showing the temperature of the summers at
the corresponding periods. This could not well be done; for there is
no relation at the periods in question between the intensity of the
sun’s heat and the temperature of the summers. One is apt to suppose,
without due consideration, that the summers ought to be then as much
warmer than they are at present, as the winters were then colder than
now. Sir Charles Lyell, in his “Principles,” has given a column of
summer temperatures calculated from my table upon this principle.
Astronomically the principle is correct, but physically, as was shown
in Chapter IV., it is totally erroneous, and calculated to convey a
wrong impression regarding the whole subject of geological climate.
The summers at those periods, instead of being much warmer than they
are at present, would in reality be much colder, notwithstanding the
great increase in the intensity of the sun’s heat resulting from the
diminished distance of the sun.
What, then, is the date of the glacial epoch? It is perfectly obvious
that if the glacial epoch resulted from a high state of eccentricity,
it must be referred either to the period included in Table III. or
to the one in Table IV. In Table III. we have a period extending from
about 980,000 to about 720,000 years ago, and in Table IV. we have a
period beginning about 240,000 years ago, and extending down to about
80,000 years ago. As the former period was of greater duration than
the latter, and the eccentricity also attained to a higher value, I at
first felt disposed to refer the glacial epoch proper (the time of the
till and boulder clay) to the former period; and the latter period, I
was inclined to believe, must have corresponded to the time of local
glaciers towards the close of the glacial epoch, the evidence for which
(moraines) is to be found in almost every one of our Highland glens.
On this point I consulted several eminent geologists, and they all
agreed in referring the glacial epoch to the former period; the reason
assigned being that they considered the latter period to be much too
recent and of too short duration to represent that epoch.
Pondering over the subject during the early part of 1866, reasons soon
suggested themselves which convinced me that the glacial epoch must
be referred to the latter and not to the former period. Those reasons
I shall now proceed to state at some length, since they have a direct
bearing, as will be seen, on the whole question of geological time.
It is the modern and philosophic doctrine of uniformity that has
chiefly led geologists to over-estimate the length of geological
periods. This philosophic school teaches, and that truly, that the
great changes undergone by the earth’s crust must have been produced,
not by convulsions and cataclysms of nature, but by those ordinary
agencies that we see at work every day around us, such as rain, snow,
frost, ice, and chemical action, &c. It teaches that the valleys
were not produced by violent dislocations, nor the hills by sudden
upheavals, but that they were actually carved out of the solid rock
by the silent and gentle agency of chemical action, frost, rain, ice,
and running water. It teaches, in short, that the rocky face of our
globe has been carved into hill and dale, and ultimately worn down
to the sea-level, by means of these apparently trifling agents, not
only once or twice, but probably dozens of times over during past
ages. Now, when we reflect that with such extreme slowness do these
agents perform their work, that we might watch their operations from
year to year, and from century to century, if we could, without being
able to perceive that they make any very sensible advance, we are
necessitated to conclude that geological periods must be enormous. And
the conclusion at which we thus arrive is undoubtedly correct. It is,
in fact, impossible to form an adequate conception of the length of
geological time. It is something too vast to be fully grasped by our
minds. But here we come to the point where the fundamental mistake
arises; Geologists do not err in forming too great a conception of the
extent of geological periods, _but in the mode in which they represent
the length of these periods in numbers_. When we speak of units, tens,
hundreds, thousands, we can form some notion of what these quantities
represent; but when we come to millions, tens of millions, hundreds
of millions, thousands of millions, the mind is then totally unable
to follow, and we can only use these numbers as representations of
quantities that turn up in calculation. We know, from the way in which
they do turn up in our process of calculation, whether they are correct
representations of things in actual nature or not; but we could not,
from a mere comparison of these quantities with the thing represented
by them, say whether they were actually too small or too great.
At present, geological estimates of time are little else than mere
conjectures. Geological science has hitherto afforded no trustworthy
means of estimating the positive length of geological epochs.
Geological phenomena tell us most emphatically that these periods
must be long; but how long they have hitherto failed to inform us.
Geological phenomena represent time to the mind under a most striking
and imposing form. They present to the eye, as it were, a sensuous
representation of time; the mind thus becomes deeply impressed with
a sense of immense duration; and when one under these feelings is
called upon to put down in figures what he believes will represent that
duration, he is very apt to be deceived. If, for example, a million of
years as represented by geological phenomena and a million of years as
represented by figures were placed before our eyes, we should certainly
feel startled. We should probably feel that a unit with six ciphers
after it was really something far more formidable than we had hitherto
supposed it to be. Could we stand upon the edge of a gorge a mile and
a half in depth that had been cut out of the solid rock by a tiny
stream, scarcely visible at the bottom of this fearful abyss, and were
we informed that this little streamlet was able to wear off annually
only 1/10 of an inch from its rocky bed, what would our conceptions be
of the prodigious length of time that this stream must have taken to
excavate the gorge? We should certainly feel startled when, on making
the necessary calculations, we found that the stream had performed this
enormous amount of work in something less than a million of years.
If, for example, we could possibly form some adequate conception of a
period so prodigious as one hundred millions of years, we should not
then feel so dissatisfied with Sir W. Thomson’s estimate that the age
of the earth’s crust is not greater than that.
Here is one way of conveying to the mind some idea of what a million
of years really is. Take a narrow strip of paper an inch broad, or
more, and 83 feet 4 inches in length, and stretch it along the wall of
a large hall, or round the walls of an apartment somewhat over 20 feet
square. Recall to memory the days of your boyhood, so as to get some
adequate conception of what a period of a hundred years is. Then mark
off from one of the ends of the strip 1/10 of an inch. The 1/10 of the
inch will then represent one hundred years, and the entire length of
the strip a million of years. It is well worth making the experiment,
just in order to feel the striking impression that it produces on the
mind.
The latter period, which we have concluded to be that of the glacial
epoch, extended, as we have seen, over a period of 160,000 years. But
as the glaciation was only on one hemisphere at a time, 80,000 years
or so would represent the united length of the cold periods. In order
to satisfy ourselves that this period is sufficiently long to account
for all the amount of denudation effected during the glacial epoch,
let us make some rough estimate of the probable rate at which the
surface of the country would be ground down by the ice. Suppose the
ice to grind off only one-tenth of an inch annually this would give
upwards of 650 feet as the quantity of rock removed during the time.
But it is probable that it did not amount to one-fourth part of that
quantity. Whether one-tenth of an inch per annum be an over-estimate or
an under-estimate of the rate of denudation by the ice, it is perfectly
evident that the period in question is sufficiently long, so far as
denudation is concerned, to account for the phenomena of the glacial
epoch.
But admitting that the period under consideration is sufficiently
_long_ to account for all the denudation which took place _during_
the glacial epoch, we have yet to satisfy ourselves that it is also
sufficiently _remote_ to account for all the denudation which has taken
place _since_ the glacial epoch. Are the facts of geology consistent
with the idea that the close of the glacial epoch does not date back
beyond 80,000 years?
This question could be answered if we knew the present rate of
subaërial denudation, for the present rate evidently does not differ
greatly from that which has obtained since the close of the glacial
epoch.
CHAPTER XX.
GEOLOGICAL TIME.—METHOD OF MEASURING THE RATE OF SUBAËRIAL
DENUDATION.
Rate of Subaërial Denudation a Measure of Time.—Rate determined
from Sediment of the Mississippi.—Amount of Sediment carried
down by the Mississippi; by the Ganges.—Professor Geikie on
Modern Denudation.—Professor Geikie on the Amount of Sediment
conveyed by European Rivers.—Rate at which the Surface of
the Globe is being denuded.—Alfred Tylor on the Sediment
of the Mississippi.—The Law which determines the Rate of
Denudation.—The Globe becoming less oblate.—Carrying Power
of our River Systems the true Measure of Denudation.—Marine
Denudation trifling in comparison to Subaërial.—Previous
Methods of measuring Geological Time.—Circumstances which
show the recent Date of the Glacial Epoch.—Professor Ramsay
on Geological Time.
It is almost self-evident that the rate of subaërial denudation must
be equal to the rate at which the materials are carried off the land
into the sea, but the rate at which the materials are carried off the
land is measured by the rate at which sediment is carried down by our
river systems. _Consequently, in order to determine the present rate
of subaërial denudation, we have only to ascertain the quantity of
sediment annually carried down by the river systems._
Knowing the quantity of sediment transported by a river, say annually,
and the area of its drainage, we have the means of determining the
rate at which the surface of this area is being lowered by subaërial
denudation. And if we know this in reference to a few of the great
continental rivers draining immense areas in various latitudes, we
could then ascertain with tolerable correctness the rate at which the
surface of the globe is being lowered by subaërial denudation, and
also the length of time which our present continents can remain above
the sea-level. Explaining this to Professor Ramsay during the winter
of 1865, I learned from him that accurate measurements had been made
of the amount of sediment annually carried down by the Mississippi
River, full particulars of which investigations were to be found
in the Proceedings of the American Association for the Advancement
of Science for 1848. These proceedings contain a report by Messrs.
Brown and Dickeson, which unfortunately over-estimated the amount of
sediment transported by the Mississippi by nearly four times what
was afterwards found by Messrs. Humphreys and Abbot to be the actual
amount. From this estimate, I was led to the conclusion that if the
Mississippi is a fair representative of rivers in general, our existing
continents would not remain longer than one million and a half years
above the sea-level.[196] This was a conclusion so startling as to
excite suspicion that there must have been some mistake in reference
to Messrs. Brown and Dickeson’s data. It showed beyond doubt, however,
that the rate of subaërial denudation, when accurately determined by
this method, would be found to be enormously greater than had been
supposed. Shortly afterwards, on estimating the rate from the data
furnished by Humphreys and Abbot, I found the rate of denudation to
be about one foot in 6,000 years. Taking the mean elevation of all
the land as ascertained by Humboldt to be 1,000 feet, the whole would
therefore be carried down into the ocean by our river systems in about
6,000,000 of years if no elevation of the land took place.[197] The
following are the data and mode of computation by which this conclusion
was arrived at. It was found by Messrs. Humphreys and Abbot that the
average amount of sediment held in suspension in the waters of the
Mississippi is about 1/1500 of the weight of the water, or 1/2900
by bulk. The annual discharge of the river is 19,500,000,000,000
cubic feet of water. The quantity of sediment carried down into the
Gulf of Mexico amounts to 6,724,000,000 cubic feet. But besides
that which is held in suspension, the river pushes down into the
sea about 750,000,000 cubic feet of earthy matter, making in all a
total of 7,474,000,000 cubic feet transferred from the land to the
sea annually. Where does this enormous mass of material come from?
Unquestionably it comes from the ground drained by the Mississippi. The
area drained by the river is 1,244,000 square miles. Now 7,474,000,000
cubic feet removed off 1,224,000 square miles of surface is equal to
1/4566 of a foot off that surface per annum, or one foot in 4,566
years. The specific gravity of the sediment is taken at 1·9, that of
rock is about 2·5; consequently the amount removed is equal to one foot
of rock in about 6,000 years. The average height of the North American
continent above the sea-level, according to Humboldt, is 748 feet;
consequently, at the present rate of denudation, the whole area of
drainage will be brought down to the sea-level in less than 4,500,000
years, if no elevation of the land takes place.
Referring to the above, Sir Charles Lyell makes the following
appropriate remarks:—“There seems no danger of our overrating the
mean rate of waste by selecting the Mississippi as our example, for
that river drains a country equal to more than half the continent of
Europe, extends through twenty degrees of latitude, and therefore
through regions enjoying a great variety of climate, and some of its
tributaries descend from mountains of great height. The Mississippi
is also more likely to afford us a fair test of ordinary denudation,
because, unlike the St. Lawrence and its tributaries, there are no
great lakes in which the fluviatile sediment is thrown down and
arrested on its way to the sea.”[198]
The rate of denudation of the area drained by the river Ganges is much
greater than that of the Mississippi. The annual discharge of that
river is 6,523,000,000,000 cubic feet of water. The sediment held in
suspension is equal to 1/510 by weight; area of drainage 432,480 square
miles. This gives one foot of rock in 2,358 years as the amount removed.
Rough estimates have been made of the amount of sediment carried down
by some eight or ten European rivers; and although those estimates
cannot be depended upon as being anything like perfectly accurate,
still they show (what there is very little reason to doubt) that it is
extremely probable that the European continent is being denuded about
as rapidly as the American.
For a full account of all that is known on this subject I must
refer to Professor Geikie’s valuable memoir on Modern Denudation
(Transactions of Geological Society of Glasgow, vol. iii.; also Jukes
and Geikie’s “Manual of Geology,” chap. xxv.) It is mainly through the
instrumentality of this luminous and exhaustive memoir that the method
under consideration has gained such wide acceptance amongst geologists.
Professor Geikie finds that at the present rate of erosion the
following is the number of years required by the undermentioned rivers
to remove one foot of rock from the general surface of their basins.
Professor Geikie thus shows that the rate of denudation, as determined
from the amount of sediment carried down the Mississippi, is certainly
not too high.
Danube 6,846 years.
Mississippi 6,000 〃
Nith 4,723 〃
Ganges 2,358 〃
Rhone 1,528 〃
Hoang Ho 1,464 〃
Po 729 〃
By means of subaërial agencies continents are being cut up into
islands, the islands into smaller islands, and so on till the whole
ultimately disappears.
No proper estimate has been made of the quantity of sediment carried
down into the sea by our British rivers. But, from the principles just
stated, we may infer that it must be as great in proportion to the area
of drainage as that carried down by the Mississippi. For example, the
river Tay, which drains a great portion of the central Highlands of
Scotland, carries to the sea three times as much water in proportion
to its area of drainage as is carried by the Mississippi. And any one
who has seen this rapidly running river during a flood, red and turbid
with sediment, will easily be convinced that the quantity of solid
material carried down by it into the German Ocean must be very great.
Mr. John Dougall has found that the waters of the Clyde during a flood
hold in suspension 1/800 by bulk of sediment. The observations were
made about a mile above the city of Glasgow. But even supposing the
amount of sediment held in suspension by the waters of the Tay to be
only one-third (which is certainly an under-estimate) of that of the
Mississippi, viz. 1/4500 by weight, still this would give the rate of
denudation of the central Highlands at one foot in 6,000 years, or
1,000 feet in 6 millions of years.
It is remarkable that although so many measurements have been made of
the amount of fluviatile sediment being transported seawards, yet that
the bearing which this has on the broad questions of geological time
and the rate of subaërial denudation should have been overlooked. One
reason for this, no doubt, is that the measurements were made, not
with a view to determine the rate at which the river basins are being
lowered, but mainly to ascertain the age of the river deltas and the
rate at which these are being formed.[199]
_The Law which determines the Rate at which any Country is being
denuded._—By means of subaërial agencies continents are being cut up
into islands, the islands into smaller islands, and so on till the
whole ultimately disappears.
So long as the present order of things remains, the rate of denudation
will continue while land remains above the sea-level; and we have no
warrant for supposing that the rate was during past ages less than it
is at the present day. It will not do to object that, as a considerable
amount of the sediment carried down by rivers is boulder clay and
other materials belonging to the Ice age, the total amount removed
by the rivers is on that account greater than it would otherwise be.
Were this objection true, it would follow that, prior to the glacial
period, when it is assumed that there was no boulder clay, the face of
the country must have consisted of bare rock; for in this case no soil
could have accumulated from the disintegration and decomposition of the
rocks, _since, unless the rocks of a country disintegrate more rapidly
than the river systems are able to carry the disintegrated materials
to the sea, no surface soil can form on that country_. The rate at
which rivers carry down sediment is evidently not determined by the
rate at which the rocks are disintegrated and decomposed, but by the
quantity of rain falling, and the velocity with which it moves off the
face of the country. Every river system possesses a definite amount of
carrying-power, depending upon the slope of the ground, the quantity of
rain falling per annum, the manner in which the rain falls, whether it
falls gradually or in torrents, and a few other circumstances. When it
so happens, as it generally does, that the amount of rock disintegrated
on the face of the country is greater than the carrying-power of the
river systems can remove, then a soil necessarily forms. But when the
reverse is the case no soil can form on that country, and it will
present nothing but barren rock. This is no doubt the reason why in
places like the Island of Skye, for example, where the rocks are
exceedingly hard and difficult to decompose and separate, the ground
steep, and the quantity of rain falling very great, there is so much
bare rock to be seen. If, prior to the glacial epoch, the rocks of
the area drained by the Mississippi did not produce annually more
material from their destruction under atmospheric agency than was being
carried down by that river, then it follows that the country must have
presented nothing but bare rock, if the amount of rain falling then was
as great as at present.
But, after all, one foot removed off the general level of the country
since the creation of man, according to Mosaic chronology, is certainly
not a very great quantity. No person but one who had some preconceived
opinions to maintain, would ever think of concluding that one foot of
soil during 6,000 years was an extravagant quantity to be washed off
the face of the country by rain and floods during that long period.
Those who reside in the country and are eye-witnesses of the actual
effects of heavy rains upon the soil, our soft country roads, ditches,
brooks, and rivers, will have considerable difficulty in actually
believing that only one foot has been washed away during the past 6,000
years.
Some may probably admit that a foot of soil may be washed off during
a period so long as 6,000 years, and may tell us that what they deny
is not that a foot of loose and soft soil, but a foot of solid rock
can be washed away during that period. But a moment’s reflection must
convince them that, unless the rocks of the country were disintegrating
and decomposing as rapidly into soil as the rain is carrying the soil
away, the surface of the country would ultimately become bare rock. It
is true that the surface of our country in many places is protected by
a thick covering of boulder clay; but when this has once been removed,
the rocks will then disintegrate far more rapidly than they are doing
at present.
But slow as is the rate at which the country is being denuded, yet
when we take into consideration a period so enormous as 6 millions of
years, we find that the results of denudation are really startling.
One thousand feet of solid rock during that period would be removed
from off the face of the country. But if the mean level of the country
would be lowered 1,000 feet in 6 millions of years, how much would our
valleys and glens be deepened during that period? This is a problem
well worthy of the consideration of those who treat with ridicule the
idea that the general features of our country have been carved out by
subaërial agency.
In consequence of the retardation of the earth’s rotation, occasioned
by the friction of the tidal wave, the sea-level must be slowly sinking
at the equator and rising at the poles. But it is probable that the
land at the equator is being lowered by denudation as rapidly as
the sea-level is sinking. _Nearly one mile must have been worn off
the equator during the past 12 millions of years_, if the rate of
denudation all along the equator be equal to that of the basin of the
Ganges. It therefore follows that we cannot infer from the present
shape of our globe what was its form, or the rate at which it was
rotating, at the time when its crust became solidified. Although it
had been as oblate as the planet Jupiter, denudation must in time have
given it its present form.
There is another effect which would result from the denudation of the
equator and the sinking of the ocean at the equator and its rise at
the poles. This, namely, that it would tend to increase the rate of
rotation; or, more properly, it would tend to _lessen_ the rate of
tidal retardation.
But if the rate of denudation be at present so great, what must it have
been during the glacial epoch? It must have been something enormous.
At present, denudation is greatly retarded by the limited power of
our river systems to remove the loose materials resulting from the
destruction of the rocks. These materials accumulate and form a thick
soil over the surface of the rocks, which protects them, to a great
extent, from the weathering effects of atmospheric agents. So long as
the amount of rock disintegrated exceeds that which is being removed
by the river systems, the soil will continue to accumulate till the
amount of rock destroyed per annum is brought to equal that which
is being removed. It therefore follows from this principle that the
CARRYING-POWER OF OUR RIVER SYSTEMS IS THE TRUE MEASURE OF DENUDATION.
But during the glacial epoch the thickness of the soil would have but
little effect in diminishing the waste of the rocks; for at that period
the rocks were not decomposed by atmospheric agency, but were ground
down by the mechanical friction of the ice. But the presence of a thick
soil at this period, instead of retarding the rate of denudation,
would tend to increase it tenfold, for the soil would then be used as
grinding-material for the ice-sheet. In places where the ice was, say,
2,000 feet in thickness, the soil would be forced along over the rocky
face of the country, exerting a pressure on the rocks equal to 50 tons
on the square foot.
It is true that the rate at which many kinds of rocks decompose and
disintegrate is far less than what has been concluded to be the mean
rate of denudation of the whole country. This is evident from the fact
which has been adduced by some writers, that inscriptions on stones
which have been exposed to atmospheric agency for a period of 2,000
years or so, have not been obliterated. But in most cases epitaphs on
monuments and tombstones, and inscriptions on the walls of buildings,
200 years old, can hardly be read. And this is not all: the stone on
which the letters were cut has during that time rotted in probably to
the depth of several inches; and during the course of a few centuries
more the whole mass will crumble into dust.
The facts which we have been considering show also how trifling is the
amount of denudation effected by the sea in comparison with that by
subaërial agents. The entire sea-coast of the globe, according to Dr.
A. Keith Johnston, is 116,531 miles. Suppose we take the average height
of the coast-line at 25 feet, and take also the rate at which the sea
is advancing on the land at one foot in 100 years, then this gives
15,382,500,000 cubic feet of rock as the total amount removed in 100
years by the action of the sea. The total amount of land is 57,600,000
square miles, or 1,605,750,000,000,000 square feet; and if one foot is
removed off the surface in 6,000 years, then 26,763,000,000,000 cubic
feet is removed by subaërial agency in 100 years, or about 1,740 times
as much as that removed by the sea. Before the sea could denude the
globe as rapidly as the subaërial agents, it would have to advance on
the land at the rate of upwards of 17 feet annually.
It will not do, however, to measure marine denudation by the rate at
which the sea is advancing on the land. There is no relation whatever
between the rate at which the sea is _advancing_ on the land and the
rate at which the sea is _denuding_ the land. For it is evident that as
the subaërial agents bring the coast down to the sea-level, all that
the sea has got to do is simply to advance, or at most to remove the
loose materials which may lie in its path. The amount of denudation
which has been effected by the sea during past geological ages,
compared with what has been effected by subaërial agency, is evidently
but trifling. Denudation is not the proper function of the sea. The
great denuding agents are land-ice, frost, rain, running-water,
chemical agency, &c. The proper work which belongs to the sea is the
transporting of the loose materials carried down by the rivers, and the
spreading of these out so as to form the stratified beds of future ages.
_Previous Methods of measuring Geological Time unreliable._—The method
which has just been detailed of estimating the rate of subaërial
denudation seems to afford the only reliable means of a geological
character of determining geological time in absolute measure. The
methods which have hitherto been adopted not only fail to give the
positive length of geological periods, but some of them are actually
calculated to mislead.
The common method of calculating the length of a period from the
thickness of the stratified rocks belonging to that period is one of
that class. Nothing whatever can be inferred from the thickness of a
deposit as to the length of time which was required to form it. The
thickness of a deposit will depend upon a great many circumstances,
such as whether the deposition took place near to land or far away in
the deep recesses of the ocean, whether it occurred at the mouth of a
great river or along the sea-shore, or at a time when the sea-bottom
was rising, subsiding, or remaining stationary. Stratified formations
10,000 feet in thickness, for example, may, under some conditions, have
been formed in as many years, while under other conditions it may have
required as many centuries. Nothing whatever can be safely inferred as
to the absolute length of a period from the thickness of the stratified
formations belonging to that period. Neither will this method give us a
trustworthy estimate of the _relative_ lengths of geological periods.
Suppose we find the average thickness of the Cambrian rocks to be,
say, 26,000 feet, the Silurian to be 28,000 feet, the Devonian to be
6,000 feet, and the Tertiary to be 10,000 feet, it would not be safe
to assume, as is sometimes done, that the relative duration of those
periods must have corresponded to these numbers. Were we sure that we
had got the correct average thickness of all the rocks belonging to
each of those formations, we might probably be able to arrive at the
relative lengths of those periods; but we can never be sure of this.
Those formations all, at one time, formed sea-bottoms; and we can only
measure such deposits as are now raised above the sea-level. But is
not it probable that the relative positions of sea and land during the
Cambrian, Silurian, Old Red Sandstone, Carboniferous, and other early
periods of the earth’s history, differed more from the present than the
distribution of sea and land during the Tertiary period differed from
that which obtains now? May not the greater portion of the Tertiary
deposits be still under the sea-bottom? And if this be the case, it may
yet be found at some day in the distant future, when these deposits
are elevated into dry land, that they are much thicker than we now
conclude them to be. Of course, it is by no means asserted that this
is so, but only that they _may_ be thicker for anything we know to the
contrary; and the possibility that they may, destroys our confidence
in the accuracy of this method of determining the relative lengths of
geological periods.
Neither does palæontology afford any better mode of measuring
geological time. In fact, the palæontological method of estimating
geological time, either absolute or relative, from the rate at which
species change, appears to be even still more unsatisfactory. If we
could ascertain by some means or other the time that has elapsed from
some given epoch (say, for example, the glacial) till the present
day, and were we sure at the same time that species have changed at a
uniform rate during all past ages, then, by ascertaining the percentage
of change that has taken place since the glacial epoch, we should
have a means of making something like a rough estimate of the length
of the various periods. But without some such period to start with,
the palæontological method is useless. It will not do to take the
historic period as a base-line. It is far too short to be used with
safety in determining the distance of periods so remote as those which
concern the geologist. But even supposing the palæontologist had a
period of sufficient length measured off correctly to begin with, his
results would still be unsatisfactory; for it is perfectly obvious,
that unless the climatic conditions of the globe during the various
periods were nearly the same, the rate at which the species change
would certainly not be uniform; but such has not been the case, as an
examination of the Tables of eccentricity will show. Take, for example,
that long epoch of 260,000 years, beginning about 980,000 years ago
and terminating about 720,000 years ago. During that long period the
changes from cold to warm conditions of climate every 10,000 or 12,000
years must have been of the most extreme character. Compare that
period with the period beginning, say, 80,000 years ago, and extending
to nearly 150,000 years into the future, during which there will be
no extreme variations of climate, and how great is the contrast! How
extensive the changes in species must have been during the first period
as compared with those which are likely to take place during the latter!
Besides, it must also be taken into consideration that organization was
of a far more simple type in the earlier Palæozoic ages than during the
Tertiary period, and would probably on this account change much more
slowly in the former than in the latter.
The foregoing considerations render it highly probable, if not
certain, that the rate at which the general surface of the globe is
being lowered by subaërial denudation cannot be much under one foot
in 6,000 years. How, if we assign the glacial epoch to that period of
high eccentricity beginning 980,000 years ago, and terminating 720,000
years ago, then we must conclude that as much as 120 feet must have
been denuded off the face of the country since the close of the glacial
epoch. But if as much as this had been carried down by our rivers into
the sea, hardly a patch of boulder clay, or any trace of the glacial
epoch, should be now remaining on the land. It is therefore evident
that the glacial epoch cannot be assigned to that remote period, but
ought to be referred to the period terminating about 80,000 years ago.
We have, in this latter case, 13 feet, equal to about 18 feet of drift,
as the amount removed from the general surface of the country since
the glacial epoch. This amount harmonizes very well with the direct
evidence of geology on this point. Had the amount of denudation since
the close of the glacial epoch been much greater than this, the drift
deposits would not only have been far less complete, but the general
appearance and outline of the surface of all glaciated countries would
have been very different from what they really are.
_Circumstances which show the Recent Date of the Glacial Epoch._—One
of the circumstances to which I refer is this. When we examine the
surface of any glaciated country, such as Scotland, we can easily
satisfy ourselves that the upper surface of the ground differs very
much from what it would have been had its external features been due
to the action of rain and rivers and the ordinary agencies which have
been at work since the close of the Ice period. Go where one will in
the Lowlands of Scotland, and he shall hardly find a single acre whose
upper surface bears the marks of being formed by the denuding agents
which are presently in operation. He will observe everywhere mounds
and hollows, the existence of which cannot be accounted for by the
present agencies at work. In fact these agencies are slowly denuding
pre-existing heights and silting up pre-existing hollows. Everywhere
one comes upon patches of alluvium which upon examination prove to be
simply old glacially formed hollows silted up. True, the main rivers,
streams, and even brooks, occupy channels which have been formed by
running water, either since or prior to the glacial epoch, but, in
regard to the general surface of the country, the present agencies may
be said to be just beginning to carve a new line of features out of
the old glacially formed surface. But so little progress has yet been
made, that the kames, gravel mounds, knolls of boulder clay, &c., still
retain in most cases their original form. Now, when we reflect that
more than a foot of drift is being removed from the general surface of
the country every 5,000 years or so, it becomes perfectly obvious that
the close of the glacial epoch must be of comparatively recent date.
There is another circumstance which shows that the glacial epoch must
be referred to the latest period of great eccentricity. If we refer the
glacial epoch to the penultimate period of extreme eccentricity, and
place its commencement one million of years back, then we must also
lengthen out to a corresponding extent the entire geological history
of the globe. Sir Charles Lyell, who is inclined to assign the glacial
epoch to this penultimate period, considers that when we go back as far
as the Lower Miocene formations, we arrive at a period when the marine
shells differed as a whole from those now existing. But only 5 per
cent. of the shells existing at the commencement of the glacial epoch
have since died out. Hence, assuming the rate at which the species
change to be uniform, it follows that the Lower Miocene period must
be twenty times as remote as the commencement of the glacial epoch.
Consequently, if it be one million of years since the commencement
of the glacial epoch, 20 millions of years, Sir Charles concludes,
must have elapsed since the time of the Lower Miocene period, and
60 millions of years since the beginning of the Eocene period, and
about 160 millions of years since the Carboniferous period, and about
240 millions of years must be the time which has elapsed since the
beginning of the Cambrian period. But, on the other hand, if we refer
the glacial epoch to the latest period of great eccentricity, and take
250,000 years ago as the beginning of that period, then, according
to the same mode of calculation, we have 15 millions of years since
the beginning of the Eocene period, and 40 millions of years since
the Carboniferous period, and 60 millions of years in all since the
beginning of the Cambrian period.
If the beginning of the glacial epoch be carried back a million years,
then it is probable, as Sir Charles Lyell concludes, that the beginning
of the Cambrian period will require to be placed 240 millions of years
back. But it is very probable that the length of time embraced by the
pre-Cambrian ages of geological history may be as great as that which
has elapsed since the close of the Cambrian period, and, if this be
so, then we shall be compelled to admit that nearly 500 millions of
years have passed away since the beginning of the earth’s geological
history. But we have evidence of a physical nature which proves that it
is absolutely impossible that the existing order of things, as regards
our globe, can date so far back as anything like 500 millions of years.
The arguments to which I refer are those which have been advanced by
Professor Sir William Thomson at various times. These arguments are
well known, and to all who have really given due attention to them must
be felt to be conclusive. It would be superfluous to state them here; I
shall, however, for reasons which will presently appear, refer briefly
to one of them, and that one which seems to be the most conclusive of
all, viz., the argument derived from the limit to the age of the sun’s
heat.
_Professor Ramsay on Geological Time._—In an interesting suggestive
memoir, “On Geological Ages as items of Geological Time,”[200]
Professor Ramsay discusses the comparative values of certain groups of
formations as representative of geological time, and arrives at the
following general conclusion, viz., “That the local continental era
which began with the Old Red Sandstone and closed with the New Red Marl
is comparable, in point of geological time, to that occupied in the
deposition of the whole of the Mesozoic, or Secondary series, later
than the New Red Marl and all the Cainozoic or Tertiary formations,
and indeed of all the time that has elapsed since the beginning of
the deposition of the Lias down to the present day.” This conclusion
is derived partly from a comparison of the physical character of
the formations constituting each group, but principally from the
zoological changes which took place during the time represented by them.
The earlier period represented by the Cambrian and Silurian rocks he
also, from the same considerations, considers to have been very long,
but he does not attempt to fix its relative length. Of the absolute
length of any or all of these great eras of geological time no
estimate or guess is given. He believes, however, that the whole time
represented by all the fossiliferous rocks, from the earliest Cambrian
to the most recent, is, geologically speaking, short compared with that
which went before it. After quoting Professor Huxley’s enumeration of
the many classes and orders of marine life (identical with those still
existing), whose remains characterize the lowest Cambrian rocks, he
says, “The inference is obvious that in this earliest known varied
life we find no evidence of its having lived near the beginning of
the zoological series. In a broad sense, compared with what must have
gone before, both biologically and physically, all the phenomena
connected with this old period seem to my mind to be quite of a recent
description, and the climates of seas and lands were of the very same
kind as those that the world enjoys at the present day.”... “In the
words of Darwin, when discussing the imperfection of the geological
record of this history, ‘we possess the last volume alone relating
only to two or three countries,’ and the reason why we know so little
of pre-Cambrian faunas and the physical characters of the more ancient
formations as originally deposited, is that below the Cambrian strata
we get at once involved in a sort of chaos of metamorphic strata.’”
It seems to me that Professor Ramsay’s results lead to the same
conclusion regarding the _positive_ length of geological periods as
those derived from physical considerations. It is true that his views
lead us back to an immense lapse of unknown time prior to the Cambrian
period, but this practically tends to shorten geological periods. For
it is evident that the geological history of our globe must be limited
by the age of the sun’s heat, no matter how long or short its age may
be. This being the case, the greater the length of time which must
have elapsed prior to the Cambrian period, the less must be the time
which has elapsed since that period. Whatever is added to the one
period must be so much taken from the other. Consequently, the longer
we suppose the pre-Cambrian periods to have been, the shorter must we
suppose the post-Cambrian to be.
CHAPTER XXI.
THE PROBABLE AGE AND ORIGIN OF THE SUN.
Gravitation Theory.—Amount of Heat emitted by the Sun.—Meteoric
Theory.—Helmholtz’s Condensation Theory.—Confusion of
Ideas.—Gravitation not the chief Source of the Sun’s
Heat.—Original Heat.—Source of Original Heat.—Original Heat
derived from Motion in Space.—Conclusion as to Date of
Glacial Epoch.—False Analogy.—Probable Date of Eocene and
Miocene Periods.
_Gravitation Theory of the Origin and Source of the Sun’s Heat._—There
are two forms in which this theory has been presented: the first, the
meteoric theory, propounded by Dr. Meyer, of Heilbronn; and the second,
the contraction theory, advocated by Helmholtz.
It is found that 83·4 foot-pounds of heat per second are incident upon
a square foot of the earth’s surface exposed to the perpendicular rays
of the sun. The amount radiated from a square foot of the sun’s surface
is to that incident on a square foot of the earth’s surface as the
square of the sun’s distance to the square of his radius, or as 46,400
to 1. Consequently 3,869,000 foot-pounds of heat are radiated off every
square foot of the sun’s surface per second—an amount equal to about
7,000 horse power. The total amount radiated from the whole surface
of the sun per annum is 8,340 × 10^{30} foot-pounds. To maintain the
present rate of radiation, it would require the combustion of about
1,500 lbs. of coal per hour on every square foot of the sun’s surface;
and were the sun composed of that material, it would be all consumed in
less than 5,000 years. The opinion that the sun’s heat is maintained
by combustion cannot be entertained for a single moment. A pound of
coal falling into the sun from an infinite distance would produce by
its concussion more than 6,000 times the amount of heat that would be
generated by its combustion.
It is well known that the velocity with which a body falling from an
infinite distance would reach the sun would be equal to that which
would be generated by a constant force equal to the weight of the body
at the sun’s surface operating through a space equal to the sun’s
radius. One pound would at the sun’s surface weigh about 28 pounds.
Taking the sun’s radius at 441,000 miles,[201] the energy of a pound
of matter falling into the sun from infinite space would equal that
of a 28-pound weight descending upon the earth from an elevation of
441,000 miles, supposing the force of gravity to be as great at that
elevation as it is at the earth’s surface. It would amount to upwards
of 65,000,000,000 foot-pounds. A better idea of this enormous amount
of energy exerted by a one-pound weight falling into the sun will be
conveyed by stating that it would be sufficient to raise 1,000 tons to
a height of 5½ miles. It would project the _Warrior_, fully equipped
with guns, stores, and ammunition, over the top of Ben Nevis.
Gravitation is now generally admitted to be the only conceivable
source of the sun’s heat. But if we attribute the energy of the sun to
gravitation as a source, we assign it to a cause the value of which can
be accurately determined. Prodigious as is the energy of a single pound
of matter falling into the sun, nevertheless a range of mountains,
consisting of 176 cubic miles of solid rock, falling into the sun,
would maintain his heat for only a single second. A mass equal to that
of the earth would maintain the heat for only 93 years, and a mass
equal to that of the sun itself falling into the sun would afford but
33,000,000 years’ sun-heat.
It is quite possible, however, that a meteor may reach the sun with a
velocity far greater than that which it could acquire by gravitation;
for it might have been moving in a direct line towards the sun with
an original velocity before coming under the sensible influence of
the sun’s attraction. In this case a greater amount of heat would
be generated by the meteor than would have resulted from its merely
falling into the sun under the influence of gravitation. But then
meteors of this sort must be of rare occurrence. The meteoric theory
of the sun’s heat has now been pretty generally abandoned for the
contraction theory advanced by Helmholtz.
Suppose, with Helmholtz, that the sun originally existed as a nebulous
mass, filling the entire space presently occupied by the solar system
and extending into space indefinitely beyond the outermost planet. The
total amount of work in foot-pounds performed by gravitation in the
condensation of this mass to an orb of the sun’s present size can be
found by means of the following formula given by Helmholtz,[202]
3 _r_^{2}M^{2}
Work of condensation = — × ———————————— × _g_
5 R_m_
M is the mass of the sun, _m_ the mass of the earth, R the sun’s
radius, and _r_ the earth’s radius. Taking M = 4230 × 10^{27} lbs.,
_m_ = 11,920 × 10^{21} lbs., R = 2,328,500,000 feet, and _r_ =
20,889,272 feet; we have then for the total amount of work performed by
gravitation in foot-pounds,
3 (20,889,272·5)^2 × (4230 × 10^{27})^2
Work = — × —————————————————————————————————————
5 2,328,500,000 × 11,920 × 10^{21}
= 168,790 × 10^{36} foot-pounds.
The amount of heat thus produced by gravitation would suffice for
nearly 20,237,500 years.
These calculations are based upon the assumption that the density of
the sun is uniform throughout. But it is highly probable that the sun’s
density increases towards the centre, in which case the amount of work
performed by gravitation would be somewhat more than the above.
Some confusion has arisen in reference to this subject by the
introduction of the question of the amount of the sun’s specific heat.
If we simply consider the sun as an incandescent body in the process
of cooling, the question of the amount of the sun’s specific heat is
of the utmost importance; because the absolute amount of heat which
the sun is capable of giving out depends wholly upon his temperature
and specific heat. In this case three things only are required: (1),
the sun’s mass; (2), temperature of the mass; (3), specific heat of
the mass. But if we are considering what is the absolute amount of
heat which could have been given out by the sun on the hypothesis that
gravitation, either according to the meteoric theory suggested by Meyer
or according to the contraction theory advocated by Helmholtz, is the
only source of his heat, then we have nothing whatever to do with any
inquiries regarding the specific heat of the sun. This is evident
because the absolute amount of work which gravitation can perform in
the pulling of the particles of the sun’s mass together, is wholly
independent of the specific heat of those particles. Consequently, the
amount of energy in the form of heat thus imparted to the particles
by gravity must also be wholly independent of specific heat. That is
to say, the amount of heat imparted to a particle will be the same
whatever may be its specific heat.
Even supposing we limit the geological history of our globe to 100
millions of years, it is nevertheless evident that gravitation will not
account for the supply of the sun’s heat during so long a period. There
must be some other source of much more importance than gravitation.
What other source of energy greater than that of gravitation can there
be? It is singular that the opinion should have become so common even
among physicists, that there is no other conceivable source than
gravitation from which a greater amount of heat could have been derived.
_The Origin and Chief Source of the Sun’s Heat._—According to the
foregoing theories regarding the source of the sun’s heat, it is
assumed that the matter composing the sun, when it existed in space as
a nebulous mass, was not originally possessed of temperature, but that
the temperature was given to it as the mass became condensed under the
force of gravitation. It is supposed that the heat given out was simply
the heat of condensation. But it is quite conceivable that the nebulous
mass might have been possessed of an original store of heat previous to
condensation.
It is quite possible that the very reason why it existed in such a
rarefied or gaseous condition was its excessive temperature, and that
condensation only began to take place when the mass began to cool down.
It seems far more probable that this should have been the case than
that the mass existed in so rarefied a condition without temperature.
For why should the particles have existed in this separated form when
devoid of the repulsive energy of heat, seeing that in virtue of
gravitation they had such a tendency to approach to one another? But
if the mass was originally in a heated condition, then in condensing
it would have to part not only with the heat generated in condensing,
but also with the heat which it originally possessed, a quantity
which would no doubt much exceed that produced by condensation. To
illustrate this principle, let us suppose a pound of air, for example,
to be placed in a cylinder and heat applied to it. If the piston be so
fixed that it cannot move, 234·5 foot-pounds of heat will raise the
temperature of the air 1° C. But if the piston be allowed to rise as
the heat is applied, then it will require 330·2 foot-pounds of heat to
raise the temperature 1° C. It requires 95·7 foot-pounds more heat in
the latter case than in the former. The same amount of energy, viz.,
234·5 foot-pounds, in both cases goes to produce temperature; but in
the latter case, where the piston is allowed to move, 95·7 foot-pounds
of additional heat are consumed in the mechanical work of raising the
piston. Suppose, now, that the air is allowed to cool under the same
conditions: in the one case 234·5 foot-pounds of heat will be given
out while the temperature of the air sinks 1° C.; in the other case,
where the piston is allowed to descend, 330·2 foot-pounds will be given
out while the temperature sinks 1° C. In the former case, the air in
cooling has simply to part with the energy which it possesses in
the form of temperature; but in the latter case it has, in addition
to this, to part with the energy bestowed upon its molecules by the
descending piston. While the temperature of the gas is sinking 1°,
95·7 foot-pounds of energy in the form of heat are being imparted to
it by the descending piston; and these have to be got rid of before
the temperature is lowered by 1°. Consequently 234·5 foot-pounds of
the heat given out previously existed in the air under the form of
temperature, and the remaining 95·7 foot-pounds given out were imparted
to the air by the descending piston while the gas was losing its
temperature. 234·5 foot-pounds represent the energy or heat which the
air previously possessed, and 95·7 the energy or heat of condensation.
In the case of the cooling of the sun from a nebulous mass, there
would of course be no external force or pressure exerted on the mass
analogous to that of the piston on the air; but there would be, what
is equivalent to the same, the gravitation of the particles to each
other. There would be the pressure of the whole mass towards the centre
of convergence. In the case of air, and all perfect gases cooling
under pressure, about 234 foot-pounds of the original heat possessed
by the gas are given out while 95 foot-pounds are being generated by
condensation. We have, however, no reason whatever to believe that in
the case of the cooling of the sun the same proportions would hold
true. The proportion of original heat possessed by the mass of the sun
to that produced by condensation may have been much greater than 234 to
95, or it may have been much less. In the absence of all knowledge on
this point, we may in the meantime assume that to be the proportion.
The total quantity of heat given out by the sun resulting from the
condensation of his mass, on the supposition that the density of the
sun is uniform throughout, we have seen to be equal to 20,237,500
years’ sun-heat. Then the quantity of heat given out, which previously
existed in the mass as original temperature, must have been 49,850,000
years’ heat, making in all 70,087,500 years’ heat as the total amount.
The above quantity represents, of course, the total amount of heat
given out by the mass since it began to condense. But the geological
history of our globe must date its beginning at a period posterior to
that. For at that time the mass would probably occupy a much greater
amount of space than is presently possessed by the entire solar system;
and consequently, before it had cooled down to within the limits of
the earth’s present orbit, our earth could not have had an existence
as a separate planet. Previously to that time it must have existed as
a portion of the sun’s fiery mass. If we assume that it existed as a
globe previously to that, and came in from space after the condensation
of the sun, then it is difficult to conceive how its orbit should be so
nearly circular as it is at present.
Let us assume that by the time that the mass of the sun had condensed
to within the space encircled by the orbit of the planet Mercury (that
is, to a sphere having, say, a radius of 18,000,000 miles) the earth’s
crust began to form; and let this be the time when the geological
history of our globe dates its commencement. The total amount of heat
generated by the condensation of the sun’s mass from a sphere of this
size to its present volume would equal 19,740,000 years’ sun-heat.
The amount of original heat given out during that time would equal
48,625,000 years’ sun-heat,—thus giving a total of 68,365,000 years’
sun-heat enjoyed by our globe since that period. The total quantity may
possibly, of course, be considerably more than that, owing to the fact
that the sun’s density may increase greatly towards his centre. But we
should require to make extravagant assumptions regarding the interior
density of the sun and the proportion of original heat to that produced
by condensation before we could manage to account for anything like the
period that geological phenomena are supposed by some to demand.
The question now arises, by what conceivable means could the mass of
the sun have become possessed of such a prodigious amount of energy
in the form of heat previous to condensation? What power could have
communicated to the mass 50,000,000 years’ heat before condensation
began to take place?
_The Sun’s Energy may have originally been derived from Motion in
Space._—There is nothing at all absurd or improbable in the supposition
that such an amount of energy might have been communicated to the
mass. The Dynamical Theory of Heat affords an easy explanation of at
least _how_ such an amount of energy _may_ have been communicated. Two
bodies, each one-half the mass of the sun, moving directly towards
each other with a velocity of 476 miles per second, would by their
concussion generate in a _single moment_ the 50,000,000 years’ heat.
For two bodies of that mass moving with a velocity of 476 miles per
second would possess 4149 × 10^{38} foot-pounds of energy in the form
of _vis viva_; and this, converted into heat by the stoppage of their
motion, would give an amount of heat which would cover the present rate
of the sun’s radiation, for a period of 50,000,000 years.
Why may not the sun have been composed of two such bodies? And why may
not the original store of heat possessed by him have all been derived
from the concussion of these two bodies? Two such bodies coming into
collision with that velocity would be dissipated into vapour by such
an inconceivable amount of heat as would thus be generated; and when
they condensed on cooling, they would form one spherical mass like the
sun. It is perfectly true that two such bodies could never attain the
required amount of velocity by their mutual gravitation towards each
other. But there is no necessity whatever for supposing that their
velocities were derived from their mutual attraction alone. They might
have been approaching towards each other with the required velocity
wholly independent of gravitation.
We know nothing whatever regarding the absolute motion of bodies in
space. And beyond the limited sphere of our observation, we know
nothing even of their relative motions. There may be bodies moving
in relation to our system with inconceivable velocity. For anything
that we know to the contrary, were one of these bodies to strike our
earth, the shock might be sufficient to generate an amount of heat that
would dissipate the earth into vapour, though the striking body might
not be heavier than a cannon-ball. There is, however, nothing very
extraordinary in the velocity which we have found would be required
in the two supposed bodies to generate the 50,000,000 years’ heat. A
comet, having an orbit extending to the path of the planet Neptune,
approaching so near the sun as to almost graze his surface in passing,
would have a velocity of about 390 miles per second, which is within 86
miles of the required velocity.
But in the original heating and expansion of the sun into a gaseous
mass, an amount of work must have been performed against gravitation
equal to that which has been performed by gravitation during his
cooling and condensation, a quantity which we have found amounts to
about 20,000,000 years’ heat. The total amount of energy originally
communicated by the concussion must have been equal to 70,000,000
years’ sun-heat. A velocity of 563 miles per second would give this
amount. It must be borne in mind, however, that the 563 miles per
second is the velocity at the moment of collision; about one-half of
this velocity would be derived from the mutual attraction of the two
bodies in their approach to each other. Suppose each body to be equal
in volume to the sun, and of course one-half the density, the amount
of velocity which they would acquire by their mutual attraction would
be 274 miles per second, consequently we have to assume an original or
projected velocity of only 289 miles per second.
If we admit that gravitation is not sufficient to account for the
amount of heat given out by the sun during the geological history of
our globe, we are compelled to assume that the mass of which the sun is
composed existed prior to condensation in a heated condition; and if
so, we are further obliged to admit that the mass must have received
its heat from some source or other. And as the dissipation of heat into
space must have been going on, in all probability, as rapidly before
as after condensation took place, we are further obliged to conclude
that the heat must have been communicated to the mass immediately
before condensation began, for the moment the mass began to lose its
heat condensation would ensue. If we confine our speculations to causes
and agencies known to exist, the cause which has been assigned appears
to be the only conceivable one that will account for the production of
such an enormous amount of heat.
The general conclusion to which we are therefore led from physical
considerations regarding the age of the sun’s heat is, that the entire
geological history of our globe must be comprised within less than
100 millions of years, and that consequently the commencement of the
glacial epoch cannot date much farther back than 240,000 years.
The facts of geology, more especially those in connection with
denudation, seem to geologists to require a period of much longer
duration than 100 millions of years, and it is this which has so
long prevented them accepting the conclusions of physical science in
regard to the age of our globe. But the method of measuring subaërial
denudation already detailed seems to me to show convincingly that the
geological data, when properly interpreted, are in perfect accord with
the deductions of physical science. Perhaps there are now few who
have fairly considered the question who will refuse to admit that 100
millions of years are amply sufficient to comprise the whole geological
history of our globe.
_A false Analogy supposed to exist between Astronomy and
Geology._—Perhaps one of the things which has tended to mislead on
this point is a false analogy which is supposed to subsist between
astronomy and geology, viz., that geology deals with unlimited _time_,
as astronomy deals with unlimited _space_. A little consideration,
however, will show that there is not much analogy between the two cases.
Astronomy deals with the countless worlds which lie spread out in the
boundless infinity of space; but geology deals with only one world.
No doubt reason and analogy both favour the idea that the age of the
material universe, like its magnitude, is immeasurable; we have no
reason, however, to conclude that it is eternal, any more than we
have to infer that it is infinite. But when we compare the age of the
material universe with its magnitude, we must not take the age of one
of its members (say, our globe) and compare it with the size of the
universe. Neither must we compare the age of all the presently existing
systems of worlds with the magnitude of the universe; but we must
compare the past history of the universe as it stretches back into the
immensity of bygone _time_, with the presently existing universe as it
stretches out on all sides into limitless _space_. For worlds precede
worlds in time as worlds lie beyond worlds in space. Each world,
each individual, each atom is evidently working out a final purpose,
according to a plan prearranged and predetermined by the Divine Mind
from all eternity. And each world, like each individual, when it
serves the end for which it was called into existence, disappears to
make room for others. This is the grand conception of the universe
which naturally impresses itself on every thoughtful mind that has not
got into confusion about those things called in science the Laws of
Nature.[203]
But the geologist does not pass back from world to world as they stand
related to each other in the order of _succession in time_, as the
astronomer passes from world to world as they stand related to each
other in the order of _coexistence in space_. The researches of the
geologist, moreover, are not only confined to one world, but it is only
a portion of the history of that one world that can come under his
observation. The oldest of existing formations, so far as is yet known,
the Laurentian Gneiss, is made up of the waste of previously existing
rocks, and it, again, has probably been derived from the degradation
of rocks belonging to some still older period. Regarding what succeeds
these old Laurentian rocks geology tells us much; but of the formations
that preceded, we know nothing whatever. For anything that geology
shows to the contrary, the time which may have elapsed from the
solidifying of the earth’s crust to the deposition of the Laurentian
strata—an absolute blank—may have been as great as the time that has
since intervened.
_Probable Date of the Eocene and Miocene Periods._—If we take into
consideration the limit which physical science assigns to the age of
our globe, and the rapid rate at which, as we have seen, denudation
takes place, it becomes evident that the enormous period of 3 millions
of years comprehended in the foregoing tables must stretch far back
into the Tertiary age. Supposing that the mean rate of denudation
during that period was not greater than the present rate of denudation,
still we should have no less than 500 feet of rock worn off the face of
the country and carried into the sea during these 3 millions of years.
This fact shows how totally different the appearance and configuration
of the country in all probability was at the commencement of this
period from what it is at the present day. If it be correct that the
glacial epoch resulted from the causes which we have already discussed,
those tables ought to aid us in our endeavour to ascertain _how_ much
of the Tertiary period may be comprehended within these 3 millions of
years.
We have already seen (Chapter XVIII.) that there is evidence of a
glacial condition of climate at two different periods during the
Tertiary age, namely, about the middle of the Miocene and Eocene
periods respectively. As has already been shown, the more severe a
glacial epoch is, the more marked ought to be the character of its warm
inter-glacial periods; the greater the extension of the ice during the
cold periods of a glacial epoch the further should that ice disappear
in arctic regions during the corresponding warm periods. Thus the
severity of a glacial epoch may in this case be indirectly inferred
from the character of the warm periods and the extent to which the
ice may have disappeared from arctic regions. Judged by this test, we
have every reason to believe that the Miocene glacial epoch was one of
extreme severity.
The Eocene conglomerate, devoid of all organic remains, and containing
numerous enormous ice-transported blocks, is, as we have seen,
immediately associated with nummulitic strata charged with fossils
characteristic of a warm climate. Referring to this Sir Charles Lyell
says, “To imagine icebergs carrying such huge fragments of stone in so
southern a latitude, and at a period immediately preceded and followed
by the signs of a warm climate, is one of the most perplexing enigmas
which the geologist has yet been called upon to solve.”[204]
It is perfectly true that, according to the generally received theories
of the cause of a glacial climate the whole is a perplexing enigma, but
if we adopt the Secular theory of change of climate, every difficulty
disappears. According to this theory the very fact of the conglomerate
being formed at a period immediately preceded and succeeded by warm
conditions of climate, is of itself strong presumptive evidence of the
conglomerate being a glacial formation. But this is not all, the very
highness of the temperature of the preceding and succeeding periods
bears testimony to the severity of the intervening glacial period.
Despite the deficiency of direct evidence regarding the character of
the Miocene and Eocene glacial periods, we are not warranted, for
reasons which have been stated in Chapter XVII., to conclude that these
periods were less severe than the one which happened in Quaternary
times. Judging from indirect evidence, we have some grounds for
concluding that the Miocene glacial epoch at least was even more severe
and protracted than our recent glacial epoch.
By referring to Table I., or the accompanying diagram, it will be seen
that prior to the period which I have assigned as that of the glacial
epoch, there are two periods when the eccentricity almost attained
its superior limit. The first period occurred 2,500,000 years ago,
when it reached 0·0721, and the second period 850,000 years ago, when
it attained a still higher value, viz., 0·0747, being within 0·0028
of the superior limit. To the first of these periods I am disposed
to assign the glacial epoch of Eocene times, and to the second that
of the Miocene age. With the view of determining the character of
these periods Tables II. and III. have been computed. They give the
eccentricity and longitude of perihelion at intervals of 10,000 years.
It will be seen from Table II. that the Eocene period extends from
about 2,620,000 to about 2,460,000 years ago; and from Table III. it
will be gathered that the Miocene period lasted from about 980,000 to
about 720,000 years ago.
In order to find whether the eccentricity attained a higher value about
850,000 years ago than 0·0747, I computed the values for one or two
periods immediately before and after that period, and satisfied myself
that the value stated was indeed the highest, as will be seen from the
subjoined table:—
851,000 0·07454
850,000 0·074664
849,500 0·07466
849,000 0·07466
How totally different must have been the condition of the earth’s
climate at that period from what it is at present! Taking the mean
distance of the sun to be 91,400,000 miles, his present distance at
midwinter is 89,864,480 miles; but at the period in question, when the
winter solstice was in perihelion, his distance at midwinter would be
no less than 98,224,289 miles. But this is not all; our winters are at
present shorter than our summers by 7·8 days, but at that period they
would be longer than the summers by 34·7 days.
At present the difference between the perihelion and aphelion distance
of the sun amounts to only 3,069,580 miles, but at the period under
consideration it would amount to no less than 13,648,579 miles!
CHAPTER XXII.
A METHOD OF DETERMINING THE MEAN THICKNESS OF THE SEDIMENTARY
ROCKS OF THE GLOBE.
Prevailing Methods defective.—Maximum Thickness of British
Rocks.—Three Elements in the Question.—Professor Huxley
on the Rate of Deposition.—Thickness of Sedimentary Rocks
enormously over-estimated.—Observed Thickness no Measure of
mean Thickness.—Deposition of Sediment principally along
Sea-margin.—Mistaken Inference regarding the Absence of a
Formation.—Immense Antiquity of existing Oceans.
Various attempts have been made to measure the positive length of
geological periods. Some geologists have sought to determine, roughly,
the age of the stratified rocks by calculations based upon their
probable thickness and the rate at which they may have been deposited.
This method, however, is worthless, because the rates which have been
adopted are purely arbitrary. One geologist will take the rate of
deposit at a foot in a hundred years, while another will assume it
to be a foot in a thousand or perhaps ten thousand years; and, for
any reasons that have been assigned, the one rate is just as likely
to be correct as the other: for if we examine what is taking place
in the ocean-bed at the present day, we shall find in some places a
foot of sediment laid down in a year, while in other places a foot
may not be deposited in a thousand years. The stratified rocks were
evidently formed at all possible rates. When we speak of the rate of
their formation, we must of course refer to the _mean rate_; and it is
perfectly true that if we knew the thickness of these rocks and the
mean rate at which they were deposited, we should have a ready means
of determining their positive age. But there appears to be nearly as
great uncertainty regarding the thickness of the sedimentary rocks as
regarding the rate at which they were formed. No doubt we can roughly
estimate their probable maximum thickness; for instance, Professor
Ramsay has found from actual measurement, that the sedimentary
formations of Great Britain have a maximum thickness of upwards of
72,000 feet; but all such measurements give us no idea of their mean
thickness. What is the mean thickness of the sedimentary rocks of
the globe? On this point geology does not afford a definite answer.
Whatever the present mean thickness of the sedimentary rocks of our
globe may be, it must be small in comparison to the mean thickness
of all the sedimentary rocks which have been formed. This is obvious
from the fact that the sedimentary rocks of one age are partly formed
from the destruction of the sedimentary rocks of former ages. From the
Laurentian age down to the present day, the stratified rocks have been
undergoing constant denudation.
Unless we take into consideration the quantity of rock removed during
past ages by denudation, we cannot—even though we knew the actual mean
thickness of the existing sedimentary rocks of the globe, and the rate
at which they were formed—arrive at an estimate regarding the length of
time represented by these rocks. For if we are to determine the age of
the stratified rocks from the rate at which they were formed, we must
have, not the present quantity of sedimentary rocks, but the present
plus the quantity which has been denuded during past ages. In other
words, we must have the absolute quantity formed. In many places the
missing beds must have been of enormous thickness. The time represented
by beds which have disappeared is, doubtless, as already remarked,
much greater than that represented by the beds which now remain. The
greater mass of the sedimentary rocks has been formed out of previously
existing sedimentary rocks, and these again out of sedimentary rocks
still older. As the materials composing our stratified beds may have
passed through many cycles of destruction and re-formation, the time
required to have deposited at a given rate the present existing mass
of sedimentary rocks may be but a fraction of the time required to
have deposited at the same rate the total mass that has actually been
formed. To measure the age of the sedimentary rocks by the present
existing rocks, assumed to be formed at some given rate, even supposing
the rate to be correct, is a method wholly fallacious.
“The aggregate of sedimentary strata in the earth’s crust,” says Sir
Charles Lyell, “can never exceed in volume the amount of solid matter
which has been ground down and washed away by rivers, waves, and
currents. How vast, then, must be the spaces which this abstraction
of matter has left vacant! How far exceeding in dimensions all the
valleys, however numerous, and the hollows, however vast, which we can
prove to have been cleared out by aqueous erosion!”[205]
I presume there are few geologists who would not admit that if all the
rocks which have in past ages been removed by denudation were restored,
the mean thickness of the sedimentary rocks of the globe would be at
least equal to their present maximum thickness, which we may take at
72,000 feet.
There are three elements in the question; of which if two are known,
the third is known in terms of the other two. If we have the mean
thickness of all the sedimentary rocks which have been formed and the
mean rate of formation, then we have the time which elapsed during the
formation; or having the thickness and the time, we have the rate; or,
having the rate and the time, we have the thickness.
One of these three, namely, the rate, can, however, be determined with
tolerable accuracy if we are simply allowed to assume—what is very
probable, as has already been shown—that the present rate at which the
sedimentary deposits are being formed may be taken as the mean rate
for past ages. If we know the rate at which the land is being denuded,
then we know with perfect accuracy the rate at which the sedimentary
deposits are being formed in the ocean. This is obvious, because all
the materials denuded from the land are deposited in the sea; and
what is deposited in the sea is just what comes off the land, with the
exception of the small proportion of calcareous matter which may not
have been derived from the land, and which in our rough estimate may be
left out of account.
Now the mean rate of subaërial denudation, we have seen, is about one
foot in 6,000 years. Taking the proportion of land to that of water
at 576 to 1,390, then one foot taken off the land and spread over the
sea-bottom would form a layer 5 inches thick. Consequently, if one foot
in 6,000 years represents the mean rate at which the land is being
denuded, one foot in 14,400 years represents the mean rate at which the
sedimentary rocks are being formed.
Assuming, as before, that 72,000 feet would represent the mean
thickness of all the sedimentary rocks which have ever been formed,
this, at the rate of one foot in 14,400 years, gives 1,036,800,000
years as the age of the stratified rocks.
Professor Huxley, in his endeavour to show that 100,000,000 years is
a period sufficiently long for all the demands of geologists, takes
the thickness of the stratified rocks at 100,000 feet, and the rate
of deposit at a foot in 1,000 years. One foot of rock per 1,000 years
gives, it is true, 100,000 feet in 100,000,000 years. But what about
the rocks which have disappeared? If it takes a hundred millions of
years to produce a mass of rock equal to that which now exists, how
many hundreds of millions of years will it require to produce a mass
equal to what has actually been produced?
Professor Huxley adds, “I do not know that any one is prepared to
maintain that the stratified rocks may not have been formed on the
average at the rate of 1/83rd of an inch per annum.” When the rate,
however, is accurately determined, it is found to be, not 1/83rd of
an inch per annum, but only 1/1200th of an inch, so that the 100,000
feet of rock must have taken 1,440,000,000 years in its formation,—a
conclusion which, according to the results of modern physics, is wholly
inadmissible.
Either the thickness of the sedimentary rocks has been over-estimated,
or the rate of their formation has been under-estimated, or both.
If it be maintained that a foot in 14,400 years is too slow a rate
of deposit, then it must be maintained that the land must have been
denuded at a greater rate than one foot in 6,000 years. But most
geologists probably felt surprised when the announcement was first
made, that at this rate of denudation the whole existing land of the
globe would be brought under the ocean in 6,000,000 of years.
The error, no doubt, consists in over-estimating the thickness of the
sedimentary rocks. Assuming, for physical reasons already stated, that
100,000,000 years limits the age of the stratified rocks, and that the
proportion of land to water and the rate of denudation have been on the
average the same as at present, the mean thickness of sedimentary rocks
formed in the 100,000,000 years amounts to only 7,000 feet.
But be it observed that this is the mean thickness on an area equal
to that of the ocean. Over the area of the globe it amounts to only
5,000 feet; and this, let it be observed also, is the total mean
thickness formed, without taking into account what has been removed
by denudation. If we wish to ascertain what is actually the present
mean thickness, we must deduct from this 5,000 feet an amount of rock
equal to all the sedimentary rocks which have been denuded during
the 100,000,000 years; for the 5,000 feet is not the present mean
thickness, but the total mean thickness formed during the whole of the
100,000,000 years. If we assume, what no doubt most geologists would be
willing to grant, that the quantity of sedimentary rocks now remaining
is not over one-half of what has been actually deposited during the
history of the globe, then the actual mean thickness of the stratified
rocks of the globe is not over 2,500 feet. This startling result would
almost necessitate us to suspect that the rate of subaërial denudation
is probably greater than one foot in 6,000 years. But, be this as it
may, we are apt, in estimating the mean thickness of the stratified
rocks of the globe from their ascertained maximum thickness, to arrive
at erroneous conclusions. There are considerations which show that
the mean thickness of these rocks must be small in proportion to their
maximum thickness. The stratified rocks are formed from the sediment
carried down by rivers and streamlets and deposited in the sea. It is
obvious that the greater quantity of this sediment is deposited near
the mouths of rivers, and along a narrow margin extending to no great
distance from the land. Did the land consist of numerous small islands
equally distributed over the globe, the sediment carried off from these
islands would be spread pretty equally over the sea-bottom. But the
greater part of the land-surface consists of two immense continents.
Consequently, the materials removed by denudation are not spread
over the ocean-bottom, but on a narrow fringe surrounding those two
continents. Were the materials spread over the entire ocean-bed, a foot
removed off the general surface of the land would form a layer of rock
only five inches thick. But in the way in which the materials are at
present deposited, the foot removed from the land would form a layer
of rock many feet in thickness. The greater part of the sediment is
deposited within a few miles of the shore.
The entire coast-line of the globe is about 116,500 miles. I should
think that the quantity of sediment deposited beyond, say, 100 miles
from this coast-line is not very great. No doubt several of the large
rivers carry sediment to a much greater distance from their mouths than
100 miles, and ocean currents may in some cases carry mud and other
materials also to great distances. But it must be borne in mind that
at many places within the 100 miles of this immense coast-line little
or no sediment is deposited, so that the actual area over which the
sediment carried off the land is deposited is probably not greater than
the area of this belt—116,500 miles long and 100 miles broad. This
area on which the sediment is deposited, on the above supposition, is
therefore equal to about 11,650,000 square miles. The amount of land on
the globe is about 57,600,000 square miles. Consequently, one foot of
rock, denuded from the surface of the land and deposited on this belt,
would make a stratum of rock 5 feet in thickness; but were the sediment
spread over the entire bed of the ocean, it would form, as has already
been stated, a stratum of rock of only 5 inches in thickness.
Suppose that no subsidence of the land should take place for a period
of, say, 3,000,000 of years. During that period 500 feet would be
removed by denudation, on an average, off the land. This would make a
formation 2,500 feet thick, which some future geologist might call the
Post-tertiary formation. But this, be it observed, would be only the
mean thickness of the formation on this area; its maximum thickness
would evidently be much greater, perhaps twice, thrice, or even four
times that thickness. A geologist in the future, measuring the actual
thickness of the formation, might find it in some places 10,000 feet
in thickness, or perhaps far more. But had the materials been spread
over the entire ocean-bed, the formation would have a mean thickness
of little more than 200 feet; and spread over the entire surface of
the globe, would form a stratum of scarcely 150 feet in thickness.
Therefore, in estimating the mean thickness of the stratified rocks of
the globe, a formation with a maximum thickness of 10,000 feet may not
represent more than 150 feet. A formation with a _mean_ thickness of
10,000 feet represents only 600 feet.
It may be objected that in taking the present rate at which the
sedimentary deposits are being formed as the mean rate for all ages,
we probably under-estimate the total amount of rock formed, because
during the many glacial periods which must have occurred in past ages
the amount of materials ground off the rocky surface of the land in a
given period would be far greater than at present. But, in reply, it
must be remembered that although the destruction in ice-covered regions
would be greater during these periods than at present, yet the quantity
of materials carried down by rivers into the sea would be less. At
the present day the greater part of the materials carried down by our
rivers is not what is being removed off the rocky face of the country,
but the boulder clay, sand, and other materials which were ground off
during the glacial epoch. It is therefore possible, on this account,
that the rate of deposit may have been less during the glacial epoch
than at present.
When any particular formation is wanting in a given area, the inference
generally drawn is, that either the formation has been denuded off
the area, or the area was a land-surface during the period when that
formation was being deposited. From the foregoing it will be seen that
this inference is not legitimate; for, supposing that the area had been
under water, the chances that materials should have been deposited on
that area are far less than are the chances that there should not.
There are sixteen chances against one that no formation ever existed in
the area.
If the great depressions of the Atlantic, Pacific, and Indian Oceans
be, for example, as old as the beginning of the Laurentian period—and
they may be so for anything which geology can show to the contrary—then
under these oceans little or no stratified rocks may exist. The
supposition that the great ocean basins are of immense antiquity, and
that consequently only a small proportion of the sedimentary strata
can possibly occupy the deeper bed of the sea, acquires still more
probability when we consider the great extent and thickness of the
Old Red Sandstone, the Permian, and other deposits, which, according
to Professor Ramsay and others, have been accumulated in vast inland
lakes.
CHAPTER XXIII.
THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE OF THE LAND
DURING THE GLACIAL EPOCH.
Displacement of the Earth’s Centre of Gravity by Polar
Ice-cap.—Simple Method of estimating Amount of
Displacement.—Note by Sir W. Thomson on foregoing
Method.—Difference between Continental Ice and
a Glacier.—Probable Thickness of the Antarctic
Ice-cap.—Probable Thickness of Greenland Ice-sheet.—The
Icebergs of the Southern Ocean.—Inadequate Conceptions
regarding the Magnitude of Continental Ice.
_Displacement of the Earth’s Centre of Gravity by Polar
Ice-cap._[206]—In order to represent the question in its most simple
elementary form, I shall assume an ice-cap of a given thickness at the
pole and gradually diminishing in thickness towards the equator in the
simple proportion of the sines of the latitudes, where at the equator
its thickness of course is zero. Let us assume, what is actually the
case, that the equatorial diameter of the globe is somewhat greater
than the polar, but that when the ice-cap is placed on one hemisphere
the whole forms a perfect sphere.
I shall begin with a period of glaciation on the southern hemisphere.
Let W N E S′ (Fig. 5) be the solid part of the earth, and _c_ its
centre of gravity. And let E S W be an ice-cap covering the southern
hemisphere. Let us in the first case assume the earth to be of the same
density as the cap. The earth with its cap forms now a perfect sphere
with its centre of gravity at _o_; for W N E S is a circle, and _o_
is its centre. Suppose now the whole to be covered with an ocean a few
miles deep, the ocean will assume the spherical form, and will be of
uniform depth. Let the southern winter solstice begin now to move round
from the aphelion. The ice-cap will also commence gradually to diminish
in thickness, and another cap will begin to make its appearance on
the northern hemisphere. As the northern cap may be supposed, for
simplicity of calculation, to increase at the same rate that the
southern will diminish, the spherical form of the earth will always be
maintained. By the time that the northern cap has reached a maximum,
the southern cap will have completely disappeared. The circle W N′ E S′
will now represent the earth with its cap on the northern hemisphere,
and _o′_ will be its centre of gravity; for _o′_ is the centre of the
circle W N′ E S′. And as the distance between the centres _o_ and
_o′_ is equal to N N′, the thickness of the cap at the pole N N′ will
therefore represent the extent to which the centre of gravity has been
displaced. It will also represent the extent to which the ocean has
risen at the north pole and sunk at the south. This is evident; for as
the sphere W N′ E S′ is the same in all respects as the sphere W N E
S, with the exception only that the cap is on the opposite side, the
surface of the ocean at the poles will now be at the same distance from
the centre _o′_ as it was from the centre _o_ when the cap covered
the southern hemisphere. Hence the distance between _o_ and _o′_ must
be equal to the extent of the submergence at the north pole and the
emergence at the south. Neglect the attraction of the altering water on
the water itself, which later on will come under our consideration.
[Illustration: Fig. 5.]
We shall now consider the result when the earth is taken at its actual
density, which is generally believed to be about 5·5. The density
of ice being ·92, the density of the cap to that of the earth will
therefore be as 1 to 6.
[Illustration: Fig. 6.]
Let Fig. 6 represent the earth with an ice-cap on the northern
hemisphere, whose thickness is, say, 6,000 feet at the pole. The centre
of gravity of the earth without the cap is at _c_. When the cap is on,
the centre of gravity is shifted to _o_, a point a little more than
500 feet to the north of _c_. Had the cap and the earth been of equal
density, the centre of gravity would have been shifted to _o′_ the
centre of the figure, a point situated, of course, 3,000 feet to the
north of _c_. Now it is very approximately true that the ocean will
tend to adjust itself as a sphere around the centre of gravity, _o_.
Thus it would of course sink at the south pole and rise to the same
extent at the north, in any opening or channel in the ice allowing the
water to enter.
Let the ice-cap be now transferred over to the southern hemisphere,
and the condition of things on the two hemispheres will in every
particular be reversed. The centre of gravity will then lie to
the south of _c_, or about 1,000 feet from its former position.
Consequently the transference of the cap from the one hemisphere to the
other will produce a total submergence of about 1,000 feet.
It is, of course, absurd to suppose that an ice-cap could ever actually
reach down to the equator. It is probable that the great ice-cap of the
glacial epoch nowhere reached even halfway to the equator. Our cap must
therefore terminate at a moderately high latitude. Let it terminate
somewhere about the latitude of the north of England, say at latitude
55°. All that we have to do now is simply to imagine our cap, up to
that latitude, becoming converted into the fluid state. This would
reduce the cap to less than one-half its former mass. But it would not
diminish the submergence to anything like that extent. For although the
cap would be reduced to less than one-half its former mass, yet its
influence in displacing the centre of gravity would not be diminished
to that extent. This is evident; for the cap now extending down to
only latitude 55°, has its centre of gravity much farther removed from
the earth’s centre of gravity than it had when it extended down to the
equator. Consequently it now possesses, in proportion to its mass, a
much greater power in displacing the earth’s centre of gravity.
There is another fact which must be taken into account. The common
centre of gravity of the earth and cap is not exactly the point
around which the ocean tends to adjust itself. It adjusts itself not
in relation to the centre of gravity of the solid mass alone, but in
relation to the common centre of gravity of the entire mass, solid and
liquid. Now the water which is pulled over from the one hemisphere to
the other by the attraction of the cap will also aid in displacing the
centre of gravity. It will co-operate with the cap and carry the true
centre of gravity to a point beyond that of the centre of gravity of
the earth and cap, and thus increase the effect.
It is of course perfectly true that when the ice-cap does not extend
down to the equator, as in the latter supposition, and is of less
density than the globe, the ocean will not adjust itself uniformly
around the centre of gravity; but the deviation from perfect uniformity
is so trifling, as will be seen from the appended note of Sir William
Thomson, that for all practical purposes it may be entirely left out of
account.
In the _Reader_ for January 13, 1866, I advanced an objection to the
submergence theory on the grounds that the lowering of the ocean-level
by the evaporation of the water to form the ice-cap, would exceed the
submergence resulting from the displacement of the earth’s centre of
gravity. But, after my letter had gone to press, I found that I had
overlooked some important considerations which seem to prove that the
objection had no real foundation. For during a glacial period, say
on the northern hemisphere, the entire mass of ice which presently
exists on the southern hemisphere would be transferred to the northern,
leaving the quantity of liquid water to a great extent unchanged.
_Note on the preceding by Sir William Thomson, F.R.S._
“Mr. Croll’s estimate of the influence of a cap of ice on the sea-level
is very remarkable in its relation to Laplace’s celebrated analysis,
as being founded on that law of thickness which leads to expressions
involving only the first term of the series of ‘Laplace’s functions,’
or ‘spherical harmonics.’ The equation of the level surface, as
altered by any given transference of solid matter, is expressed by
equating the altered potential function to a constant. This function,
when expanded in the series of spherical harmonics, has for its first
term the potential due to the whole mass supposed collected at its
altered centre of gravity. Hence a spherical surface round the altered
centre of gravity is the _first_ approximation in Laplace’s method of
solution for the altered level surface. Mr. Croll has with admirable
tact chosen, of all the arbitrary suppositions that may be made
foundations for rough estimates of the change of sea-level due to
variations in the polar ice-crusts, _the_ one which reduces to zero all
terms after the first in the harmonic series, and renders that first
approximation (which always expresses the _essence_ of the result) the
whole solution, undisturbed by terms irrelevant to the great physical
question.
“Mr. Croll, in the preceding paper, has alluded with remarkable
clearness to the effect of the change in the distribution of the
water in increasing, by its own attraction, the deviation of the
level surface above that which is due to the _given_ change in the
distribution of solid matter. The remark he makes, that it is round
the centre of gravity of the altered solid and altered liquid that
the altering liquid surface adjusts itself, expresses the essence of
Laplace’s celebrated demonstration of the stability of the ocean, and
suggests the proper elementary solution of the problem to find the
true alteration of sea-level produced by a given alteration of the
solid. As an assumption leading to a simple calculation, let us suppose
the solid earth to rise out of the water in a vast number of small
flat-topped islands, each bounded by a perpendicular cliff, and let the
proportion of water area to the whole be equal in all quarters. Let all
of these islands in one hemisphere be covered with ice, of thickness
according to the law assumed by Mr. Croll—that is, varying in simple
proportion of the sine of the latitude. Let this ice be removed from
the first hemisphere and similarly distributed over the islands of
the second. By working out according to Mr. Croll’s directions, it is
easily found that the change of sea-level which this will produce will
consist in a sinking in the first hemisphere and rising in the second,
through heights varying according to the same law (that is, simple
proportionality to sines of latitudes), and amounting at each pole to
(1 - ω)it
—————————,
1 - ωw
where _t_ denotes the thickness of the ice-crust at the pole; _i_ the
ratio of the density of ice, and _w_ that of sea-water to the earth’s
mean density; and ω the ratio of the area of ocean to the whole surface.
“Thus, for instance, if we suppose ω = ⅔, and _t_ = 6,000 feet, and
take ⅙ and 1/(5½) as the densities of ice and water respectively, we
find for the rise of sea-level at one pole, and depression at the other,
⅓ × ⅙ × 6000
————————————,
2 1
1 - — × —
3 5½
or approximately 380 feet.
“I shall now proceed to consider roughly what is the probable extent
of submergence which, during the glacial epoch, may have resulted from
the displacement of the earth’s centre of gravity by means of the
transferrence of the polar ice from the one hemisphere to the other.”
_Difference between Continental-ice and a Glacier._—An ordinary
glacier descends in virtue of the slope of its bed, and, as a general
rule, it is on this account thin at its commencement, and thickens
as it descends into the lower valleys, where the slope is less and
the resistance to motion greater. But in the case of continental ice
matters are entirely different. The slope of the ground exercises
little or no influence on the motion of the ice. In a continent of one
or two thousand miles across, the general slope of the ground may be
left out of account; for any slight elevation which the centre of such
a continent may have will not compensate for the resistance offered to
the flow of the ice by mountain ridges, hills, and other irregularities
of its surface. The ice can move off such a surface only in consequence
of pressure acting from the interior. In order to produce such a
pressure, there must be a piling up of the ice in the interior; or, in
other words, the ice-sheet must thicken from the edge inwards to the
centre. We are necessarily led to the same conclusion, though we should
not admit that the ice moves in consequence of pressure from behind,
but should hold, on the contrary, that each particle of ice moves by
gravity in virtue of its own weight; for in order to have such a motion
there must be a slope, and as the slope is not on the ground, it must
be on the ice itself: consequently we must conclude that the upper
surface of the ice slopes upwards from the edge to the interior. What,
then, is the least slope at which the ice will descend? Mr. Hopkins
found that ice barely moves on a slope of one degree. We have therefore
some data for arriving at least at a rough estimate of the probable
thickness of an ice-sheet covering a continent, such, for example, as
Greenland or the Antarctic Continent.
_Probable Thickness of the Antarctic Ice-cap._—The antarctic continent
is generally believed to extend, on an average, from the South Pole
down to about, at least, lat. 70°. In round numbers, we may take the
diameter of this continent at 2,800 miles. The distance from the
edge of this ice-cap to its centre, the South Pole, will, therefore,
be 1,400 miles. The whole of this continent, like Greenland, is
undoubtedly covered with one continuous sheet of ice gradually
thickening inwards from its edge to its centre. A slope of one degree
continued for 1,400 miles will give twenty-four miles as the thickness
of the ice at the pole. But suppose the slope of the upper surface
of the cap to be only one-half this amount, viz., a half degree,—and
we have no evidence that a slope so small would be sufficient to
discharge the ice,—still we have twelve miles as the thickness of the
cap at the pole. To those who have not been accustomed to reflect on
the physical conditions of the problem, this estimate may doubtless
be regarded as somewhat extravagant; but a slight consideration
will show that it would be even more extravagant to assume that a
slope of less than half a degree would be sufficient to produce the
necessary outflow of the ice. In estimating the thickness of a sheet of
continental ice of one or two thousand miles across, our imagination
is apt to deceive us. We can easily form a pretty accurate sensuous
representation of the thickness of the sheet; but we can form no
adequate representation of its superficial area. We can represent
to the mind with tolerable accuracy a thickness of a few miles, but
we cannot do this in reference to the area of a surface 2,800 miles
across. Consequently, in judging what proportion the thickness of the
sheet should bear to its superficial area, we are apt to fall into the
error of under-estimating the thickness. We have a striking example
of this in regard to the ocean. The thing which impresses us most
forcibly in regard to the ocean is its profound depth. A mean depth
of, say, three miles produces a striking impression; but if we could
represent to the mind the vast area of the ocean as correctly as we can
do its depth, _shallowness_ rather than _depth_ would be the impression
produced. A sheet of water 100 yards in diameter, and only one inch
deep, would not be called a _deep_ but a very _shallow_ pool or thin
layer of water. But such a layer would be a correct representation of
the ocean in miniature. Were we in like manner to represent to the eye
in miniature the antarctic ice-cap, we would call it a _thin crust of
ice_. Taking the mean thickness of the ice at four miles, the antarctic
ice-sheet would be represented by a carpet covering the floor of an
ordinary-sized dining-room. Were those who consider the above estimate
of the thickness of the antarctic ice-cap as extravagantly great called
upon to sketch on paper a section of what they should deem a cap of
moderate thickness, ninety-nine out of every hundred would draw one of
much greater thickness than twelve miles at the centre.
The diagram on following page (Fig. 7) represents a section across the
cap drawn to a natural scale; the upper surface of the sheet having
a slope of half a degree. No one on looking at the section would
pronounce it to be too thick at the centre, unless he were previously
made aware that it represented a thickness of twelve miles at that
place. It may be here mentioned that had the section been drawn upon
a much larger scale—had it, for instance, been made seven feet long,
instead of seven inches—it would have shown to the eye in a more
striking manner the thinness of the cap.
But to avoid all objections on the score of over-estimating the
thickness of the cap, I shall assume the angle of the upper surface to
be only a quarter of a degree, and the thickness of the sheet one-half
what it is represented in the section. The thickness at the pole will
then be only six miles instead of twelve, and the mean thickness of the
cap two instead of four miles.
[Illustration: Fig. 7.
Section across Antarctic Ice-cap, drawn to a natural scale.
Length represented by section = 2,800 miles. Thickness at centre
(South Pole) = 12 miles.
Slope of upper surface = half-degree.]
Is there any well-grounded reason for concluding the above to be an
over-estimate of the actual thickness of the antarctic ice? It is not
so much in consequence of any _à priori_ reason that can be urged
against the probability of such a thickness of ice, but rather because
it so far transcends our previous experience that we are reluctant to
admit such an estimate. If we never had any experience of ice thicker
than what is found in England, we should feel startled on learning for
the first time that in the valleys of Switzerland the ice lay from 200
to 300 feet in depth. Again, if we had never heard of glaciers thicker
than those of Switzerland, we could hardly credit the statement that
in Greenland they are actually from 2,000 to 3,000 feet thick. We, in
this country, have long been familiar with Greenland; but till very
lately no one ever entertained the idea that that continent was buried
under one continuous mass of ice, with scarcely a mountain top rising
above the icy mantle. And had it not been that the geological phenomena
of the glacial epoch have for so many years accustomed our minds to
such an extraordinary condition of things, Dr. Rink’s description of
the Greenland ice would probably have been regarded as the extravagant
picture of a wild imagination.
Let us now consider whether or not the facts of observation and
experience, so far as they go, bear out the conclusions to which
physical considerations lead us in reference to the magnitude of
continental ice; and more especially as regards the ice of the
antarctic regions.
_First._ In so far as the antarctic ice-sheet is concerned, observation
and experience to a great extent may be said to be a perfect blank. One
or two voyagers have seen the outer edge of the sheet at a few places,
and this is all. In fact, we judge of the present condition of the
interior of the antarctic continent in a great measure from what we
know of Greenland. But again, our experience of Greenland ice is almost
wholly confined to the outskirts.
Few have penetrated into the interior, and, with the exception of Dr.
Hayes and Professor Nordenskjöld, none, as far as I know, have passed
to any considerable distance over the inland ice. Dr. Robert Brown
in his interesting memoir on “Das Innere von Grönland,”[207] gives
an account of an excursion made in 1747 by a Danish officer of the
name of Dalager, from Fredrikshaab, near the southern extremity of
the continent, into the interior. After a journey of a day or two, he
reached an eminence from which he saw the inland ice stretching in an
unbroken mass as far as the eye could reach, but was unable to proceed
further. Dr. Brown gives an account also of an excursion made in the
beginning of March, 1830, by O. B. Kielsen, a Danish whale-fisher, from
Holsteinborg (lat. 67° N.). After a most fatiguing journey of several
days, he reached a high point from which he could see the ice of the
interior. Next morning he got up early, and towards midday reached
an extensive plain. From this the land sank inwards, and Kielsen now
saw fully in view before him the enormous ice-sheet of the interior.
He drove rapidly over all the little hills, lakes, and streams, till
he reached a pretty large lake at the edge of the ice-sheet. This was
the end of his journey, for after vainly attempting to climb up on the
ice-sheet, he was compelled to retrace his steps, and had a somewhat
difficult return. When he arrived at the fiord, he found the ice broken
up, so that he had to go round by the land way, by which he reached
the depôt on the 9th of March. The distance which he traversed in a
straight line from Holsteinborg into the interior measured eighty
English miles.
Dr. Hayes’s excursion was made, however, not upon the real inland
ice, but upon a smaller ice-field connected with it; while Professor
Nordenskjöld’s excursion was made at a place too far south to
afford an accurate idea of the actual condition of the interior of
North Greenland, even though he had penetrated much farther than he
actually did. However, the state of things as recorded by Hayes and by
Nordenskjöld affords us a glimpse into the condition of things in the
interior of the continent. They both found by observation, what follows
as a necessary result from physical considerations, that the upper
surface of the ice plain, under which hills and valleys are buried,
gradually _slopes upwards towards the interior of the continent_.
Professor Nordenskjöld states that when at the extreme point at which
he reached, thirty geographical miles from the coast, he had attained
an elevation of 2,200 feet, and that the inland ice _continued
constantly to rise_ towards the interior, so that the horizon towards
the east, north, and south, was terminated by an ice-border almost as
smooth as that of the ocean.”[208]
Dr. Hayes and his party penetrated inwards to the distance of about
seventy miles. On the first day they reached the foot of the great Mer
de Glace; the second day’s journey carried them to the upper surface
of the ice-sheet. On the third day they travelled 30 miles, and the
ascent, which had been about 6°, diminished gradually to about 2°. They
advanced on the fourth day about 25 miles; the temperature being 30°
below zero (Fah.). “Our station at the camp,” he says, “was sublime as
it was dangerous. We had attained an altitude of 5,000 feet above the
sea-level, and were 70 miles from the coast, in the midst of a vast
frozen Sahara immeasurable to the human eye. There was neither hill,
mountain, nor gorge, anywhere in view. We had completely sunk the
strip of land between the Mer de Glace and the sea, and no object met
the eye but our feeble tent, which bent to the storm. Fitful clouds
swept over the face of the full-orbed moon, which, descending towards
the horizon, glimmered through the drifting snow that scudded over the
icy plain—to the eye in undulating lines of downy softness, to the
flesh in showers of piercing darts.”[209]
Dr. Rink, referring to the inland ice, says that the elevation or
height above the sea of this icy plain at its junction with the
outskirts of the country, and where it begins to lower itself through
the valleys to the firths, is, in the ramifications of the Bay of
Omenak, found to be 2,000 feet, from which level _it gradually rises
towards the interior_.[210]
Dr. Robert Brown, who, along with Mr. Whymper in 1867, attempted a
journey to some distance over the inland ice, is of opinion that
Greenland is not traversed by any ranges of mountains or high land,
but that the entire continent, 1,200 miles in length and 400 miles in
breadth, is covered with one continuous unbroken field of ice, the
upper surface of which, he says, _rises by a gentle slope towards the
interior_.[211]
Suppose now the point reached by Hayes to be within 200 miles of
the centre of dispersion of the ice, and the mean slope from that
point to the centre, as in the case of the antarctic cap, to be only
half a degree; this would give 10,000 feet as the elevation of the
centre above the point reached. But the point reached was 5,000 feet
above sea-level, consequently the surface of the ice at the centre
of dispersion would be 15,000 feet above sea-level, which is about
one-fourth what I have concluded to be the elevation of the surface
of the antarctic ice-cap at its centre. And supposing we assume
the general surface of the ground to have in the central region an
elevation as great as 5,000 feet, which is not at all probable, still
this would give 10,000 feet for the thickness of the ice at the centre
of the Greenland continent. But if we admit this conclusion in
reference to the thickness of the Greenland ice, we must admit that
the antarctic ice is far thicker, because the thickness, other things
being equal, will depend upon the size, or, more properly, upon the
diameter of the continent; for the larger the surface the greater is
the thickness of ice required to produce the pressure requisite to make
the rate of discharge of the ice equal to the rate of increase. Now
the area of the antarctic continent must be at least a dozen of times
greater than that of Greenland.
_Second._ That the antarctic ice must be far thicker than the arctic
is further evident from the dimensions of the icebergs which have been
met with in the Southern Ocean. No icebergs over three hundred feet in
height have been found in the arctic regions, whereas in the antarctic
regions, as we shall see, icebergs of twice and even thrice that height
have been reported.
_Third._ We have no reason to believe that the thickness of the ice
at present covering the antarctic continent is less than that which
covered a continent of a similar area in temperate regions during the
glacial epoch. Take, for example, the North American continent, or,
more properly, that portion of it covered by ice during the glacial
epoch. Professor Dana has proved that during that period the thickness
of the ice on the American continent must in many places have been
considerably over a mile. He has shown that over the northern border of
New England the ice had a mean thickness of 6,500 feet, while its mean
thickness over the Canada watershed, between St. Lawrence and Hudson’s
Bay, was not less than 12,000 feet, or upwards of two miles and a
quarter (see _American Journal of Science and Art_ for March, 1873).
_Fourth._ Some may object to the foregoing estimate of the amount of
ice on the antarctic continent, on the grounds that the quantity of
snowfall in that region cannot be much. But it must be borne in mind
that, no matter however small the annual amount of snowfall may be, if
more falls than is melted, the ice must continue to accumulate year by
year till its thickness in the centre of the continent be sufficiently
great to produce motion. The opinion that the snowfall of the antarctic
regions is not great does not, however, appear to be borne out by the
observation and experience of those who have visited those regions.
Captain Wilkes, of the American Exploring Expedition, estimated it at
30 feet per annum; and Sir James Ross says, that during a whole month
they had only three days free from snow. The fact that perpetual snow
is found at the sea-level at lat. 64° S. proves that the snowfall
must be great. But there is another circumstance which must be taken
into account, viz., that the currents carrying moisture move in from
all directions towards the pole, consequently the area on which they
deposit their snow becomes less and less as the pole is reached, and
this must, to a corresponding extent, increase the quantity of snow
falling on a given area. Let us assume, for example, that the clouds
in passing from lat. 60° to lat. 80° deposit moisture sufficient to
produce, say, 30 feet of snow per annum, and that by the time they
reach lat. 80° they are in possession of only one-tenth part of their
original store of moisture. As the area between lat. 80° and the
pole is but one-eighth of that between lat. 60° and 80°, this would,
notwithstanding, give 24 feet as the annual amount of snowfall between
lat. 80° and the pole.[212]
_Fifth._ The enormous size and thickness of the icebergs which have
been met with in the Southern Ocean testify to the thickness of the
antarctic ice-cap.
We know from the size of some of the icebergs which have been met with
in the southern hemisphere that the ice at the edge of the cap where
the bergs break off must in some cases be considerably over a mile in
thickness, for icebergs of more than a mile in thickness have been
found in the southern hemisphere. The following are the dimensions of
a few of these enormous bergs taken from the Twelfth Number of the
Meteorological Papers published by the Board of Trade, and from the
excellent paper of Mr. Towson on the Icebergs of the Southern Ocean,
published also by the Board of Trade.[213] With one or two exceptions,
the heights of the bergs were accurately determined by angular
measurement:—
Sept. 10th, 1856.—The _Lightning_, when in lat. 55° 33′ S.,
long. 140° W., met with an iceberg 420 feet high.
Nov., 1839.—In lat. 41° S., long. 87° 30′ E., numerous icebergs
400 feet high were met with.
Sept., 1840.—In lat. 37° S., long. 15° E., an iceberg 1,000
feet long and 400 feet high was met with.
Feb., 1860.—Captain Clark, of the _Lightning_, when in lat. 55°
20′ S., long. 122° 45′ W., found an iceberg 500 feet high and 3
miles long.
Dec. 1st, 1859.—An iceberg, 580 feet high, and from two and a
half to three miles long, was seen by Captain Smithers, of the
_Edmond_, in lat. 50° 52′ S., long. 43° 58′ W. So strongly did
this iceberg resemble land, that Captain Smithers believed it
to be an island, and reported it as such, but there is little
or no doubt that it was in reality an iceberg. There were
pieces of drift-ice under its lee.
Nov., 1856.—Three large icebergs, 500 feet high, were found in
lat. 41° 0′ S., long. 42° 0′ E.
Jan., 1861.—Five icebergs, one 500 feet high, were met with in
lat. 55° 46′ S., long. 155° 56′ W.
Jan., 1861.—In lat. 56° 10′ S., long. 160° 0′ W., an iceberg
500 feet high and half a mile long was found.
Jan., 1867.—The barque _Scout_, from the West Coast of
America, on her way to Liverpool, passed some icebergs 600 feet
in height, and of great length.
April, 1864.—The _Royal Standard_ came in collision with an
iceberg 600 feet in height.
Dec., 1856.—Four large icebergs, one of them 700 feet high, and
another 500 feet, were met with in lat. 50° 14′ S., long. 42°
54′ E.
Dec. 25th, 1861.—The _Queen of Nations_ fell in with an iceberg
in lat. 53° 45′ S., long. 170° 0′ W., 720 feet high.
Dec., 1856.—Captain P. Wakem, ship _Ellen Radford_, found, in
lat. 52° 31′ S., long. 43° 43′ W., two large icebergs, one at
least 800 feet high.
Mr. Towson states that one of our most celebrated and talented
naval surveyors informed him that he had seen icebergs in the
southern regions 800 feet high.
March 23rd, 1855.—The _Agneta_ passed an iceberg in lat. 53°
14′ S., long. 14° 41′ E., 960 feet in height.
Aug. 16th, 1840.—The Dutch ship, _General Baron von Geen_,
passed an iceberg 1,000 feet high in lat. 37° 32′ S., long. 14°
10′ E.
May 15th, 1859.—The _Roseworth_ found in lat. 53° 40′ S., long.
123° 17′ W., an iceberg as large as “Tristan d’Acunha.”
In the regions where most of these icebergs were met with, the mean
density of the sea is about 1·0256. The density of ice is ·92. The
density of icebergs to that of the sea is therefore as 1 to 1·115;
consequently every foot of ice above water indicates 8·7 feet below
water. It therefore follows that those icebergs 400 feet high had 3,480
feet under water,—3,880 feet would consequently be the total thickness
of the ice. The icebergs which were 500 feet high would be 4,850 feet
thick, those 600 feet high would have a total thickness of 5,820 feet,
and those 700 feet high would be no less than 6,790 feet thick, which
is more than a mile and a quarter. The iceberg 960 feet high, sighted
by the _Agneta_, would be actually 9,312 feet thick, which is upwards
of a mile and three-quarters.
Although the mass of an iceberg below water compared to that above
may be taken to be about 8·7 to 1, yet it would not be always safe
to conclude that the thickness of the ice below water bears the same
proportion to its height above. If the berg, for example, be much
broader at its base than at its top, the thickness of the ice below
water would bear a less proportion to the height above water than
as 8·7 to 1. But a berg such as that recorded by Captain Clark, 500
feet high and three miles long, must have had only 1/8·7 of its total
thickness above water. The same remark applies also to the one seen by
Captain Smithers, which was 580 feet high, and so large that it was
taken for an island. This berg must have been 5,628 feet in thickness.
The enormous berg which came in collision with the _Royal Standard_
must have been 5,820 feet thick. It is not stated what length the
icebergs 730, 960, and 1,000 feet high respectively were; but supposing
that we make considerable allowance for the possibility that the
proportionate thickness of ice below water to that above may have been
less than as 8·7 to 1, still we can hardly avoid the conclusion that
the icebergs were considerably above a mile in thickness. But if there
are icebergs above a mile in thickness, then there must be land-ice
somewhere on the southern hemisphere of that thickness. In short, the
great antarctic ice-cap must in some places be over a mile in thickness
at its edge.
_Inadequate Conceptions regarding the Magnitude of Continental
Ice._—Few things have tended more to mislead geologists in the
interpretation of glacial phenomena than inadequate conceptions
regarding the magnitude of continental ice. Without the conception
of continental ice the known facts connected with glaciation would
be perfectly inexplicable. It was only when it was found that the
accumulated facts refused to be explained by any other conception,
that belief in the very existence of such a thing as continental ice
became common. But although most geologists now admit the existence of
continental ice, yet, nevertheless, adequate conceptions of its real
magnitude are by no means so common. Year by year, as the outstanding
facts connected with glaciation accumulate, we are compelled to extend
our conceptions of the magnitude of land-ice. Take the following as
an example. It was found that the transport of the Wastdale Crag
blocks, the direction of the striæ on the islands of the Baltic, on
Caithness and on the Orkney, Shetland, and Faroe, islands, the boulder
clay with broken shells in Caithness, Holderness, and other places,
were inexplicable on the theory of land-ice. But it was so only in
consequence of the inadequacy of our conceptions of the magnitude of
the ice; for a slight extension of our ideas of its thickness has
explained not only these phenomena,[214] but others of an equally
remarkable character, such as the striation of the Long Island and
the submerged rock-basins around our coasts described by Mr. James
Geikie. In like manner, if we admit the theory of the glacial epoch
propounded in former chapters, all that is really necessary to account
for the submergence of the land is a slight extension of our hitherto
preconceived estimate of the thickness of the ice on the antarctic
continent. If we simply admit a conclusion to which all physical
considerations, as we have seen, necessarily lead us, viz., that the
antarctic continent is covered with a mantle of ice at least two miles
in thickness, we have then a complete explanation of the cause of the
submergence of the land during the glacial epoch.
Although of no great importance to the question under consideration, it
may be remarked that, except during the severest part of the glacial
epoch, we have no reason to believe that the total quantity of ice
on the globe was much greater than at present, only it would then be
all on one hemisphere. Remove two miles of ice from the antarctic
continent, and place it on the northern hemisphere, and this, along
with the ice that now exists on this hemisphere, would equal, in all
probability, the quantity existing on our hemisphere during the glacial
epoch; at least, before it reached its maximum severity.
CHAPTER XXIV.
THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE OF THE LAND
DURING THE GLACIAL EPOCH.—_Continued._
Extent of Submergence from Displacement of Earth’s Centre
of Gravity.—Circumstances which show that the Glacial
Submergence resulted from Displacement of the Earth’s
Centre of Gravity.—Agreement between Theory and observed
Facts.—Sir Charles Lyell on submerged Areas during
Tertiary Period.—Oscillations of Sea-level in Relation to
Distribution.—Extent of Submergence on the Hypothesis that
the Earth is fluid in the Interior.
_Extent of Submergence from Displacement of Earth’s Centre of
Gravity._—How much, then, would the transference of the two miles of
ice from the southern to the northern hemisphere raise the level of the
ocean on the latter hemisphere? This mass, be it observed, is equal to
only one-half that represented in our section. A considerable amount
of discussion has arisen in regard to the method of determining this
point. According to the method already detailed, which supposes the
rise at the pole to be equal to the extent of the displacement of the
earth’s centre of gravity, the rise at the North Pole would be about
380 feet, taking into account the effect produced by the displaced
water; and the rise in the latitude of Edinburgh would be 312 feet. The
fall of level on the southern hemisphere would, of course, be equal to
the rise of level on the northern. According to the method advanced
by Mr. D. D. Heath,[215] the rise of level at the North Pole would be
about 650 feet. Archdeacon Pratt’s method[216] makes the rise still
greater; while according to Rev. O. Fisher’s method[217] the rise would
be no less than 2,000 feet. There is, however, another circumstance
which must be taken into account, which will give an additional rise of
upwards of one hundred feet.
The greatest extent of the displacement of the earth’s centre of
gravity, and consequently the greatest rise of the ocean resulting
from that displacement, would of course occur at the time of maximum
glaciation, when the ice was all on one hemisphere. But owing to the
following circumstance, a still greater rise than that resulting from
the displacement of the earth’s centre of gravity alone might take
place at some considerable time, either before or after the period of
maximum glaciation.
It is not at all probable that the ice would melt on the warm
hemisphere at exactly the same rate as it would form on the cold
hemisphere. It is probable that the ice would melt more rapidly on the
warm hemisphere than it would form on the cold. Suppose that during
the glacial epoch, at a time when the cold was gradually increasing on
the northern and the warmth on the southern hemisphere, the ice should
melt more rapidly off the antarctic continent than it was being formed
on the arctic and subarctic regions; suppose also that, by the time
a quantity of ice, equal to one-half what exists at present on the
antarctic continent, had accumulated on the northern hemisphere, the
whole of the antarctic ice had been melted away, the sea would then be
fuller than at present by the amount of water resulting from the one
mile of melted ice. The height to which this would raise the general
level of the sea would be as follows:—
The antarctic ice-cap is equal in area to 1/23·46 of that covered by
the ocean. The density of ice to that of water being taken at ·92 to
1, it follows that 25 feet 6 inches of ice melted off the cap would
raise the general level of the ocean one foot, and the one mile of
ice melted off would raise the level 200 feet. This 200 feet of rise
resulting from the melted ice we must add to the rise resulting from
the displacement of the earth’s centre of gravity. The removal of the
two miles of ice from the antarctic continent would displace the
centre of gravity 190 feet, and the formation of a mass of ice equal
to the one-half of this on the arctic regions would carry the centre
of gravity 95 feet farther; giving in all a total displacement of 285
feet, thus producing a rise of sea-level at the North Pole of 285 feet,
and in the latitude of Edinburgh of 234 feet. Add to this the rise of
200 feet resulting from the melted ice, and we have then 485 feet of
submergence at the pole, and 434 feet in the latitude of Edinburgh. A
rise to a similar extent might probably take place after the period
of maximum glaciation, when the ice would be melting on the northern
hemisphere more rapidly than it would be forming on the southern.
If we assume the antarctic ice-cap to be as thick as is represented in
the diagram, the extent of the submergence would of course be double
the above, and we might have in this case a rise of sea-level in the
latitude of Edinburgh to the extent of from 800 to 1,000 feet. But be
this as it may, it is evident that the quantity of ice on the antarctic
continent is perfectly sufficient to account for the submergence of
the glacial epoch, for we have little evidence to conclude that the
_general_ submergence much exceeded 400 or 500 feet.[218] We have
evidence in England and other places of submergence to the extent of
from 1,000 to 2,000 feet, but these may be quite local, resulting
from subsidence of the land in those particular areas. Elevations and
depressions of the land have taken place in all ages, and no doubt
during the glacial epoch also.
_Circumstances which show that the Glacial Submergence resulted from
Displacement of the Earth’s Centre of Gravity._—In favour of this
view of the cause of the submergence of the glacial epoch, it is a
circumstance of some significance, that in every part of the globe
where glaciation has been found evidence of the submergence of the
land has also been found along with it. The invariable occurrence of
submergence along with glaciation points to some physical connection
between the two. It would seem to imply, either that the two were the
direct effects of a common cause, or that the one was the cause of the
other; that is, the submergence the cause of the glaciation, or the
glaciation the cause of the submergence. There is, I presume, no known
cause to which the two can be directly related as effects. Nor do I
think that there is any one who would suppose that the submergence of
the land could have been the cause of its glaciation, even although he
attributed all glacial effects to floating ice. The submergence of our
country would, of course, have allowed floating ice to pass over it had
there been any to pass over; but submergence would not have produced
the ice, neither would it have brought the ice from the arctic regions
where it already existed. But although submergence could not have been
the cause of the glacial epoch, yet we can, as we have just seen,
easily understand how the ice of the glacial epoch could have been the
cause of the submergence. If the glacial epoch was brought about by an
increase in the eccentricity of the earth’s orbit, then a submergence
of the land as the ice accumulated was a physical necessity.
There is another circumstance connected with glacial submergence which
it is difficult to reconcile with the idea that it resulted from a
subsidence of the land. It is well known that during the glacial
epoch the land was not once under water only, but several times; and,
besides, there were not merely several periods when the land stood
at a lower level in relation to the sea than at present, but there
were also several periods when it stood at a much higher level than
now. And this holds true, not merely of our own country, but of every
country on the northern hemisphere where glaciation has yet been found.
All this follows as a necessary consequence from the theory that the
oscillations of sea-level resulted from the transference of the ice
from the one hemisphere to the other; but it is wholly inconsistent
with the idea that they resulted from upheavals and subsidence of the
land during a very recent period.
But this is not all, there is more still to be accounted for. It has
been the prevailing opinion that at the time when the land was covered
with ice, it stood at a much greater elevation than at present. It
is, however, not maintained that the facts of geology establish such
a conclusion. The greater elevation of the land is simply assumed as
an hypothesis to account for the cold.[219] The facts of geology,
however, are fast establishing the opposite conclusion, viz., that
when the country was covered with ice, the land stood in relation to
the sea at a lower level than at present, and that the continental
periods or times when the land stood in relation to the sea at a higher
level than now were the warm inter-glacial periods, when the country
was free of snow and ice, and a mild and equable condition of climate
prevailed. This is the conclusion towards which we are being led by the
more recent revelations of surface geology, and also by certain facts
connected with the geographical distribution of plants and animals
during the glacial epoch.
The simple occurrence of a rise and fall of the land in relation to
the sea-level in one or in two countries during the glacial epoch,
would not necessarily imply any physical connection. The coincidence
of these movements with the glaciation of the land might have been
purely accidental; but when we find that a succession of such movements
occurred, not merely in one or in two countries, but in every glaciated
country where proper observations have been made, we are forced to the
conclusion that the connection between the two is not accidental, but
the result of some fixed cause.
If we admit that an increase in the eccentricity of the earth’s orbit
was the cause of the glacial epoch, then we must admit that all those
results followed as necessary consequences. For if the glacial epoch
lasted for upwards of one hundred thousand years or so, there would be
a succession of cold and warm periods, and consequently a succession
of elevations and depressions of sea-level. And the elevations of
the sea-level would take place during the cold periods, and the
depressions during the warm periods.
But the agreement between theory and observed facts does not terminate
here. It follows from theory that the greatest oscillations of
sea-level would take place during the severest part of the glacial
epoch, when the eccentricity of the earth’s orbit would be at its
highest value, and that the oscillations would gradually diminish
in extent as the eccentricity diminished and the climate gradually
became less severe. Now it is well known that this is actually what
took place; the great submergence, as well as the great elevation or
continental period, occurred during the earlier or more severe part of
the glacial epoch, and as the climate grew less severe these changes
became of less extent, till we find them terminating in our submerged
forests and 25-foot raised beach.
It follows, therefore, according to the theory advanced, that the mere
fact of an area having been under sea does not imply that there has
been any subsidence or elevation of the land, and that consequently the
inference which has been drawn from these submerged areas as to changes
in physical geography may be in many cases not well founded.
Sir Charles Lyell, in his “Principles,” publishes a map showing the
extent of surface in Europe which has been covered by the sea since
the earlier part of the Tertiary period. This map is intended to show
the extraordinary amount of subsidence and elevation of the land which
has taken place during that period. It is necessary for Sir Charles’s
theory of the cause of the glacial epoch that changes in the physical
geography of the globe to an enormous extent should have taken place
during a very recent period, in order to account for the great change
of climate which occurred at that epoch. But if the foregoing results
be anything like correct, it does not necessarily follow that there
must have been great changes in the physical geography of Europe,
simply because the sea covered those areas marked in the map, for this
may have been produced by oscillations of sea-level, and not by changes
in the land. In fact, the areas marked in Sir Charles’s map as having
been covered by the sea, are just those which would be covered were the
sea-level raised a few hundred feet. No doubt there were elevations and
subsidences in many of the areas marked in the map during the Tertiary
period, and to this cause a considerable amount of the submergence
might be due; but I have little doubt that by far the greater part
must be attributed to oscillations of sea-level. It is no objection
that the greater part of the shells and other organic remains found
in the marine deposits of those areas are not indicative of a cold
or glacial condition of climate, for, as we have seen, the greatest
submergence would probably have taken place either before the more
severe cold had set in or after it had to a great extent passed away.
That the submergence of those areas probably resulted from elevations
of sea-level rather than depressions of the land, is further evident
from the following considerations. If we suppose that the climate of
the glacial epoch was brought about mainly by changes in the physical
geography of the globe, we must assume that these great changes took
place, geologically speaking, at a very recent date. Then when we ask
what ground is there for assuming that any such change in the relations
of sea and land as is required actually took place, the submergence
of those areas is adduced as the proof. Did it follow as a physical
necessity that all submergence must be the result of subsidence of the
land, and not of elevations of the sea, there would be some force in
the reasons adduced. But such a conclusion by no means follows, and,
_à priori_, it is just as likely that the appearance of the ice was
the cause of the submergence as that the submergence was the cause
of the appearance of the ice. Again, a subsidence of the land to the
extent required would to a great extent have altered the configuration
of the country, and the main river systems of Europe; but there is no
evidence that any such change has taken place. All the main valleys
are well known to have existed prior to the glacial epoch, and our
rivers to have occupied the same channels then as they do now. In the
case of some of the smaller streams, it is true, a slight deviation
has resulted at some points from the filling up of their channels with
drift during the glacial epoch; but as a general rule all the principal
valleys and river systems are older than the glacial epoch. This, of
course, could not be the case if a subsidence of the land sufficiently
great to account for the submergence of the areas in question, or
changes in the physical geography of Europe necessary to produce a
glacial epoch, had actually taken place. The total absence of any
geological evidence for the existence of any change which could explain
either the submergence of the areas in question or the climate of the
glacial epoch, is strong evidence that the submergence of the glacial
epoch, as well as of the areas in question, was the result of a simple
oscillation of sea-level resulting from the displacement of the earth’s
centre of gravity by the transferrence of the ice-cap from the southern
to the northern hemisphere.
_Oscillations of Sea-level in relation to Distribution._—The
oscillations of sea-level resulting from the displacement of the
earth’s centre of gravity help to throw new light on some obscure
points connected with the subject of the geographical distribution
of plants and animals. At the time when the ice was on the southern
hemisphere during the glacial epoch, and the northern hemisphere was
enjoying a warm and equable climate, the sea-level would be several
hundred feet lower than at present, the North Sea would probably be
dry land, and Great Britain and Ireland joined to the continent, thus
opening up a pathway from the continent to our island. As has been
shown in former chapters, during the inter-glacial periods the climate
would be much warmer and more equable than now, so that animals from
the south, such as the hippopotamus, hyæna, lion, _Elephas antiquus_
and _Rhinoceros megarhinus_, would migrate into this country, where
at present they could not live in consequence of the cold. We have
therefore an explanation, as was suggested on a former occasion,[220]
of the fact that the bones of these animals are found mingled in the
same grave with those of the musk-ox, mammoth, reindeer, and other
animals which lived in this country during the cold periods of the
glacial epoch; the animals from the north would cross over into this
country upon the frozen sea during the cold periods, while those from
the south would find the English Channel dry land during the warm
periods.
The same reasoning will hold equally true in reference to the old
and new world. The depth of Behring Straits is under 30 fathoms;
consequently a lowering of the sea-level of less than 200 feet would
connect Asia with America, and thus allow plants and animals, as Mr.
Darwin believes, to pass from the one continent to the other.[221]
During this period, when Behring Straits would be dry land, Greenland
would be comparatively free from ice, and the arctic regions enjoying a
comparatively mild climate. In this case plants and animals belonging
to temperate regions could avail themselves of this passage, and thus
we can explain how plants belonging to temperate regions may have,
during the Miocene period, passed from the old to the new continent,
and _vice versâ_.
As has already been noticed, during the time of the greatest extension
of the ice, the quantity of ice on the southern hemisphere might be
considerably greater than what exists on the entire globe at present.
In that case there might, in addition to the lowering of the sea-level
resulting from the displacement of the earth’s centre of gravity, be a
considerable lowering resulting from the draining of the ocean to form
the additional ice. This decrease and increase in the total quantity
of ice which we have considered would affect the level of the ocean as
much at the equator as at the poles; consequently during the glacial
epoch there might have been at the equator elevations and depressions
of sea-level to the extent of a few hundred feet.
_Extent of Submergence on the Hypothesis that the Earth is fluid in
the Interior._—But we have been proceeding upon the supposition that
the earth is solid to its centre. If we assume, however, what is the
general opinion among geologists, that it consists of a fluid interior
surrounded by a thick and rigid crust or shell, then the extent of the
submergence resulting from the displacement of the centre of gravity
for a given thickness of ice must be much greater than I have estimated
it to be. This is evident, because, if the interior of the globe be in
a fluid state, it, in all probability, consists of materials differing
in density. The densest materials will be at the centre, and the least
dense at the outside or surface. Now the transferrence of an ice-cap
from the one pole to the other will not merely displace the ocean—the
fluid mass on the outside of the shell—but it will also displace the
heavier fluid materials in the interior of the shell. In other words,
the heavier materials will be attracted by the ice-cap more forcibly
than the lighter, consequently they will approach towards the cap to a
certain extent, sinking, as it were, into the lighter materials, and
displacing them towards the opposite pole. This displacement will of
course tend to shift the earth’s centre of gravity in the direction
of the ice-cap, because the heavier materials are shifted in this
direction, and the lighter materials in the opposite direction. This
process will perhaps be better understood from the following figures.
[Illustration: Fig. 8. Fig. 9.
O. The Ocean.
S. Solid Crust or Shell.
F, F^1, F^2, F^3. The various concentric layers of the fluid
interior. The layers increase in density towards the centre.
I. The Ice-cap.
C. Centre of gravity.
C^1. The displaced centre of gravity.]
In Fig. 8, where there is no ice-cap, the centre of gravity of the
earth coincides with the centre of the concentric layers of the fluid
interior. In Fig. 9, where there is an ice-cap placed on one pole, the
concentric layer F^1 being denser than layer F, is attracted towards
the cap more forcibly than F, and consequently sinks to a certain depth
in F. Again, F^2 being denser than F^1, it also sinks to a certain
extent in F^1. And again F^3, the mass at the centre, being denser than
F^2, it also sinks in F^2. All this being combined with the effects
of the ice-cap, and the displaced ocean outside the shell, the centre
of gravity of the entire globe will no longer be at C, but at C^1, a
considerable distance nearer to the side of the shell on which the
cap rests than C, and also a considerable distance nearer than it
would have been had the interior of the globe been solid. There are
here three causes tending to shift the centre of gravity, (1) the
ice-cap, (2) the displaced ocean, and (3) the displaced materials in
the interior. Two of the three causes mutually react on each other in
such a way as to increase each other’s effect. Thus the more the ocean
is drawn in the direction of the ice-cap, the more effect it has in
drawing the heavier materials in the interior in the same direction;
and in turn the more the heavier materials in the interior are drawn
towards the cap, the greater is the displacement of the earth’s centre
of gravity, and of course, as a consequence, the greater is the
displacement of the ocean. It may be observed also that, other things
being equal, the thinner the solid crust or shell is, and the greater
the difference in the density of the fluid materials in the interior,
the greater will be the extent of the displacement of the ocean,
because the greater will be the displacement of the centre of gravity.
It follows that if we knew (1) the extent of the general submergence of
the glacial epoch, and (2) the present amount of ice on the southern
hemisphere, we could determine whether or not the earth is fluid in the
interior.
CHAPTER XXV.
THE INFLUENCE OF THE OBLIQUITY OF THE ECLIPTIC ON CLIMATE AND ON
THE LEVEL OF THE SEA.
The direct Effect of Change of Obliquity on Climate.—Mr.
Stockwell on the maximum Change of Obliquity.—How Obliquity
affects the Distribution of Heat over the Globe.—Increase of
Obliquity diminishes the Heat at the Equator and increases
that at the Poles.—Influence of Change of Obliquity on the
Level of the Sea.—When the Obliquity was last at its superior
Limit.—Probable Date of the 25-foot raised Beach.—Probable
Extent of Rise of Sea-level resulting from Increase of
Obliquity.—Lieutenant-Colonel Drayson’s and Mr. Belt’s
Theories.—Sir Charles Lyell on Influence of Obliquity.
_The direct Effect of Change in the Obliquity of the Ecliptic on
Climate._—There is still another cause which, I feel convinced, must to
a very considerable extent have affected climate during past geological
ages. I refer to the change in the obliquity of the ecliptic. This
cause has long engaged the attention of geologists and physicists,
and the conclusion generally come to is that no great effect can be
attributed to it. After giving special attention to the matter, I have
been led to the very opposite conclusion. It is quite true, as has
been urged, that the changes in the obliquity of the ecliptic cannot
sensibly affect the climate of temperate regions; but it will produce
a slight change on the climate of tropical latitudes, and a very
considerable effect on that of the polar regions, especially at the
poles themselves. We shall now consider the matter briefly.
It was found by Laplace that the obliquity of the ecliptic will
oscillate to the extent of 1° 22′ 34″ on each side of 23° 28′, the
obliquity in the year 1801.[222] This point has lately been examined
by Mr. Stockwell, and the results at which he has arrived are almost
identical with those of Laplace. “The mean value of the obliquity,” he
says, “of both the apparent and fixed ecliptics to the equator is 23°
17′ 17″. The limits of the obliquity of the apparent ecliptic to the
equator are 24° 35′ 58″ and 21° 58′ 36″; whence it follows that the
greatest and least declinations of the sun at the solstices can never
differ from each other to any greater extent than 2° 37′ 22″.”[223]
This change will but slightly affect the climate of the temperate
regions, but it will exercise a very considerable influence on
the climate of the polar regions. According to Mr. Meech,[224] if
365·24 thermal days represent the present total annual quantity of
heat received at the equator from the sun, 151·59 thermal days will
represent the quantity received at the poles. Adopting his method of
calculation, it turns out that when the obliquity of the ecliptic is at
the maximum assigned by Laplace the quantity received at the equator
would be 363·51 thermal days, and at the poles 160·04 thermal days. The
equator would therefore receive 1·73 thermal days less heat, and the
poles 8·45 thermal days more heat than at present.
ANNUAL AMOUNT OF SUN’S HEAT.
+-------------------+-------------+-----------+
| Amount in 1801. | Amount at | |
| Obliquity 23° 28′.| maximum, |Difference.|
| | 24° 50′ 34“.| |
+---------+---------+-------------+-----------+
|Latitude.| Thermal | Thermal | Thermal |
| | days. | days. | days. |
| 0 | 365·24 | 363·51 | −1·73 |
| 40 | 288·55 | 288·32 | −0·23 |
| 70 | 173·04 | 179·14 | +6·10 |
| 80 | 156·63 | 164·63 | +8·00 |
| 90 | 151·59 | 160·04 | +8·45 |
+---------+---------+-------------+-----------+
When the obliquity was at a maximum, the poles would therefore be
receiving 19 rays for every 18 they are receiving at present. The
poles would then be receiving nearly as much heat as latitude 76° is
receiving at present.
The increase of obliquity would not sensibly affect the polar winter.
It is true that it would slightly increase the breadth of the
frigid zone, but the length of the winter at the poles would remain
unaffected. After the sun disappears below the horizon his rays are
completely cut off, so that a further descent of 1° 22′ 34″ would make
no material difference in the climate. In the temperate regions, the
sun’s altitude at the winter solstice would be 1° 22′ 34″ less than
at present. This would slightly increase the cold of winter in those
regions. But the increase in the amount of heat received by the polar
regions would materially affect the condition of the polar summer.
What, then, is the rise of temperature at the poles which would result
from the increase of 8·45 thermal days in the total amount received
from the sun?
An increase of 8·45 thermal days, or 1/18th of the total quantity
received from the sun, according to the mode of calculation adopted in
Chap. II. would produce, all other things being equal, a rise in the
mean annual temperature equal to 14° or 15°.
According to Professor Dove[225] there is a difference of 7°·6
between the mean annual temperature of latitude 76° and the pole;
the temperature of the former being 9°·8, and that of the latter
2°·2. Since it follows that when the obliquity of the ecliptic is
at a maximum the poles would receive about as much heat per annum
as latitude 76° receives at present, it may be supposed that the
temperature of the poles at that period ought to be no higher than
that of latitude 76° at the present time. A little consideration will,
however, show that this by no means follows. Professor Dove’s Tables
represent correctly the mean annual temperature corresponding to every
tenth degree of latitude from the equator to the pole. But it must be
observed that the rate at which the temperature diminishes from the
equator to the pole is not proportionate to the decrease in the total
quantity of heat received from the sun as we pass from the equator to
the pole. Were the mean annual temperature of the various latitudes
proportionate to the amount of direct heat received, the equator
would be much warmer than it actually is at present, and the poles
much colder. The reason of this, as has been shown in Chapter II., is
perfectly obvious. There is a constant transferrence of _heat_ from
the equator to the poles, and of _cold_ from the poles to the equator.
The warm water of the equator is constantly flowing towards the poles,
and the cold water at the poles is constantly flowing to the equator.
The same is the case in regard to the aërial currents. Consequently
a great portion of the direct heat of the sun goes, not to raise the
temperature of the equator, but to heat the poles. And, on the other
hand, the cold materials at the poles are transferred to the equator,
and thus lower the temperature of that part of the globe to a great
extent. The present difference of temperature between lat. 76° and the
pole, determined according to the rate at which the temperature is
found to diminish between the equator and the pole, amounts to only
about 7° or 8°. But were there no mutual transferrence of warm and
cold materials between the equatorial and polar regions, and were the
temperature of each latitude to depend solely upon the direct rays of
the sun, the difference would far exceed that amount.
Now, when the obliquity of the ecliptic was at its superior limit, and
the poles receiving about 1/18th more direct heat from the sun than
at present, the increase of temperature due to this increase of heat
would be far more than 7° or 8. It would probably be nearly double that
amount.
“We may, therefore, conclude that when the obliquity of the ecliptic
was at a maximum, and the poles were receiving 1/18th more heat than
at present, the temperature of the poles ought to have been about 14°
or 15° warmer than at the present day, _provided, of course, that
this extra heat was employed wholly in raising the temperature_. Were
the polar regions free from snow and ice, the greater portion of the
extra heat would go to raise the temperature. But as those regions
are covered with snow and ice, the extra heat would have no effect in
raising the temperature, but would simply melt the snow and ice. The
ice-covered surface upon which the rays fell could never rise above
32°. At the period under consideration, the total annual quantity of
ice melted at the poles would be 1/18th more than at present.
The general effect which the change in the obliquity of the ecliptic
would have upon the climate of the polar regions when combined with the
effects resulting from the eccentricity of the earth’s orbit, would be
this:—When the eccentricity was at a very high value, the hemisphere
whose winter occurred in the aphelion (for physical reasons, which have
already been discussed)[226] would be under a condition of glaciation,
while the other hemisphere, having its winter in perihelion, would be
enjoying a warm and equable climate. When the obliquity of the ecliptic
was at a maximum, and 1/18th more heat falling at the poles than at
present, the effect would be to modify to a great extent the rigour
of the glaciation in the polar zone of the hemisphere under a glacial
condition, and, on the other hand, to produce a more rapid melting
of the ice on the other hemisphere enjoying the equable climate. The
effects of eccentricity and obliquity thus combined would probably
completely remove the polar ice-cap from off the latter hemisphere,
and forest trees might then grow at the pole. Again, when the obliquity
was at its minimum condition and less heat reaching the poles than at
present, the glaciation of the former hemisphere would be increased and
the warmth of the latter diminished.
_The Influence of Change in the Obliquity of the Ecliptic on the
Level of the Sea._—One very remarkable effect which seems to result
indirectly from a variation of the obliquity under certain conditions,
is an influence on the level of the sea. As this probably may have had
something to do with those recent changes of sea-level with which the
history of the submarine forests and raised beaches have made us all so
familiar, it may be of interest to enter at some length into this part
of this subject.
It appears almost certain that at the time when the northern winter
solstice was in the aphelion last, a rise of the sea on the northern
hemisphere to a considerable number of feet must have taken place from
the combined effect of eccentricity and obliquity. About 11,700 years
ago, the northern winter solstice was in the aphelion. The eccentricity
at that time was ·0187, being somewhat greater than it is now; but the
winters occurring in aphelion instead of, as now, in perihelion, they
would on that account be probably 10° or 15° colder than they are at
the present day. It is probable, also, for reasons stated in a previous
chapter, that the Gulf-stream at that time would be considerably less
than now. This would tend to lower the temperature to a still greater
extent. As snow instead of rain must have fallen during winter to a
greater extent than at present, this no doubt must have produced a
slight increase in the quantity of ice on the northern hemisphere had
no other cause come into operation. But the condition of things, we
have every reason to believe, must have been affected by the greater
obliquity of the ecliptic at that period. We have no formula, except,
perhaps, that given by Mr. Stockwell, from which to determine with
perfect accuracy the extent of the obliquity at a period so remote as
the one under consideration. If we adopt the formula given by Struve
and Peters, which agrees pretty nearly with that obtained from Mr.
Stockwell’s formula, we have the obliquity at a maximum about the time
that the solstice-point was in the aphelion. The formula given by
Leverrier places the maximum somewhat later. At all events, we cannot
be far from the truth in assuming that at the time the northern winter
solstice was in the aphelion, the obliquity of the ecliptic would be
about a maximum, and that since then it has been gradually diminishing.
It is evident, then, that the annual amount of heat received by the
arctic regions, and especially about the pole, would be considerably
greater than at present. And as the heat received on those regions is
chiefly employed in melting the ice, it is probable that the extra
amount of ice which would then be melted in the arctic regions would
prevent that slight increase of ice which would otherwise have resulted
in consequence of the winter occurring in the aphelion. The winters at
that period would be colder than they are at present, but the total
quantity of ice on the northern hemisphere would not probably be
greater.
Let us now turn to the southern hemisphere. As the southern winter
would then occur in the perihelion, this would tend to produce a slight
decrease in the quantity of ice on the southern hemisphere. But on this
hemisphere the effects of eccentricity would not, as on the northern
hemisphere, be compensated by those of obliquity; for both causes would
here tend to produce the same effect; namely, a melting of the ice in
the antarctic regions.
It is probable that at this time the quantity of warm water flowing
from the equatorial regions into the Southern Ocean would be much
greater than at present. This would tend to raise the temperature of
the air of the antarctic regions, and thus assist in melting the ice.
These causes, combined with the great increase of heat resulting from
the change of obliquity, would tend to diminish to a considerable
extent the quantity of ice on the southern hemisphere. I think we may
assume that the slight increase of eccentricity at that period, the
occurrence of the southern winter in perihelion, and the extra quantity
of warm water flowing from the equatorial to the antarctic regions,
would produce an effect on the south polar ice-cap equal to that
produced by the increase in the obliquity of the ecliptic. It would,
therefore, follow that for every eighteen pounds of ice melted annually
at present at the south pole twenty pounds would then be melted.
Let us now consider the effect that this condition of things would
have upon the level of the sea. It would evidently tend to produce an
elevation of the sea-level on the northern hemisphere in two ways. 1st.
The addition to the sea occasioned by the melting of the ice from off
the antarctic land would tend to raise the general level of the sea.
2ndly. The removal of the ice would also tend to shift the earth’s
centre of gravity to the north of its present position—and as the sea
must shift along with the centre, a rise of the sea on the northern
hemisphere would necessarily take place.
The question naturally suggests itself, might not the last rise of the
sea, relative to the land, have resulted from this cause? We know that
during the period of the 25-foot beach, the time when the estuarine
mud, which now forms the rich soil of the Carses of the Forth and
Tay, was deposited, the sea, in relation to the land, stood at least
20 or 30 feet higher than at present. But immediately prior to this
period, we have the age of the submarine forests and peat-beds, when
the sea relative to the land stood lower than it does now. We know
also that these changes of level were not mere local affairs. There
seems every reason to believe that our Carse clay, as Mr. Fisher
states, is the equivalent of the marine mud, with _Scrobicularia_,
which covers the submarine forests of England.[227] And on the other
hand, those submarine forests are not confined to one locality. “They
may be traced,” says Mr. Jamieson, “round the whole of Britain and
Ireland, from Orkney to Cornwall, from Mayo to the shores of Fife, and
even, it would seem, along a great part of the western sea-board of
Europe, as if they bore witness to a period of widespread elevation,
when Ireland and Britain, with all its numerous islands, formed one
mass of dry land, united to the continent, and stretching out into the
Atlantic.”[228] “These submarine forests”“ remarks De la Beche, also,
“are to be found under the same general condition from the shores of
Scandinavia to those of Spain and Portugal, and around the British
islands.”[229] Those buried forests are not confined to Europe, but
are found in the valley of the Mississippi and in Nova Scotia, and
other parts of North America. And again, the strata which underlie
those forests and peat-beds bear witness to the fact of a previous
elevation of the sea-level. In short, we have evidence of a number of
oscillations of sea-level during post-tertiary times.[230]
Had there been only one rise of the land relative to the sea-level, or
one depression, it might quite reasonably, as already remarked, have
been attributed to an upheaval or a sinking of the ground, occasioned
by some volcanic, chemical, or other agency. But certainly those
repeated oscillations of sea-level, extending as they do over so wide
an area, look more like a rising and sinking of the sea than of the
land. But, be this as it may, since it is now established, I presume,
beyond controversy, that the old notion that the general level of the
sea remains permanent, and that the changes must be all attributed to
the land is wholly incorrect, and that the sea, as well as the land,
is subject to changes of level, it is certainly quite legitimate to
consider whether the last elevation of the sea-level relatively to the
land may not have resulted from the rising of the sea rather than from
the sinking of the land, in short, whether it may not be attributed
to the cause we are now considering. The fact that those raised
beaches and terraces are found at so many different heights, and also
so discontinuously along our coasts, might be urged as an objection
to the opinion that they were due to changes in the level of the sea
itself. Space will not permit me to enter upon the discussion of this
point at present; but it may be stated that this objection is more
apparent than real. It by no means follows that beaches of the same
age must be at the same level. This has been shown very clearly by Mr.
W. Pengelly in a paper on “Raised Beaches,” read before the British
Association at Nottingham, 1866.
We have, as I think, evidence amounting to almost absolute certainty
that 11,700 years ago the general sea-level on the northern hemisphere
must have been higher than at present. And in order to determine the
question of the 25-foot beach, we have merely to consider whether a
rise to something like this extent probably took place at the period in
question. We have at present no means of determining the exact extent
of the rise which must have taken place at that period, for we cannot
tell what quantity of ice was then melted off the antarctic regions.
But we have the means of making a very rough estimate, which, at least,
may enable us to determine whether a rise of some 20 or 30 feet may not
possibly have taken place.
If we assume that the southern ice-cap extends on an average down
to lat. 70°, we shall have an area equal to 1/33·163 of the entire
surface of the globe. The proportion of land to that of water, taking
into account the antarctic continent, is as 526 to 1272. The southern
ice-cap will therefore be equal to 1/23·46 of the area covered by
water. The density of ice to that of water being taken at ·92 to 1,
it follows that 25 feet 6 inches of ice melted from off the face of
the antarctic continent would raise the level of the ocean one foot.
If 470 feet were melted off—and this is by no means an extravagant
supposition, when we reflect that for every 18 pounds of ice presently
melted an additional pound or two pounds, or perhaps more, would then
be melted, and that for many ages in succession—the water thus produced
from the melted ice would raise the level of the sea 18 feet 5 inches.
The removal of the 470 feet of solid ice— which must be but a very
small fraction of the total quantity of ice lying upon the antarctic
continent—would shift the earth’s centre of gravity about 7 feet to the
north of its present position. The shifting of the centre of gravity
would cause the sea to sink on the southern hemisphere and rise on the
northern. And the quantity of water thus transferred from the southern
hemisphere to the northern would carry the centre of gravity about one
foot further, and thus give a total displacement of the centre to the
extent of about 8 feet. The sea would therefore rise about 8 feet at
the North Pole, and in the latitude of Edinburgh about 6 feet 7 inches.
This, added to the rise of 18 feet 5 inches, occasioned by the melting
of the ice, would give 25 feet as the total rise in the latitude of
Scotland 11,700 years ago.
Each square foot of surface at the poles 11,700 years ago would be
receiving 18,223,100 foot-pounds more of heat annually than at present.
If we deduct 22 per cent. as the amount absorbed in passing through the
atmosphere, we have 14,214,000 foot-pounds. This would be sufficient
to melt 2·26 feet of ice. But if 50, instead of 22, per cent. were cut
off, 1·45 cubic feet would be melted. In this case the 470 feet of ice
would be melted, independently of the effects of eccentricity, in about
320 years. And supposing that only one-fourth part of the extra heat
reached the ground, 470 feet of ice would be removed in about 640 years.
As to the exact time that the obliquity was at a maximum, previous
to that of 11,700 years ago, our uncertainty is still greater. If we
are permitted to assume that the ecliptic passes from its maximum to
its minimum state and back to its maximum again with anything like
uniformity, at the rate assigned by Leverrier and others, the obliquity
would not be far from a maximum about 60,000 years ago. Taking the
rate of precession at 50″·21129, and assuming it to be uniform—which
it probably is not—the winter solstice would be in the aphelion about
61,300 years ago.[231] In short, it seems not at all improbable that
at the time the solstice-point was in the aphelion, the obliquity of
the ecliptic would not be far from its maximum state. But at that time
the value of the eccentricity was 0·023, instead of 0·0187, its value
at the last period. Consequently the rise of the sea would probably
be somewhat greater than it was 11,700 years ago. Might not this be
the period of the 40-foot beach? In this case 11,000 or 12,000 years
would be the age of the 25-foot beach, and 60,000 years the age of the
40-foot beach.
About 22,000 years ago, the winter solstice was in the perihelion, and
as the eccentricity was then somewhat greater than it is at present,
the winters would be a little warmer and the climate more equable than
it is at the present day. This perhaps might be the period of the
submarine forests and lower peat-beds which underlie the Carse clays,
_Scrobicularia_ mud, and other deposits belonging to the age of the
25-foot beach. At any rate, it is perfectly certain that a condition
of climate at this period prevailed exceedingly favourable to the
growth of peat. It follows also that at this time, owing to a greater
accumulation of ice on the southern hemisphere, the sea-level would be
a few feet lower than at present, and that forests and peat may have
then grown on places which are now under the sea-level.
For a few thousand years before and after 11,700 years ago, when the
winter solstice was evidently not far from the aphelion, and the sea
standing considerably above its present level, would probably, as we
have already stated, be the time when the Carse clays and other recent
deposits lying above the present level of the river were formed.
And it is also a singular fact that the condition of things at that
period must have been exceedingly favourable to the formation of
such estuarine deposits; for at that time the winter temperature of
our island, as has been already shown, would be considerably lower
than at present, and, consequently, during that season, snow, to a
much larger extent than now, would fall instead of rain. The melting
of the winter’s accumulation of snow on the approach of summer would
necessarily produce great floods, similar to what occur in the northern
parts of Asia and America at the present day from this very same
cause. The loose upper soil would be carried down by those floods and
deposited in the estuaries of our rivers.
The foregoing is a rough and imperfect sketch of the history of the
climate and the physical conditions of our globe for the past 60,000
years, in so far as physical and cosmical considerations seem to afford
us information on the subject, and its striking agreement with that
derived from geological sources is an additional evidence in favour
of the opinion that geological and cosmical phenomena are physically
related by a bond of causation.
_Lieutenant-Colonel Drayson’s Theory of the Cause of the Glacial
Epoch._—In a paper read before the Geological Society by
Lieutenant-Colonel Drayson, R.A., on the 22nd February, 1871,[232] that
author states, that after calculating from the recorded positions of
the pole of the heavens during the last 2,000 years, he finds the pole
of the ecliptic is not the centre of the circle traced by the pole of
the heavens. The pole of the heavens, he considers, describes a circle
round a point 6° distant from the pole of the ecliptic and 29° 25′ 47″
from the pole of the heavens, and that about 13,700 years b.c. the
angular distance of the two poles was 35° 25′ 47″. This would bring
the Arctic Circle down to latitude 54° 34′ 13″ N. I fear that this is
a conclusion that will not be generally accepted by those familiar with
celestial mechanics. But, be this as it may, my present object is not
to discuss the astronomical part of Colonel Drayson’s theory, but to
consider whether the conclusions which he deduces from his theory in
regard to the cause of the glacial epoch be legitimate or not. Assuming
for argument’s sake that the obliquity of the ecliptic can possibly
reach to 35° or 36°, so as to bring the Arctic Circle down to the
centre of England, would this account for the glacial epoch? Colonel
Drayson concludes that the shifting of the Arctic Circle down to the
latitude of England would induce here a condition of climate similar
to that which obtains in arctic regions. This seems to be the radical
error of the theory. It is perfectly true that were the Arctic Circle
brought down to latitude 54° 35′ part of our island would be in the
arctic regions, but it does not on that account follow that our island
would be subjected to an arctic climate.
The polar regions owe their cold not to the obliquity of the ecliptic,
but to their distance from the equator. Indeed were it not for
obliquity those regions would be much colder than they really are,
and an increase of obliquity, instead of increasing their cold, would
really make them warmer. The general effect of obliquity, as we
have seen, is to diminish the amount of heat received in equatorial
and tropical regions, and to increase it in the polar and temperate
regions. The greater the obliquity, and, consequently, the farther
the sun recedes from the equator, the smaller is the quantity of heat
received by equatorial regions, and the greater the amount bestowed on
polar and temperate regions. If, for example, we represent the present
amount of heat received from the sun at the equator on a given surface
at 100 parts, 42·47 parts will then represent the amount received at
the poles on the same given surface. But were the obliquity increased
to 35° the amount received at the equator would be reduced to 94·93
parts, and that at the poles increased to 59·81; being an increase at
the poles of nearly one half. At latitude 60° the present quantity
is equal to 57 parts; but about 63 parts would be received were the
obliquity increased to 35°. It therefore follows that although the
Arctic Circle were brought down to the latitude of London so that the
British islands would become a part of the arctic regions, the mean
temperature of these islands would not be lowered, but the reverse.
The winters would no doubt be colder than they are at present, but the
cold of winter would be far more than compensated for by the heat of
summer. It is not a fair representation of the state of things, merely
to say that an increase of obliquity tends to make the winters colder
and the summers hotter, for it affects the summer heat far more than
it does the winter cold. And the greater the obliquity the more does
the increase of heat during summer exceed the decrease during winter.
This is obvious because the greater the obliquity the greater the total
annual amount of heat received.
If an increase of obliquity tended to produce an increase of ice in
temperate and polar regions, and thus to lead to a glacial epoch, then
the greater the obliquity the greater would be the tendency to produce
such an effect. Conceive, then, the obliquity to go on increasing until
it ultimately reached its absolute limit, 90°, and the earth’s axis to
coincide with the plane of the ecliptic. The Arctic Circle would then
extend to the equator. Would this produce a glacial epoch? Certainly
not. A square foot of surface at the poles would then be receiving
as much heat per annum as a square foot at the equator at present,
supposing the sun remained on the equator during the entire year. Less
heat, however, would be reaching the equatorial regions than now. At
present, as we have just seen, the annual quantity of heat received at
either pole is to that received at the equator as 42·47 to 100; but at
the period under consideration the poles would be actually obtaining
one-half more heat than the equator. The amount received per square
foot at the poles, to that received per square foot at the equator,
would be in the ratio of half the circumference of a circle to its
diameter, or as 1·5708 to 1. But merely to say that the poles would be
receiving more heat per annum than the equator is at present, does not
convey a correct idea of the excessive heat which the poles would then
have to endure; for it must be borne in mind that the heat reaching
the equator is spread over the whole year, whereas the poles would get
their total amount during the six months of their summer. Consequently,
for six months in the year the poles would be obtaining far more than
double the quantity of heat received at present by the equator during
the same length of time, and more than three times the quantity then
received by the equator. The amount reaching the pole during the six
months to that reaching the equator would be as 3·1416 to 1.
At the equator twelve hours’ darkness alternates with twelve hours’
sunshine, and this prevents the temperature from rising excessively
high; but at the poles it would be continuous sunshine for six months
without the ground having an opportunity of cooling for a single
hour. At the summer solstice, when the sun would be in the zenith of
the pole, the amount of heat received there every twenty-four hours
would actually be nearly three-and-a-quarter times greater than that
presently received at the equator. Now what holds true with regard to
the poles would hold equally true, though to a lesser extent, of polar
and temperate regions. We can form but a very inadequate idea of the
condition of things which would result from such an enormous increase
of heat. Nothing living on the face of the globe could exist in polar
regions under so fearful a temperature as would then prevail during
summer months. How absurd would it be to suppose that this condition
of things would tend to produce a glacial epoch! Not only would every
particle of ice in polar regions be dissipated, but the very seas
around the pole would be, for several months in the year, at the
boiling point.
If it could be shown from _physical principles_—which, to say the
least, is highly improbable—that the obliquity of the ecliptic could
ever have been as great as 35°, it would to a very considerable
extent account for the comparative absence of ice in Greenland and
other regions in high latitudes, such as we know was the case during
the Carboniferous, Miocene, and other periods. But although a great
increase of obliquity might cause a melting of the ice, yet it could
not produce that mild condition of climate which we know prevailed in
high latitudes during those periods; while no increase of obliquity,
however great, could in any way tend to produce a glacial epoch.
Colonel Drayson, however, seems to admit that this great increase of
obliquity would make our summers much warmer than they are at present.
How, then, according to his theory, is the glacial epoch accounted for?
The following is the author’s explanation as stated in his own words:—
“At the date 13,700 B.C. the same conditions appear to have prevailed
down to about 54° of latitude during winter as regards the sun being
only a few degrees above the horizon. We are, then, warranted in
concluding that the same climate prevailed down to 54° of latitude as
now exists in winter down to 67° of latitude.
“Thus in the greater part of England and Wales, and in the whole of
Scotland, icebergs of large size would be _formed each winter_; every
river and stream would be frozen and blocked with ice, the whole
country would be covered with a mantle of snow and ice, and those
creatures which could neither migrate nor endure the cold of an arctic
climate would be exterminated.”—“The Last Glacial Epoch,” p. 146.
“At the summer solstice the midday altitude of the sun for the latitude
54° would be about 71½°, an altitude equal to that which the sun
now attains in the south of Italy, the south of Spain, and in all
localities having a latitude of about 40°.”
“There would, however, be this singular difference from present
conditions, that in latitude 54° the sun at the period of the summer
solstice would remain the whole twenty-four hours above the horizon;
a fact which would give extreme heat to those very regions which, six
months previously, had been subjected to an arctic cold. Not only
would this greatly increased heat prevail in the latitude of 54°, but
the sun’s altitude would be 12° greater at midday in midsummer, and
also 12° greater at midnight in high northern latitudes, than it
ever attains now; consequently the heat would be far greater than at
present, and high northern regions, even around the pole itself, would
be subjected to a heat during summer far greater than any which now
ever exists in those localities. The natural consequence would be, that
the icebergs and ice which had during the severe winter accumulated in
high latitudes would be rapidly thawed by this heat” (p. 148).
“Each winter the whole northern and southern hemispheres would be one
mass of ice; each summer nearly the whole of the ice of each hemisphere
would be melted and dispersed” (p. 150).
According to this theory, not only is the whole country covered each
winter with a continuous mass of ice, but large icebergs are formed
during that short season, and when the summer heat sets in all is
melted away. Here we have a misapprehension not only as to the actual
condition of things during the glacial epoch, but even as to the way
in which icebergs and land-ice are formed. Icebergs are formed from
land-ice, but land-ice is not formed during a single winter, much
less a mass of sufficient thickness to produce icebergs. Land-ice of
this thickness requires the accumulated snows of centuries for its
production. All that we could really have, according to this theory,
would be a thick covering of snow during winter, which would entirely
disappear during summer, so that there could be no land-ice.
_Mr. Thomas Belt’s Theory._—The theory that the glacial epoch resulted
from a great increase in the obliquity of the ecliptic has recently
been advocated by Mr. Thomas Belt.[233] His conceptions on the subject,
however, appear to me to be even more irreconcilable with physics than
those we have been considering. Lieutenant-Colonel Drayson admits that
the increase of heat to polar regions resulting from the great increase
of obliquity would dissipate the ice there, but Mr. Belt does not even
admit that an increase of obliquity would bring with it an increase of
heat, far less that it would melt the polar ice. On the contrary, he
maintains that the tendency of obliquity is to increase the rigour of
polar climate, and that this is the reason “that now around the poles
some lands are being glaciated, for excepting for that obliquity snow
and ice would not accumulate, excepting on mountain chains.” “Thus,”
he says, “there exist glacial conditions at present around the poles,
due primarily to the obliquity of the ecliptic.” And he also maintains
that if there were no obliquity and the earth’s axis were perpendicular
to the plane of its orbit, an eternal “spring would reign around the
arctic circle,” and that “under such circumstances the piling up of
snow, or even its production at the sea-level, would be impossible,
excepting perhaps in the immediate neighbourhood of the poles, where
the rays of the sun would have but little heating power from its small
altitude.”
Mr. Belt has apparently been led to these strange conclusions by the
following singular misapprehension of the effects of obliquity on
the distribution of the sun’s heat over the globe. “The obliquity of
the ecliptic,” he remarks, “_does not affect the mean amount of heat
received at any one point from the sun_, but it causes the heat and the
cold to predominate at different seasons of the year.”
It is not necessary to dwell further on the absurdity of the
supposition that an increase of obliquity can possibly account for the
glacial epoch, but we may in a few words consider whether a decrease
of obliquity would mitigate the climate and remove the snow from
polar regions. Supposing obliquity to disappear and the earth’s axis
to become perpendicular to the plane of its orbit, it is perfectly
true that day and night would be equal all over the globe, but then
the quantity of heat received by the polar regions would be far less
than at present. It is well known that at present at the equinoxes,
when day and night are equal, snow and not rain prevails in the arctic
regions, and can we suppose it could be otherwise in the case under
consideration? How, we may well ask, could these regions, deprived of
their summer, get rid of their snow and ice?
But even supposing it could be shown that a change in the obliquity of
the ecliptic to the extent assumed by Mr. Belt and Lieutenant-Colonel
Drayson would produce a glacial epoch, still the assumption of such a
change is one which physical astronomy will not permit. Mr. Belt does
not appear to dispute the accuracy of the methods by which it is proved
that the variations of obliquity are confined within narrow limits; but
he maintains that physical astronomers, in making their calculations
have left out of account some circumstances which materially affect the
problem. These, according to Mr. Belt, are the following:—(1) Upheavals
and subsidences of the land which may have taken place in past ages.
(2) The unequal distribution of sea and land on the globe. (3) The fact
that the equatorial protuberance is not a regular one, “but approaches
in a general outline to an ellipse, of which the greater diameter is
two miles longer than the other.” (4) The heaping up of ice around the
poles during the glacial period.
We may briefly consider whether any or all of these can sensibly affect
the question at issue. In reference to the last-mentioned element, it
is no doubt true that if an immense quantity of water were removed
from the ocean and placed around the poles in the form of ice it would
affect the obliquity of the ecliptic; but this is an element of change
which is not available to Mr. Belt, because according to his theory
the piling up of the ice is an effect which results from the change of
obliquity.
In reference to the difference of two miles in the equatorial diameters
of the earth, the fact must be borne in mind that the longer diameter
passes through nearly the centre of the great depression of the Pacific
Ocean,[234] whereas the shorter diameter passes through the opposite
continents of Asia and America. Now, when we take into consideration
the fact that these continents are not only two-and-a-half times denser
than the ocean, but have a mean elevation of about 1,000 feet above
the sea-level, it becomes perfectly obvious that the earth’s mass must
be pretty evenly distributed around its axis of rotation, and that
therefore the difference in the equatorial diameters can exercise no
appreciable effect on the change of obliquity. It follows also that the
present arrangement of sea and land is the best that could be chosen to
prevent disturbance of motion.
That there ever were upheavals and depressions of the land of so
enormous a magnitude as to lead to a change of obliquity to the extent
assumed by Lieutenant-Colonel Drayson and Mr. Belt is what, I presume,
few geologists would be willing to admit. Suppose the great table-land
of Thibet, with the Himalaya Mountains, were to sink under the sea,
it would hardly produce any sensible effect on the obliquity of the
ecliptic. Nay more; supposing that all the land in the globe were sunk
under the sea-level, or the ocean beds converted into dry land, still
this would not materially affect obliquity. The reason is very obvious.
The equatorial bulge is so immense that those upheavals and depressions
would not to any great extent alter the oblate form of the earth. The
only cause which could produce any sensible effect on obliquity, as has
already been noticed, would be the removal of the water of the ocean
and the piling of it up in the form of ice around the poles; but this
is a cause which is not available to Mr. Belt.
_Sir Charles Lyell’s Theory._—I am also unable to agree with Sir
Charles Lyell’s conclusions in reference to the influence of the
obliquity of the ecliptic on climate. Sir Charles says, “It may be
remarked that if the obliquity of the ecliptic could ever be diminished
to the extent of four degrees below its present inclination, such a
deviation would be of geological interest, in so far as it would cause
the sun’s light to be disseminated over a broader zone inside of the
arctic and antarctic circles. Indeed, if the date of its occurrence in
past time could be ascertained, this greater spread of the solar rays,
implying a shortening of the polar night, might help in some slight
degree to account for a vegetation such as now characterizes lower
latitudes, having had in the Miocene and Carboniferous periods a much
wider range towards the pole.”[235]
The effects, as we have seen, would be directly the reverse of what is
here stated, viz., the more the obliquity was diminished the _less_
would the sun’s rays spread over the arctic and antarctic regions, and
conversely the more the obliquity was increased the _greater_ would
be the amount of heat spread over polar latitudes. The farther the
sun recedes from the equator, the greater becomes the amount of heat
diffused over the polar regions; and if the obliquity could possibly
attain its absolute limit (90°), it is obvious that the poles would
then be receiving more heat than the equator is now.
CHAPTER XXVI.
COAL AN INTER-GLACIAL FORMATION.
Climate of Coal Period Inter-glacial in Character.—Coal Plants
indicate an Equable, not a Tropical Climate.—Conditions
necessary for Preservation of Coal Plants.—Oscillations
of Sea-level necessarily implied.—Why our Coal-fields
contain more than One Coal-seam.—Time required to form a
Bed of Coal.—Why Coal Strata contain so little evidence of
Ice-action.—Land Flat during Coal Period.—Leading Idea of the
Theory.—Carboniferous Limestones.
_An Inter-glacial Climate the one best suited for the Growth of the
Coal Plants._—No assertion, perhaps, could appear more improbable,
or is more opposed to all hitherto received theories, than the one
that the plants which form our coal grew during a glacial epoch. But,
nevertheless, if the theory of secular changes of climate, discussed
in the foregoing chapters, be correct, we have in warm inter-glacial
periods (as was pointed out several years ago)[236] the very condition
of climate best suited for the growth of those kinds of trees and
vegetation of which our coal is composed. It is the generally received
opinion among both geologists and botanists that the flora of the Coal
period does not indicate the existence of a tropical, but a moist,
equable, and temperate climate. “It seems to have become,” says Sir
Charles Lyell, “a more and more received opinion that the coal plants
do not on the whole indicate a climate resembling that now enjoyed in
the equatorial zone. Tree-ferns range as far south as the southern
parts of New Zealand, and Araucanian pines occur in Norfolk Island.
A great preponderance of ferns and lycopodiums indicates moisture,
equability of temperature, and freedom from frost, rather than intense
heat.”[237]
Mr. Robert Brown, the eminent botanist, considers that the rapid and
great growth of many of the coal plants showed that they grew in swamps
and shallow water of equable and genial temperature.
“Generally speaking,” says Professor Page, “we find them resembling
equisetums, marsh-grasses, reeds, club-mosses, tree-ferns, and
coniferous trees; and these in existing nature attain their maximum
development in warm, temperate, and subtropical, rather than in
equatorial regions. The Wellingtonias of California and the pines of
Norfolk Island are more gigantic than the largest coniferous tree yet
discovered in the coal-measures.”[238]
The Coal period was not only characterized by a great preponderance
over the present in the quantity of ferns growing, but also in the
number of different species. Our island possesses only about 50
species, while no fewer than 140 species have been enumerated as having
inhabited those few isolated places in England over which the coal has
been worked. And Humboldt has shown that it is not in the hot, but in
the mountainous, humid, and shady parts of the equatorial regions that
the family of ferns produces the greatest number of species.
“Dr. Hooker thinks that a climate warmer than ours now is, would
probably be indicated by the presence of an increased number of
flowering plants, which would doubtless have been fossilized with
the ferns; whilst a lower temperature, _equal to the mean of the
seasons now prevailing_, would assimilate our climate to that of such
cooler countries as are characterized by a disproportionate amount of
ferns.”[239]
“The general opinion of the highest authorities,” says Professor Hull,
“appears to be that the climate did not resemble that of the equatorial
regions, but was one in which the temperature was free from extremes;
the atmosphere being warm and moist, somewhat resembling that of New
Zealand and the surrounding islands, which we endeavour to imitate
artificially in our hothouses.”[240]
The enormous quantity of the carboniferous vegetation shows also that
the climate under which it grew could not have been of a tropical
character, or it must have been decomposed by the heat. Peat, so
abundant in temperate regions, is not to be found in the tropics.
The condition most favourable to the preservation of vegetable remains,
at least under the form of peat, is a cool, moist, and equable climate,
such as prevails in the Falkland Islands at the present day. “In these
islands,” says Mr. Darwin, “almost every kind of plant, even the coarse
grass which covers the whole surface of the land, becomes converted
into this substance.”[241]
From the evidence of geology we may reasonably infer that were
the difference between our summer and winter temperature nearly
annihilated, and were we to enjoy an equable climate equal to, or
perhaps a little above, the present mean annual temperature of our
island, we should then have a climate similar to what prevailed during
the Carboniferous epoch.
But we have already seen that such must have been the character of our
climate at the time that the eccentricity of the earth’s orbit was at
a maximum, and winter occurred when the earth was in the perihelion of
its orbit. For, as we have already shown, the earth would in such a
case be 14,212,700 miles nearer to the sun in winter than in summer.
This enormous difference, along with other causes which have been
discussed, would almost extinguish the difference between summer and
winter temperature. The almost if not entire absence of ice and snow,
resulting from this condition of things, would, as has already been
shown, tend to raise the mean annual temperature of the climate higher
than it is at present.
_Conditions necessary for the Preservation of the Coal Plants._—But
in order to the formation of coal, it is not simply necessary to have
a condition of climate suitable for the growth, but also for the
preservation, of a luxuriant vegetation. The very existence of coal is
as much due to the latter circumstance as to the former; nay more, as
we shall yet see, the fact that a greater amount of coal belongs to the
Carboniferous period than to any other, was evidently due not so much
to a more extensive vegetable growth during that age, suited to form
coal, as to the fact that that flora has been better preserved. Now,
as will be presently shown, we have not merely in the warm periods of
a glacial epoch a condition of climate best suited for the growth of
coal plants, but we have also in the cold periods of such an epoch the
condition most favourable for the preservation of those plants.
One circumstance necessary for the preservation of plants is that they
should have been covered over by a thick deposit of sand, mud, or clay,
and for this end it is necessary that the area upon which the plants
grew should have become submerged. It is evident that unless the area
had become submerged, the plants could not have been covered over with
a thick deposit; and, even supposing they had been covered over, they
could not have escaped destruction from subaërial denudation unless
the whole had been under water. Another condition favourable, if not
essential, to the preservation of the plants, is that they should have
been submerged in a cold and not in a warm sea. Assuming that the
coal plants grew during a warm period of a glacial epoch, we have in
the cold period which succeeded all the above conditions necessarily
secured.
It is now generally admitted that the coal trees grew near broad
estuaries and on immense flat plains but little elevated above
sea-level. But that the _Lepidodendra_, _Sigillariæ_, and other trees,
of which our coal is almost wholly composed, grew on dry ground,
elevated above sea-level, and not in swamps and shallow water, as
was at one time supposed, has been conclusively established by the
researches of Principal Dawson and others. After the growth of many
generations of trees, the plain is eventually submerged under the sea,
and the whole, through course of time, becomes covered over with thick
deposits of sand, gravel, and other sediments carried down by streams
from the adjoining land. After this the submerged plain becomes again
elevated above the sea-level, and forms the site of a second forest,
which, after continuing to flourish for long centuries, is in turn
destroyed by submergence, and, like the former, becomes covered over
with deposits from the land. This alternate process of submergence
and emergence goes on till we have a succession of buried forests
one above another, with immense stratified deposits between. These
buried forests ultimately become converted into beds of coal. This,
I presume, is a fair representation of the generally admitted way in
which our coal-beds had their origin. It is also worthy of notice that
the stratified beds between the coal-seams are of marine and not of
lacustrine origin. On this point I may quote the opinion of Professor
Hull, a well-known authority on the subject: “Whilst admitting,” he
says, “the occasional presence of lacustrine strata associated with the
coal-measures, I think we may conclude that the whole formation has
been essentially of marine and estuarine origin.”[242]
_Coal-beds necessarily imply Oscillations of Sea-level._—It may also
be observed that each coal-seam indicates both an elevation and a
depression of the land. If, for example, there are six coal-seams,
one above the other, this proves that the land must have been, at
least, six times below and six times above sea-level. This repeated
oscillation of the land has been regarded as a somewhat puzzling and
singular circumstance. But if we assume coal to be an inter-glacial
formation, this difficulty not only disappears, but all the various
circumstances which we have been detailing are readily explained.
We have to begin with a warm inter-glacial period, with a climate
specially suited for the growth of the coal trees. During this period,
as has been shown in the chapter on Submergence, the sea would be
standing at a lower level than at present, laying bare large tracts
of sea-bottom, on which would flourish the coal vegetation. This
condition of things would continue for a period of 8,000 or 10,000
years, allowing the growth of many generations of trees. When the warm
period came to a close, and the cold and glacial condition set in, the
climate became unsuited for the growth of the coal plants. The sea
would begin to rise, and the old sea-bottoms on which, during so long
a period, the forests grew, would be submerged and become covered by
sedimentary deposits brought down from the land. These forests becoming
submerged in a cold sea, and buried under an immense mass of sediment,
were then now protected from destruction, and in a position to become
converted into coal. The cold continuing for a period of 10,000 years,
or thereby, would be succeeded by another warm period, during which the
submerged areas became again a land-surface, on which a second forest
flourished for another 10,000 years, which in turn became submerged
and buried under drift on the approach of the second cold period.
This alternate process of submergence and emergence of the land,
corresponding to the rise and fall of sea-level during the cold and
warm periods, would continue so long as the eccentricity of the earth’s
orbit remained at a high value, till we might have, perhaps, five or
six submerged forests, one above the other, and separated by great
thicknesses of stratified deposits, these submerged forests being the
coal-beds of the present day.
[Illustration: Fig. 10.]
It is probable that the forests of the Coal period would extend inland
over the country, but only such portions as were slightly elevated
above sea-level would be submerged and covered over by sediment and
thus be preserved, and ultimately become coal-seams. The process will
be better understood from the following diagram. Let A B represent the
surface of the ground prior to a glacial epoch, and to the formation
of the beds of coal and stratified deposits represented in the
diagram. Let S S′ be the normal sea-level. Suppose the eccentricity
of the earth’s orbit begins to increase, and the winter solstice
approaches the perihelion, we have then a moderately warm period. The
sea-level sinks to 1, and forests of sigillariæ and other coal trees
cover the country from the sea-shore at 1, stretching away inland in
the direction of B. In course of time the winter solstice moves round
to aphelion and a cold period follows. The sea begins to rise and
continues rising till it reaches 1′. Denudation and the severity of
the climate destroy every vestige of the forest from 1′ backwards into
the interior; but the portion 1 1′ being submerged and covered over
by sediment brought down from the land is preserved. The eccentricity
continuing to increase in extent, the second inter-glacial period is
more warm and equable than the first, and the sea this time sinks to 2.
A second forest now covers the country down to the sea-shore at 2. This
second warm period is followed by the second cold period, more severe
than the first, and the sea-level rises to 2′. Denudation and severity
of climate now destroy every remnant of the forest, from 2′ inland,
but of course the submerged portion of 2 2′, like the former portion 1
1′, is preserved. During the third warm period (the eccentricity being
still on the increase) the sea-level sinks to 3, and the country for
the third time is covered by forests, which extend down to 3. This
third warm period is followed by a cold glacial period more severe than
the preceding, and the sea-level rises to 3′, and the submerged portion
of the forest from 3 to 3′ becomes covered with drift,—the rest as
before being destroyed by denudation and the severity of the climate.
We shall assume that the eccentricity has now reached a maximum, and
that during the fourth inter-glacial period the sea-level sinks only to
4, the level to which it sank during the second inter-glacial period.
The country is now covered for the fourth time by forests. The cold
period which succeeds not being so severe as the last, the sea rises
only to 4′, which, of course, marks the limit of the fourth forest. The
eccentricity continuing to diminish, the fifth forest is only submerged
up to 5′, and the sixth and last one up to 6′. The epoch of cold and
warm periods being now at a close, the sea-level remains stationary at
its old normal position S S′. Here we have six buried forests, the one
above the other, which, through course of ages, become transformed into
coal-beds.
It does not, however, necessarily follow that each separate coal-seam
represents a warm period. It is quite possible that two or more seams
separated from each other by thin partings or a few feet of sedimentary
strata might have been formed during one warm period; for during a warm
period minor oscillations of sea-level sufficient to submerge the land
to some depth might quite readily have taken place from the melting of
polar ice, as was shown in the chapter on Submergence.
It may be noticed that in order to make the section more distinct, its
thickness has been greatly exaggerated. It will also be observed that
beds 4, 5, and 6 extend considerably to the left of what is represented
in the section.
But it is not to be supposed that the whole phenomena of the
coal-fields can be explained without supposing a subsidence of the
land. The great depth to which the coal-beds have been sunk, in many
cases, must be attributed to a subsidence of the level. A series of
beds formed during a glacial epoch, may, owing to a subsidence of the
land, be sunk to a great depth, and become covered over with thousands
of feet of sediment; and then on the occurrence of another glacial
epoch, a new series of coal-beds may be formed on the surface. Thus
the upper series may be separated from the lower by thousands of feet
of sedimentary rock. There is another consequence resulting from the
sinking of the land, which must be taken into account. Had there been
no sinking of the land during the Carboniferous age, the quantity of
coal-beds now remaining would be far less than it actually is, for it
is in a great measure owing to their being sunk to a great depth that
they have escaped destruction by the enormous amount of denudation
which has taken place since that remote age. It therefore follows that
only a very small fraction of the submerged forests of the Coal period
do actually now exist in the form of coal. Generally it would only be
those areas which happened to be sunk to a considerable depth, by a
subsidence of the land, that would escape destruction from denudation.
But no doubt the areas which would thus be preserved bear but a small
proportion to those destroyed.
_Length of Inter-glacial Period, as indicated by the Thickness of a
Bed of Coal._—A fact favourable to the idea that the coal-seams were
formed during inter-glacial periods is, that the length of those
periods agrees pretty closely with the length of time supposed to be
required to form a coal-seam of average thickness. Other things being
equal, the thickness of a coal-seam would depend upon the length
of the inter-glacial period. If the rate of precession and motion
of the perihelion were always uniform the periods would all be of
equal length. But although the rate of precession is not subject to
much variation, such is not the case in regard to the motion of the
perihelion, as will be seen from the tables of the longitude of the
perihelion given in Chapter XIX. Sometimes the motion of the perihelion
is rapid, at other times slow, while in some cases its motion is
retrograde. In consequence of this, an inter-glacial period may not be
more than some six or seven thousand years in length, while in other
cases its length may be as much as fifteen or sixteen thousand years.
According to Boussingault, luxuriant vegetation at the present day
takes from the atmosphere about a half ton of carbon per acre
annually, or fifty tons per acre in a century. Fifty tons of carbon of
the specific gravity of coal, about 1·5, spread evenly over the surface
of an acre, would make a layer nearly one-third of an inch.[243]
Humboldt makes the estimate a little higher, viz., one half-inch.
Taking the latter estimate, it would require 7,200 years to form a
bed of coal a yard thick. Dr. Heer, of Zurich, thinks that it would
not require more than 1,400 years to form a bed of coal one yard
thick;[244] while Mr. Maclaren thinks that a bed of coal one yard thick
would be formed in 1,000 years.[245] Professor Phillip, calculating
from the amount of carbon taken from the atmosphere, as determined by
Liebig, considers that if it were converted into ordinary coal with
about 75 per cent. of carbon, it would yield one inch in 127·5 years,
or a yard in 4,600 years.[246]
There is here a considerable amount of difference in regard to the time
required to form a yard of coal. The truth, however, may probably be
somewhere between the two extremes, and we may assume 5,000 years to be
about the time. In a warm period of 15,000 years we should then have
deposited a seam of coal 9 feet thick, while during a warm period of
7,000 years we would have a seam of only 4 feet.
_Reason why the Coal Strata present so little Evidence of
Ice-action._—There are two objections which will, no doubt, present
themselves to the reader’s mind. (1.) If coal be an inter-glacial
formation, why do the coal strata present so little evidence of
ice-action? If the coal-seams represent warm inter-glacial periods, the
intervening beds must represent cold or glacial periods, and if so,
they ought to contain more abundant evidence of ice-action than they
really do. (2.) In the case of the glacial epoch, almost every vestige
of the vegetation of the warm periods was destroyed by the ice of the
cold periods; why then did not the same thing take place during the
glacial epoch of the Carboniferous period?
During the glacial epoch the face of the country was in all
probability covered for ages with the most luxuriant vegetation; but
scarcely a vestige of that vegetation now remains, indeed the very soil
upon which it grew is not to be found. All that now remains is the
wreck and desolation produced by the ice-sheet that covered the country
during the cold periods of that epoch, consisting of transported blocks
of stones, polished and grooved rocks, and a confused mass of boulder
clay. Here we have in this epoch nothing tangible presenting itself
but the destructive effects of the ice which swept over the land. Why,
then, in reference to the glacial epochs of the Carboniferous age
should we have such abundant evidence of the vegetation of the warm
periods, and yet so little evidence of the effect of the ice of the
cold periods? The answer to these two objections will go a great way
to explain why we have so much coal belonging to the Carboniferous
age, and so little belonging to any other age; and it will, I think,
be found in the peculiar physical character of the country during
the Carboniferous age. The areas on which the forests of the Coal
period grew escaped the destructive power of glaciers and land-ice on
account of the flat nature of the ground. There are few points on which
geologists are more unanimous than in regard to the flat character of
the country during the Coal period.
There does not seem to be any very satisfactory evidence that the
interior of the country rose to any very great elevation. Mr.
Godwin-Austen thinks that during the Coal period there must have
been “a vast expanse of continuous horizontal surface at very slight
elevations above the sea-level.”[247] Of the widely spread terrestrial
surface of the Coal-measure period, portions, he believes, attained
a considerable elevation. But in contrast to this he states, “There
is a feature which seems to distinguish this period physically from
all subsequent periods, and which consists in the vast expanse of
continuous horizontal surface which the land area presented, bordering
on, and at very slight elevations above, the sea-level.”[248] Hugh
Miller, describing in his usual graphic way the appearance of the
country during the Coal period, says:—“It seems to have been a land
consisting of immense flats, unvaried, mayhap, by a _single hill_,
in which dreary swamps, inhabited by doleful creatures, spread out
on every hand for hundreds and thousands of miles; and a gigantic
and monstrous vegetation formed, as I have shown, the only prominent
features of the scenery.”[249]
Now, if this is in any way like a just representation of the general
features of the country during the Coal period, it was physically
impossible, no matter however severe the climate may have been,
that there could have been in this country at that period anything
approaching to continental ice, or perhaps even to glaciers of such
dimensions as would reach down to near the sea-level, where the coal
vegetation now preserved is supposed chiefly to have grown. The
condition of things which would prevail would more probably resemble
that of Siberia than that of Greenland.
The absence of all traces of ice-action in the strata of the
coal-measures can in this case be easily explained. For as by
supposition there were no glaciers, there could have been no
scratching, grooving, or polishing of the rocks; neither could there
have been any icebergs, for the large masses known as icebergs are
the terminal portions of glaciers which have reached down to the sea.
Again, there being no icebergs, there of course could have been no
grinding or scratching of the rocks forming the floor of the ocean.
True, during summer, when the frozen sea broke up, we should then
have immense masses of floating ice, but these masses would not be of
sufficient thickness to rub against the sea-bottom. But even supposing
that they did occasionally touch the bottom here and there, we could
not possibly find the evidence of this in any of the strata of the
coal-measures. We could not expect to find any scratchings or markings
on the sandstone or shale of those strata indicating the action of
ice, for at that period there were no beds of sandstone or shale, but
simply beds of sand and mud, which in future ages became consolidated
into sandstone and shale. A mass of ice might occasionally rub along
the sea-bottom, and leave its markings on the loose sand or soft mud
forming that bottom, but the next wave that passed over it would
obliterate every mark, and leave the surface as smooth as before.
Neither could we expect to find any large erratics or boulders in the
coal strata, for these must come from the land, and as by supposition
there were no glaciers or land-ice at that period, there was therefore
no means of transporting them. In Greenland the icebergs sometimes
carry large boulders, which are dropped into the sea as the icebergs
melt away; but these blocks have all either been transported on the
backs of glaciers from inland tracts, or have fallen on the field-ice
along the shore from the face of crags and overhanging precipices.
But as there were probably neither glaciers reaching to the sea, nor
perhaps precipitous cliffs along the sea-shore, there could have been
few or no blocks transported by ice and dropped into the sea of the
Carboniferous period, and of course we need not expect to find them in
the sandstone and shale which during that epoch formed the bed of the
ocean. There would no doubt be coast-line ice and ground-ice in rivers,
carrying away large quantities of gravel and stones; but these gravels
and stones would of course be all water-worn, and although found in the
strata of the coal-measures, as no doubt they actually are, they would
not be regarded as indicating the action of ice. The simple absence of
relics of ice-action in the coal-measures proves nothing whatever in
regard to whether there were cold periods during their formation or not.
This comparative absence of continental ice might be one reason why
the forests of the Carboniferous period have been preserved to a much
greater extent than those of any other age.
It must be observed, however, that the conclusions at which we have
arrived in reference to the comparative absence of continental ice
applies only to the areas which now constitute our coal-fields. The
accumulation of ice on the antarctic regions, and on some parts of
the arctic regions, might have been as great during that age as it
is at present. Had there been no continental ice there could have
been no such oscillations of sea-level as is assumed in the foregoing
theory. The leading idea of the theory, expressed in a few words,
is, that the glacial epochs of the Carboniferous age were as severe,
and the accumulation of ice as great, as during any other age, only
there were large tracts of flat country, but little elevated above the
sea-level, which were not covered by ice. These plains, during the
warm inter-glacial periods, were covered with forests of sigillariæ
and other coal trees. Portions of those forests were protected by the
submergence which resulted from the rise of the sea-level during the
cold or glacial periods and the subsequent subsidence of the land.
Those portions now constitute our coal-beds.
But that coal may be an inter-glacial formation is no mere hypothesis,
for we have in the well-known Dürnten beds—described in Chapter XV.—an
actual example of such a formation.
_Carboniferous Limestones._—As a general rule the limestones of the
Carboniferous period, like the coal, are found in beds separated by
masses of sandstone and other stratified deposits, which proves that
the corals, crinoids, and other creatures, of the remains of which it
is composed, did not live continuously on during the entire Limestone
period. These limestones are a marine formation. If the land was
repeatedly submerged the coal must of necessity have been produced in
seams with stratified deposits between, but there is no reason why the
same should have been the case with the limestones. If the climatic
condition of the sea continued the same we should not have expected
this alternate succession of life and death; but, according to the
theory of alternate cold and warm periods, such a condition follows
as a necessary consequence, for during the warm periods, when the
land was covered with a luxuriant vegetation, the sea-bottom would be
covered with mollusca, crinoids, corals, &c., fitted to live only in a
moderately warm sea; but when the cold came on those creatures would
die, and their remains, during the continuance of the cold period,
would become slowly covered over with deposits of sand and clay. On the
return of the warm period those deposits would soon become covered with
life as before, forming another bed of limestone, and this alternation
of life and death would go on as long as the glacial epoch continued.
It is true that in Derbyshire, and in the south of Ireland and some
other places, the limestone is found in one mass of several hundred
feet in thickness without any beds of sandstone or shale, but then it
is nowhere found in one continuous mass from top to bottom without any
lines of division. These breaks or divisions may as distinctly mark
a cold period as though they had been occupied by beds of sandstone.
The marine creatures ceased to exist, and when the rough surface left
by their remains became smoothed down by the action of the waves into
a flat plain, another bed would begin to form upon this floor so
soon as life again appeared. Two agencies working together probably
conspired to produce these enormous masses of limestone divided only
by breaks marking different periods of elaboration. Corals grow in
warm seas, and there only in water of a depth ranging from 20 to 30
fathoms. The cold of a period of glaciation would not only serve to
destroy them, but they would be submerged so much beyond the depth
proper for their existence that even were it possible that with the
submergence a sufficient temperature was left, they would inevitably
perish from the superincumbent mass of water. We are therefore, as
it seems to me, warranted in concluding that the separate masses of
Derbyshire limestone were formed during warm inter-glacial periods,
and that the lines of division represent cold periods of glaciation
during which the animals perished by the combined influence of cold and
pressure of water. The submergence of the coral banks in deep water on
a sea-bottom, which, like the land, was characteristically flat and
even, implies its carrying away far into the bosom of the ocean, and
consequently remote from any continent and the river-borne detritus
thereof.
CHAPTER XXVII.
PATH OF THE ICE-SHEET IN NORTH-WESTERN EUROPE AND ITS RELATIONS
TO THE BOULDER CLAY OF CAITHNESS.[250]
Character of Caithness Boulder Clay.—Theories of the Origin
of the Caithness Clay.—Mr. Jamieson’s Theory.—Mr. C. W.
Peach’s Theory.—The proposed Theory.—Thickness of Scottish
Ice-sheet.—Pentlands striated on their Summits.—Scandinavian
Ice-sheet.—North Sea filled with Land-ice.—Great Baltic
Glacier.—Jutland and Denmark crossed by Ice.—Sir R.
Murchison’s Observations.—Orkney, Shetland, and Faroe Islands
striated across.—Loess accounted for.—Professor Geikie’s
Suggestion.—Professor Geikie and B. N. Peach’s Observations
on East Coast of Caithness.—Evidence from Chalk Flints and
Oolitic Fossils in Boulder Clay.
_The Nature of the Caithness Boulder Clay._—A considerable amount of
difficulty has been felt by geologists in accounting for the origin of
the boulder clay of Caithness. It is an unstratified clay, of a deep
grey or slaty colour, resembling much that of the Caithness flags on
which it rests. It is thus described by Mr. Jamieson (Quart. Jour.
Geol. Soc., vol. xxii., p. 261):—
“The glacial drift of Caithness is particularly interesting as an
example of a boulder clay which in its mode of accumulation and
ice-scratched _débris_ very much resembles that unstratified stony mud
which occurs underneath glaciers—the ‘_moraine profonde_,’ as some call
it.
“The appearance of the drift along the Haster Burn, and in many other
places in Caithness, is in fact precisely the same as that of the old
boulder clay of the rest of Scotland, except that it is charged with
remains of sea-shells and other marine organisms.
“If want of stratification, hardness of texture, and abundance of
well-glaciated stones and boulders are to be the tests for what we call
genuine boulder clay, then much of the Caithness drift will stand the
ordeal.”
So far, therefore, as the mere appearance of the drift is concerned,
it would at once be pronounced to be true Lower Till, the product of
land-ice. But there are two circumstances connected with it which have
been generally regarded as fatal to this conclusion.
(1) The striæ on the rocks show that the ice which formed the clay
must have come from the sea, and not from the interior of the country;
for their direction is almost at right angles to what it would have
been had the ice come from the interior. Over the whole district, the
direction of the grooves and scratches, not only of the rocks but
even of the stones in the clay, is pretty nearly N.W. and S.E. “When
examining the sections along the Haster Burn,” says Mr. Jamieson, “in
company with Mr. Joseph Anderson, I remarked that the striæ on the
imbedded fragments generally agreed in direction with those of the
rocks beneath. The scratches on the boulders, as usual, run lengthways
along the stones when they are of an elongated form; and the position
of these stones, as they lie imbedded in the drift, is, as a rule, such
that their longer axes point in the same direction as do the scratches
on the solid rock beneath; showing that the same agency that scored the
rocks also ground and pushed along the drift.”
Mr. C. W. Peach informs me that he seldom or never found a stone with
two sets of striæ on it, a fact indicating, as Mr. Jamieson remarks,
that the drift was produced by one great movement invariably in the
same direction. Let it be borne in mind that the ice, which thus moved
over Caithness in this invariable track, must either have come from the
Atlantic to the N.W., or from the Moray Firth to the S.E.
(2) The boulder clay of Caithness is full of sea-shells and other
marine remains. The shells are in a broken condition, and are
interspersed like the stones through the entire mass of the clay.
Mr. Jamieson states that he nowhere observed any instance of shells
being found in an undisturbed condition, “nor could I hear,” he says,
“of any such having been found; there seems to be no such thing as a
bed of laminated silt with shells _in situ_.” The shell-fragments are
scratched and ice-worn, the same as the stones found in the clay. Not
only are the shells glaciated, but even the foraminifera, when seen
through the microscope, have a rubbed and worn appearance. The shells
have evidently been broken, striated, and pushed along by the ice at
the time the boulder clay was being formed.
_Theories regarding the Origin of the Caithness Clay._—Mr. Jamieson, as
we have seen, freely admits that the boulder clay of Caithness has the
appearance of true land-ice till, but from the N.W. and S.E. direction
of the striæ on the rocks, and the presence of sea-shells in the clay,
he has come to the conclusion that the glaciation of Caithness has been
effected by floating ice at a time when the district was submerged. I
have always felt convinced that Mr. Jamieson had not hit upon the true
explanation of the phenomena.
(1) It is physically impossible that any deposit formed by icebergs
could be wholly unstratified. Suppose a mass of the materials which
would form boulder clay is dropped into the sea from, say an iceberg,
the heavier parts, such as stones, will reach the bottom first. Then
will follow lighter materials, such as sand, then clay, and last of all
the mud will settle down over the whole in fine layers. The different
masses dropped from the various icebergs, will, no doubt, lie in
confusion one over the other, but each separate mass will show signs of
stratification. A good deal of boulder clay evidently has been formed
in the sea, but if the clay be unstratified, it must have been formed
under glaciers moving along the sea-bottom as on dry ground. Whether
_unstratified_ boulder clay may happen to be formed under water or on
dry land, it must in either case be the product of land-ice.[251] Those
who imagine that materials, differing in specific gravity like those
which compose boulder clay, dropped into water, can settle down without
assuming the stratified form, should make the experiment, and they
would soon satisfy themselves that the thing is physically impossible.
The notion that unstratified boulder clay could be formed by deposits
from floating ice, is not only erroneous, but positively pernicious,
for it tends to lead those who entertain it astray in regard to the
whole question of the origin of drift.
(2) It is also physically impossible that ice-markings, such as those
everywhere found on the rocky face of the district, and on the pebbles
and shells imbedded in the clay, could have been effected by any other
agency than that of land-ice. I need not here enter into any discussion
on this point, as this has been done at considerable length in another
place.[252] In the present case, however, it is unnecessary, because
if it can be shown that all the facts are accounted for in the most
natural manner by the theory of land-ice, no one will contend for the
floating-ice theory; for it is admitted that, with the exception of the
direction of the striæ and the presence of the shells, all the facts
agree better with the land-ice than with the floating-ice theory.
My first impression on the subject was that the glaciation of Caithness
had been effected by the polar ice-cap, which, during the severer part
of the glacial epoch, must have extended down to at least the latitude
of the north of Scotland.
On a former occasion (see the _Reader_ for 14th October, 1865) it was
shown that all the northern seas, owing to their shallowness, must,
at that period, have been blocked up with solid ice, which displaced
the water and moved along the sea-bottoms the same as on dry land. In
fact, the northern seas, including the German Ocean, being filled at
the time with glacier-ice, might be regarded as dry land. Ice of this
sort, moving along the bed of the German Ocean or North Sea, and over
Caithness, could not fail to push before it the shells and other animal
remains lying on the sea-bottom, and to mix them up with the clay
which now remains upon the land as evidence of its progress.
About two years ago I had a conversation with Mr. C. W. Peach on the
subject. This gentleman, as is well known, has long been familiar with
the boulder clay of Caithness. He felt convinced that the clay of that
country is the true Lower Till, and not a more recent deposit, as Mr.
Jamieson supposes. He expressed to me his opinion that the glaciation
of Caithness had been effected by masses of land-ice crossing the
Moray Firth from the mountain ranges to the south-east, and passing
over Caithness in its course. The difficulty which seems to beset
this theory is, that a glacier entering the Firth would not leave it
and ascend over the Caithness coast. It would take the path of least
resistance and move into the North Sea, where it would find a free
passage into deeper water. Mr. Peach’s theory is, however, an important
step in the right direction. It is a part of the truth, but I believe
not the whole truth. The following is submitted as a solution of the
question.
_The Proposed Theory._—It may now be regarded as an established fact
that, during the severer part of the glacial period, Scotland was
covered with one continuous mantle of ice, so thick as to bury under
it the Ochil, Sidlaw, Pentland, Campsie, and other moderately high
mountain ranges. For example, Mr. J. Geikie and Mr. B. N. Peach found
that the great masses of the ice from the North-west Highlands, came
straight over the Ochils of Perthshire and the Lomonds of Fife. In
fact, these mountain ridges were not sufficiently high to deflect the
icy stream either to the right hand or to the left; and the flattened
and rounded tops of the Campsie, Pentland, and Lammermoor ranges bear
ample testimony to the denuding power of ice.
Further, to quote from Mr. Jamieson, “the detached mountain of
Schehallion in Perthshire, 3,500 feet high, is marked near the top as
well as on its flanks, and this not by ice flowing down the sides of
the hill itself, but by ice pressing over it from the north. On the top
of another isolated hill, called Morven, about 3,000 feet high, and
situated a few miles to the north of the village of Ballater, in the
county of Aberdeen, I found granite boulders unlike the rock of the
hill, and apparently derived from the mountains to the west. Again,
on the highest watersheds of the Ochils, at altitudes of about 2,000
feet, I found this summer (1864) pieces of mica schist full of garnets,
which seem to have come from the Grampian Hills to the north-west,
showing that the transporting agent had overflowed even the highest
parts of the Ochil ridge. And on the West Lomonds, in Fifeshire, at
Clattering-well Quarry, 1,450 feet high, I found ice-worn pebbles of
Red Sandstone and porphyry in the _débris_ covering the Carboniferous
Limestone of the top of the Bishop Hill. Facts like these meet us
everywhere. Thus on the Perthshire Hills, between Blair Athol and
Dunkeld, I found ice-worn surfaces of rocks on the tops of hills, at
elevations of 2,200 feet, as if caused by ice pressing over them from
the north-west, and transporting boulders at even greater heights.”[253]
Facts still more important, however, in their bearing on the question
before us were observed on the Pentland range by Mr. Bennie and myself
during the summer of 1870. On ascending Allermuir, one of the hills
forming the northern termination of the Pentland range, we were not a
little surprised to find its summit ice-worn and striated. The top of
the hill is composed of a compact porphyritic felstone, which is very
much broken up; but wherever any remains of the original surface could
be seen, it was found to be polished and striated in a most decided
manner. These striæ are all in one uniform direction, nearly east and
west; and on minutely examining them with a lens we had no difficulty
whatever in determining that the ice which effected them came from the
west and not from the east, a fact which clearly shows that they must
have been made at the time when, as is well known, the entire Midland
valley was filled with ice, coming from the North-west Highlands. On
the summit of the hill we also found patches of boulder clay in hollow
basins of the rock. At one spot it was upwards of a foot in depth, and
rested on the ice-polished surface. The clay was somewhat loose and
sandy, as might be expected of a layer so thin, exposed to rain, frost,
and snow, during the long course of ages which must have elapsed since
it was deposited there. Of 100 pebbles collected from the clay, just as
they turned up, every one, with the exception of three or four composed
of hard quartz, presented a flattened and ice-worn surface; and
forty-four were distinctly striated: in short, every stone which was
capable of receiving and retaining scratches was striated. A number of
these stones must have come from the Highlands to the north-west.[254]
The height of Allermuir is 1,617 feet, and, from its position, it is
impossible that the ice could have gone over its summit, unless the
entire Midland valley, at this place, had been filled with ice to the
depth of more than 1,600 feet. The hill is situated about four or
five miles to the south of Edinburgh, and forms, as has already been
stated, the northern termination of the Pentland range. Immediately
to the north lies the broad valley of the Firth of Forth, more than
twelve miles across, offering a most free and unobstructed outlet for
the great mass of ice coming along the Midland valley from the west.
Now, when we reflect how easily ice can accommodate itself to the
inequalities of the channel along which it moves, how it can turn to
the right hand or to the left, so as to find for itself the path of
least resistance, it becomes obvious that the ice never would have gone
over Allermuir, unless not only the Midland valley at this point, but
also the whole surrounding country had been covered with one continuous
mass of ice to a depth of more than 1,600 feet. But it must not be
supposed that the height of Allermuir represents the thickness of the
ice; for on ascending Scald Law, a hill four miles to the south-west
of Allermuir, and the highest of the Pentland range, we found, in
the _débris_ covering its summit, hundreds of transported stones of
all sizes, from one to eighteen inches in diameter. We also dug up a
Greenstone boulder about eighteen inches in diameter, which was finely
polished and striated. As the height of this hill is 1,898 feet, the
mass of ice covering the surrounding country must have been at least
1,900 feet deep. But this is not all. Directly to the north of the
Pentlands, in a line nearly parallel with the east coast, and at right
angles to the path of ice from the interior, there is not, with the
exception of the solitary peak of East Lomond, and a low hill or two of
the Sidlaw range, an eminence worthy of the name of a hill nearer than
the Grampians in the north of Forfarshire, distant upwards of sixty
miles. This broad plain, extending from almost the Southern to the
Northern Highlands, was the great channel through which the ice of the
interior of Scotland found an outlet into the North Sea. If the depth
of the ice in the Firth of Forth, which forms the southern side of this
broad hollow, was at least 1,900 feet, it is not at all probable that
its depth in the northern side, formed by the Valley of Strathmore
and the Firth of Tay, which lay more directly in the path of the ice
from the North Highlands, could have been less. Here we have one vast
glacier, more than sixty miles broad and 1,900 feet thick, coming from
the interior of the country.
It is, therefore, evident that the great mass of ice entering the North
Sea to the east of Scotland, especially about the Firths of Forth
and Tay, could not have been less, and was probably much more, than
from 1,000 to 2,000 feet in thickness. The grand question now to be
considered is, What became of the huge sheet of ice after it entered
the North Sea? Did it break up and float away as icebergs? This appears
to have been hitherto taken for granted; but the shallowness of the
North Sea shows such a process to have been utterly impossible. The
depth of the sea in the English Channel is only about twenty fathoms,
and although it gradually increases to about forty fathoms at the
Moray Firth, yet we must go to the north and west of the Orkney and
Shetland Islands ere we reach the 100 fathom line. Thus the average
depth of the entire North Sea is not over forty fathoms, which is even
insufficient to float an iceberg 300 feet thick.
No doubt the North Sea, for two reasons, is now much shallower than
it was during the period in question. (1.) There would, at the time
of the great extension of the ice on the northern hemisphere, be a
considerable submergence, resulting from the displacement of the
earth’s centre of gravity.[255] (2.) The sea-bed is now probably
filled up to a larger extent with drift deposits than it was at the
ice period. But, after making the most extravagant allowance for the
additional depth gained on this account, still there could not possibly
have been water sufficiently deep to float a glacier of 1,000 or 2,000
feet in thickness. Indeed, the North Sea would have required to be
nearly ten times deeper than it is at present to have floated the
ice of the glacial period. We may, therefore, conclude with the most
perfect certainty that the ice-sheet of Scotland could not possibly
have broken up into icebergs in such a channel, but must have moved
along on the bed of the sea in one unbroken mass, and must have found
its way to the deep trough of the Atlantic, west of the Orkney and
Shetland Islands, ere it broke up and floated away in the iceberg form.
It is hardly necessary to remark that the waters of the North Sea would
have but little effect in melting the ice. A shallow sea like this,
into which large masses of ice were entering, would be kept constantly
about the freezing-point, and water of this temperature has but little
melting power, for it takes 142 lbs. of water, at 33°, to melt one
pound of ice. In fact, an icy sea tends rather to protect the ice
entering it from being melted than otherwise. And besides, owing to
fresh acquisitions of snow, the ice-sheet would be accumulating more
rapidly upon its upper surface than it would be melting at its lower
surface, supposing there were sea-water under that surface. The ice of
Scotland during the glacial period must, of necessity, have found its
way into warmer water than that of the North Sea before it could have
been melted. But this it could not do without reaching the Atlantic,
and in getting there it would have to pass round by the Orkney Islands,
along the bed of the North Sea, as land-ice.
This will explain how the Orkney Islands may have been glaciated by
land-ice; but it does not, however, explain how Caithness should have
been glaciated by that means. These islands lay in the very track of
the ice on its way to the Atlantic, and could hardly escape being
overridden; but Caithness lay considerably to the left of the path
which we should expect the ice to have taken. The ice would not leave
its channel, turn to the left, and ascend upon Caithness, unless it
were forced to do so. What, then, compelled the ice to pass over
Caithness?
_Path of the Scandinavian Ice._—We must consider that the ice from
Scotland and England was but a fraction of that which entered the
North Sea. The greater part of the ice of Scandinavia must have gone
into this sea, and if the ice of our island could not find water
sufficiently deep in which to float, far less would the much thicker
ice of Scandinavia do so. The Scandinavian ice, before it could break
up, would thus, like the Scottish ice, have to cross the bed of the
North Sea and pass into the Atlantic. It could not pass to the north,
or to the north-west, for the ocean in these directions would be
blocked up by the polar ice. It is true that along the southern shore
of Norway there extends a comparatively deep trough of from one to two
hundred fathoms. But this is evidently not deep enough to have floated
the Scandinavian ice-sheet; and even supposing it had been sufficiently
deep, the floating ice must have found its way to the Atlantic, and
this it could not have done without passing along the coast. Now, its
passage would not only be obstructed by the mass of ice continually
protruding into the sea directly at right angles to its course, but it
would be met by the still more enormous masses of ice coming off the
entire Norwegian coast-line. And, besides this, the ice entering the
Arctic Ocean from Lapland and the northern parts of Siberia, except
the very small portion which might find an outlet into the Pacific
through Behring’s Straits, would have to pass along the Scandinavian
coast in its way to the Atlantic. No matter, then, what the depth of
this trough may have been, if the ice from the land, after entering
it, could not make its escape, it would continue to accumulate till
the trough became blocked up; and after this, the great mass from the
land would move forward as though the trough had no existence. Thus,
the only path for the ice would be by the Orkney and Shetland Islands.
Its more direct and natural path would, no doubt, be to the south-west,
in the direction of our shores; and in all probability, had Scotland
been a low flat island, instead of being a high and mountainous one,
the ice would have passed completely over it. But its mountainous
character, and the enormous masses of ice at the time proceeding from
its interior, would effectually prevent this, so that the ice of
Scandinavia would be compelled to move round by the Orkney Islands.
Consequently, these two huge masses of moving ice—the one from Scotland
and the much greater one from Scandinavia—would meet in the North Sea,
probably not far from our shores, and would move, as represented in
the diagram, side by side northwards into the Atlantic as one gigantic
glacier.
Nor can this be regarded as an anomalous state of things; for in
Greenland and the antarctic continent the ice does not break up into
icebergs on reaching the sea, but moves along the sea-bottom in a
continuous mass until it reaches water sufficiently deep to float
it. It is quite possible that the ice at the present day may nowhere
traverse a distance of three or four hundred miles of sea-bottom, but
this is wholly owing to the fact that it finds water sufficiently deep
to float it before having travelled so far. Were Baffin’s Bay and
Davis’s Straits, for example, as shallow as the North Sea, the ice of
Greenland would not break up into icebergs in these seas, but cross in
one continuous mass to and over the American continent.
The median line of the Scandinavian and Scottish ice-sheets would be
situated not far from the east coast of Scotland. The Scandinavian ice
would press up as near to our coast as the resistance of the ice from
this side permitted. The enormous mass of ice from Scotland, pressing
out into the North Sea, would compel the Scandinavian ice to move round
by the Orkneys, and would also keep it at some little distance from
Scotland. Where, on the other hand, there was but little resistance
offered by ice from the interior of this country (and this might be the
case along many parts of the English coast), the Scandinavian ice might
reach the shores, and even overrun the country for some distance inland.
We have hitherto confined our attention to the action of ice proceeding
from Norway; but if we now consider what took place in Sweden and the
Baltic, we shall find more conclusive proof of the downward pressure
of Scandinavian ice on our own shores. The western half of Gothland
is striated in the direction of N.E. and S.W., and that this has been
effected by a huge mass of ice covering the country, and not by local
glaciers, is apparent from the fact observed by Robert Chambers,[256]
and officers of the Swedish Geological Survey, that the general
direction of the groovings and striæ on the rocks bears little or no
relation to the conformation of the surface, showing that the ice was
of sufficient thickness to move straight forward, regardless of the
inequalities of the ground.
At Gottenburg, on the shores of the Cattegat, and all around Lake
Wener and Lake Wetter, the ice-markings are of the most remarkable
character, indicating, in the most decided manner, that the ice came
from the interior of the country to the north-east in one vast mass.
All this mass of ice must have gone into the shallow Cattegat, a sea
not sufficiently deep to float even an ordinary glacier. The ice coming
off Gothland would therefore cross the Cattegat, and thence pass over
Jutland into the North Sea. After entering the North Sea, it would be
obliged to keep between our shores and the ice coming direct from the
western side of Scandinavia.
But this is not all. A very large proportion of the Scandinavian ice
would pass into the Gulf of Bothnia, where it could not possibly float.
It would then move south into the Baltic as land-ice. After passing
down the Baltic, a portion of the ice would probably move south into
the flat plains in the north of Germany, but the greater portion
would keep in the bed of the Baltic, and of course turn to the right
round the south end of Gothland, and thence cross over Denmark into
the North Sea. That this must have been the path of the ice is, I
think, obvious from the observations of Murchison, Chambers, Hörbye,
and other geologists. Sir Roderick Murchison found—though he does not
attribute it to land-ice—that the Aland Islands, which lie between the
Gulf of Bothnia and the Baltic, are all striated in a north and south
direction.[257]
Upsala and Stockholm, a tract of flat country projecting for some
distance into the Baltic, is also grooved and striated, not in the
direction that would be effected by ice coming from the interior of
Scandinavia, but north and south, in a direction parallel to what must
have been the course of the ice moving down the Baltic.[258] This part
of the country must have been striated by a mass of ice coming from
the direction of the Gulf of Bothnia. And that this mass must have
been great is apparent from the fact that Lake Malar, which crosses
the country from east to west, at right angles to the path of the ice,
does not seem to have had any influence in deflecting the icy stream.
That the ice came from the north and not from the south is also evident
from the fact that the northern sides of rocky eminences are polished,
rounded, and ice-worn, while the southern sides are comparatively
rough. The northern banks of Lake Malar, for example, which, of course,
face the south, are rough, while the southern banks, which must have
offered opposition to the advance of the ice, are smoothed and rounded
in a most singular manner.
Again, that the ice, after passing down the Baltic, turned to the
right along the southern end of Gothland, is shown by the direction
of the striæ and ice-groovings observed on such islands as Gothland,
Öland, and Bornholm. Sir R. Murchison found that the island of
Gothland is grooved and striated in one uniform direction from N.E.
to S.W. “These groovings,” says Sir Roderick, “so perfectly resemble
the flutings and striæ produced in the Alps by the actual movement
of glaciers, that neither M. Agassiz nor any one of his supporters
could detect a difference.” He concludes, however, that the markings
could not have been made by land-ice, because Gothland is not only a
low, flat island in the middle of the Baltic, but is “at least 400
miles distant from any elevation to which the term of mountain can be
applied.” This, of course, is conclusive against the hypothesis that
Gothland and the other islands of the Baltic could have been glaciated
by ordinary glaciers; but it is quite in harmony with the theory
that the Gulf of Bothnia and the entire Baltic were filled with one
continuous mass of land-ice, derived from the drainage of the greater
part of Sweden, Lapland, and Finland. In fact, the whole glacial
phenomena of Scandinavia are inexplicable on the hypothesis of local
glaciers.
That the Baltic was completely filled by a mass of ice moving from the
north is further evidenced by the fact that the mainland, not only at
Upsala, but at several places along the coast of Gothland, is grooved
and striated parallel to the shore, and often at right angles to the
markings of the ice from the interior, showing that the present bed of
the Baltic was not large enough to contain the icy stream. For example,
along the shores between Kalmar and Karlskrona, as described by Sir
Roderick Murchison and by M. Hörbye, the striations are parallel to the
shore. Perhaps the slight obstruction offered by the island of Öland,
situated so close to the shore, would deflect the edge of the stream at
this point over on the land. The icy stream, after passing Karlskrona,
bent round to the west along the present entrance to the Baltic, and
again invaded the mainland, and crossed over the low headland of
Christianstadt, and thence passed westward in the direction of Zealand.
[Illustration: PLATE V.
W. & A. K. Johnston, Edinb^r. and London.
CHART SHOWING THE PROBABLE PATH OF THE ICE IN NORTH-WESTERN EUROPE
DURING THE PERIOD OF MAXIMUM GLACIATION.
_The lines also represent the actual direction of the striae on the
rocks._]
This immense Baltic glacier would in all probability pass over Denmark,
and enter the North Sea somewhere to the north of the River Elbe, and
would then have to find an outlet to the Atlantic through the English
Channel, or pass in between our eastern shores and the mass from
Gothland and the north-western shores of Europe. The entire probable
path of the ice may be seen by a reference to the accompanying chart
(Plate V.) That the ice crossed over Denmark is evident from the fact
that the surface of that country is strewn with _débris_ derived from
the Scandinavian peninsula.
Taking all these various considerations into account, the conclusion is
inevitable that the great masses of ice from Scotland would be obliged
to turn abruptly to the north, as represented in the diagram, and pass
round into the Atlantic in the direction of Caithness and the Orkney
Islands.
If the foregoing be a fair representation of the state of matters,
it is physically impossible that Caithness could have escaped being
overridden by the land-ice of the North Sea. Caithness, as is well
known, is not only a low, flat tract of land, little elevated above the
sea-level, and consequently incapable of supporting large glaciers;
but, in addition, it projects in the form of a headland across the
very path of the ice. Unless Caithness could have protected itself by
pushing into the sea glaciers of one or two thousand feet in thickness,
it could not possibly have escaped the inroads of the ice of the
North Sea. But Caithness itself could not have supported glaciers of
this magnitude, neither could it have derived them from the adjoining
mountainous regions of Sutherland, for the ice of this county found a
more direct outlet than along the flat plains of Caithness.
The shells which the boulder clay of Caithness contains have thus
evidently been pushed out of the bed of the North Sea by the land-ice,
which formed the clay itself.
The fact that these shells are not so intensely arctic as those found
in some other quarters of Scotland, is no evidence that the clay was
not formed during the most severe part of the glacial epoch, for the
shells did not live in the North Sea at the time that it was filled
with land-ice. The shells must have belonged to a period prior to the
invasion of the ice, and consequently before the cold had reached its
greatest intensity. Neither is there any necessity for supposing the
shells to be pre-glacial, for these shells may have belonged to an
inter-glacial period. In so far as Scotland is concerned, it would be
hazardous to conclude that a plant or an animal is either pre-glacial
or post-glacial simply because it may happen not to be of an arctic or
of a boreal type.
The same remarks which apply to Caithness apply to a certain extent
to the headland at Fraserburgh. It, too, lay in the path of the ice,
and from the direction of the striæ on the rocks, and the presence of
shells in the clay, as described by Mr. Jamieson, it bears evidence
also of having been overridden by the land-ice of the North Sea.
In fact, we have, in the invasion of Caithness and the headland at
Fraserburgh by the land-ice of the North Sea, a repetition of what we
have seen took place at Upsala, Kalmar, Christianstadt, and other flat
tracts along the sides of the Baltic.
The scarcity, or perhaps entire absence of Scandinavian boulders in
the Caithness clay is not in any way unfavourable to the theory, for
it would only be the left edge of the North Sea glacier that could
possibly pass over Caithness; and this edge, as we have seen, was
composed of the land-ice from Scotland. We might expect, however, to
find Scandinavian blocks on the Shetland and Faroe Islands, for, as we
shall presently see, there is pretty good evidence to prove that the
Scandinavian ice passed over these islands.
_The Shetland and Faroe Islands glaciated by Land-ice._—It is also
worthy of notice that the striæ on the rocks in the Orkney, Shetland,
and Faroe Islands, all point in the direction of Scandinavia, and are
what would be effected by land-ice moving in the paths indicated
in the diagram. And it is a fact of some significance, that when we
proceed north to Iceland, the striæ, according to the observations
of Robert Chambers, seem to point towards North Greenland. Is it
possible that the entire Atlantic, from Scandinavia to Greenland, was
filled with land-ice? Astounding as this may at first appear, there
are several considerations which render such a conclusion probable.
The observations of Chambers, Peach, Hibbert, Allan, and others, show
that the rocky face of the Shetland and Faroe Islands has been ground,
polished, and striated in a most remarkable manner. That this could not
have been done by ice belonging to the islands themselves is obvious,
for these islands are much too small to have supported glaciers of any
size, and the smallest of them is striated as well as the largest.
Besides, the uniform direction of the striæ on the rocks shows that
it must have been effected by ice passing over the islands. That the
striations could not have been effected by floating icebergs at a time
when the islands were submerged is, I think, equally obvious, from the
fact that not only are the tops of the highest eminences ice-worn,
but the entire surface down to the present sea-level is smoothed and
striated; and these striations conform to all the irregularities of the
surface. This last fact Professor Geikie has clearly shown is wholly
irreconcilable with the floating-ice theory.[259] Mr. Peach[260] found
vertical precipices in the Shetlands grooved and striated, and the
same thing was observed by Mr. Thomas Allan on the Faroe Islands.[261]
That the whole of these islands have been glaciated by a continuous
sheet of ice passing over them was the impression left on the mind of
Robert Chambers after visiting them.[262] This is the theory which
alone explains all the facts. The only difficulty which besets it is
the enormous thickness of the ice demanded by the theory. But this
difficulty is very much diminished when we reflect that we have good
evidence, from the thickness of icebergs which have been met with
in the Southern Ocean,[263] that the ice moving off the antarctic
continent must be in some places considerably over a mile in thickness.
It is then not so surprising that the ice of the glacial epoch, coming
off Greenland and Northern Europe, should not have been able to float
in the North Atlantic.
_Why the Ice of Scotland was of such enormous Thickness._—The enormous
thickness of the ice in Scotland, during the glacial epoch, has been
a matter of no little surprise. It is remarkable how an island, not
more than 100 miles across, should have been covered with a sheet
of ice so thick as to bury mountain ranges more than 1,000 feet in
height, situated almost at the sea-shore. But all our difficulties
disappear when we reflect that the seas around Scotland, owing to their
shallowness, were, during the glacial period, blocked up with solid
ice. Scotland, Scandinavia, and the North Sea, would form one immense
table-land of ice, from 1,000 to 2,000 feet above the sea-level. This
table-land would terminate in the deep waters of the Atlantic by a
perpendicular wall of ice, extending probably from the west of Ireland
away in the direction of Iceland. From this barrier icebergs would be
continually breaking off, rivalling in magnitude those which are now to
be met with in the antarctic seas.
_The great Extension of the Loess accounted for._—An effect which would
result from the blocking up of the North Sea with land-ice, would be
that the waters of the Rhine, Elbe, and Thames would have to find
an outlet into the Atlantic through the English Channel. Professor
Geikie has suggested to me that if the Straits of Dover were not then
open—quite a possible thing—or were they blocked up with land-ice, say
by the great Baltic glacier crossing over from Denmark, the consequence
would be that the waters of the Rhine and Elbe would be dammed back,
and would inundate all the low-lying tracts of country to the south;
and this might account for the extraordinary extension of the Loess in
the basin of the Rhine, and in Belgium and the north of France.[264]
[Illustration: PLATE VI.
CHART SHOWING PATH OF THE ICE
W. & A. K. Johnston, Edinb^r and London.
Note.
_Curved lines shew path of Ice.
Arrows shew direction of striae
as observed by Prof. Geikie & B. N. Peach.
Short thick lines shew direction of
striae by other observers._]
_Note on the Glaciation of Caithness._
I have very lately received a remarkable confirmation of the path of
the Caithness ice in observations communicated to me by Professor
Geikie and Mr. B. N. Peach. The latter geologist says, “Near the Ord
of Caithness and on to Berriedale the striæ pass off the land and out
to sea; but near Dunbeath, 6 miles north-east of Berriedale, they
begin to creep up out of the sea on to the land and range from about
15° to 10° east of north. _Where the striæ pass out to the sea_ the
boulder clay is made up of the materials from inland and contains no
shells, but _immediately the striæ begin to creep up on to the land_
then shells begin to make their appearance; and there is a difference,
moreover, in the colour of the clay, for in the former case it is
red and incoherent, and in the latter hard and dark-coloured.” The
accompanying chart (Plate VI.) shows the outline of the Caithness coast
and the direction of the striæ as observed by Professor Geikie and Mr.
Peach, and no demonstration could be more conclusive as to the path of
the ice and the obstacles it met than these observations, supplemented
and confirmed as they are by other recorded facts to which I shall
presently allude. Had the ice-current as it entered the North Sea off
the Sutherland coast met with no obstacle it would have ploughed its
way outwards till it broke off in glaciers and floated away. But it is
clear that the great press of Scandinavian ice and the smaller mass of
land-ice from the Morayshire coast converging in the North Sea filled
up its entire bed, and these, meeting the opposing current from the
Sutherland coast, turned it back upon itself, and forced it over the
north-east part of Caithness. The farther south on the Sutherland
coast that the ice entered the sea the deeper would it be able to
penetrate into the ocean-bed before it met an opposition sufficiently
strong to turn its course, and the wider would be its sweep; but when
we come to the Sutherland coast we reach a point where the land-ice—as,
for example, near Dunbeath—is forced to bend round before it even
reaches the sea-shore, as will be seen from the accompanying diagram.
We are led to the same conclusions regarding the path of the ice in the
North Sea from the presence of oolitic fossils and chalk flints found
likewise in the boulder clay of Caithness, for these, as we shall see,
evidently must have come from the sea. At the meeting of the British
Association, Edinburgh, 1850, Hugh Miller exhibited a collection of
boreal shells with fragments of oolitic fossils, chalk, and chalk
flints from the boulder clay of Caithness collected by Mr. Dick, of
Thurso. My friend, Mr. C. W. Peach, found that the chalk flints in
the boulder clay of Caithness become more abundant as we proceed
northward, while the island of Stroma in the Pentland Firth he found
to be completely strewn with them. This same observer found, also, in
the Caithness clay stones belonging to the Oolitic and Lias formations,
with their characteristic fossils, while ammonites, belemnites, fossil
wood, &c., &c., were also found loose in the clay.[265] The explanation
evidently is, that these remains were derived from an outcrop of
oolitic and cretaceous beds in the North Sea. It is well known that
the eastern coast of Sutherlandshire is fringed with a narrow strip
of oolite, which passes under the sea, but to what distance is not
yet ascertained. Outside the Oolitic formation the chalk beds in all
probability crop out. It will be seen from a glance at the accompanying
chart (Plate VI.) that the ice which passed over the north-eastern part
of Caithness must have crossed the out-cropping chalk beds.
As has already been stated in the foregoing chapter, the headland of
Fraserburgh, north-eastern corner of Aberdeenshire, bears evidence,
both from the direction of the striæ and broken shells in the boulder
clay, of having been overridden also by land-ice from the North Sea.
This conclusion is strengthened by the fact that chalk flints and
oolitic fossils have also been abundantly met with in the clay by Dr.
Knight, Mr. James Christie, Mr. W. Ferguson, Mr. T. F. Jamieson, and
others.
CHAPTER XXVIII.
NORTH OF ENGLAND ICE-SHEET, AND TRANSPORT OF WASTDALE CRAG
BLOCKS.[266]
Transport of Blocks; Theories of.—Evidence of Continental
Ice.—Pennine Range probably striated on Summit.—Glacial
Drift in Centre of England.—Mr. Lacy on Drift of Cotteswold
Hills.—England probably crossed by Land-ice.—Mr. Jack’s
Suggestion.—Shedding of Ice North and South.—South of England
Ice-sheet.—Glaciation of West Somerset.—Why Ice-markings are
so rare in South of England.—Form of Contortion produced by
Land-ice.
Considerable difficulty has been felt in accounting for the transport
of the Wastdale granite boulders across the Pennine chain to the east.
Professors Harkness,[267] and Phillips,[268] Messrs. Searles Wood,
jun.,[269] Mackintosh,[270] and I presume all who have written on
the subject, agree that these blocks could not have been transported
by land-ice. The agency of floating ice under some form or other is
assumed by all.
We have in Scotland phenomena of an exactly similar nature. The summits
of the Ochils, the Pentlands, and other mountain ranges in the east
of Scotland, at elevations of from 1,500 to 2,000 feet, are not only
ice-marked, but strewn over with boulders derived from rocks to the
west and north-west. Many of them must have come from the Highlands
distant some 50 or 60 miles. It is impossible that these stones could
have been transported, or the summits of the hills striated, by means
of ordinary glaciers. Neither can the phenomena be attributed to the
agency of icebergs carried along by currents. For we should require to
assume not merely a submergence of the land to the extent of 2,000
feet or so,—an assumption which might be permitted,—but also that the
currents bearing the icebergs took their rise in the elevated mountains
of the Highlands (a most unlikely place), and that these currents
radiated in all directions from that place as a centre.
In short, the glacial phenomena of Scotland are wholly inexplicable
upon any other theory than that, during at least a part of the
glacial epoch, the entire island from sea to sea was covered with one
continuous mass of ice of not less than 2,000 feet in thickness.
In my paper on the Boulder Clay of Caithness (see preceding chapter),
I have shown that if the ice was 2,000 feet or so in thickness, it
must, in its motion seawards, have followed the paths indicated by the
curved lines in the chart accompanying that paper (See Plate V.). In
so far as Scotland is concerned [and Scandinavia also], these lines
represent pretty accurately not only the paths actually taken by the
boulders, but also the general direction of the ice-markings on all the
elevated mountain ridges. But if Scotland was covered to such an extent
with ice, it is not at all probable that Westmoreland and the other
mountainous districts of the North of England could have escaped being
enveloped in a somewhat similar manner. Now if we admit the supposition
of a continuous mass of ice covering the North of England, all our
difficulties regarding the transport of the Wastdale blocks across the
Pennine chain disappear. An inspection of the chart above referred to
will show that these blocks followed the paths which they ought to have
done upon the supposition that they were conveyed by continental ice.
That Wastdale Crag itself suffered abrasion by ice moving over it, in
the direction indicated by the lines in the diagram, is obvious from
what has been recorded by Dr. Nicholson and Mr. Mackintosh. They both
found the Crag itself beautifully _moutonnée_ up to its summit, and
striated in a W.S.W. and E.N.E. direction. Mr. Mackintosh states that
these scorings run obliquely up the sloping face of the crag. Ice
scratches crossing valleys and running up the sloping faces of hills
and over their summits are the sure marks of continental ice, which
meet the eye everywhere in Scotland. Dr. Nicholson found in the drift
covering the lower part of the crag, pebbles of the Coniston flags and
grits from the west.[271]
The fact that in Westmoreland the direction of the ice-markings, as a
general rule, corresponds with the direction of the main valleys, is
no evidence whatever that the country was not at one period covered
with a continuous sheet of ice; because, for long ages after the period
of continental ice, the valleys would be occupied by glaciers, and
these, of course, would necessarily leave the marks of their presence
behind. This is just what we have everywhere in Scotland. It is on
the summits of the hills and elevated ridges, where no glacier could
possibly reach, that we find the sure evidence of continental ice.
But that land-ice should have passed over the tops of hills 1,000 or
2,000 feet in height is a thing hitherto regarded by geologists as
so unlikely that few of them ever think of searching in such places
for ice-markings, or for transported stones. Although little has been
recorded on this point, I hardly think it likely that there is in
Scotland a hill under 2,000 feet wholly destitute of evidence that ice
has gone over it. If there were hills in Scotland that should have
escaped being overridden by ice, they were surely the Pentland Hills;
but these, as was shown on a former occasion,[272] were completely
buried under the mass of ice covering the flat surrounding country.
I have no doubt whatever that if the summits of the Pennine range
were carefully examined, say under the turf, evidence of ice-action,
in the form of transported stones or scratches on the rock, would be
found.[273]
Nor is the fact that the Wastdale boulders are not rounded and
ice-marked, or found in the boulder clay, but lie on the surface, any
evidence that they were not transported by land-ice. For it would not
be the stones _under_ the ice, but those falling on the upper surface
of the sheet, that would stand the best chance of being carried over
mountain ridges. But such blocks would not be crushed and ice-worn;
and it is on the surface of the clay, and not imbedded in it, that we
should expect to find them.
It is quite possible that the dispersion of the Wastdale boulders took
place at various periods. During the period of local glaciers the
blocks would be carried along the line of the valleys.
All I wish to maintain is that the transport of the blocks across
the Pennine chain is easily accounted for if we admit, what is very
probable, that the great ice-covering of Scotland overlapped the high
grounds of the North of England. The phenomenon is the same in both
places, and why not attribute it to the same cause?
There is another curious circumstance connected with the drift of
England which seems to indicate the agency of an ice-covering.
As far back as 1819, Dr. Buckland, in his Memoir on the Quartz Rock
of Lickey Hill,[274] directed attention to the fact, that on the
Cotteswold Hills there are found pebbles of hard red chalk which must
have come from the Wolds of Yorkshire and Lincolnshire. He pointed
out also that the slaty and porphyritic pebbles probably came from
Charnwood Forest, near Leicester. Professor Hull, of the Geological
Survey, considers that “almost all the Northern Drift of this part
of the country had been derived from the _débris_ of the rocks of
the Midland Counties.”[275] He came also to the conclusion that the
slate fragments may have been derived from Charnwood Forest. In the
Vale of Moreton he found erratic boulders from two feet to three
feet in diameter. The same northern character of the drift of this
district is remarked by Professor Ramsay and Mr. Aveline, in their
Memoir of the Geology of parts of Gloucestershire. In Leicestershire
and Northamptonshire the officers of the Geological Survey found in
abundance drift which must have come from Lincolnshire and Yorkshire to
the north-east.
Mr. Lucy, who has also lately directed attention to the fact that
the Cotteswold Hills are sprinkled over with boulders from Charnwood
Forest, states also that, on visiting the latter place, he found that
many of the stones contained in it had come from Yorkshire, still
further to the north-east.[276]
Mr. Searles Wood, jun., in his interesting paper on the Boulder Clay
of the North of England,[277] states that enormous quantities of the
chalk _débris_ from the Yorkshire Wold are found in Leicester, Rutland,
Warwick, Northampton, and other places to the south and south-west.
Mr. Wood justly concludes that this chalk _débris_ could not have been
transported by water. “If we consider,” he says, “the soluble nature
of chalk, it must be evident that none of this débris can have been
detached from the parent mass, either by water-action, or by any other
atmospheric agency than moving ice. The action of the sea, of rivers,
or of the atmosphere, upon chalk, would take the form of dissolution,
the degraded chalk being taken up in minute quantities by the water,
and held in suspension by it, and in that form carried away; so that
it seems obvious that this great volume of rolled chalk can have been
produced in no other way than by the agency of moving ice; and for that
agency to have operated to an extent adequate to produce a quantity
that I estimate as exceeding a layer 200 feet thick over the entire
Wold, nothing less than the complete envelopment of a large part of the
Wold by ice for a long period would suffice.”
I have already assigned my reasons for disbelieving the opinion that
such masses of drift could have been transported by floating ice; but
if we refer it to land-ice, it is obvious that the ice could not have
been in the form of local glaciers, but must have existed as a sheet
moving in a south and south-west direction, from Yorkshire, across the
central part of England. But how is this to harmonize with the theory
of glaciation, which is advanced to explain the transport of the Shap
boulders?
The explanation has, I think, been pointed out by a writer in the
_Glasgow Herald_,[278] of the 26th November, 1870, in a review of Mr.
Lucy’s paper.
In my paper on the Boulder Clay of Caithness, I had represented the ice
entering the North Sea from the east coast of Scotland and England,
as all passing round the north of Scotland. But the reviewer suggests
that the ice entering at places to the south of, say, Flamborough Head,
would be deflected southwards instead of northwards, and thus pass over
England. “It is improbable, however,” says the writer, “that this joint
ice-sheet would, as Mr. Croll supposes, all find its way round the
north of Scotland into the deep sea. The southern uplands of Scotland,
and probably also the mountains of Northumberland, propelled, during
the coldest part of the glacial period, a land ice-sheet in an eastward
direction. This sheet would be met by another streaming outward from
the south-western part of Norway—in a diametrically opposite direction.
In other words, an imaginary line might be drawn representing the
course of some particular boulder in the _moraine profonde_ from
England met by a boulder from Norway, in the same straight line. With
a dense ice-sheet to the north of this line, and an open plain to the
south, it is clear that all the ice travelling east or west from points
to the south of the starting-points of our two boulders would be ‘shed’
off to the south. There would be a point somewhere along the line, at
which the ice would turn as on a pivot—this point being nearer England
or Scandinavia, as the degree of pressure exercised by the respective
ice-sheets should determine. There is very little doubt that the point
in question would be nearer England. Further, the direction of the
joint ice-sheet could not be _due_ south unless the pressure of the
component ice-sheets should be exactly equal. In the event of that from
Scandinavia pressing with greater force, the direction would be to the
south-west. This is the direction in which the drifts described by Mr.
Lucy have travelled.”
I can perceive no physical objection to this modification of the
theory. What the ice seeks is the path of least resistance, and along
this path it will move, whether it may lie to the south or to the
north. And it is not at all improbable that an outlet to the ice would
be found along the natural hollow formed by the valleys of the Trent,
Avon, and Severn. Ice moving in this direction would no doubt pass down
the Bristol Channel and thence into the Atlantic.
Might not the shedding of the north of England ice-sheet to the north
and south, somewhere not far from Stainmoor, account for the remarkable
fact pointed out by Mr. Searles Wood, that the boulder clay, with
Shap boulders, to the north of the Wold is destitute of chalk; while,
on the other hand, the chalky boulder clay to the south of the Wold
is destitute of Shap boulders? The ice which passed over Wastdale
Crag moved to the E.N.E., and did not cross the chalk of the Wold;
while the ice which bent round to the south by the Wold came from the
district lying to the south of Wastdale Crag, and consequently did not
carry with it any of the granite from that Crag. In fact, Mr. Searles
Wood has himself represented on the map accompanying his Memoir this
shedding of the ice north and south.
These theoretical considerations are, of course, advanced for what
they are worth. Hitherto geologists have been proceeding upon the
supposition of an ice-sheet and an open North Sea; but the latter is
an impossibility. But if we suppose the seas around our island to have
been filled with land-ice during the glacial epoch, the entire glacial
problem is changed, and it does not then appear so surprising that ice
should have passed over England.
_Note on the South of England Ice-sheet._
If what has already been stated regarding the north of England be
anything like correct, it is evident that the south of England
could not possibly have escaped glaciation. If the North Sea was so
completely blocked up by Scandinavian ice, that the great mass of ice
from the Cumberland mountains entering the sea on the east coast was
compelled to bend round and find a way of escape across the centre
of England in the direction of the Bristol Channel, it is scarcely
possible that the immense mass of ice filling the Baltic Sea and
crossing over Denmark could help passing across at least a portion
of the south of England. The North Sea being blocked up, its natural
outlet into the Atlantic would be through the English Channel; and it
is not likely that it could pass through without impinging to some
extent upon the land. Already geologists are beginning to recognise the
evidence of ice in this region.
Mr. W. C. Lucy, in the _Geological Magazine_ for June, 1874, records
the finding by himself of evidences of glaciation in West Somerset,
in the form of “rounded rocky knolls,” near Minehead, like those of
glaciated districts; of a bed of gravel and clay 70 feet deep, which
he considered to be boulder clay. He also mentions the occurrence near
Portlock of a large mass of sandstone well striated, only partially
detached from the parent rock. In the same magazine for the following
month Mr. H. B. Woodward records the discovery by Mr. Usher of some
“rum stuff” near Yarcombe, in the Black Down Hills of Devonshire,
which, on investigation, proved to be boulder clay; and further, that
it was not a mere isolated patch, but occurred in several other places
in the same district. Mr. C. W. Peach informs me that on the Cornwall
coast, near Dodman Point, at an elevation of about 60 feet above
sea-level, he found the rock surface well striated and ice-polished.
In a paper on the Drift Deposits of the Bath district, read before the
Bath Natural History and Antiquarian Field Club, March 10th, 1874,
Mr. C. Moore describes the rock surfaces as grooved, with deep and
long-continued furrows similar to those usually found on glaciated
rocks, and concludes that during the glacial period they were subjected
to ice-action. This conclusion is confirmed by the fact of there being
found, immediately overlying these glaciated rocks, beds of gravel
with intercalated clay-beds, having a thickness of 30 feet, in which
mammalian remains of arctic types are abundant. The most characteristic
of which are _Elephas primigenius_, _E. antiquus_, _Rhinoceros
tichorhinus_, _Bubalus moschatus_, and _Cervus tarandus_.
There is little doubt that when the ground is better examined many
other examples will be found. One reason, probably, why so little
evidence of glaciation in the south of England has been recorded,
is the comparative absence of rock surfaces suitable for retaining
ice-markings. There is, however, one class of evidence which might
determine the question of the glaciation of the south of England as
satisfactorily as markings on the rock. The evidence to which I refer
is that of contorted beds of sand or clay. In England contortions from
the sinking of the beds are, of course, quite common, but a thoughtful
observer, who has had a little experience of ice-formed contortions,
can easily, without much trouble, distinguish the latter from the
former. Contortions resulting from the lateral pressure of the ice
assume a different form from those produced by the sinking of the beds.
In Scotland, for example, there is one well-marked form of contortion,
which not only proves the existence of land-ice, but also the direction
in which it moved. The form of contortion to which I refer is the
bending back of the stratified beds upon themselves, somewhat in
the form of a fishing-hook. This form of contortion will be better
understood from the accompanying figure.
[Illustration: Fig. 11.
SECTION OF CONTORTED DRIFT NEAR MUSSELBURGH.
_a_ Boulder Clay; _b_ Laminated Clay; _c_ Sand, Gravel, and Clay,
contorted.
Depth of Section, twenty-two feet.—H. SKAE.]
CHAPTER XXIX.
EVIDENCE FROM BURIED RIVER CHANNELS OF A CONTINENTAL PERIOD IN
BRITAIN.[279]
Remarks on the Drift Deposits.—Examination of Drift
by Borings.—Buried River Channel from Kilsyth to
Grangemouth.—Channels not excavated by Sea nor by
Ice.—Section of buried Channel at Grangemouth.—Mr. Milne
Home’s Theory.—German Ocean dry Land.—Buried River Channel
from Kilsyth to the Clyde.—Journal of Borings.—Marine
Origin of the Drift Deposits.—Evidence of Inter-glacial
Periods.—Oscillations of Sea-level.—Other buried River
Channels.
_Remarks on the Drift Deposits._—The drift and other surface deposits
of the country have chiefly been studied from sections observed on the
banks of streams, railway cuttings, ditches, foundations of buildings,
and other excavations. The great defect of such sections is that they
do not lay open a sufficient depth of surface. They may, no doubt,
represent pretty accurately the character and order of the more recent
deposits which overlie the boulder clay, but we are hardly warranted
in concluding that the succession of deposits belonging to the earlier
part of the glacial epoch, the period of the true till, is fully
exhibited in such limited sections.
Suppose, for example, the glacial epoch proper—the time of the lower
boulder clay—to have consisted of a succession of alternate cold and
warm periods, there would, in such a case, be a series of separate
formations of boulder clay; but we could hardly expect to find on the
flat and open face of the country, where the surface deposits are
generally not of great depth, those various formations of till lying
the one superimposed upon the other. For it is obvious that the till
formed during one ice-period would, as a general rule, be either swept
away or re-ground and laid down by the ice of the succeeding period.
If the very hardest rocks could not withstand the abrading power
of the enormous masses of ice which passed over the surface of the
country during the glacial epoch, it is hardly to be expected that the
comparatively soft boulder clay would be able to do so. It is probable
that the boulder clay of one period would be used as grinding materials
by the ice of the succeeding periods. The boulder clay which we find in
one continuous mass may, therefore, in many cases, have been ground off
the rocks underneath at widely different periods.
If we wish to find the boulder clays belonging to each of the
successive cold periods lying, the one superimposed on the other in
the order of time in which they were formed, we must go and search in
some deep gorge or valley, where the clay has not only accumulated
in enormous masses, but has been partially protected from the
destructive power of the ice. But it is seldom that the geologist has
an opportunity of seeing a complete section down to the rock-head in
such a place. In fact, excepting by bores for minerals, or by shafts
of pits, the surface, to a depth of one or two hundred feet, is never
passed through or laid open.
_Examination of Drift by Borings._—With the view of ascertaining if
additional light would be cast on the sequence of events, during the
formation of the boulder clay, by an examination of the journals of
bores made through a great depth of surface deposits, a collection
of about 250 bores, put down in all parts of the mining districts
of Scotland, was made. An examination of these bores shows most
conclusively that the opinion that the boulder clay, or lower till, is
one great undivided formation, is wholly erroneous.
These 250 bores, as already stated,[280] represent a total thickness
of 21,348 feet, giving 86 feet as the mean thickness of the deposits
passed through. Twenty of these bores have one boulder clay, with beds
of stratified sand or gravel beneath the clay; 25 have 2 boulder clays,
with stratified beds of sand and gravel between; 10 have 3 boulder
clays; one has 4 boulder clays; 2 have 5 boulder clays; and one has no
fewer than 6 separate masses of boulder clay, with stratified beds of
sand and gravel between; 16 have two or three separate boulder clays,
differing altogether in colour and hardness, without any stratified
beds between. We have, therefore, out of 250 bores, 75 of them
representing a condition of things wholly different from that exhibited
to the geologist in ordinary sections.
These bores bear testimony to the conclusion that the glacial epoch
consisted of a succession of cold and warm periods, and not of one
continuous and unbroken period of ice, as was at one time generally
supposed.
The full details of the character of the deposits passed through by
these bores, and their bearing on the history of the glacial epoch,
have been given by Mr. James Bennie, in an interesting paper read
before the Glasgow Geological Society,[281] to which I would refer
all those interested in the subject of surface geology. But it is not
to the mere contents of the bores that I wish at present to direct
attention, but to a new and important result, to which they have
unexpectedly led.
_Buried River Channel, Kilsyth to Grangemouth, Firth of Forth._—These
borings reveal the existence of a deep pre-glacial, or perhaps
inter-glacial, trough or hollow, extending from the Clyde above Bowling
across the country by Kilsyth, along the valley of the Forth and Clyde
Canal, to the Firth of Forth at Grangemouth. This trough is filled up
with immense deposits of mud, sand, gravel, and boulder clay. These
deposits not only fill it up, but they cover it over to such an extent
that it is absolutely impossible to find on the surface a single trace
of it; and had it not been for borings, and other mining operations,
its existence would probably never have been known. In places where the
bottom of the trough is perhaps 200 feet below the sea-level, we find
on the surface not a hollow, but often an immense ridge or elliptical
knoll of sand, gravel, or boulder clay, rising sometimes to 150 or 200
feet above the present sea-level.
I need not here enter into any minute details regarding the form,
depth, and general outline of this trough, or of the character of
the deposits covering it, these having already been described by Mr.
Bennie, but shall proceed to the consideration of circumstances which
seem to throw light on the physical origin of this curious hollow,
and to the proof which it unexpectedly affords that Scotland, during
probably an early part of the glacial epoch, stood higher in relation
to the sea-level than it does at present; or rather, as I would be
disposed to express it, the sea stood much lower than at present.
From the fact that all along the line of this trough the surface of the
country is covered with enormous beds of stratified sands and gravels
of marine origin, which proves that the sea must have at a recent
period occupied the valley, my first impression was that this hollow
had been scooped out by the sea. This conclusion appeared at first
sight quite natural, for at the time that the sea filled the valley,
owing to the Gulf-stream impinging on our western shores, a strong
current would probably then pass through from the Atlantic on the west
to the German Ocean on the east. However, considerations soon began to
suggest themselves wholly irreconcilable with this hypothesis.
The question immediately arose, if the tendency of the sea occupying
the valley is to deepen it, by wearing down its rocky bottom, and
removing the abraded materials, then why is the valley filled up to
such a prodigious extent with marine deposits? Does not the fact of the
whole valley being filled up from sea to sea with marine deposits to a
depth of from 100 to 200 feet, and in some places, to even 400 feet,
show that the tendency of the sea filling this valley is to silt it up
rather than to deepen it? What conceivable change of conditions could
account for operations so diverse?
That the sea could not have cut out this trough, is, however,
susceptible of direct proof. The height of the surface of the valley
at the watershed or highest part, about a mile to the east of
Kilsyth, where the Kelvin and the Bonny Water, running in opposite
directions,—the one west into the Clyde, and the other east into the
Carron,—take their rise, is 160 feet above the sea-level. Consequently,
before the sea could pass through the valley at present, the sea-level
would require to be raised 160 feet.
But in discussing the question as to the origin of this pre-glacial
hollow, we must suppose the surface deposits of the valley all removed,
for this hollow was formed before these deposits were laid down. Let
us take the average depth of these deposits at the watershed to be 50
feet. It follows that, assuming the hollow in question to have been
formed by the sea, the sea-level at the time must have been at least
110 feet higher than at present.
Were the surface deposits of the country entirely removed, the district
to the west and north-west of Glasgow would be occupied by a sea
which would stretch from the Kilpatrick Hills, north of Duntocher,
to Paisley, a distance of about five miles, and from near Houston to
within a short way of Kirkintilloch, a distance of more than twelve
miles. This basin would contain a few small islands and sunken rocks,
but its mean depth, as determined from a great number of surface bores
obtained over its whole area, would be not much under 70 or 80 feet.
But we shall, however, take the depth at only 50 feet. Now, if we raise
the sea-level so as to allow the water just barely to flow over the
watershed of the valley, the sea in this basin would therefore be 160
feet deep. Let us now see what would be the condition of things on the
east end of the valley. The valley, for several miles to the east of
Kilsyth, continues very narrow, but on reaching Larbert it suddenly
opens into the broad and flat carse lands through which the Forth and
Carron wind. The average depth at which the sea would stand at present
in this tract of country, were the surface removed, as ascertained from
bores, would be at least 100 feet, or about double that in the western
basin. Consequently, when the sea was sufficiently high to pass over
the watershed, the water would be here 210 feet in depth, and several
miles in breadth.
[Illustration: PLATE VII.
W. & A. K. Johnston Edinb^r. and London.
CHART OF THE MIDLAND VALLEY, SHOWING BURIED RIVER CHANNELS.
_The blue parts represent the area which would be covered by sea
were the land submerged to the extent of 200 feet. The heavy black
lines A and B represent the buried River Channels._]
But in order to have a current of some strength passing through the
valley, let us suppose the sea at the time to have stood 150 feet
higher in relation to the land than at present. This would give 40 feet
as the depth of the sea on the watershed, and 200 feet as the depth in
the western basin, and 250 feet as the depth in the eastern.
An examination of the Ordnance Survey map of the district will show
that the 200 feet contour lines which run along each side of the valley
from Kilsyth to Castlecary come, in several places, to within one-third
of a mile of each other. From an inspection of the ground, I found
that, even though the surface deposits were removed off the valley, it
would not sensibly affect the contours at those places. It is therefore
evident that though the sea may have stood even 200 feet higher than at
present, the breadth of the strait at the watershed and several other
points could not have exceeded one-third of a mile. It is also evident
that at those places the current would be flowing with the greatest
velocity, for here it was not only narrowest, but also shallowest. A
reference to Plate VII. will show the form of the basins. The stippled
portion, coloured blue, represents the area which would be covered by
the sea were the land submerged to the extent of 200 feet.
Let us take the breadth of the current in the western basin at, say,
three miles. This is two miles less than the breadth of the basin
itself. Suppose the current at the narrow parts between Kilsyth and
Castlecary to have had a velocity of, say, five miles an hour. Now, as
the mean velocity of the current at the various parts of its course
would be inversely proportionate to the sectional areas of those parts,
it therefore follows that the mean velocity of the current in the
western basin would be only 1/45th of what it was in the narrow pass
between Kilsyth and Castlecary. This would give a mile in nine hours
as the velocity of the water in the western basin. In the eastern
basin the mean velocity of the current, assuming its breadth to be the
same as in the western, would be only a mile in eleven hours. In the
central part of the current the velocity at the surface would probably
be considerably above the mean, but at the sides and bottom it would,
no doubt, be under the mean. In fact, in these two basins the current
would be almost insensible.
The effect of such a current would simply be to widen and deepen the
valley all along that part between Kilsyth and Castlecary where the
current would be flowing with considerable rapidity. But it would
have little or no effect in deepening the basins at each end, but the
reverse. It would tend rather to silt them up. If the current flowed
from west to east, the materials removed from the narrow part between
Kilsyth and Castlecary, where the velocity of the water was great,
would be deposited when the current almost disappeared in the eastern
part of the valley. Sediment carried by a current flowing at the rate
of five miles an hour, would not remain in suspension when the velocity
became reduced to less than five miles a day.
But even supposing it were shown that the sea under such conditions
could have deepened the valley along the whole distance from the Clyde
to the Forth, still this would not explain the origin of the trough
in question. What we are in search of is not the origin of the valley
itself, but the origin of a deep and narrow hollow running along
the bottom of it. A sea filling the whole valley, and flowing with
considerable velocity, would, under certain conditions, no doubt deepen
and widen it, but it would not cut out along its bottom a deep, narrow
trough, with sides often steep, and in some places perpendicular and
even overhanging.
This hollow is evidently an old river-bed scooped out of the rocky
valley by a stream, flowing probably during an early part of the
glacial period.
During the latter part of the summer of 1868, I spent two or three
weeks of my holidays in tracing the course of this buried trough from
Kilsyth to the river Forth at Grangemouth, and I found unmistakable
evidence that the eastern portion of it, stretching from the watershed
to the Forth, had been cut out, not by the sea, but by a stream which
must have followed almost the present course of the Bonny Water.
I found that this deep hollow enters the Forth a few hundred yards to
the north of Grangemouth Harbour, at the extraordinary depth of 260
feet below the present sea-level. At the period when the sea occupied
the valley of the Forth and Clyde Canal, the bottom of the trough at
this spot would therefore be upwards of 400 feet below the level of the
sea.
A short distance to the west of Grangemouth, and also at Carron,
several bores were put down in lines almost at right angles across
the trough, and by this means we have been enabled to form a pretty
accurate estimate of its depth, breadth, and shape at those places. I
shall give the details of one of those sections.
Between Towncroft Farm and the river Carron, a bore was put down to
the depth of 273 feet before the rock was reached. About 150 yards to
the north of this there is another bore, giving 234 feet as the depth
to the rock; 150 yards still further north the depth of the surface
deposits, as determined by a third bore, is 155 feet. This last bore is
evidently outside of the hollow, for one about 150 yards north of it
gives the same depth of surface, which seems to be about its average
depth for a mile or two around. About half a mile to the south of the
hollow at this place the surface deposits are 150 feet deep. From a
number of bores obtained at various points within a circuit of 1½
miles, the surface appears to have a pretty uniform depth of 150 feet
or thereby. For the particulars of these “bores” I am indebted to the
kindness of Mr. Mackay, of Grangemouth.
To the south of the trough (see Fig. 12) there is a fault running
nearly parallel to it, having a down-throw to the north, and cutting
off the coal and accompanying strata to the south. But an inspection of
the section will show that the hollow in question is no way due to the
fault, but has been scooped out of the solid strata.
[Illustration: Fig. 12.
SECTION OF BURIED RIVER-BED NEAR TOWNCROFT FARM, GRANGEMOUTH.]
The main coal wrought extensively here is cut off by the trough,
as will be seen from the section. Mr. Dawson, of Carron Iron Works,
informs me that at Carronshore pit, about a mile and a quarter above
where this section is taken, the coal was found to be completely cut
off by this trough. In one of the workings of this pit, about forty
years ago, the miners cut into the trough at 40 fathoms below the
surface, when the sand rushed in with irresistible pressure, and filled
the working. Again, about a mile below where the section is taken,
or about two miles below Carronshore, and just at the spot where the
trough enters the Firth, it was also cut into in one of the workings of
the Heuck pit at a depth of 40 fathoms from the surface. Fortunately,
however, at this point the trough is filled with boulder clay instead
of sand, and no damage was sustained. Here, for a distance of two
miles, the Main coal and “Upper Coxroad” are cut off by this hollow; or
rather, I should say this hollow has been cut through the coal-seams.
The “Under Coxroad,” lying about 14 fathoms below the position of the
“Main” coal, as will be seen in the descriptive section (Fig. 12), is
not reached by the trough, and passes undisturbed under it.
This hollow would seem to narrow considerably as it recedes westwards,
for at Carronshore pit-shaft the surface is 138 feet deep; but not much
over 150 yards to the south of this is the spot where the coal was cut
off by the trough at a depth of 40 fathoms or 240 feet. Here it deepens
upwards of 100 feet in little more than 150 yards. That it is narrow at
this place is proved by the fact, that a bore put down near Carronbank,
a little to the south, shows the surface to be only 156 feet deep.
In the section (Fig. 12) the line described as “150 _feet above
sea-level_” registers the height of the sea-level at the time when
the central valley was occupied by sea 40 feet deep at the watershed.
Now, if this hollow, which extends right along the whole length of
the valley, had been cut out by the sea, the surface of the rock 150
feet below the present surface of the ground would be the sea-bottom
at the time, and the line marked “150 _feet above sea-level_” would be
the surface of the sea. The sea would therefore be here 300 feet deep
for several miles around. It cannot be supposed that the sea acting on
a broad flat plain of several miles in extent should cut out a deep,
narrow hollow, like the one exhibited in the section, and leave the
rest of the plain a flat sea-bottom.
And it must be observed, that this is not a hollow cut merely in a
sea-beach, but one extending westward to Kilsyth. Now, if this hollow
was cut out by the sea, it must have been done, not by the waves
beating on the beach, but by a current flowing through the valley.
The strongest current that could possibly pass through the narrow
part between Kilsyth and Castlecary would be wholly insensible when it
reached Grangemouth, where the water was 300 feet deep, and several
miles broad. Consequently, it is impossible that the current could have
scooped out the hollow represented in the section.
Again, if this hollow had been scooped out by the sea, it ought to
be deepest between Kilsyth and Castlecary, where the current was
narrowest; but the reverse is actually the case. It is shallowest at
the place where the current was narrowest, and deepest at the two
ends where the current was broadest. In the case of a trough cut by
a sea current, we must estimate its depth from the level of the sea.
Its depth is the depth of the water in it while it was being scooped
out. The bottom of the trough in the highest and narrowest part of
the valley east of Kilsyth is 40 feet above the present sea-level.
Consequently, its depth at this point at the period in question, when
the sea-level was 150 feet higher than at present, would be 110 feet.
The bottom of the trough at Grangemouth is 260 feet below the present
sea-level; add to this 150 feet, and we have 410 feet as its depth here
at the time in question. If this hollow was scooped out by the sea,
how then does it thus happen that at the place where the current was
strongest and confined to a narrow channel by hills on each side, it
cut its channel to a depth of only 110 feet, whereas at the place where
it had scarcely any motion it has cut, on a flat and open plain several
miles broad, a channel to a depth of 410 feet?
But, suppose we estimate the relative amount of work performed by the
sea at Kilsyth and Grangemouth, not by the actual depth of the bottom
of the trough at these two places below the sea-level at the time that
the work was performed, but by the present actual depth of the bottom
of the trough below the rocky surface of the valley, this will still
not help us out of the difficulty. Taking, as before, the height of the
rocky bed of the valley at the watershed at 110 feet above the present
sea-level, and the bottom of the trough at 40 feet, this gives 70 feet
as the depth scooped out of the rock at that place. The depth of the
trough at Grangemouth below the rocky surface is 118 feet. Here we have
only 70 feet cut out at the only place where there was any resistance
to the current, as well as the place where it possessed any strength;
whereas at Grangemouth, where there was no resistance, and no strength
of current, 118 feet has been scooped out. Such a result as this is
diametrically opposed to all that we know of the dynamics of running
water.
We may, therefore, conclude that it is physically impossible that this
hollow could have been cut out by the sea.
Owing to the present tendency among geologists to attribute effects
of this kind to ocean-currents, I have been induced to enter thus at
much greater length than would otherwise have been necessary into the
facts and arguments against the possibility of the hollow having been
excavated by the sea. In the present case the discussion is specially
necessary, for here we have positive evidence of the sea having
occupied the valley for ages, along which this channel has been cut.
Consequently, unless it is proved that the sea could not possibly have
scooped out the channel, most geologists would be inclined to attribute
it to the sea-current which is known to have passed through the valley
rather than to any other cause.
But that it is a hollow of denudation, and has been scooped out by some
agent, is perfectly certain. By what agent, then, has the erosion been
made? The only other cause to which it can possibly be attributed is
either land-ice or river-action.
The supposition that this hollow was scooped out by ice is not more
tenable than the supposition that the work has been done by the sea.
A glacier filling up the entire valley and descending into the German
Ocean would unquestionably not only deepen the valley, but would grind
down the surface over which it passed all along its course. But such a
glacier would not cut a deep and narrow channel along the bottom of the
valley. A glacier that could do this would be a small and narrow one,
just sufficiently large to fill this narrow trough; for if it were
much broader than the trough, it would grind away its edges, and make a
broad trough instead of a narrow one. But a glacier so small and narrow
as only to fill the trough, descending from the hills at Kilsyth to the
sea at Grangemouth, a distance of fifteen miles, is very improbable
indeed. The resistance to the advance of the ice along such a slope
would cause the ice to accumulate till probably the whole valley would
be filled.[282]
There is no other way of explaining the origin of this hollow, but
upon the supposition of its being an old river-bed. But there is
certainly nothing surprising in the fact of finding an old watercourse
under the boulder clay and other deposits. Unless the present contour
of the country be very different from what it was at the earlier
part of the glacial epoch, there must have then been watercourses
corresponding to the Bonny Water and the river Carron of the present
day; and that the remains of these should be found under the present
surface deposits is not surprising, seeing that these deposits are of
such enormous thickness. When water began to flow down our valleys, on
the disappearance of the ice at the close of the glacial epoch, the
Carron and the Bonny Water would not be able to regain their old rocky
channels, but would be obliged to cut, as they have done, new courses
for themselves on the surface of the deposits under which their old
ones lay buried.
Although an old pre-glacial or inter-glacial river-bed is in itself an
object of much interest and curiosity, still, it is not on that account
that I have been induced to enter so minutely into the details of this
buried hollow. There is something of far more importance attached to
this hollow than the mere fact of its being an old watercourse. For the
fact that it enters the Firth of Forth at a depth of 260 feet below the
present sea-level, proves incontestably that at the time this hollow
was occupied by a stream, _the land must have stood at least between
200 and 300 feet higher in relation to the sea-level than at present_.
We have seen that the old surface of the country in the neighbourhood
of Grangemouth, out of which this ancient stream cut its channel,
is at least 150 feet below the present sea-level. Now, unless this
surface had been above the sea-level at that time, the stream would
not have cut a channel in it. But it has not merely cut a channel, but
cut one to a depth of 120 feet. It is impossible that this channel
could have been occupied by a river of sufficient volume to fill it.
It is not at all likely that the river which scooped it out could have
been much larger than the Carron of the present day, for the area of
drainage, from the very formation of the country, could not have been
much greater above Grangemouth than at present. An elevation of the
land would, no doubt, increase the area of the drainage of the stream
measured from its source to where it might then enter the sea, because
it would increase the length of the stream; but it would neither
increase the area of drainage, nor the length of the stream above
Grangemouth. Kilsyth would be the watershed then as it is now.
What we have here is not the mere channel which had been occupied by
the ancient Carron, but the valley in which the channel lay. It may,
perhaps, be more properly termed a buried river valley; formed, no
doubt, like other river valleys by the denuding action of rain and
river.
The river Carron at present is only a few feet deep. Suppose the
ancient Carron, which flowed in this old channel, to have been say 10
feet deep. This would show that the land in relation to the sea at that
time must have stood at least 250 feet higher than at present. If 10
feet was the depth of this old river, and Grangemouth the place where
it entered the sea, then 250 feet would be the extent of the elevation.
But it is probable that Grangemouth was not the mouth of the river; it
would likely be merely the place where it joined the river Forth of
that period. We have every reason to believe that the bed of the German
Ocean was then dry land, and that the Forth, Tay, Tyne, and other
British rivers flowing eastward, as Mr. Godwin-Austin supposes, were
tributaries to the Rhine, which at that time was a huge river passing
down the bed of the German Ocean, and entering the Atlantic to the west
of the Orkney Islands. That the German Ocean, as well as the sea-bed of
the Western Hebrides, was dry land at a very recent geological period,
is so well known, that, on this point, I need not enter into details.
We may, therefore, conclude that the river Forth, after passing
Grangemouth, would continue to descend until it reached the Rhine. If,
by means of borings, we could trace the old bed of the Forth and the
Rhine up to the point where the latter entered the Atlantic, in the
same way as we have done the Bonny Water and the Carron, we should no
doubt obtain a pretty accurate estimate as to the height at which the
land stood at that remote period. Nothing whatever, I presume, is known
as to the depth of the deposits covering the bed of the German Ocean
along what was then the course of the Rhine. It must, no doubt, be
something enormous. We are also in ignorance as to the thickness of the
deposits covering the ancient bed of the Forth. A considerable number
of bores have been put down at various parts of the Firth of Forth in
connection with the contemplated railway bridge across the Firth, but
in none of those bores has the rock been reached. Bores to a depth of
175 feet have been made without even passing through the deposits of
silt which probably overlie an enormous thickness of sand and boulder
clay. Even in places where the water is 40 fathoms deep and quite
narrow, the bottom is not rock but silt.
It is, however, satisfactory to find on the land a confirmation of
what has long been believed from evidence found in the seas around our
island, that at a very recent period the sea-level in relation to the
land must have been some hundreds of feet lower than at the present
day, and that our island must have at that time formed a part of the
great eastern continent.
A curious fact was related to me by Mr. Stirling, the manager of the
Grangemouth collieries, which seems to imply a great elevation of the
land at a period long posterior to the time when this channel was
scooped out.
In sinking a pit at Orchardhead, about a mile to the north of
Grangemouth, the workmen came upon the boulder clay after passing
through about 110 feet of sand, clay, and gravel. On the upper surface
of the boulder clay they found cut out what Mr. Stirling believes
to have been an old watercourse. It was 17 feet deep, and not much
broader. The sides of the channel appear to have been smooth and
water-worn, and the whole was filled with a fine sharp sand beautifully
stratified. As this channel lay about 100 feet below the present
sea-level, it shows that if it actually be an old watercourse, it must
have been scooped out at a time when the land in relation to the sea
stood at least 100 feet higher than at present.
_Buried River Channel from Kilsyth to the Clyde._—In all probability
the western half of this great hollow, extending from the watershed
at Kilsyth to the Clyde, is also an old river channel, probably
the ancient bed of the Kelvin. This point cannot, however, be
satisfactorily settled until a sufficient number of bores have been
made along the direct line of the hollow, so as to determine with
certainty its width and general form and extent. That the western
channel is as narrow as the eastern is very probable. It has been
found that its sides at some places, as, for example, at Garscadden,
are very steep. At one place the north side is actually an overhanging
buried precipice, the bottom of which is about 200 feet below the
sea-level. We know also that the coal and ironstone in that quarter are
cut through by the trough, and the miners there have to exercise great
caution in driving their workings, in case they might cut into it. The
trough along this district is filled with sand, and is known to the
miners of the locality as the “sand-dyke.” To cut into running sand at
a depth of 40 or 50 fathoms is a very dangerous proceeding, as will be
seen from the details given in Mr. Bennie’s paper[283] of a disaster
which occurred about twenty years ago to a pit near Duntocher, where
this trough was cut into at a depth of 51 fathoms from the surface.
The depth of this hollow, below the present sea-level at Drumry, as
ascertained by a bore put down, is 230 feet. For several miles to the
east the depth is nearly as great. Consequently, if this hollow be an
old river-bed, the ancient river that flowed in it must have entered
the Clyde at a depth of more than 200 feet below the present sea-level;
and if so, then it follows that the rocky bed of the ancient Clyde must
lie buried under more than 200 feet of surface deposits from Bowling
downwards to the sea. Whether this is the case or not we have no means
at present of determining. The manager to the Clyde Trustees informs
me, however, that in none of the borings or excavations which have
been made has the rock ever been reached from Bowling downwards. The
probability is, that this deep hollow passes downwards continuously to
the sea on the western side of the island as on the eastern.[284]
The following journals of a few of the borings will give the reader an
idea of the character of the deposits filling the channels. The beds
which are believed to be boulder clay are printed in italics:—
BORINGS MADE THROUGH THE DEPOSITS FILLING THE WESTERN CHANNEL.
Bore, Drumry Farm, on Lands of Garscadden.
ft. ins.
Surface soil 2 6
Sand and gravel 3 6
Dry sand 11 0
Blue mud 8 6
Light mud and sand beds 13 0
Sand 31 6
Sand and mud 8 0
Sand and gravel 19 6
Sand 8 6
Gravel 24 4
Sand 5 0
Gravel 9 6
Sand 71 6
Sand (coaly) 1 0
Sand 9 0
Sand (coaly) 1 0
Sand 10 3
Red clay and gravel 4 8
Sand 1 5
Gravel 2 0
Sand 2 8
Gravel 10 6
Sand 1 6
Gravel 8 10
_Clay stones and gravel_ 33 3
————————
297 10
Bore on Mains of Garscadden, one mile north-east of Drumry.
ft. ins.
Surface soil 1 0
Blue clay and stones 60 0
Red clay and stones 18 0
Soft clay and sand beds 7 0
Gravel 6 0
Large gravel 9 0
Sand and gravel 7 0
Hard gravel 1 6
Sand and gravel 16 6
Dry sand 30 0
Black sand 2 0
Dry sand 33 0
Wet sand 8 0
Light mud 5 0
Sand 3 0
Gravel 5 6
Sandstone, black 0 6
Blue clay and stones 1 4
Whin block 0 10
Sandy clay 4 6
————————
219 8
Bore nearly half a mile south-west of Millichen.
ft. ins.
Sandy clay 5 0
_Brown clay and stones_ 17 0
Mud 15 0
Sandy mud 31 0
Sand and gravel with water 28 0
Sandy clay and gravel 17 0
Sand 5 0
Mud 6 0
Sand 14 0
Gravel 30 0
_Brown sandy clay and stones_ 30 0
Hard red gravel 4 6
Light mud and sand 1 8
_Light clay and stones_ 6 6
_Light clay and whin block_ 26 0
Fine sandy mud 36 0
_Brown clay and gravel and stones_ 14 4
_Bark clay and stones_ 68 0
————————
355 0
Bore at West Millichen, about 100 yards east of farm-house.
ft. ins.
Soil 1 6
_Muddy sand and stones_ 4 6
Soft mud 4 0
Sand and gravel 45 0
_Sandy mud and stones_ 20 6
Coarse gravel 11 6
Clay and gravel 1 4
Fine mud 7 0
Sand and gravel 2 0
Sandy mud 30 6
_Brown sandy clay and stones_ 25 0
Sand and gravel 6 0
_Brown sandy clay and stones_ 12 0
Sand 2 0
_Brown sandy clay and stones_ 4 0
Mud 5 0
Mud and sand 10 9
Sand and stones 2 9
_Blue clay and stones_ 5 0
————————
200 4
BORINGS MADE THROUGH THE DEPOSITS FILLING THE EASTERN CHANNEL.
No. 1. Between Towncroft Farm and Carron River—200 yards from
river. Height of surface, 12 feet above sea-level.
Feet.
Surface sand 6
Blue mud 4
Sand 4
Gravel 3
Sand 33
Red clay 46
_Soft blue till_ 17
_Hard blue till_ 140
Sand 20
———
273
No. 2. About 150 yards north of No. 1. Height of surface, 12
feet above sea-level.
Feet.
Surface sand 6
Blue mud 3
Shell bed 1
Gravel 2
Blue mud 8
Gravel 3
Blue muddy sand 15
Red clay 49
_Blue till and stones_ 20
Sand 20
_Hard blue till and stones_ 24
Sand 2
_Hard blue till and stones_ 40
Sand 7
_Hard blue till_ 24
———
234
No. 3. About 150 yards north of No. 2. Height of surface,
12 feet above sea-level.
Feet.
Surface sand 6
Soft mud with shells 11
Blue mud and sand (hard) 3
Channel (rough gravel) 3
Fine sand 8
Running sand (red and fine) 17
Red clay 30
_Soft till_ 36
Sand (pure) 2
_Soft till and sand_ 17
Gravel 8
_Hard blue till_ 14
———
155
No. 4. About 100 yards from No. 1.
Feet.
Surface 5
Blue mud 5
Black sand 3
Gravel 3
_Red clay and stones_ 34
Red clay 44
_Soft blue till_ 32
_Hard blue till and stones_ 104
Grey sand not passed through 22
———
252
Rock-head not reached.
No. 5. About 50 yards north of No. 4.
Feet.
Surface 6
Blue mud 3
Shell bed 1
Channel 2
Blue mud 8
Channel 3
Blue mud and sand 15
Red clay and sand 10
Red clay 49
_Blue till and stones_ 20
Sand 20
_Hard blue till and stones_ 24
Sand 2
_Hard blue till and stones_ 40
Sand 7
_Hard blue till_ 24
———
211
No. 6. Between Heuck and Carron River.
Feet.
Sandy clay 7
Mud 16
_Brown sandy clay and stones_ 3
Mud 36
Brown clay 39
_Blue till and stones_ 54
———
155
The question arises as to what is the origin of the stratified sands
and gravels filling up the buried river channels. Are they of marine or
of freshwater origin? Mr. Dugald Bell[285] and Mr. James Geikie[286]
are inclined to believe that as far as regards those filling the
western channel they are of lacustrine origin; that they were formed
in lakes, produced by the damming back of the water resulting from the
melting of the ice. I am, however, for the following reasons, inclined
to agree with Mr. Bennie’s opinion that they are of marine origin.
It will be seen, by a comparison of the journals of the borings made
through the deposits in the eastern channel with those in the western,
that they are of a similar character; so that, if we suppose those in
the western channel to be of freshwater origin, we may from analogy
infer the same in reference to the origin of those in the eastern
channel. But, as we have already seen, the deposits extend to the Firth
of Forth at Grangemouth, where they are met with at a depth of 260 feet
below sea-level. Consequently, if we conclude them to be of freshwater
origin, we are forced to the assumption, not that the water formed by
the melted ice was dammed back, but that the sea itself was dammed
back, and that by a wall extending to a depth of not less than two or
three hundred feet, so as to allow of a lake being formed in which the
deposits might accumulate; assuming, of course, that the absolute level
of the land was the same then as it is now.
But as regards the stratified deposits of Grangemouth, we have direct
evidence of their marine origin down to the bottom of the Red Clay that
immediately overlies the till and its intercalated beds, which on an
average is no less than 85 feet, and in some cases 100 feet, below the
present surface. From this deposit, Foraminifera, indicating an arctic
condition of sea, were determined by Mr. David Robertson. Marine shells
were also found in this bed, and along with them the remains of a
seal, which was determined by Professor Turner to be of an exceedingly
arctic type, thus proving that these deposits were not only marine but
glacial.
Direct fossil evidence as to the character of the deposits occupying
the western basin, is, however, not so abundant, but this may be owing
to the fact that during the sinking of pits, no special attention
has been paid to the matter. At Blairdardie, in sinking a pit-shaft
through these deposits, shells were found in a bed of sand between two
immense masses of boulder clay. The position of this bed will be better
understood from the following section of the pit-shaft:—
Feet.
Surface soil 4½
Blue clay 9
Hard stony clay 69
Sand with, a few _shells_ 3
Stony clay and boulders 46½
Mud and running sand 11
Hard clay, boulders, and broken rock 27
———
170
But as the shells were not preserved, we have, of course, no means of
determining whether they were of marine or of freshwater origin.
In another pit, at a short distance from the above, _Cyprina Islandica_
was found in a bed at the depth of 54 feet below the surface.[287]
In a paper read by Mr. James Smith, of Jordanhill, to the Geological
Society, April 24th, 1850,[288] the discovery is recorded of a
stratified bed containing _Tellina proxima_ intercalated between two
distinct boulder clays. The bed was discovered by Mr. James Russell in
sinking a well at Chapelhall, near Airdrie. Its height above sea-level
was 510 feet. The character of the shell not only proves the marine
origin of the bed, but also the existence of a submergence to that
extent during an inter-glacial period.
On the other hand, the difficulty besetting the theory of the marine
origin of the deposits is this. The intercalated boulder clays bear
no marks of stratification, and are evidently the true unstratified
till formed when the country was covered by ice. But the fact that
these beds are both underlaid and overlaid by stratified deposits
would, on the marine theory, imply not merely the repeated appearance
and disappearance of the ice, but also the repeated submergence and
emergence of the land. If the opinion be correct that the submergences
and emergences of the glacial epoch were due to depressions and
elevations of the land, and not to oscillations of sea-level, then
the difficulty in question is, indeed, a formidable one. But, on the
other hand, if the theory of submergences propounded in Chapters XXIII.
and XXIV. be the true one, the difficulty entirely disappears. The
explanation is as follows, viz., during a cold period of the glacial
epoch, when the winter solstice was in aphelion, the low grounds would
be covered with ice, under which a mass of till would be formed.
After the cold began to decrease, and the ice to disappear from the
plains, the greatest rise of the ocean, for reasons already stated,
would take place. The till covering the low grounds would be submerged
to a considerable depth and would soon be covered over by mud, sand,
and gravel, carried down by streams from the high ground, which, at
the time, would still be covered with snow and ice. In course of time
the sea would begin to sink and a warm and continental period of,
perhaps, from 6,000 to 10,000 years, would follow, when the sea would
be standing at a much lower level than at present. The warm period
would be succeeded by a second cold period, and the ice would again
cover the land and form a second mass of till, which, in some places,
would rest directly on the former till, while in other places it would
be laid down upon the surface of the sands and gravels overlying the
first mass. Again, on the disappearance of the ice the second mass of
till would be covered over in like manner by mud, sand, and gravel, and
so on, while the eccentricity of the earth’s orbit continued at a high
value. In this way we might have three, four, five, or more masses of
till separated by beds of sand and gravel.
It will be seen from Table IV. of the eccentricity of the earth’s
orbit, given in Chapter XIX., that the former half of that long
succession of cold and warm periods, known as the glacial epoch,
was much more severe than the latter half. That is to say, in the
former half the accumulation of ice during the cold periods, and its
disappearance in polar regions during the warm periods, would be
greater than in the latter half. It was probable that it was during
the warm periods of the earlier part of the glacial epoch that the two
buried channels of the Midland valley were occupied by rivers, and that
it was during the latter and less severe part of the glacial epoch that
these channels became filled up with that remarkable series of deposits
which we have been considering.
_Other buried River Channels._—A good many examples of buried river
channels have been found both in Scotland and in England, though none
of them of so remarkable a character as the two occupying the valley
of the Forth and Clyde Canal which have been just described. I may,
however, briefly refer to one or two localities where some of these
occur.
(1.) An ancient buried river channel, similar to the one extending
from Kilsyth to Grangemouth, exists in the coal-fields of Durham,
and is known to miners in the district as the “Wash.” Its course was
traced by Mr. Nicholas Wood, F.G.S., and Mr. E. F. Boyd, from Durham
to Newcastle, a distance of fourteen miles.[289] It traverses, after
passing the city of Durham, a portion of the valley of the Wear, passes
Chester-le-Street, and then follows the valley of the river Team, and
terminates at the river Tyne. And what is remarkable, it enters the
Tyne at a depth of 140 feet below the present level of the sea. This
curious hollow lies buried, like the Scottish one just alluded to,
under an enormous mass of drift, and it is only through means of boring
and other mining operations that its character has been revealed. The
bottom and sides of this channel everywhere bear evidence of long
exposure to the abrading influence of water in motion; the rocky bottom
being smoothed, furrowed, and water-worn. The river Wear of the present
day flows to the sea over the surface of the drift at an elevation of
more than 100 feet above this buried river-bed. At the time that this
channel was occupied by running water the sea-level must have been at
least 140 feet lower than at present. This old river evidently belongs
to the same continental period as those of Scotland.
(2.) From extensive borings and excavations, made at the docks of Hull
and Grimsby, it is found that the ancient bed of the Humber is buried
under more than 100 feet of silt, clay, and gravel. At Hull the bottom
of this buried trough was found to be 110 feet below the sea-level.
And what is most interesting at both these places, the remains of a
submerged forest was found at a depth of from thirty to fifty feet
below the sea-level. In some places two forests were found divided by a
bed of leafy clay from five to fifteen feet thick.
(3.) In the valleys of Norfolk we also find the same conditions
exhibited. The ancient bed of the Yare and other rivers of this
district enter the sea at a depth of more than 100 feet below the
present sea-level. At Yarmouth the surface was found 170 feet thick,
and the deep surface extends along the Yare to beyond Norwich. Buried
forests are also found here similar to those on the Humber.
It is probable that all our British rivers flow into the sea over their
old buried channels, except in cases where they may have changed their
courses since the beginning of the glacial epoch.
(4.) In the Sanquhar Coal Basin, at the foot of the Kello Water, an
old buried river course was found by Mr. B. N. Peach. It ran at right
angles to the Kello, and was filled with boulder clay which cut off the
coal; but, on driving the mine through the clay, the coal was found in
position on the other side.
(5.) An old river course, under the boulder clay, is described by Mr.
Milne Home in his memoir on the Mid-Lothian coal-fields. It has been
traced out from Niddry away in a N.E. direction by New Craighall. At
Niddry, the hollow is about 100 yards wide and between 60 and 70 feet
deep. It seems to deepen and widen as it approaches towards the sea,
for at New Craighall it is about 200 yards wide and 97 feet deep. This
old channel will probably enter the sea about Musselburgh. Like the
channels in the Midland Valley of Scotland already described, it is so
completely filled up by drift that not a trace of it is to be seen on
the surface. And like these, also, it must have belonged to a period
when the sea-level stood much lower than at present.
(6.) At Hailes’ Quarry, near Edinburgh, there is to be seen a portion
of an ancient watercourse under the boulder drift. A short account
of it was given by Dr. Page in a paper read before the Edinburgh
Geological Society.[290] The superincumbent sandstone, he says, has
been cut to a depth of 60 feet. The width of the channel at the surface
varies from 12 to 14 feet, but gradually narrows to 2 or 3 feet at the
bottom. The sides and bottom are smoothed and polished, and the whole
is now filled with till and boulders.
(7.) One of the most remarkable buried channels is that along the
Valley of Strathmore, supposed to be the ancient bed of the Tay. It
extends from Dunkeld, the south of Blairgowrie, Ruthven, and Forfar,
and enters the German Ocean at Lunan Bay. Its length is about 34 miles.
“No great river,” says Sir Charles Lyell, “follows this course, but
it is marked everywhere by lakes or ponds, which afford shell-marl,
swamps, and peat moss, commonly surrounded by ridges of detritus from
50 to 70 feet high, consisting in the lower part of till and boulders,
and in the upper of stratified gravels, sand, loam, and clay, in some
instances curved or contorted.”[291]
“It evidently marks an ancient line, by which, first, a great glacier
descended from the mountains to the sea, and by which, secondly, at
a later period, the principal water drainage of this country was
effected.”[292]
(8.) A number of examples of ancient river courses, underneath the
boulder clay, are detailed by Professor Geikie in his glacial drift of
Scotland. Some of the cases described by him have acquired additional
interest from the fact of their bearing decided testimony to the
existence of inter-glacial warm periods. I shall briefly refer to a few
of the cases described by him.
In driving a trial mine in a pit at Chapelhall, near Airdrie, the
workmen came upon what they believed to be an old river course. At
the end of the trial mine the ironstone, with its accompanying coal
and fire-clay, were cut off at an angle of about 20° by a stiff,
dark-coloured earth, stuck full of angular pieces of white sandstone,
coal, and shale, with rounded pebbles of greenstone, basalt, quartz,
&c. Above this lay a fine series of sand and clay beds. Above these
stratified beds lay a depth of 50 or 60 feet of true boulder clay. The
channel ran in the direction of north-east and south-west. Mr. Russell,
of Chapelhall, informs Professor Geikie that another of the same kind,
a mile farther to the north-west, had been traced in some of the pit
workings.
“It is clear,” says Professor Geikie, “that whatever may be the true
explanation of these channels and basins, they unquestionably belong to
the period of the boulder clay. The Chapelhall basin lies, indeed, in a
hollow of the carboniferous rocks, but its stratified sands and clays
rest on an irregular floor of true till. The old channel near the banks
of the Calder is likewise scooped out of sandstones and shales; but
it has a coating of boulder clay, on which its finely-laminated sands
and clays repose, _as if the channel itself had once been filled with
boulder clay, which was re-excavated to allow of the deposition of the
stratified deposits. In all cases, a thick mantle of coarse, tumultuous
boulder clay buries the whole._”[293]
Professor Geikie found between the mouth of the Pease Burn and St.
Abb’s Head, Berwickshire, several ancient buried channels. One at
the Menzie Cleuch, near Redheugh Shore, was filled to the brim with
boulder clay. Another, the Lumsden Dean, half a mile to the east of
Fast Castle, on the bank of the Carmichael Burn, near the parish church
of Carmichael,—an old watercourse of the boulder clay period—is to
be seen. The valley of the Mouse Water he instances as a remarkable
example.
One or two he found in Ayrshire, and also one on the banks of the Lyne
Water, a tributary of the Tweed.
(9.) In the valley of the Clyde, above Hamilton, several buried river
channels have been observed. They are thus described by Mr. James
Geikie:—[294]
“In the Wishaw district, two deep, winding troughs, filled with sand
and fine gravel, have been traced over a considerable area in the coal
workings.[295] These troughs form no feature at the surface, but are
entirely concealed below a thick covering of boulder clay. They appear
to be old stream courses, and are in all probability the pre-glacial
ravines of the Calder Water and the Tillon Burn. The ‘sand-dyke’ that
represents the pre-glacial course of the Calder Water runs for some
distance parallel to the present course of the stream down to Wishaw
House, where it is intersected by the Calder, and the deposits which
choke it up are well seen in the steep wooded banks below the house
and in the cliff on the opposite side. It next strikes to south-east,
and is again well exposed on the road-side leading down from Wishaw
to the Calder Water. From this point it has been traced underground,
more or less continuously, as far as Wishaw Ironworks. Beyond this
place the coal-seams sink to a greater depth, and therefore cease to
be intersected by the ancient ravine, the course of which, however,
may still be inferred from the evidence obtained during the sinking
of shafts and trial borings. In all probability it runs south, and
enters the old course of the Clyde a little below Cambusnethan House.
Only a portion of the old ravine of the Tillon Burn is shown upon the
Map. It is first met with in the coal-workings of Cleland Townhead
(Sheet 31). From this place it winds underground in a southerly
direction until it is intersected by the present Tillon Burn, a little
north of Glencleland (Sheet 31). It now runs to south-west, keeping
parallel to the burn, and crosses the valley of the Calder just
immediately above the mouth of the Tillon. From this point it can be
traced in pit-shafts, open-air sections, borings, and coal-workings,
by Ravenscraig, Nether Johnstone, and Robberhall Belting, on to the
Calder Water below Coursington Bridge (Sheet 31). It would thus appear
that in pre-glacial times the Calder and the Tillon were independent
streams, and that since glacial times the Calder Water, forsaking its
pre-glacial course, has cut its way across the intervening ground,
ploughing out deep ravines in the solid rocks, until eventually it
united with the Tillon. Similar buried stream courses occur at other
places. Thus, at Fairholme, near Larkhall, as already mentioned (par.
94), the pre-glacial course of the Avon has been traced in pit-shafts
and borings for some distance to the north. Another old course, filled
up with boulder clay, is exposed in a burn near Plotcock, a mile
south-west from Millheugh; and a similar pre-glacial ravine was met
with in the cement-stone workings at Calderwood.[296] Indeed, it might
be said with truth that nearly all the rocky ravines through which the
waters flow, especially in the carboniferous areas, are of post-glacial
age—the pre-glacial courses lying concealed under masses of drift. Most
frequently, however, the present courses of the streams are partly
pre-glacial and partly post-glacial. In the pre-glacial portions the
streams flow through boulder clay, in the post-glacial reaches their
course, as just mentioned, is usually in rocky ravines. The Avon and
the Calder, with their tributaries, afford numerous illustrations of
these phenomena.”
The question naturally arises, When were those channels scooped out?
To what geological period must those ancient rivers be referred? It
will not do to conclude that those channels must be pre-glacial simply
because they contain boulder clay. Had the glacial epoch been one
unbroken period of cold, and the boulder clay one continuous formation,
then the fact of finding boulder clay in those channels would show that
they were pre-glacial. But when we find undoubted geological evidence
of a warm condition of climate of long continuance, during the severest
part of the glacial epoch, when the ice, to a great extent, must have
disappeared, and water began to flow as usual down our valleys, all
that can reasonably be inferred from the fact of finding till in those
channels, is that they must be older than the till they contain. We
cannot infer that they are older than all the till lying on the face
of the country. The probability, however, is, that some of them are
of pre-glacial and others of inter-glacial origin. That many of these
channels have been used as watercourses during the glacial epoch, or
rather during warm periods of that epoch, is certain, from the fact
that they have been filled with boulder clay, then re-excavated, and
finally filled up again with the clay.
CHAPTER XXX.
THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.—THEORIES OF
GLACIER-MOTION.
Why the Question of Glacier-motion has been found to be so
difficult.—The Regelation Theory.—It accounts for the
Continuity of a Glacier, but not for its Motion.—Gravitation
proved by Canon Moseley insufficient to shear the Ice
of a Glacier.—Mr. Mathew’s Experiment.—No Parallel
between the bending of an Ice Plank and the shearing
of a Glacier.—Mr. Ball’s Objection to Canon Moseley’s
Experiment.—Canon Moseley’s Method of determining the Unit
of Shear.—Defect of Method.—Motion of a Glacier in some
Way dependent on Heat.—Canon Moseley’s Theory.—Objections
to his Theory.—Professor James Thomson’s Theory.—This
Theory fails to explain Glacier-motion.—De Saussure and
Hopkins’s “Sliding” Theories.—M. Charpentier’s “Dilatation”
Theory.—Important Element in the Theory.
The cause of the motion of glaciers has proved to be one of the most
difficult and perplexing questions within the whole domain of physics.
The main difficulty lies in reconciling the motion of the glacier with
the physical properties of the ice. A glacier moves down a valley
very much in the same way as a river, the motion being least at the
sides and greatest at the centre, and greater at the surface than at
the bottom. In a cross section scarcely two particles will be moving
with the same velocity. Again, a glacier accommodates itself to the
inequalities of the channel in which it moves exactly as a semifluid
or plastic substance would do. So thoroughly does a glacier behave
in the manner of a viscous or plastic body that Professor Forbes was
induced to believe that viscosity was a property of the ice, and that
in virtue of this property it was enabled to move with a differential
motion and accommodate itself to all the inequalities of its channel
without losing its continuity just as a mass of mud or putty would do.
But experience proves that ice is a hard and brittle substance far
more resembling glass than putty. In fact it is one of the most brittle
and unyielding substances in nature. So unyielding is a glacier that
it will snap in two before it will stretch to any perceptible extent.
This is proved by the fact that crevasses resulting from a strain on
the glacier consist at first of a simple crack scarcely wide enough to
admit the blade of a penknife.
All the effects which were considered to be due to the viscosity of
the ice have been fully explained and accounted for on the principle
of fracture and regelation discovered by Faraday. The principle of
regelation explains why the ice moving with a differential motion and
accommodating itself to the inequalities of its channel is yet enabled
to retain its continuity, but it does not account for the _cause_ of
glacier motion. In fact it rather involves the question in deeper
mystery than before. For it is far more difficult to conceive how the
particles of a hard and brittle solid like that of ice can move with
a differential motion, than it is to conceive how this may take place
in the case of a soft and yielding substance. The particles of ice
have all to be displaced one over another and alongside each other,
and as those particles are rigidly fixed together this connection must
be broken before the one can slide over the other. _Shearing-force_,
as Canon Moseley shows, comes into play. Were ice a plastic substance
there would not be much difficulty in understanding how the particles
should move the one over the other, but it is totally different when
we conceive ice to be a solid and unyielding substance. The difficulty
in connection with glacier-motion is not to account for the continuity
of the ice, for the principle of regelation fully explains this, but
to show how it is that one particle succeeds in sliding over the over.
The principle of regelation, instead of assisting to remove this
difficulty, increases it tenfold. Regelation does not explain the cause
of glacier-motion, but the reverse. It rather tends to show that a
glacier should not move. What, then, is the cause of glacier-motion?
According to the regelation theory, gravitation is the impelling
cause. But is gravitation sufficient to _shear_ the ice in the manner
in which it is actually done in a glacier?
I presume that few who have given much thought to the subject of
glacier-motion have not had some slight misgivings in regard to the
commonly received theory. There are some facts which I never could
harmonize with this theory. For example, boulder clay is a far looser
substance than ice; its shearing-force must be very much less than
that of ice; yet immense masses of boulder clay will lie immovable for
ages on the slope of a hill so steep that one can hardly venture to
climb it, while a glacier will come crawling down a valley which by
the eye we could hardly detect to be actually off the level. Again, a
glacier moves faster during the day than during the night, and about
twice as fast during summer as during winter. Professor Forbes, for
example, found that the Glacier des Bois near its lower extremity moved
sometimes in December only 11·5 inches daily, while during the month
of July its rate of motion sometimes reached 52·1 inches per day. Why
such a difference in the rate of motion between day and night, summer
and winter? The glacier is not heavier during the day than it is
during the night, or during the summer than it is during the winter;
neither is the shearing-force of the great mass of the ice of a glacier
sensibly less during day than night, or during summer than winter;
for the temperature of the great mass of the ice does not sensibly
vary with the seasons. If this be the case, then gravitation ought to
be as able to shear the ice during the night as during the day, or
during the winter as during the summer. At any rate, if there should
be any difference it ought to be but trifling. It is true that, owing
to the melting of the ice, the crevices of the glacier are more gorged
with water during summer than winter; and this, as Professor Forbes
maintains,[297] may tend to make the glacier move faster during the
former than the latter season. But the advocates of the regelation
theory cannot conclude, with Professor Forbes, that the water favours
the motion of the glacier by making the ice more soft and plastic. The
melting of the ice, according to the regelation theory, cannot very
materially aid the motion of the glacier.
The theory which has led to the general belief that the ice of a
glacier is sheared by the force of gravity appears to be the following.
It is supposed that the only forces to which the motion of a glacier
can be referred are _gravitation_ and _heat_; but as the great mass
of a glacier remains constantly at the same uniform temperature it
is concluded to be impossible that the motion of the glacier can be
due to this cause, and therefore of course it must be attributed to
gravitation, there being no other cause.
That gravitation is insufficient to shear the ice of a glacier has been
clearly demonstrated by Canon Moseley.[298] He determined by experiment
the amount of force required to shear one square inch of ice, and found
it to be about 75 lbs. By a process of calculation which will be found
detailed in the Memoir referred to, he demonstrated that to descend
by its own weight at the rate at which Professor Tyndall observed the
ice of the Mer de Glace to be descending at the Tacul, the unit of
shearing force of the ice could not have been more than 1·31931 lbs.
Consequently it will require a force more than 34 times the weight of
the glacier to shear the ice and cause it to descend in the manner in
which it is found to descend.
It is now six years since Canon Moseley’s results were laid before the
public, and no one, as far as I am aware, has yet attempted to point
out any serious defect in his mathematical treatment of the question.
Seeing the great amount of interest manifested in the question of
glacier-motion, I think we are warranted to conclude that had the
mathematical part of the memoir been inconclusive its defects would
have been pointed out ere this time. The question, then, hinges on
whether the experimental data on which his calculations are based
be correct. Or, in other words, is the unit of shear of ice as much
as 75 lbs.? This part of Mr. Moseley’s researches has not passed
unquestioned. Mr. Ball and Mr. Mathews, both of whom have had much
experience among glaciers, and have bestowed considerable attention on
the subject of glacier-motion, have objected to the accuracy of Mr.
Moseley’s unit of shear. I have carefully read the interesting memoirs
of Mr. Mathews and Mr. Ball in reply to Canon Moseley, but I am unable
to perceive that anything which they have advanced materially affects
his general conclusions as regards the commonly received theory. Mr.
Mathews objects to Canon Moseley’s experiments on the grounds that
extraneous forces are brought to bear upon the substance submitted
to operation, and that conditions are thus introduced which do not
obtain in the case of an actual glacier. “It would throw,” he says,
“great light upon our inquiry if we were to change this method of
procedure and simply to observe the deportment of masses of ice under
the influence of no external forces but the gravitation of their own
particles.”[299] A plank of ice six inches wide and 2⅜ inches in
thickness was supported at each end by bearers six feet apart. From the
moment the plank was placed in position it began to sink, and continued
to do so until it touched the surface over which it was supported. Mr.
Mathews remarks that with this property of ice, viz., its power to
change its form under strains produced by its own gravitation, combined
with the sliding movement demonstrated by Hopkins, we have an adequate
cause for glacier-motion. Mr. Mathews concludes from this experiment
that the unit of shear in ice, instead of being 75 lbs., is less than
1¾ lbs.
There is, however, no parallel between the bending of the ice-plank and
the shearing of a glacier. Mr. Mathews’ experiment appears to prove too
much, as will be seen from the following reply of Canon Moseley:—
“Now I will,” he says, “suggest to Mr. Mathews a parallel experiment
and a parallel explanation. If a bar of wrought iron 1 inch square and
20 feet long were supported at its extremities, it would _bend_ by its
weight alone, and would therefore shear. Now the weight of such a
rod would be about 67 lbs. According to Mr. Mathews’s explanation in
the case of the ice-plank, the unit of shear in wrought-iron should
therefore be 67 lbs. per square inch. It is actually 50,000 lbs.”[300]
Whatever theory we may adopt as to the cause of the motion of glaciers,
the deflection of the plank in the way described by Mr. Mathews
_follows as a necessary consequence_. Although no weight was placed
upon the plank, it does not necessarily follow that the deflection
was caused by the weight of the ice alone; for, according to Canon
Moseley’s own theory of the motion of glaciers by heat, the plank
ought to be deflected in the middle, just as it was in Mr. Mathews’s
experiment. A solid body, when exposed to variations of temperature,
will expand and contract transversely as well as longitudinally. Ice,
according to Canon Moseley’s theory, expands and contracts by heat.
Then if the plank expands transversely, the upper half of the plank
must rise and the lower half descend. But the side which rises has
to perform work against gravity, whereas the side which descends has
work performed upon it by gravity; consequently more of the plank will
descend than rise, and this will, of course, tend to lower or deflect
the plank in the middle. Again, when the plank contracts, the lower
half will rise and the upper half will descend; but as gravitation,
in this case also, favours the descending part and opposes the rising
part, more of the plank will descend than rise, and consequently
the plank will be lowered in the middle by contraction as well as
by expansion. Thus, as the plank changes its temperature, it must,
according to Mr. Moseley’s theory, descend or be deflected in the
middle, step by step—and this not by gravitation alone, but chiefly
by the motive power of heat. I do not, of course, mean to assert that
the descent of the plank was caused by heat; but I assert that Mr.
Mathews’s experiment does not necessarily prove (and this is all that
is required in the meantime) that gravitation alone was the cause of
the deflection of the plank. Neither does this experiment prove that
the ice was deflected without shearing; for although the weight of the
plank was not sufficient to shear the ice, as Mr. Mathews, I presume,
admits, yet Mr. Moseley would reply that the weight of the ice,
assisted by the motive power of heat, was perfectly sufficient.
I shall now briefly refer to Mr. Ball’s principal objections to Canon
Moseley’s proof that a glacier cannot shear by its weight alone. One
of his chief objections is that Mr. Moseley has assumed the ice to be
homogeneous in structure, and that pressures and tensions acting within
it, are not modified by the varying constitution of the mass.[301]
Although there is, no doubt, some force in this objection (for we have
probably good reason to believe that ice will shear, for example, more
easily along certain planes than others), still I can hardly think that
Canon Moseley’s main conclusion can ever be materially affected by this
objection. The main question is this, Can the ice of the glacier shear
by its own weight in the way generally supposed? Now the shearing force
of ice, take it in whatever direction we may, so enormously exceeds
that required by Mr. Moseley in order to allow a glacier to descend by
its weight only, that it is a matter of indifference whether ice be
regarded as homogeneous in structure or not. Mr. Ball objects also to
Mr. Moseley’s imaginary glacier lying on an even slope and in a uniform
rectangular channel. He thinks that an irregular channel and a variable
slope would be more favourable to the descent of the ice. But surely
if the work by the weight of the ice be not equal to the work by the
resistance in a glacier of uniform breadth and slope, it must be much
less so in the case of one of irregular shape and slope.
That a relative displacement of the particles of the ice is involved
in the motion of a glacier, is admitted, of course, by Mr. Ball; but
he states that the amount of this displacement is but small, and that
it is effected with extreme slowness. This may be the case; but if the
weight of the ice be not able to overcome the mutual cohesion of the
particles, then the weight of the ice cannot produce the required
displacement, however small it may be. Mr. Ball then objects to Mr.
Moseley’s method of determining the unit of shear on this ground:—The
shearing of the ice in a glacier is effected with extreme slowness;
but the shearing in Canon Moseley’s experiment was effected with
rapidity; and although it required 75 lbs. to shear one square inch of
surface in his experiment, it does not follow that 75 lbs. would be
required to shear the ice if done in the slow manner in which it is
effected in the glacier. “In short,” says Mr. Ball, “to ascertain the
resistance opposed to very slow changes in the relative positions of
the particles, so slight as to be insensible at short distances, Mr.
Moseley measures the resistance opposed to rapid disruption between
contiguous portions of the same substance.”
There is force in this objection; and here we arrive at a really weak
point in Canon Moseley’s reasoning. His experiments show that if we
want to shear ice quickly a weight of nearly 120 lbs. is required; but
if the thing is to be done more slowly, 75 lbs. will suffice.[302] In
short, the number of pounds required to shear the ice depends, to a
large extent, on the length of time that the weight is allowed to act;
the longer it is allowed to act, the less will be the weight required
to perform the work. “I am curious to know,” says Mr. Mathews, when
referring to this point, “what weight would have sheared the ice
if a _day_ had been allowed for its operation.” I do not know what
would have been the weight required to shear the ice in Mr. Moseley’s
experiments had a day been allowed; but I feel pretty confident that,
should the ice remain unmelted, and sufficient time be allowed,
shearing would be produced without the application of any weight
whatever. There are no weights placed upon a glacier to make it move,
and yet the ice of the glacier shears. If the shearing is effected by
weight, the only weight applied is the weight of the ice; and if the
weight of the ice makes the ice shear in the glacier, why may it not
do the same thing in the experiment? Whatever may be the cause which
displaces the particles of the ice in a glacier, they, as a matter of
fact, are displaced without any weight being applied beyond that of
the ice itself; and if so, why may not the particles of the ice in
the experiment be also displaced without the application of weights?
Allow the ice of the glacier to take its own time and its own way, and
the particles will move over each other without the aid of external
weights, whatever may be the cause of this; well, then, allow the ice
in the experiment to take its own time and its own way, and it will
probably do the same thing. There is something here unsatisfactory.
If, by the unit of shear, be meant the pressure in pounds that must
be applied to the ice to break the connection of one square inch of
two surfaces frozen together and cause the one to slip over the other,
then the amount of pressure required to do this will depend upon the
time you allow for the thing being done. If the thing is to be done
rapidly, as in some of Mr. Moseley’s experiments, it will take, as he
has shown, a pressure of about 120 lbs.; but if the thing has to be
done more slowly, as in some other of his experiments, 75 lbs. will
suffice. And if sufficient time be allowed, as in the case of glaciers,
the thing may be done without any weight whatever being applied to the
ice, and, of course, Mr. Moseley’s argument, that a glacier cannot
descend by its weight alone, falls to the ground. But if, by the unit
of shear, be meant not the _weight_ or _pressure_ necessary to shear
the ice, but the amount of _work_ required to shear a square inch of
surface _in a given time or at a given rate_, then he might be able
to show that in the case of a glacier (say the Mer de Glace) the work
of all the resistances which are opposed to its descent at the _rate_
at which it is descending is greater than the work of its weight, and
that consequently there must be some cause, in addition to the weight,
urging the glacier forward. But then he would have no right to affirm
that the glacier would not descend by its weight only; all that he
could affirm would simply be that it could not descend by its weight
alone at the _rate_ at which it is descending.
Mr. Moseley’s unit of shear, however, is not the amount of work
performed in shearing a square inch of ice in a given time, but the
amount of _weight_ or _pressure_ requiring to be applied to the ice
to shear a square inch. But this amount of pressure depends upon the
length of time that the pressure is applied. Here lies the difficulty
in determining what amount of pressure is to be taken as the real unit.
And here also lies the radical defect in Canon Moseley’s result. Time
as well as pressure enters as an element into the process. The key to
the explanation of this curious circumstance will, I think, be found in
the fact that the rate at which a glacier descends depends in some way
or other upon the amount of heat that the ice is receiving. This fact
shows that heat has something to do in the shearing of the ice of the
glacier. But in the communication of heat to the ice _time_ necessarily
enters as an element. There are two different ways in which heat may be
conceived to aid in shearing the ice: (1.) we may conceive that heat
acts as a force along with gravitation in producing displacement of the
particles of the ice; or (2.) we may conceive that heat does not act as
a force in pushing the particles over each other, but that it assists
the shearing processes by diminishing the cohesion of the particles of
the ice, and thus allowing gravitation to produce displacement. The
former is the function attributed to heat in Canon Moseley’s theory
of glacier-motion; the latter is the function attributed to heat in
the theory of glacier-motion which I ventured to advance some time
ago.[303] It results, therefore, from Canon Moseley’s own theory, that
the longer the time that is allowed for the pressure to shear the
ice, the less will be the pressure required; for, according to his
theory, a very large proportion of the displacement is produced by the
motive power of heat entering the ice; and, as it follows of course,
other things being equal, the longer the time during which the heat
is allowed to act, the greater will be the proportionate amount of
displacement produced by the heat; consequently the less will require
to be done by the weight applied. In the case of the glacier, Mr.
Moseley concludes that at least thirty or forty times as much work is
done by the motive power of heat in the way of shearing the ice as is
done by mere pressure or weight. Then, if sufficient time be allowed,
why may not far more be done by heat in shearing the ice in his
experiment than by the weight applied? In this case how is he to know
how much of the shearing is effected by the heat and how much by the
weight? If the greater part of the shearing of the ice in the case of a
glacier is produced, not by pressure, but by the heat which necessarily
enters the ice, it would be inconceivable that in his experiments the
heat entering the ice should not produce, at least to some extent, a
similar effect. And if a portion of the displacement of the particles
is produced by heat, then the weight which is applied cannot be
regarded as the measure of the force employed in the displacement, any
more than it could be inferred that the weight of the glacier is the
measure of the force employed in the shearing of it. If the weight
is not the entire force employed in shearing, but only a part of the
force, then the weight cannot, as in Mr. Moseley’s experiment, be taken
as the measure of the force.
How, then, are we to determine what is the amount of force required to
shear ice? in other words, how is the unit of shear to be determined?
If we are to measure the unit of shear by the weight required to
produce displacement of the particles of the ice, we must make sure
that the displacement is wholly effected by the weight. We must be
certain that heat does not enter as an element in the process. But
if time be allowed to elapse during the experiment, we can never
be certain that heat has not been at work. It is impossible to
prevent heat entering the ice. We may keep the ice at a constant
temperature, but this would not prevent heat from entering the ice and
producing molecular work. True that, according to Moseley’s theory
of glacier-motion, if the temperature of the ice be not permitted
to _vary_, then no displacement of the particles can take place
from the influence of heat; but according to the molecular theory of
glacier-motion, which will shortly be considered, heat will aid the
displacement of the particles whether the temperature be kept constant
or not. In short, it is absolutely impossible in our experiments to
be certain that heat is not in some way or other concerned in the
displacement of the particles of the ice. But we can shorten the time,
and thus make sure that the amount of heat entering the ice during the
experiments is too small to affect materially the result. We cannot in
this case say that all the displacement has been effected by the weight
applied to the ice, but we can say that so little has been done by heat
that, practically, we may regard it as all done by the weight.
This consideration, I trust, shows that the unit of shear adopted by
Canon Moseley in his calculations is not too large. For if in half an
hour, after all the work that may have been done by heat, a pressure of
75 lbs. is still required to displace the particles of one square inch,
it is perfectly evident that if no work had been done by heat during
that time, the force required to produce the displacement could not
have been less than 75 lbs. It might have been more than that; but it
could not have been less. Be this, however, as it may, in determining
the unit of shear we cannot be permitted to prolong the experiment for
any considerable length of time, because the weight under which the
ice might then shear could not be taken as the measure of the force
which is required to shear ice. By prolonging the experiment we might
possibly get a unit smaller than that required by Canon Moseley for
a glacier to descend by its own weight. But it would be just as much
begging the whole question at issue to assume that, because the ice
sheared under such a weight, a glacier might descend by its weight
alone, as it would be to assume that, because a glacier shears without
a weight being placed upon it, the glacier descends by its weight alone.
But why not determine the unit of shear of ice in the same way as we
would the unit of shear of any other solid substance, such, as iron,
stone, or wood? If the shearing force of ice be determined in this
manner, it will be found to be by far too great to allow of the ice
shearing by its weight alone. We shall be obliged to admit either
that the ice of the glacier does not shear (in the ordinary sense of
the term), or if it does shear, that there must, as Canon Moseley
concludes, be some other force in addition to the weight of the ice
urging the glacier forward.
The fact that the rate of descent of a glacier depends upon the amount
of heat which it receives, proves that heat must be regarded either as
a cause or as a necessary condition of its motion; what, then, is the
necessary relationship between heat and the motion of the glacier? If
heat is to be regarded as a cause, in what way does the heat produce
motion? I shall now briefly refer to one or two theories which have
been advanced on the subject. Let us consider first that of Canon
Moseley.
_Canon Moseley’s Theory._—He found, from observations and experiments,
that sheets of lead, placed upon an inclined plane, when subjected to
variations of temperature, tend to descend even when the slope is far
less than that which would enable it to slide down under the influence
of gravitation. The cause of the descent he shows to be this. When the
temperature of the sheet is raised, it expands, and, in expanding, its
upper portion moves up the slope, and its lower portion down the slope;
but as gravitation opposes the upward and favours the downward motion,
more of the sheet moves down than up, and consequently the centre
of gravity of the sheet is slightly lowered. Again, when the sheet
is cooled, it contracts, and in contracting the upper portion moves
downwards and the lower portion upwards, and here again, for the same
reason, more of the sheet moves downwards than upwards. Consequently,
at every change of temperature there is a slight displacement of the
sheet downwards. “Now a theory of the descent of glaciers,” says
Canon Moseley, “which I have ventured to propose myself, is that they
descend, as the lead in this experiment does, by reason of the passage
into them and the withdrawal of the sun’s rays, and that the dilatation
and contraction of the ice so produced is the proximate cause of their
descent, as it is of that of the lead.”[304]
The fundamental condition in Mr. Moseley’s theory of the descent of
solid bodies on an incline, is, not that heat should maintain these
bodies at a high temperature, but that the temperature should vary.
The rate of descent is proportionate, not simply to the amount of
heat received, but to the extent and frequency of the variations of
temperature. As a proof that glaciers are subjected to great variations
of temperature, he adduces the following:—“All alpine travellers,” he
says, “from De Saussure to Forbes and Tyndall, have borne testimony
to the intensity of the solar radiation on the surfaces of glaciers.
‘I scarcely ever,’ says Forbes, ‘remember to have found the sun more
piercing than at the Jardin.’ This heat passes abruptly into a state
of intense cold when any part of the glacier falls into shadow by an
alteration of the position of the sun, or even by the passing over it
of a cloud.”[305]
Mr. Moseley is here narrating simply what the traveller feels, and
not what the glacier experiences. The traveller is subjected to great
variations of temperature; but there is no proof from this that the
glacier experiences any changes of temperature. It is rather because
the temperature of the glacier is not affected by the sun’s heat that
the traveller is so much chilled when the sun’s rays are cut off. The
sun shines down with piercing rays and the traveller is scorched; the
glacier melts on the surface, but it still remains “cold as ice.” The
sun passes behind a cloud or disappears behind a neighbouring hill; the
scorching rays are then withdrawn, and the traveller is now subjected
to radiation on every side from surfaces at the freezing-point.
It is also a necessary condition in Mr. Moseley’s theory that the heat
should pass easily into and out of the glacier; for unless this were
the case sudden changes of temperature could produce little or no
effect on the great mass of the glacier. How, then, is it possible that
during the heat of summer the temperature of the glacier could vary
much? During that season, in the lower valleys at least, everything,
with the exception of the glacier, is above the freezing-point;
consequently when the glacier goes into the shade there is nothing
to lower the ice below the freezing-point; and as the sun’s rays do
not raise the temperature of the ice above the freezing-point, the
temperature of the glacier must therefore remain unaltered during that
season. It therefore follows that, instead of a glacier moving more
rapidly during the middle of summer than during the middle of winter,
it should, according to Moseley’s theory, have no motion whatever
during summer.
The following, written fifteen years ago by Professor Forbes on this
very point, is most conclusive:—“But how stands the fact? Mr. Moseley
quotes from De Saussure the following _daily ranges_ of the temperature
of the air in the month of July at the Col du Géant and at Chamouni,
between which points the glacier lies:
°
At the Col du Géant 4·257 Réaumur.
At Chamouni 10·092 〃
And he assumes ‘the same mean daily variation of temperature to obtain
throughout the length’ [and depth?] ‘of the Glacier du Géant which De
Saussure observed in July at the Col du Géant.’ But between what limits
does the temperature of the air oscillate? We find, by referring to
the third volume of De Saussure’s ‘Travels,’ that the mean temperature
of the coldest hour (4 A.M.) during his stay at the Col du Géant was
33°·03 Fahrenheit, and of the warmest (2 P.M.) 42°·61 F. So that even
upon that exposed ridge, between 2,000 and 3,000 feet above where the
glacier can be properly said to commence, the air does not, on an
average of the month of July, reach the freezing-point at any hour
of the night. Consequently the _range of temperature attributed to
the glacier is between limits absolutely incapable of effecting the
expansion of the ice in the smallest degree_.”[306]
Again, during winter, as Mr. Ball remarks, the glacier is completely
covered with snow and thus protected both from the influence of
cold and of heat, so that there can be nothing either to raise the
temperature of the ice above the freezing-point or to bring it below
that point; and consequently the glacier ought to remain immovable
during that season also.
“There can be no doubt, therefore,” Mr. Moseley states, “that the
rays of the sun, which in those alpine regions are of such remarkable
intensity, find their way into the depths of the glacier. They are
a _power_, and there is no such thing as the loss of power. The
mechanical work which is their equivalent, and into which they are
converted when received into the substance of a solid body, accumulates
and stores itself up in the ice under the form of what we call
elastic force or tendency to dilate, until it becomes sufficient to
produce actual dilatation of the ice in the direction in which the
resistance is weakest, and by its withdrawal to produce contraction.
From this expansion and contraction follows of necessity the descent
of the glacier.”[307] When the temperature of the ice is below
the freezing-point, the rays which are absorbed will, no doubt,
produce dilatation; but during summer, when the ice is not below the
freezing-point, no dilatation can possibly take place. All physicists,
so far as I am aware, agree that the rays that are then absorbed go to
melt the ice, and not to expand it. But to this Mr. Moseley replied
as follows:—“To this there is the obvious answer that radiant heat
does find its way into ice as a matter of common observation, and
that it does not melt it except at its surface. Blocks of ice may be
seen in the windows of ice-shops with the sun shining full upon them,
and melting nowhere but on their surfaces. And the experiment of the
ice-lens shows that heat may stream through ice in abundance (of which
a portion is necessarily stopped in the passage) without melting it,
except on its surface.” But what evidence is there to conclude that
if there is no melting of the ice in the interior of the lens there
is a portion of the rays “necessarily stopped” in the interior? It
will not do to assume a point so much opposed to all that we know of
the physical properties of ice as this really is. It is absolutely
essential to Mr. Moseley’s theory of the motion of glaciers, during
summer at least, that ice should continue to expand after it reaches
the melting-point; and it has therefore to be shown that such is the
case; or it need not be wondered at that we cannot accept his theory,
because it demands the adoption of a conclusion contrary to all our
previous conceptions. But, as a matter of fact, it is not strictly true
that when rays pass through a piece of ice there is no melting of the
ice in the interior. Experiments made by Professor Tyndall show the
contrary.[308]
There is, however, one fortunate circumstance connected with Canon
Moseley’s theory. It is this: its truth can be easily tested by direct
experiment. The ice, according to this theory, descends not simply
in virtue of heat, but in virtue of _change of temperature_. Try,
then, Hopkins’s famous experiment, but keep the ice at a _constant
temperature_; then, according to Moseley’s theory, the ice will not
descend. Let it be observed, however, that although the ice under this
condition should descend (as there is little doubt but it would),
it would show that Mr. Moseley’s theory of the descent of glaciers
is incorrect, still it would not in the least degree affect the
conclusions which he lately arrived at in regard to the generally
received theory of glacier-motion. It would not prove that the ice
sheared, in the way generally supposed, by its weight only. It might be
the heat, after all, entering the ice, which accounted for its descent,
although gravitation (the weight of the ice) might be the impelling
cause.
According to this theory, the glacier, like the sheet of lead, must
expand and contract as one entire mass, and it is difficult to
conceive how this could account for the differential motion of the
particles of the ice.
_Professor James Thomson’s Theory._—It was discovered by this physicist
that the freezing-point of water is lowered by pressure. The extent
of the lowering is equal to ·0075° centigrade for every atmosphere
of pressure. As glacier ice is generally about the melting-point,
it follows that when enormous pressure is brought to bear upon any
given point of a glacier a melting of the ice at that particular spot
will take place in consequence of the lowering of the melting-point.
The melting of the ice will, of course, tend to favour the descent
of the glacier, but I can hardly think the liquefaction produced by
pressure can account for the motion of glaciers. It will help to
explain the giving way of the ice at particular points subjected to
great pressure, but I am unable to comprehend how it can account for
the general descent of the glacier. Conceive a rectangular glacier of
uniform breadth and thickness, and lying upon an even slope. In such a
glacier the pressure at each particular point would remain constant,
for there would be no reason why it should be greater at one time than
at another. Suppose the glacier to be 500 feet in thickness; the ice
at the lower surface of the glacier, owing to pressure, would have its
melting-point permanently lowered one-tenth of a degree centigrade
below that of the upper surface; but the ice at the lower surface would
not, on this account, be in the fluid state. It would simply be ice at
a slightly lower temperature. True, when pressure is exerted the ice
melts in consequence of the lowering of the melting-point, but in the
case under consideration there would, properly speaking, be no exertion
of pressure, but a constant statical pressure resulting from the weight
of the ice. But this statical condition of pressure would not produce
fluidity any more than a statical condition of pressure would produce
heat, and consequently motion could not take place as a result of
fluidity. In short, motion itself is required to produce the fluidity.
I need not here wait to consider the sliding theories of De Saussure
and Hopkins, as they are now almost universally admitted to be
inadequate to explain the phenomena of glacier-motion, seeing that they
do not account for the displacement of the particles of the ice over
one another.
According to the dilatation theory of M. Charpentier, a glacier is
impelled by the force exerted by water freezing in the fissures of the
ice. A glacier he considers is full of fissures into which water is
being constantly infiltrated, and when the temperature of the air sinks
below the freezing-point it converts the water into ice. The water, in
passing into ice, expands, and in expanding tends to impel the glacier
in the direction of least resistance. This theory, although it does not
explain glacier-motion, as has been clearly shown by Professor J. D.
Forbes, nevertheless contains one important element which, as we shall
see, must enter into the true explanation. The element to which I refer
is the expansive force exerted on the glacier by water freezing.
CHAPTER XXXI.
THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.—THE
MOLECULAR THEORY.
Present State of the Question.—Heat necessary to the Motion of
a Glacier.—Ice does not shear in the Solid State.—Motion
of a Glacier _molecular_.—How Heat is transmitted through
Ice.—Momentary Loss of Shearing Force.—The _Rationale_
of Regelation.—The Origin of “Crevasses.”—Effects of
Tension.—Modification of Theory.—Fluid Molecules crystallize
in Interstices.—Expansive Force of crystallizing Molecules
a Cause of Motion.—Internal molecular Pressure the chief
Moving Power.—How Ice can excavate a Rock Basin.—How Ice can
ascend a Slope.—How deep River Valleys are striated across.—A
remarkable Example in the Valley of the Tay.—How Boulders can
be carried from a lower to a higher Level.
The condition which the perplexing question of the cause of the descent
of glaciers has now reached seems to be something like the following.
The ice of a glacier is not in a soft and plastic state, but is solid,
hard, brittle, and unyielding. It nevertheless behaves in some respects
in a manner very like what a soft and plastic substance would do if
placed in similar circumstances, inasmuch as it accommodates itself
to all the inequalities of the channel in which it moves. The ice of
the glacier, though hard and solid, moves with a differential motion;
the particles of the ice are displaced over each other, or, in other
words, the ice shears as it descends. It had been concluded that the
mere weight of the glacier is sufficient to shear the ice. Canon
Moseley has investigated this point, and shown that it is not. He has
found that for a glacier to shear in the way that it is supposed to
do, it would require a force some thirty or forty times as great as
the weight of the glacier. Consequently, for the glacier to descend,
a force in addition to that of gravitation is required. What, then,
is this force? It is found that the rate at which the glacier descends
depends upon the amount of heat which it is receiving. This shows that
the motion of the glacier is in some way or other dependent upon heat.
Is heat, then, the force we are in search of? The answer to this, of
course, is, since heat is a force necessarily required, we have no
right to assume any other till we see whether or not heat will suffice.
In what way, then, does heat aid gravitation in the descent of the
glacier? In what way does heat assist gravitation in the shearing of
the ice? There are two ways whereby we may conceive the thing to be
done: the heat may assist gravitation to shear, by pressing the ice
forward, or it may assist gravitation by diminishing the cohesion of
the particles, and thus allow gravitation to produce motion which it
otherwise could not produce. Every attempt which has yet been made
to explain how heat can act as a force in pushing the ice forward,
has failed. The fact that heat cannot expand the ice of the glacier
may be regarded as a sufficient proof that it does not act as a force
impelling the glacier forward; and we are thus obliged to turn our
attention to the other conception, viz., that heat assists gravitation
to shear the ice, not by direct pressure, but by diminishing the
cohesive force of the particles, so as to enable gravitation to push
the one past the other. But how is this done? Does heat diminish the
cohesion by acting as an expansive force in separating the particles?
Heat cannot do this, because it cannot expand the ice of a glacier;
and besides, were it to do this, it would destroy the solid and firm
character of the ice, and the ice of the glacier would not then, as
a mass, possess the great amount of shearing-force which observation
and experiment show that it does. In short it is because the particles
are so firmly fixed together at the time the glacier is descending,
that we are obliged to call in the aid of some other force in addition
to the weight of the glacier to shear the ice. Heat does not cause
displacement of the particles by making the ice soft and plastic; for
we know that the ice of the glacier is not soft and plastic, but
hard and brittle. The shearing-force of the ice of the moving glacier
is found to be by at least from thirty to forty times too great to
permit of the ice being sheared by the mere force of gravitation;
how, then, is it that gravitation, without the direct assistance of
any other force, can manage to shear the ice? Or to put the question
under another form: heat does not reduce the shearing-force of the ice
of a glacier to something like 1·3193 lb. per square inch of surface,
the unit required by Mr. Moseley to enable a glacier to shear by
its weight; the shearing-force of the ice, notwithstanding all the
heat received, still remains at about 75 lbs.; how, then, can the
glacier shear without any other force than its own weight pushing it
forward? _This is the fundamental question; and the true answer to it
must reveal the mystery of glacier-motion._ We are compelled in the
present state of the problem to admit that glaciers do descend with
a differential motion without any other force than their own weight
pushing them forward; and yet the shearing-force of the ice is actually
found to be thirty or forty times the maximum that would permit of the
glacier shearing by its weight only. _The explanation of this apparent
paradox will remove all our difficulties in reference to the cause of
the descent of glaciers._
There seems to be but one explanation (and it is a very obvious
one), viz. that the motion of the glacier is _molecular_. The ice
descends molecule by molecule. The ice of a glacier is in the hard
crystalline state, but it does not descend in this state. Gravitation
is a constantly acting force; if a particle of the ice lose its
shearing-force, though but for the moment, it will descend by its
weight alone. But a particle of the ice will lose its shearing-force
for a moment if the particle loses its crystalline state for the
moment. The passage of heat through ice, whether by conduction or by
radiation, in all probability is a molecular process; that is, the
form of energy termed heat is transmitted from molecule to molecule
of the ice. A particle takes the energy from its neighbour A on the
one side and hands it over to its neighbour B on the opposite side.
But the particle must be in a different state at the moment it is in
possession of the energy from what it was before it received it from
A, and from what it will be after it has handed it over to B. Before
it became possessed of the energy, it was in the crystalline state—it
was ice; and after it loses possession of the energy it will be ice;
but at the moment that it is in possession of the passing energy is
it in the crystalline or icy state? If we assume that it is not, but
that in becoming possessed of the energy, it loses its crystalline form
and for the moment becomes water, all our difficulties regarding the
cause of the motion of glaciers are removed. We know that the ice of a
glacier in the mass cannot become possessed of energy in the form of
heat without becoming fluid; _if it can be shown that the same thing
holds true of the ice particle, we have the key to the mystery of
glacier-motion_. A moment’s reflection will suffice to convince any one
that if the glacier ice in the mass cannot receive energy in the form
of heat without melting, the same must hold true of the ice particles,
for it is inconceivable that the ice in the mass could melt and yet
the ice particles themselves remain in the solid state. It is the
solidity of the particles which constitutes the solidity of the mass.
If the particles lose their solid form the mass loses its solid form,
for the mass has no other solidity than that which is possessed by the
particles.
The correctness of the conclusion, that the weight of the ice is
not a sufficient cause, depends upon the truth of a certain element
taken for granted in the reasoning, viz. that the _shearing-force_ of
the molecules of the ice remains _constant_. If this force remains
constant, then Canon Moseley’s conclusion is undoubtedly correct,
but not otherwise; for if a molecule should lose its shearing-force,
though it were but for a moment, if no obstacle stood in front of the
molecule, it would descend in virtue of its weight.
The fact that the shearing-force of a mass of ice is found to be
constant does not prove that the same is the case in regard to the
individual molecules. If we take a mass of molecules in the aggregate,
the shearing-force of the mass taken thus collectively may remain
absolutely constant, while at the same time each individual molecule
may be suffering repeated momentary losses of shearing-force. This is
so obvious as to require no further elucidation. The whole matter,
therefore, resolves itself into this one question, as to whether or not
the shearing-force of a crystalline molecule of ice remains constant.
In the case of ordinary solid bodies we have no reason to conclude that
the shearing-force of the molecules ever disappears, but in regard to
ice it is very different.
If we analyze the process by which heat is conducted through ice, we
shall find that we have reason to believe _that while a molecule of
ice is in the act of transmitting the energy received (say from a
fire), it loses for the moment its shearing-force if the temperature of
the ice be not under_ 32° F. If we apply heat to the end of a bar of
iron, the molecules at the surface of the end have their temperatures
raised. Molecule A at the surface, whose temperature has been raised,
instantly commences to transfer to B a portion of the energy received.
The tendency of this process is to lower the temperature of A and raise
that of B. B then, with its temperature raised, begins to transfer
the energy to C. The result here is the same; B tends to fall in
temperature, and C to rise. This process goes on from molecule to
molecule until the opposite end of the bar is reached. Here in this
case the energy or heat applied to the end of the bar is transmitted
from molecule to molecule under the form of _heat or temperature_.
The energy applied to the bar does _not change its character; it
passes right along from molecule to molecule under the form of heat or
temperature_. But the nature of the process must be wholly different if
the transferrence takes place through a bar of ice at the temperature
of 32°. Suppose we apply the heat of the fire to the end of the bar
of ice at 32°, the molecules of the ice cannot possibly have their
temperatures raised in the least degree. How, then, can molecule A
take on, _under the form of heat_, the energy received from the fire
without being heated or having its _temperature_ raised? The thing is
impossible. The energy of the fire must appear in A under a different
form from that of heat. The same process of reasoning is equally
applicable to B. The molecule B cannot accept of the energy from A
under the form of heat; it must receive it under some other form. The
same must hold equally true of all the other molecules till we reach
the opposite end of the bar of ice. And yet, strange to say, the last
molecule transmits in the form of heat its energy to the objects
beyond; for we find that the heat applied to one side of a piece of ice
will affect the thermal pile on the opposite side.
The question is susceptible of a clear and definite answer. When
heat is applied to a molecule of ice at 32°, the heat applied
does not raise the temperature of the molecule, it is consumed in
work against the cohesive forces binding the atoms or particles
together into the crystalline form. The energy then must exist in
the dissolved crystalline molecule, under the statical form of an
affinity—crystalline affinity, or whatever else we may call it. That is
to say, the energy then exists in the particles as a power or tendency
to rush together again into the crystalline form, and the moment they
are allowed to do so they give out the energy that was expended upon
them in their separation. This energy, when it is thus given out again,
assumes the dynamical form of heat; in other words, the molecule gives
out _heat_ in the act of freezing. The heat thus given out may be
employed to melt the next adjoining molecule. The ice-molecules take
on energy from a heated body by melting. That peculiar form of motion
or energy called heat disappears in forcing the particles of the
crystalline molecule separate, and for the time being exists in the
form of a tendency in the separated particles to come together again
into the crystalline form.
But it must be observed that although the crystalline molecule, when
it is acting as a conductor, takes on energy under this form from the
heated body, it only exists in the molecule under such a form during
the moment of transmission; that is to say, the molecule is melted, but
only for the moment. When B accepts of the energy from A, the molecule
A instantly assumes the crystalline form. B is now melted; and when C
accepts of the energy from B, then B also in turn assumes the solid
state. This process goes on from molecule to molecule till the energy
is transmitted through to the opposite side and the ice is left in its
original solid state. This, as will be shown in the Appendix, is the
_rationale_ of Faraday’s property of regelation.
This is no mere theory or hypothesis; it is a necessary consequence
from known facts. We know that ice at 32° cannot take on energy from
a heated body without melting; and we know also equally well that a
slab of ice at 32°, notwithstanding this, still, as a mass, retains its
solid state while the heat is being transmitted through it. This proves
that every molecule resumes its crystalline form the moment after the
energy is transferred to the adjoining molecule.
This point being established, every difficulty regarding the descent
of the glacier entirely disappears; for a molecule the moment that
it assumes the fluid state is completely freed from shearing-force,
and can descend by virtue of its own weight without any impediment.
All that the molecule requires is simply room or space to advance in.
If the molecule were in absolute contact with the adjoining molecule
below, it would not descend unless it could push that molecule before
it, which it probably would not be able to do. But the molecule
actually has room in which to advance; for in passing from the solid
to the liquid state its volume is diminished by about 1/10, and it
consequently can descend. True, when it again assumes the solid form
it will regain its former volume; but the question is, will it go back
to its old position? If we examine the matter thoroughly we shall find
that it cannot. If there were only this one molecule affected by the
heat, this molecule would certainly not descend; but all the molecules
are similarly affected, although not all at the same moment of time.
Let us observe what takes place, say, at the lower end of the glacier.
The molecule A at the lower end, say, of the surface, receives heat
from the sun’s rays; it melts, and in melting not only loses its
shearing-force and descends by its own weight, but it contracts also.
B immediately above it is now, so far as A is concerned, at liberty to
descend, and will do so the moment that it assumes the liquid state. A
by this time has become solid, and again fixed by shearing-force; but
it is not fixed in its old position, but a little below where it was
before. If B has not already passed into the fluid state in consequence
of heat derived from the sun, the additional supply which it will
receive from the solidifying of A will melt it. The moment that B
becomes fluid it will descend till it reaches A. B then is solidified
a little below its former position. The same process of reasoning is
in a similar manner applicable to every molecule of the glacier. Each
molecule of the glacier consequently descends step by step as it melts
and solidifies, and hence the glacier, considered as a mass, is in a
state of constant motion downwards. The fact observed by Professor
Tyndall that there are certain planes in the ice along which melting
takes place more readily than others will perhaps favour the descent of
the glacier.
We have in this theory a satisfactory explanation of the origin of
“crevasses” in glaciers. Take, for example, the transverse crevasses
formed at the point where an increase in the inclination of the glacier
takes place. Suppose a change of inclination from, say, 4° to 8° in
the bed of the glacier. The molecules on the slope of 8° will descend
more rapidly than those above on the slope of 4°. A state of tension
will therefore be induced at the point where the change of inclination
occurs. The ice on the slope of 8° will tend to pull after it the mass
of the glacier moving more slowly on the slope above. The pull being
continued, the glacier will snap asunder the moment that the cohesion
of the ice is overcome. The greater the change of inclination is, the
more readily will the rupture of the ice take place. Every species of
crevasse can be explained upon the same principle.[309]
This theory explains also why a glacier moves at a greater rate during
summer than during winter; for as the supply of heat to the glacier is
greater during the former season than during the latter, the molecules
will pass oftener into the liquid state.
As regards the denuding power of glaciers, I may observe that, though
a glacier descends molecule by molecule, it will grind the rocky bed
over which it moves as effectually as it would do did it slide down in
a rigid mass in the way generally supposed; for the grinding-effect
is produced not by the ice of the glacier, but by the stones, sand,
and other materials forced along under it. But if all the resistances
opposing the descent of a glacier, internal and external, are overcome
by the mere weight of the ice alone, it can be proved that in the case
of one descending with a given velocity the amount of work performed
in forcing the grinding materials lying under the ice forward must be
as great, supposing the motion of the ice to be molecular in the way
I have explained, as it would be supposing the ice descended in the
manner generally supposed.
Of course, a glacier could not descend by means of its weight as
rapidly in the latter case as in the former; for, in fact, as Canon
Moseley has shown, it would not in the latter case descend at all; but
assuming for the sake of argument the rate of descent in both cases to
be the same, the conclusion I have stated would follow. Consequently
whatever denuding effects may have been attributed to the glacier,
according to the ordinary theory, must be equally attributable to it
according to the present explanation.
This theory, however, explains, what has always hitherto excited
astonishment, viz., why a glacier can descend a slope almost
horizontal, or why the ice can move off the face of a continent
perfectly level.
This is the form in which my explanation was first stated about
half-a-dozen years ago.[310] There is, however another element
which must be taken into account. It is one which will help to cast
additional light on some obscure points connected with glacial
phenomena.
Ice is evidently not absolutely solid throughout. It is composed of
crystalline particles, which, though in contact with one another, are,
however, not packed together so as to occupy the least possible space,
and, even though they were, the particles would not fit so closely
together as to exclude interstices. The crystalline particles are,
however, united to one another at special points determined by their
polarity, and on this account they require more space; and this in
all probability is the reason, as Professor Tyndall remarks, why ice,
volume for volume, is less dense than water.
“They (the molecules) like the magnets,” says Professor Tyndall, “are
acted upon by two distinct forces; for a time, while the liquid is
being cooled, they approach each other, in obedience to their general
attraction for each other. But at a certain point new forces, some
attractive some repulsive, _emanating from special points_ of the
molecules, come into play. The attracted points close up, the repelled
points retreat. Thus the molecules turn and rearrange themselves,
demanding as they do so more space, and overcoming all ordinary
resistance by the energy of their demand. This, in general terms, is an
explanation of the expansion of water in solidifying.”[311]
It will be obvious, then, that when a crystalline molecule melts, it
will not merely descend in the manner already described, but capillary
attraction will cause it to flow into the interstices between the
adjoining molecules. The moment that it parts with the heat received,
it will of course resolidify, as has been shown, but it will not
solidify so as to fit the cavity which it occupied when in the fluid
state. For the liquid molecule in solidifying assumes the crystalline
form, and of course there will be a definite proportion between the
length, breadth, and thickness of the crystal; consequently it will
always happen that the interstice in which it solidifies will be too
narrow to contain it. The result will be that the fluid molecule
in passing into the crystalline form will press the two adjoining
molecules aside in order to make sufficient room for itself between
them, and this it will do, no matter what amount of space it may
possess in all other directions. The crystal will not form to suit the
cavity, the cavity must be made to contain the crystal. And what holds
true of one molecule, holds true of every molecule which melts and
resolidifies. This process is therefore going on incessantly in every
part of the glacier, and in proportion to the amount of heat which the
glacier is receiving. This internal molecular pressure, resulting from
the solidifying of the fluid molecules in the interstices of the ice,
acts on the mass of the ice as an expansive force, tending to cause the
glacier to widen out laterally in all directions.
Conceive a mass of ice lying on a flat horizontal surface, and
receiving heat on its upper surface, say from the sun; as the heat
passes downwards through the mass, the molecules, acting as conductors,
melt and resolidify. Each fluid molecule solidifies in an interstice,
which has to be widened in order to contain it. The pressure thus
exerted by the continual resolidifying of the molecules will cause the
mass to widen out laterally, and of course as the mass widens out it
will grow thinner and thinner if it does not receive fresh acquisition
on its surface. In the case of a glacier lying in a valley, motion,
however, will only take place in one direction. The sides of the
valley prevent the glacier from widening; and as gravitation opposes
the motion of the ice up, and favours its motion down the valley, the
path of least resistance to molecular pressure will always be down
the slope, and consequently in this direction molecular displacement
will take place. Molecular pressure will therefore produce motion in
the same direction as that of gravity. In other words, it will tend to
cause the glacier to descend the valley.
The lateral expansion of the ice from internal molecular pressure
explains in a clear and satisfactory manner how rock-basins may be
excavated by means of land-ice. It also removes the difficulties
which have been felt in accounting for the ascent of ice up a steep
slope. The main difficulty besetting the theory of the excavation of
rock-basins by ice is to explain how the ice after entering the basin
manages to get out again—how the ice at the bottom is made to ascend
the sloping sides of the basin. Pressure acting from behind, it has
been argued by some; but if the basin be deep and its sides steep, this
will simply cause the ice lying above the level of the basin to move
forward over the surface of the mass filling it. This conclusion is,
however, incorrect. The ice filling the basin and the glacier overlying
it are united in one solid mass, so that the latter cannot move over
the former without shearing; and although the resistance to motion
offered by the sloping sides of the basin may be much greater than the
resistance to shear, still the ice will be slowly dragged out of the
basin. However, in order to obviate this objection to which I refer,
the advocates of the glacial origin of lake-basins point out that
the length of those basins in proportion to their depth is so great
that the slope up which the ice has to pass is in reality but small.
This no doubt is true of lake-basins in general, but it does not hold
universally true. But the theory does not demand that an ice-formed
lake-basin cannot have steep sides. We have incontestable evidence that
ice will pass up a steep slope; and, if ice can pass up a steep slope,
it can excavate a basin with a steep slope. That ice will pass up a
steep slope is proved by the fact that comparatively deep and narrow
river valleys are often found striated across, while hills which stood
directly in the path of the ice of the glacial epoch are sometimes
found striated _upwards_ from their base to their summit. Some striking
examples of striæ running up hill are given by Professor Geikie in
his “Glacial Drift of Scotland.” I have myself seen a slope striated
upwards so steep that one could not climb it.
A very good example of a river valley striated across came under
my observation during the past summer. The Tay, between Cargill
and Stanley (in the centre of the broad plain of Strathmore), has
excavated, through the Old Red Sandstone, a channel between 200 and
300 feet in depth. The channel here runs at right angles to the path
taken during the glacial epoch by the great mass of ice coming from
the North-west Highlands. At a short distance below Cargill, the trap
rising out of the bed of the river is beautifully ice-grooved and
striated, at right angles to the stream. A trap-dyke, several miles in
length, crosses the river about a mile above Stanley, forming a rapid,
known as the Linn of Campsie. This dyke is _moutonnée_ and striated
from near the Linn up the sloping bank to the level of the surrounding
country, showing that the ice must have ascended a gradient of one in
seven to a height of 300 feet.
From what has been already stated in reference to the resolidifying of
the molecules in the interstices of the ice, the application of the
molecular theory to the explanation of the effects under consideration
will no doubt be apparent. Take the case of the passage of the
ice-sheet across a river valley. As the upper surface of the ice-sheet
is constantly receiving heat from the sun and the air in contact with
it, there is consequently a transferrence of heat from above downwards
to the bottom of the sheet. This transferrence of heat from molecule
to molecule is accompanied by the melting and resolidifying of the
successive molecules in the manner already detailed. As the fluid
molecules tend to flow into adjoining interstices before solidifying
and assuming the crystalline form, the interstices of the ice at the
bottom of the valley are constantly being filled by fluid molecules
from above. These molecules no sooner enter the interstices than they
pass into the crystalline form, and become, of course, separated from
their neighbours by fresh interstices, which new interstices become
filled by fluid molecules, which, in turn, crystallize, forming fresh
interstices, and so on. The ice at the bottom of the valley, so long as
this process continues, is constantly receiving fresh additions from
above. The ice must therefore expand laterally to make room for these
additions, which it must do unless the resistance to lateral expansion
be greater than the force exerted by the molecules in crystallizing.
But a resistance sufficient to do this must be enormous. The ice at the
bottom of the valley cannot expand laterally without passing up the
sloping sides. In expanding it will take the path of least resistance,
but the path of least resistance will always be on the side of the
valley towards which the general mass of the ice above is flowing.
It has been shown (Chapter XXVII.) that the ice passing over Strathmore
must have been over 2,000 feet in thickness. An ice-sheet 2,000 feet
in thickness exerts on its bed a pressure of upwards of 51 tons per
square foot. When we reflect that ice under so enormous a pressure,
with grinding materials lying underneath, was forced by irresistible
molecular energy up an incline of one in seven, it is not at all
surprising that the hard trap should be ground down and striated.
We can also understand how the softer portions of the rocky surface
over which the ice moved should have been excavated into hollow basins.
We have also an explanation of the transport of boulders from a lower
to a higher level, for if ice can move from a lower to a higher level,
it of course can carry boulders along with it.
The bearing which the foregoing considerations of the manner in which
heat is transmitted through ice have on the question of the cause of
regelation will be considered in the Appendix.
APPENDIX.
I.
OPINIONS EXPRESSED PREVIOUS TO 1864 REGARDING
THE INFLUENCE OF THE ECCENTRICITY OF THE EARTH’S
ORBIT ON CLIMATE.[312]
M. DE MAIRAN.
M. de Mairan, in an article in the _Memoirs of the Royal Academy of
France_[313] “On the General Cause of Heat in Summer and Cold in
Winter, in so far as depends on the internal and permanent Heat of the
Earth,” makes the following remarks on the influence of the difference
of distance of the sun in apogee and perigee:—
“Cet élément est constant pour les deux solstices; tandis que les
autres (height of the sun and obliquity of his rays) y varient à raison
des latitudes locales; et il y a encore cela de particulier, qu’il
tend à diminuer la valeur de notre été, et à augmenter celle de notre
hiver dans l’hémisphère boréal où nous sommes, et tout au contraire
dans l’austral. Remarquons cependant que de ces mêmes distances, qui
constituent ce troisième élément, naît en partie un autre principe
de chaleur tout opposé, et qui semble devoir tempérer les effets du
précédent; sçavoir, la lenteur et la vitesse réciproques du mouvement
annuel apparent, en vertu duquel et du réel qui s’y mêle, le soleil
emploie 8 jours de plus à parcourir les signes septentrionaux.
C’est-à-dire, que le soleil passe 186½ jours dans notre hémisphère, et
seulement 178½ dans l’hémisphère opposé. Ce qui, en général, ne peut
manquer de répandre un pen plus de chaleur sur l’été du premier, et un
peu moins sur son hiver.”
MR. RICHARD KIRWAN.
“Œpinus,[314] reasoning on astronomical principles, attributes the
inferior temperature of the southern hemisphere to the shorter abode of
the sun in the southern tropic, shorter by seven days, which produces
a difference of fourteen days in favour of the northern hemisphere,
during which more heat is accumulated, and hence he infers that the
temperature of the northern hemisphere is to that of the southern, as
189·5 to 175·5, or as 14 to 13.”—_Trans. of the Royal Irish Academy_,
vol. viii., p. 417. 1802.
SIR CHARLES LYELL.
“Before the amount of difference between the temperature of the two
hemispheres was ascertained, it was referred by astronomers to the
acceleration of the earth’s motion in its perihelion; in consequence of
which the spring and summer of the southern hemisphere are shorter by
nearly eight days than those seasons north of the equator. A sensible
effect is probably produced by this source of disturbance, but it is
quite inadequate to explain the whole phenomena. It is, however, of
importance to the geologist to bear in mind that in consequence of the
precession of the equinoxes, the two hemispheres receive alternately,
each for a period of upwards of 10,000 years, a greater share of
solar light and heat. This cause may sometimes tend to counterbalance
inequalities resulting from other circumstances of a far more
influential nature; but, on the other hand, it must sometimes tend to
increase the extreme of deviation, which certain combinations of causes
produce at distant epochs.”—_Principles_, First Edition, 1830, p. 110,
vol. i.
SIR JOHN F. HERSCHEL, BART.
The following, in so far as it relates to the effects of eccentricity,
is a copy of Sir John Herschel’s memoir, “On the Astronomical Causes
which may influence Geological Phenomena,” read before the Geological
Society, Dec. 15th, 1830.—_Trans. Geol. Soc._, vol. iii., p. 293,
Second Series:—
“... Let us next consider the changes arising in the orbit of the earth
itself about the sun, from the disturbing action of the planets. In so
doing it will be obviously unnecessary to consider the effect produced
on the solar tides, to which the above reasoning applies much more
forcibly than in the case of the lunar. It is, therefore, only the
variations in the supply of light and heat received from the sun that
we have now to consider.
“Geometers having demonstrated the absolute invariability of the _mean_
distance of the earth from the sun, it would seem to follow that
the mean annual supply of light and heat derived from that luminary
would be alike invariable; but a closer consideration of the subject
will show that this would not be a legitimate conclusion, but that,
on the contrary, the _mean_ amount of solar radiation is dependent
on the eccentricity of the orbit, and therefore liable to variation.
Without going at present into any geometrical investigations, it will
be sufficient for the purpose here to state it as a theorem, of which
any one may easily satisfy himself by no very abstruse geometrical
reasoning, that ‘_the eccentricity of the orbit varying, the_ total
_quantity of heat received by the earth from the sun in one revolution
is inversely proportional to the_ minor _axis of the orbit_.’ Now since
the major axis is, as above observed, invariable, and therefore, of
course, the absolute length of the year, it will follow that the _mean
annual_ average of heat will also be in the same inverse ratio of the
_minor_ axis; and thus we see that the very circumstance which on a
cursory view we should have regarded as demonstrative of the constancy
of our supply of solar heat, forms an essential link in the chain of
strict reasoning by which its variability is proved.
“The eccentricity of the earth’s orbits is actually diminishing, and
has been so for ages, beyond the records of history. In consequence,
the ellipse is in a state of approach to a circle, and its minor
axis being, therefore, on the increase, the annual average of solar
radiation is actually on the _decrease_.
“So far this is in accordance with the testimony of geological
evidence, which indicates a general refrigeration of climate; but when
we come to consider the amount of diminution which the eccentricity
must be supposed to have undergone to render an account of the
variation which has taken place, we have to consider that, in the first
place, a great diminution of the eccentricity is required to produce
any sensible increase of the minor axis. This is a purely geometrical
conclusion, and is best shown by the following table:—
Eccentricity. Minor Axis. Reciprocal or Ratio of
Heat received.
0·00 1·000 1·000
0·05 0·999 1·002
0·10 0·995 1·005
0·15 0·989 1·011
0·20 0·980 1·021
0·25 0·968 1·032
0·30 0·954 1·048
By this it appears that a variation of the eccentricity of the orbit
from the circular form to that of an ellipse, having an eccentricity
of one-fourth of the major axis, would produce only a variation of 3
per cent. on the _mean_ annual amount of solar radiation, and this
variation takes in the whole range of the planetary eccentricities,
from that of Pallas and Juno downwards.
“I am not aware that the limit of increase of the eccentricity of the
earth’s orbit has ever been determined. That it has a limit has been
satisfactorily proved; but the celebrated theorem of Laplace, which
is usually cited as demonstrating that none of the planetary orbits
can ever deviate materially from the circular form, leads to no such
conclusion, except in the case of the great preponderant planets
Jupiter and Saturn, while for anything that theorem proves to the
contrary, the orbit of the earth may become elliptic to any amount.
“In the absence of calculations which though practicable have, I
believe, never been made,[315] and would be no slight undertaking, we
may assume that eccentricities which exist in the orbits of planets,
both interior and exterior to that of the earth, may _possibly_
have been attained, and may be attained again by that of the earth
itself. It is clear that such eccentricities _existing_ they cannot
be incompatible with the stability of the system generally, and that,
therefore, the question of the possibility of such an amount in the
particular case of the earth’s orbit will depend on the particular
data belonging to that case, and can only be determined by executing
the calculations alluded to, having regard to the simultaneous effects
of at least the four most influential planets, Venus, Mars, Jupiter,
and Saturn, _not only on the orbit of the earth, but on those of each
other_. The principles of this calculation are detailed in the article
of Laplace’s work cited. But before entering on a work of so much
labour, it is quite necessary to inquire what prospect of advantage
there is to induce any one to undertake it.
“Now it certainly at first sight seems clear that a variation of 3
per cent. only in the mean annual amount of solar radiation, and
that arising from an extreme supposition, does _not_ hold out such a
prospect. Yet it might be argued that the effects of the sun’s heat is
to maintain the temperature of the earth’s surface at its actual mean
height, not above the zero of Fahrenheit’s or any other thermometer,
but above the temperature of the celestial spaces, out of the reach of
the sun’s influence, and what that temperature is may be a matter of
much discussion. M. Fourier has considered it as demonstrated that it
is not greatly inferior to that of the polar regions of our own globe,
but the grounds of this decision appear to me open to considerable
objection.[316] If those regions be really void of matter, their
temperature can only arise, according to M. Fourier’s own view of
the subject, from the radiation of the stars. It ought, therefore,
to be as much inferior to that due to solar radiation, as the light
of a starlight night is to that of the brightest noon day, in other
words it should be very nearly a total privation of heat—almost the
_absolute zero_ respecting which so much difference of opinion exists,
some placing it at 1,000°, some at 5,000° of Fahrenheit below the
freezing-point, and some still lower, in which case a single unit per
cent. in the mean annual amount of radiation would suffice to produce a
change of climate fully commensurate to the demands of geologists.[317]
“Without attempting, however, to enter further into the perplexing
difficulties in which this point is involved, which are far greater
than appear on a cursory view, let us next consider, not the _mean_,
but the _extreme_ effects which a variation in the eccentricity of
the earth’s orbit may be expected to produce in the summer and winter
climates in particular regions of its surface, and under the influence
of circumstances favouring a difference of effect. And here, if I
mistake not, it will appear that an amount of variation, which we need
not hesitate to admit (at least, provisionally) as a possible one, may
be productive of considerable diversity of climate, and may operate
during great periods of time either to mitigate or to exaggerate
the difference of winter and summer temperatures, so as to produce
alternately, in the same latitude of either hemisphere, a perpetual
spring, or the extreme vicissitudes of a burning summer and a rigorous
winter.
“To show this, let us at once take the extreme case of an orbit as
eccentric as that of Juno or Pallas, in which the greatest and least
distances of the sun are to each other as 5 to 3, and consequently the
radiations at those distances as 25 to 9, or very nearly as 3 to 1. To
conceive what would be the _extreme_ effects of this great variation
of the heat received at different periods of the year, let us first
imagine in our latitude the place of the perigee of the sun to coincide
with the summer solstice. In that case, the difference between the
summer and winter temperature would be exaggerated in the same degree
as if three suns were placed side by side in the heavens in the former
season and only one in the latter, which would produce a climate
perfectly intolerable. On the other hand, were the perigee situated
in the winter solstice our three suns would combine to warm us in the
winter, and would afford such an excess of winter radiation as would
probably more than counteract the effect of short days and oblique
sunshine, and throw the summer season into the winter months.
“The actual diminution of the eccentricity is so slow, that the
transition from a state of the orbit such as we have assumed to the
present nearly circular figure would occupy upwards of 600,000 years,
supposing it uniformly changeable—this, of course, would not be the
case; when near the maximum, however, it would vary slower still, so
that at that point it is evident a period of 10,000 years would elapse
without any perceptible change in the state of the data of the case we
are considering.
“Now this adopting the very ingenious idea of Mr. Lyell[318] would
suffice, by reason of the combined effect of the precession of the
equinoxes and the motion of the apsides of the orbit itself, to
transfer the perigee from the summer to the winter solstice, and thus
to produce a transition from the one to the other species of climate in
a period sufficiently great to give room for a material change in the
botanical character of country.
“The supposition above made is an extreme, but it is not demonstrated
to be an impossible one, and should even an approach to such a state
of things be possible, the same consequences, in a mitigated degree,
would follow. But if, on executing the calculations, it should appear
that the limits of the eccentricity of the earth’s orbit are really
narrow, and if, on a full discussion of the very difficult and delicate
point of the actual effect of solar radiation, it should appear that
the mean, as well as the extreme, temperature of our climates would
_not_ be materially affected,—it will be at least satisfactory to
_know_ that the causes of the phenomena in question are to be sought
elsewhere than in the relations of our planet to the system to which
it belongs, since there does not appear to exist any other conceivable
connections between these relations and the facts of geology than
those we have enumerated, the obliquity of the ecliptic being, as we
know, confined within too narrow limits for its variation to have any
sensible influence.”—_J. F. W. Herschel._
The influence which this paper might have had on the question as
to whether eccentricity may be regarded as a cause of changes in
geological climate appears to have been completely neutralized by the
following, which appeared shortly afterwards both in his “Treatise” and
“Outlines of Astronomy,” showing evidently that he had changed his mind
on the subject.
“It appears, therefore, from what has been shown, the supplies of heat
received from the sun will be equal in the two segments, in whatever
direction the line PTQ be drawn. They will, indeed, be described in
unequal times: that in which the perihelion A lies in a shorter, and
the other in a longer, in proportion to their unequal area; but the
greater proximity of the sun in the smaller segment compensates exactly
for its more rapid description, and thus an equilibrium of heat is, as
it were, maintained.
“Were it not for this the eccentricity of the orbit would materially
influence the transition of seasons. The fluctuation of distance
amounts to nearly 1/30th of the mean quantity, and, consequently,
the fluctuation of the sun’s direct heating power to double this, or
1/15th of the whole.... Were it not for the compensation we have just
described, the effect would be to exaggerate the difference of summer
and winter in the southern hemisphere, and to moderate it in the
northern; thus producing a more violent alternation of climate in the
one hemisphere and an approach to perpetual spring in the other. _As it
is, however, no such inequality subsists_, but an equal and impartial
distribution of heat and light is accorded to both.”—“_Treatise of
Astronomy_,” _Cabinet Cyclopædia_, § 315; _Outlines of Astronomy_, §
368.
“The fact of a great change in the general climate of large tracts
of the globe, if not of the whole earth, and of a diminution of
general temperature, having been recognised by geologists, from
their examination of the remains of animals and vegetables of former
ages enclosed in the strata, various causes for such diminution of
temperature have been assigned.... It is evident that the _mean_
temperature of the whole surface of the globe, in so far as it is
maintained by the action of the sun at a higher degree than it would
have were the sun extinguished, must depend on the mean quantity of
the sun’s rays which it receives, or, which comes to the same thing,
on the _total_ quantity received in a given invariable time; and the
length of the year being unchangeable in all the fluctuations of the
planetary system, it follows that the total _annual_ amount of solar
radiation will determine, _cæteris paribus_, the general climate
of the earth. Now, it is not difficult to show that this amount is
inversely proportional to the minor axis of the ellipse described
by the earth about the sun, regarded as slowly variable; and that,
therefore, the major axis remaining, as we know it to be, constant,
and the orbit being actually in a state of approach to a circle,
and consequently the minor axis being on the _increase_, the mean
annual amount of solar radiation received by the whole earth must
be actually on the _decrease_. We have here, therefore, an evident
real cause of sufficient universality, and acting _in the right
direction_, to account for the phenomenon. Its adequacy is another
consideration.”[319]—_Discourse on the Study of Natural Philosophy_,
pp. 145−147 (1830).
SIR CHARLES LYELL, BART.
“_Astronomical Causes of Fluctuations in Climate._—Sir John Herschel
has lately inquired, whether there are any astronomical causes which
may offer a possible explanation of the difference between the actual
climate of the earth’s surface, and those which formerly appear to
have prevailed. He has entered upon this subject, he says, ‘impressed
with the magnificence of that view of geological revolutions, which
regards them rather as regular and necessary effects of great and
general causes, than as resulting from a series of convulsions
and catastrophes, regulated by no laws, and reducible to no fixed
principles.’ Geometers, he adds, have demonstrated the absolute
invariability of the mean distance of the earth from the sun; whence
it would seem to follow that the mean annual supply of light and heat
derived from that luminary would be alike invariable; but a closer
consideration of the subject will show that this would not be a
legitimate conclusion, but that, on the contrary, the _mean_ amount of
solar radiation is dependent on the eccentricity of the earth’s orbit,
and, therefore, liable to variation.
“Now, the eccentricity of the orbit, he continues, is actually
diminishing, and has been so for ages beyond the records of history.
In consequence, the ellipse is in a state of approach to a circle, and
the annual average of solar heat radiated to the earth is actually on
the _decrease_. So far, this is in accordance with geological evidence,
which indicates a general refrigeration of climate; but the question
remains, whether the amount of diminution which the eccentricity may
have ever undergone can be supposed sufficient to account for any
sensible refrigeration.[320] The calculations necessary to determine
this point, though practicable, have never yet been made, and would be
extremely laborious; for they must embrace all the perturbations which
the most influential planets, Venus, Mars, Jupiter, and Saturn, would
cause in the earth’s orbit and in each other’s movements round the sun.
“The problem is also very complicated, inasmuch as it depends not
merely on the ellipticity of the earth’s orbit, but on the assumed
temperature of the celestial spaces beyond the earth’s atmosphere;
a matter still open to discussion, and on which M. Fourier and Sir
J. Herschel have arrived at very different opinions. But if, says
Herschel, we suppose an extreme case, as if the earth’s orbit should
ever become as eccentric as that of the planet Juno or Pallas, a great
change of climate might be conceived to result, the winter and summer
temperatures being sometimes mitigated and at others exaggerated, in
the same latitudes.
“It is much to be desired that the calculations alluded to were
executed, as even if they should demonstrate, as M. Arago thinks highly
probable, that the mean of solar radiation can never be materially
affected by irregularities in the earth’s motion, it would still be
satisfactory to ascertain the point.”—_Principles of Geology_, Ninth
Edition, 1853, p. 127.
M. ARAGO.
“_Can the variations which certain astronomical elements undergo
sensibly modify terrestrial climates?_
“The sun is not always equally distant from the earth. At this time
its least distance is observed in the first days of January, and the
greatest, six months after, or in the first days of July. But, on the
other hand, a time will come when the _minimum_ will occur in July,
and the _maximum_ in January. Here, then, this interesting question
presents itself,—Should a summer such as those we now have, in which
the _maximum_ corresponds to the solar distance, differ sensibly, from
a summer with which the _minimum_ of this distance should coincide?
“At first sight every one probably would answer in the affirmative;
for, between the _maximum_ and the _minimum_ of the sun’s distance
from the earth there is a remarkable difference, a difference in round
numbers of a thirtieth of the whole. Let, however, the consideration of
the velocities be introduced into the problem, elements which cannot
fairly be neglected, and the result will be on the side opposite to
that we originally imagined.
“The part of the orbit where the sun is found nearest the earth, is, at
the same time, the point where the luminary moves most rapidly along.
The demi-orbit, or, in other words, the 180° comprehended betwixt the
two equinoxes of spring-time and autumn, will then be traversed in the
least possible time, when, in moving from the one of the extremities
of this arc to the other, the sun shall pass, near the middle of
this course of six months, at the point of the smallest distance. To
resume—the hypothesis we have just adopted would give, on account of
the lesser distance, a spring-time and summer hotter than they are in
our days; but on account of the greater rapidity, the sum of the two
seasons would be shorter by about seven days. Thus, then, all things
considered, the compensation is mathematically exact. After this it is
superfluous to add, that the point of the sun’s orbit corresponding to
the earth’s least distance changes very gradually; and that since the
most distant periods, the luminary has always passed by this point,
either at the end of autumn or beginning of winter.
“We have thus seen that the changes which take place in the _position_
of the solar orbit, _have no power in modifying the climate of our
globe_. We may now inquire, if it be the same concerning the variations
which this orbit experiences in its _form_....
“Herschel, who has recently been occupying himself with this problem,
in the hope of discovering the explanation of several geological
phenomena, allows that the succession of ages might bring the
eccentricity of the terrestrial orbit to the proportion of that of
the planet Pallas, that is to say, to be the 25/100 of a semi-greater
axis. It is exceedingly improbable that in these periodical changes
the eccentricity of our orbit should ever experience such enormous
variations, and even then these twenty-five hundredth parts (25/100),
would not augment the _mean_ annual solar radiation except by about one
hundredth part (1/100). To repeat, an eccentricity of 25/100 _would not
alter in any appreciated manner the mean thermometrical state of the
globe_....
“The changes of the form, and of the position, of the terrestrial
orbit are mathematically inoperative, or, at most, their influence is
so minute that it is not indicated by the most delicate instruments.
For the explanation of the changes of climates, then, there only
remains to us either the local circumstances, or some alteration in
the heating or illuminating power of the sun. But of these two causes,
we may continue to reject the last. And thus, in fact, all the changes
would come to be attributed to agricultural operations, to the clearing
of plains and mountains from wood, the draining of morasses, &c.
“Thus, at one swoop, to confine, the whole earth, the variations
of climates, past and future, within the limits of the naturally
very narrow influence which the labour of man can effect, would be
a meteorological result of the very last importance.”—pp. 221−224,
_Memoir on the “Thermometrical State of the Terrestrial Globe,” in the
Edinburgh New Philosophical Journal_, vol. xvi., 1834.
BARON HUMBOLDT.
“The question,” he says, “has been raised as to whether the increasing
value of this ellipticity is capable during thousands of years of
modifying to any considerable extent the temperature of the earth,
in reference to the daily and annual quantity and distribution of
heat? Whether a partial solution of the great geological problem of
the imbedding of tropical vegetable and animal remains in the now
cold zones may not be found in these _astronomical_ causes proceeding
regularly in accordance with eternal laws?... It might at the first
glance be supposed that the occurrence of the perihelion at an opposite
time of the year (instead of the winter, as, is now the case, in
summer) must necessarily produce great climatic variations; but, on the
above supposition, the sun will no longer remain seven days longer in
the northern hemisphere; no longer, as is now the case, traverse that
part of the ecliptic from the autumnal equinox to the vernal equinox,
in a space of time which is one week shorter than that in which it
traverses the other half of its orbit from the vernal to the autumnal
equinox.
“The difference of temperature which is considered as the consequence
to be apprehended from the turning of the major axis, _will on the
whole disappear_, principally from the circumstance that the point of
our planet’s orbit in which it is nearest to the sun is at the same
time always that over which it passes with the greatest velocity....
“As the altered position of the major axis is capable of exerting
only a very _slight influence upon the temperature of the earth;_ so
likewise the _limit_ of the probable changes in the elliptical form of
the earth’s orbit are, according to Arago and Poisson, so narrow that
these changes could _only very slightly_ modify the climates of the
individual zones, and that in very long periods.”[321]—_Cosmos_, vol.
iv., pp. 458, 459. Bohn’s Edition. 1852.
SIR HENRY T. DE LA BECHE.
“Mr. Herschel, viewing this subject with the eye of an astronomer,
considers that a diminution of the surface-temperature might arise from
a change in ellipticity of the earth’s orbit, which, though slowly,
gradually becomes more circular. No calculations having yet been made
as to the probable amount of decreased temperature from this cause,
it can at present be only considered as a possible explanation of
those geological phenomena which point to considerable alterations in
climates.”—_Geological Manual_. Third Edition. 1833. p. 8.
PROFESSOR PHILLIPS.
“_Temperature of the Globe._—_Influence of the Sun._—No proposition is
more certain than the fundamental dependence of the temperature of the
surface of the globe on the solar influence.
“It is, therefore, very important for geologists to inquire whether
this be variable or constant; whether the amount of solar heat
communicated to the earth is and has always been the same in every
annual period, or what latitude the laws of planetary movements permit
in this respect.
“Sir John Herschel has examined this question in a satisfactory manner,
in a paper read to the Geological Society of London. The total amount
of solar radiation which determines the general climate of the earth,
the year being of invariable length, is inversely proportional to
the minor axis of the ellipse described by the earth about the sun,
regarded as slowly variable; the major axis remaining constant and
the orbit being actually in a state of approach to a circle, and,
consequently, the minor axis being on the increase, it follows that
the mean annual amount of solar radiation received by the whole earth
must be actually on the decrease. The limits of the variation in the
eccentricity of the earth’s orbit are not known. It is, therefore,
impossible to say accurately what may have been in former periods of
time, the amount of solar radiation; it is, however, certain that
if the ellipticity has ever been so great as that of the orbit of
Mercury or Pallas, the temperature of the earth must have been sensibly
higher than it is at present. But the difference of a few degrees of
temperature thus occasioned, is of too small an order to be employed
in explaining the growth of tropical plants and corals in the polar
or temperate zones, and other great phenomena of Geology.”—_From A
Treatise on Geology_, p. 11, _forming the article under that head in
the seventh edition of the Encyclopædia Britannica_. 1837.
MR. ROBERT BAKEWELL.
“A change in the form of the earth’s orbit, if considerable, might
change the temperature of the earth, by bringing it nearer to the
sun in one part of its course. The orbit of the earth is an ellipsis
approaching nearly to a circle; the distance from the centre of the
orbit to either focus of the ellipsis is called by astronomers ‘the
eccentricity of the orbit.’ This eccentricity has been for ages slowly
decreasing, or, in other words, the orbit of the earth has been
approaching nearer to the form of a perfect circle; after a long period
it will again increase, and the possible extent of the variation has
not been yet ascertained. From what is known respecting the orbits of
Jupiter and Saturn, it appears highly probable that the eccentricity of
the earth’s orbit is confined within limits that preclude the belief
of any great change in the mean annual temperature of the globe ever
having been occasioned by this cause.”—_Introduction to Geology_, p.
600. 1838. Fifth Edition.
MRS. SOMERVILLE.
“Sir John Herschel has shown that the elliptical form of the earth’s
orbit has but a trifling share in producing the variation of
temperature corresponding to the difference of the seasons.”—_Physical
Geography_, vol. ii., p. 20. Third Edition.
MR. L. W. MEECH, A.M.
“Let us, then, look back to that primeval epoch when the earth
was in aphelion at midsummer, and the eccentricity at its maximum
value—assigned by Leverrier near to ·0777. Without entering into
elaborate computation, it is easy to see that the extreme values
of diurnal intensity, in Section IV., would be altered as by the
multiplier ((1 ± _e_)/(1 ± _e′_))^2, that is 1 - 0·11 in summer, and 1
+ 0·11 in winter. This would diminish the midsummer intensity by about
9°, and increase the midwinter intensity by 3° or 4°; the temperature
of spring and autumn being nearly unchanged. But this does not appear
to be of itself adequate to the geological effects in question.
“It is not our purpose, here, to enter into the inquiry whether the
atmosphere was once more dense than now, whether the earth’s axis
had once a different inclination to the orbit, or the sun a greater
emissive power of heat and light. Neither shall we attempt to speculate
upon the primitive heat of the earth, nor of planetary space, nor of
the supposed connection of terrestrial heat and magnetism; nor inquire
how far the existence of coal-fields in this latitude, of fossils,
and other geological remains, have depended upon existing causes. The
preceding discussion seems to prove simply that, under the present
system of physical astronomy, the sun’s intensity could never have been
materially different from what is manifested upon the earth at the
present day. _The causes of notable geological changes must be other
than the relative position of the sun and earth, under their present
laws of motion._”—_“On the Relative Intensity of the Heat and Light of
the Sun.” Smithsonian Contributions to Knowledge_, vol. ix.
M. JEAN REYNAUD.
“La révolution qui pourrait y causer les plus grands changements
thermométriques, celle qui porte l’orbite à s’élargir et à se rétrécir
alternativement et, par suite, la planète à passer, aux époques de
périhélie, plus ou moins près du soleil, embrasse une période de plus
de cent mille années terrestres et demeure comprise dans de si étroites
limites que les habitants doivent être à peine avertis que la chaleur
décroît, par cette raison, depuis une haute antiquité et décroîtra
encore pendant des siècles en variant en même temps dans sa répartition
selon les diverses époques de l’année.... Enfin, le tournoiement de
l’axe du globe s’empreint également d’une manière particulière sur
l’ètablissement des saisons qui, à tour de rôle, dans chacun des deux
hémisphères, deviennent graduellement, durant une période d’environ
vingt-cinq mille ans, de plus en plus uniformes, ou, à l’inverse, de
plus en plus dissemblables. C’est actuellement dans l’hémisphère boréal
que règne l’uniformité, et quoique les étés et les hivers y tendent,
dès à présent, à se trancher de plus en plus, il ne paraît pas douteux
que la modération des saisons n’y produise, pendant longtemps encore,
des effets appréciables. En résumé, de tous ces changements il n’en est
donc aucun ni qui suive un cours précipité, ni qui s’élève jamais à des
valeurs considérables; ils se règlent tous sur un mode de développement
presque insensible, et il s’ensuit que les années de la terre, malgré
leur complexité virtuelle, se distinguent par le constance de leurs
caractères non-seulement de ce qui peut avoir lieu, en vertu des mêmes
principes, dans les autres systèmes planétaires de l’univers, mais
même de ce qui s’observe dans plusieurs des mondes qui composent le
nôtre.”—_Philosophie Religieuse: Terre et Ciel._
M. ADHÉMAR.
Adhémar does not consider the effects which ought to result from a
change in the eccentricity of the earth’s orbit; he only concerns
himself with those which, in his opinion, arise from the present amount
of such eccentricity. He admits, of course, that both hemispheres
receive from the sun equal quantities of heat per annum; but, as
the southern hemisphere has a winter longer by 168 hours than the
corresponding season in the northern hemisphere, an accumulation of
heat necessarily takes place in the latter, and an accumulation of
cold in the former. Adhémar also measures the loss of heat sustained
by the southern hemisphere in a year by the number of hours by which
the southern exceeds the northern winter. “The south pole,” he says,
“loses in one year more heat than it receives, because the total
duration of its nights surpasses that of the days by 168 hours; and the
contrary takes place for the north pole. If, for example, we take for
unity the mean quantity of heat which the sun sends off in one hour,
the heat accumulated at the end of the year at the north pole will be
expressed by 168, while the heat lost by the south pole will be equal
to 168 times what the radiation lessens it by in one hour; so that at
the end of the year the difference in the heat of the two hemispheres
will be represented by 336 times what the earth receives from the sun
or loses in an hour by radiation,”[322] and at the end of 100 years the
difference will be 33,600 times, and at the end of 1,000 years 336,000
times, or equal to what the earth receives from the sun in 38½ years,
and so on during the 10,000 years that the southern winter exceeds in
length the northern. This, in his opinion, is all that is required to
melt the ice off the arctic regions, and cover the antarctic regions
with an enormous ice-cap. He further supposes that in about 10,000
years, when our northern winter will occur in aphelion and the southern
in perihelion, the climatic conditions of the two hemispheres will be
reversed; that is to say, the ice will melt at the south pole, and the
northern hemisphere will become enveloped in one continuous mass of
ice, leagues in thickness, extending down to temperate regions.
This theory, as shown in Chapter V., is based upon a misconception
regarding the laws of radiant heat. The loss of heat sustained by the
southern hemisphere from radiation, resulting from the greater length
of the southern winter, is vastly over-estimated by M. Adhémar, and
could not possibly produce the effects which he supposes. But I need
not enter into this subject here, as the reader will find the whole
question discussed at length in the chapter above referred to. By far
the most important part of Adhemar’s theory, however, is his conception
of the submergence of the land by means of a polar ice-cap. He appears
to have been the first to put forth the idea that a mass of ice placed
on the globe, say, for example, at the south pole, will shift the
earth’s centre of gravity a little to the south of its former position,
and thus, as a physical consequence, cause the sea to sink at the
north pole and to rise at the south. According to Adhémar, as the one
hemisphere cools and the other grows warmer, the ice at the pole of the
former will increase in thickness and that at the pole of the latter
diminish.
The sea, as a consequence, will sink on the warm hemisphere where the
ice is decreasing and rise on the cold hemisphere where the ice is
increasing. And, again, in 10,000 years, when the climatic conditions
of the two hemispheres are reversed, the sea will sink on the
hemisphere where it formerly rose, and rise on the hemisphere where it
formerly sank, and so on in like manner through indefinite ages.
Adhémar, however, acknowledges to have derived the grand conception
of a submergence of the land from the shifting of the earth’s centre
of gravity from the following wild speculation of one Bertrand, of
Hamburgh:—
“Bertrand de Hambourg, dans un ouvrage imprimé en 1799 et qui a
pour titre: _Renouvellement périodique des Continents_, avait déjà
émis cette idée, que la masse des eaux pouvait être alternativement
entraînée d’un hémisphère à l’autre par le déplacement du centre de
gravité du globe. Or, pour expliquer ce déplacement, il supposait que
la terre était creuse et qu’il y avait dans son intérieur un gros noyau
d’aimant auquel les comètes par leur attraction communiquaient un
mouvement de va-et-vient analogue à celui du pendule.”—_Révolutions de
la Mer_, p. 41.
The somewhat extravagant notions which Adhémar has advanced in
connection with his theory of submergence have very much retarded
its acceptance. Amongst other remarkable views he supposes the polar
ice-cap to rest on the bottom of the ocean, and to rise out of the
water to the enormous height of twenty leagues. Again, he holds that
on the winter approaching perihelion and the hemisphere becoming warm
the ice waxes soft and rotten from the accumulated heat, and the sea
now beginning to eat into the base of the cap, this is so undermined
as, at last, to be left standing upon a kind of gigantic pedestal. This
disintegrating process goes on till the fatal moment at length arrives,
when the whole mass tumbles down into the sea in huge fragments which
become floating icebergs. The attraction of the opposite ice-cap, which
has by this time nearly reached its maximum thickness, becomes now
predominant. The earth’s centre of gravity suddenly crosses the plain
of the equator, dragging the ocean along with it, and carrying death
and destruction to everything on the surface of the globe. And these
catastrophes, he asserts, occur alternately on the two hemispheres
every 10,000 years.—_Révolutions de la Mer_, pp. 316−328.
Adhémar’s theory has been advocated by M. Le Hon, of Brussels, in a
work entitled _Périodicité des Grands Déluges_. Bruxelles et Leipzig,
1858.
II.
ON THE NATURE OF HEAT-VIBRATIONS.[323]
From the _Philosophical Magazine_ for May, 1864.
In a most interesting paper on “Radiant Heat,” by Professor Tyndall,
read before the Royal Society in March last, it is shown conclusively
that the _period_ of heat-vibrations is not affected by the state
of aggregation of the molecules of the heated body; that is to say,
whether the substance be in the gaseous, the liquid, or, perhaps, the
solid condition, the tendency of its molecules to vibrate according to
a given period remains unchanged. The force of cohesion binding the
molecules together exercises no effect on the rapidity of vibration.
I had arrived at the same conclusion from theoretical considerations
several years ago, and had also deduced some further conclusions
regarding the nature of heat-vibrations, which seem to be in a measure
confirmed by the experimental results of Professor Tyndall. One of
these conclusions was, that the heat-vibration does not consist in
a motion of an aggregate mass of molecules, but in a motion of the
individual molecules themselves. Each molecule, or rather we should
say each atom, acts as if there were no other in existence but
itself. Whether the atom stands by itself as in the gaseous state,
or is bound to other atoms as in the liquid or the solid state, it
behaves in exactly the same manner. The deeper question then suggested
itself, viz., what is the nature of that mysterious motion called heat
assumed by the atom? Does it consist in excursions across centres
of equilibrium external to the atom itself? It is the generally
received opinion among physicists that it does. But I think that the
experimental results arrived at by Professor Tyndall, as well as some
others which will presently be noticed, are entirely hostile to such an
opinion. The relation of an atom to its centre of equilibrium depends
entirely on the state of aggregation. Now if heat-vibrations consist in
excursions to and fro across these centres, then the _period_ ought to
be affected by the state of aggregation. The higher the _tension_ of
the atom in regard to the centre, the more rapid ought its movement to
be. This is the case in regard to the vibrations constituting sound.
The harder a body becomes, or, in other words, the more firmly its
molecules are bound together, the higher is the _pitch_. Two harp-cords
struck with equal force will vibrate with equal force, however much
they may differ in the rapidity of their vibrations. The _vis viva_
of vibration depends upon the force of the stroke; but the rapidity
depends, not on the stroke, but upon the tension of the cord.
That heat-vibrations do not consist in excursions of the molecules
or atoms across centres of equilibrium, follows also as a necessary
consequence from the fact that the real specific heat of a body remains
unchanged under all conditions. All changes in the specific heat of a
body are due to differences in the amount of heat consumed in molecular
work against cohesion or other forces binding the molecules together.
Or, in other words, to produce in a body no other effect than a given
rise of temperature, requires the same amount of force, whatever may be
the physical condition of the body. Whether the body be in the solid,
the fluid, or the gaseous condition, the same rise of temperature
always indicates the same quantity of force consumed in the simple
production of the rise. Now, if heat-vibrations consist in excursions
of the atom to and fro across a centre of equilibrium _external to
itself_, as is generally supposed, then the _real_ specific heat of a
solid body, for example, _ought to decrease with the hardness of the
body_, because an increase in the strength of the force binding the
molecules together would in such a case tend to favour the rise in the
rapidity of the vibrations.
These conclusions not only afford us an insight into the hidden nature
of heat-vibrations, but they also appear to cast some light on the
physical constitution of the atom itself. They seem to lead to the
conclusion that the ultimate atom itself is _essentially elastic_.[324]
For if heat-vibrations do not consist in excursions of the atom, then
it must consist in alternate expansions and contractions of the atom
itself. This again is opposed to the ordinary idea that the atom is
essentially solid and impenetrable. But it favours the modern idea,
that matter consists of forces of resistance acting from a centre.
Professor Tyndall in a memoir read before the Royal Society “On a new
Series of Chemical Reactions produced by Light,” has subsequently
arrived at a similar conclusion in reference to the atomic nature of
heat-vibrations. The following are his views on the subject:—
“A question of extreme importance in molecular physics here
arises:—What is the real mechanism of this absorption, and where is its
seat?
“I figure, as others do, a molecule as a group of atoms, held together
by their mutual forces, but still capable of motion among themselves.
The vapour of the nitrite of amyl is to be regarded as an assemblage
of such molecules. The question now before us is this:—In the act
of absorption, is it the _molecules_ that are effective, or is it
their constituent _atoms?_ Is the _vis viva_ of the intercepted waves
transferred to the molecule as a whole, or to its constituent parts?
“The molecule, as a whole, can only vibrate in virtue of the forces
exerted between it and its neighbour molecules. The intensity of these
forces, and consequently the rate of vibration, would, in this case,
be a function of the distance between the molecules. Now the identical
absorption of the liquid and of the vaporous nitrite of amyl indicates
an identical vibrating period on the part of liquid and vapour, and
this, to my mind, amounts to an experimental demonstration that the
absorption occurs in the main _within_ the molecule. For it can hardly
be supposed, if the absorption were the act of the molecule as a whole,
that it could continue to affect waves of the same period after the
substance had passed from the vaporous to the liquid state.”—_Proc. of
Roy. Soc._, No. 105. 1868.
Professor W. A. Norton, in his memoir on “Molecular Physics,”[325] has
also arrived at results somewhat similar in reference to the nature of
heat-vibrations. “It will be seen,” he says, “that these (Mr. Croll’s)
ideas are in accordance with the conception of the constitution of a
molecule adopted at the beginning of the present memoir (p. 193), and
with the theory of heat-vibrations or heat-pulses deduced therefrom (p.
196).”[326]
III.
ON THE REASON WHY THE DIFFERENCE OF READING BETWEEN A
THERMOMETER EXPOSED TO DIRECT SUNSHINE AND ONE SHADED
DIMINISHES AS WE ASCEND IN THE ATMOSPHERE.[327]
From the _Philosophical Magazine_ for March, 1867.
The remarkable fact was observed by Mr. Glaisher, that the difference
of reading between a black-bulb thermometer exposed to the direct rays
of the sun and one shaded diminishes as we ascend in the atmosphere.
On viewing the matter under the light of Professor Tyndall’s important
discovery regarding the influence of aqueous vapour on radiant heat,
the fact stated by Mr. Glaisher appears to be in perfect harmony with
theory. The following considerations will perhaps make this plain.
The shaded thermometer marks the temperature of the surrounding
air; but the exposed thermometer marks not the temperature of the
air, but that of the bulb heated by the direct rays of the sun. The
temperature of the bulb depends upon two elements: (1) the rate at
which it receives heat by _direct radiation_ from the sun above, the
earth beneath, and all surrounding objects, and by _contact_ with the
air; (2) the rate at which it loses heat by radiation and by contact
with the air. As regards the heat gained and lost by contact with the
surrounding air, both thermometers are under the same conditions,
or nearly so. We therefore require only to consider the element of
radiation.
We begin by comparing the two thermometers at the earth’s surface, and
we find that they differ by a very considerable number of degrees.
We now ascend some miles into the air, and on again comparing the
thermometers we find that the difference between them has greatly
diminished. It has been often proved, by direct observation, that the
intensity of the sun’s rays increases as we rise in the atmosphere.
How then does the exposed thermometer sink more rapidly than the
shaded one as we ascend? The reason is obviously this. The temperature
of the thermometers depends as much upon the rate at which they are
losing their heat as upon the rate at which they are gaining it.
The higher temperature of the exposed thermometer is the result of
_direct radiation_ from the sun. Now, although this thermometer
receives by radiation more heat from the sun at the upper position
than at the lower, it does not necessarily follow on this account
that its temperature ought to be higher. Suppose that at the upper
position it should receive one-fourth more heat from the sun than at
the lower, yet if the rate at which it loses its heat by radiation
into space be, say, one-third greater at the upper position than at
the lower, the temperature of the bulb would sink to a considerable
extent, notwithstanding the extra amount of heat received. Let us now
reflect on how matters stand in this respect in regard to the actual
case under our consideration. When the exposed thermometer is at the
higher position, it receives more heat from the sun than at the lower,
but it receives less from the earth; for a considerable part of the
radiation from the earth is cut off by the screen of aqueous vapour
intervening between the thermometer and the earth. But, on the whole,
it is probable that the total quantity of radiant heat reaching the
thermometer is greater in the higher position than in the lower.
Compare now the two positions in regard to the rate at which the
thermometer loses its heat by radiation. When the thermometer is at the
lower position, it has the warm surface of the ground against which to
radiate its heat downwards. The high temperature of the ground thus
tends to diminish the rate of radiation. Above, there is a screen of
aqueous vapour throwing back upon the thermometer a very considerable
part of the heat which the instrument is radiating upwards. This, of
course, tends greatly to diminish the loss from radiation. But at
the upper position this very screen, which prevented the thermometer
from throwing off its heat into the cold space above, now affects
the instrument in an opposite manner; for the thermometer has now to
radiate its heat downwards, not upon the warm surface of the ground
as before, but upon the cold upper surface of the aqueous screen
intervening between the instrument and the earth. This of course tends
to lower the mercury. We are now in a great measure above the aqueous
screen, with nothing to protect the thermometer from the influence of
cold stellar space. It is true that the air above is at a temperature
little below that of the thermometer itself; but then the air is dry,
and, owing to its diathermancy, it does not absorb the heat radiated
from the thermometer, and consequently the instrument radiates its heat
directly into the cold stellar space above, some hundreds of degrees
below zero, almost the same as it would do were the air entirely
removed. The enormous loss of heat which the thermometer now sustains
causes it to fall in temperature to a great extent. The molecules of
the comparatively dry air at this elevation, being very bad radiators,
do not throw off their heat into space so rapidly as the bulb of the
exposed thermometer; consequently their temperature does not (for this
reason) tend to sink so rapidly as that of the bulb. Hence the shaded
thermometer, which indicates the temperature of those molecules, is
not affected to such an extent as the exposed one. Hence also the
difference of reading between the two instruments must diminish as we
rise in the atmosphere.
This difference between the temperature of the two thermometers
evidently does not go on diminishing to an indefinite extent. Were we
able to continue our ascent in the atmosphere, we should certainly
find that a point would be reached beyond which the difference of
reading would begin to increase, and would continue to do so till the
outer limits of the atmosphere were reached. The difference between
the temperatures of the two thermometers beyond the limits of the
atmosphere would certainly be enormous. The thermometer exposed to
the direct rays of the sun would no doubt be much colder than it had
been when at the earth’s surface; but the shaded thermometer would
now indicate the temperature of space, which, according to Sir John
Herschel and M. Pouillet, is more than 200° Fahrenheit below zero.
It follows also, from what has been stated, that even under direct
sunshine the removal of the earth’s atmosphere would tend to lower the
temperature of the earth’s surface to a great extent. This conclusion
also follows as an immediate inference from the fact that the earth’s
atmosphere, as it exists at present charged with aqueous vapour,
affects terrestrial radiation more than it does radiation from the sun;
for the removal of the atmosphere would increase the rate at which the
earth throws off its heat into space more than it would increase the
rate at which it receives heat from the sun; therefore its temperature
would necessarily fall until the rate of radiation _from_ the earth’s
surface exactly equalled the rate of radiation _to_ the surface. Let
the atmosphere again envelope the earth, and terrestrial radiation
would instantly be diminished; the temperature of the earth’s surface
would therefore necessarily begin to rise, and would continue to do so
till the rate of radiation from the surface would equal the rate of
radiation received by the surface. Equilibrium being thus restored, the
temperature would remain stationary. It is perfectly obvious that if we
envelope the earth with a substance such as our atmosphere, that offers
more resistance to terrestrial radiation than to solar, the temperature
of the earth’s surface must necessarily rise until the heat which is
being radiated off equals that which is being received from the sun.
Remove the air and thus get quit of the resistance, and the temperature
of the surface would fall, because in this case a lower temperature
would maintain equilibrium.
It follows, therefore, that the moon, which has no atmosphere, must
be much colder than our earth, even on the side exposed to the sun.
Were our earth with its atmosphere as it exists at present removed to
the orbit of Venus or Mars, for example, it certainly would not be
habitable, owing to the great change of temperature that would result.
But a change in the physical constitution of the atmospheric envelope
is really all that would be necessary to retain the earth’s surface at
its present temperature in either position.
IV.
REMARKS ON MR. J. Y. BUCHANAN’S THEORY OF THE VERTICAL
DISTRIBUTION OF TEMPERATURE OF THE OCEAN.[328]
Since the foregoing was in type, a paper on the “Vertical Distribution
of Temperature of the Ocean,” by Mr. J. Y. Buchanan, chemist on board
the _Challenger_, has been read before the Royal Society.[329] In that
paper Mr. Buchanan endeavours to account for the great depth of warm
water in the middle of the North Atlantic compared with that at the
equator, without referring it to horizontal circulation of any kind.
The following is the theory as stated by Mr. Buchanan:—
“Let us assume the winter temperature of the surface-water to be 60° F.
and the summer temperature to be 70° F. If we start from midwinter, we
find that, as summer approaches, the surface-water must get gradually
warmer, and that the temperature of the layers below the surface must
decrease at a very rapid rate, until the stratum of winter temperature,
or 60° F., is reached; in the language of the isothermal charts, the
isothermal line for degrees between 70° F. (if we suppose that we have
arrived at midsummer) and 60° F. open out or increase their distance
from each other as the depth increases. Let us now consider the
conditions after the summer heat has begun to waver. During the whole
period of heating, the water, from its increasing temperature, has been
always becoming lighter, so that heat communication by convection with
the water below has been entirely suspended during the whole period.
The heating of the surface-water has, however, had another effect,
besides increasing its volume; it has, by evaporation, rendered it
denser than it was before, at the same temperature. Keeping in view
this double effect of the summer heat upon the surface-water, let us
consider the effect of the winter cold upon it. The superficial water
having assumed the atmospheric temperature of, say 60° F., will sink
through the warmer water below it, until it reaches the stratum of
water having the same temperature as itself. Arrived here, however,
although it has the same temperature as the surrounding water,
the two are no longer in equilibrium, for the water which has come
from the surface, has a greater density than that below at the same
temperature. It will therefore not be arrested at the stratum of the
same temperature, as would have been the case with fresh water; but it
will continue to sink, carrying of course its higher temperature with
it, and distributing it among the lower layers of colder water. At
the end of the winter, therefore, and just before the summer heating
recommences, we shall have at the surface a more or less thick stratum
of water having a nearly uniform temperature of 60° F., and below this
the temperature decreasing at a considerable but less rapid rate than
at the termination of the summer heating. If we distinguish between
_surface-water_, the temperature of which rises with the atmospheric
temperature (following thus, in direction at least, the variation of
the seasons), and _subsurface_-water, or the stratum immediately below
it, we have for the latter the, at first sight, paradoxical effect of
summer cooling and winter heating. The effect of this agency is to
diffuse the same heat to a greater depth in the ocean, the greater the
yearly range of atmospheric temperature at the surface. This effect
is well shown in the chart of isothermals, on a vertical section,
between Madeira and a position in lat. 3° 8′ N., long. 14° 49′ W. The
isothermal line for 45° F. rises from a depth of 740 fathoms at Madeira
to 240 fathoms at the above-mentioned position. In equatorial regions
there is hardly any variation in the surface-temperature of the sea;
consequently we find cold water very close to the surface all along the
line. On referring to the temperature section between the position lat.
3° 8′ N., long. 14° 49′ W., and St. Paul’s Rocks, it will be seen that,
with a surface-temperature of from 75° F. to 79° F., water at 55° F. is
reached at distances of less than 100 fathoms from the surface. Midway
between the Azores and Bermuda, with a surface-temperature of 70° F.,
it is only at a depth of 400 fathoms that we reach water of 55° F.”
What Mr. Buchanan states will explain why the mean annual temperature
of the water at the surface extends to a greater depth in the middle
of the North Atlantic than at the equator. It also explains why the
temperature from the surface downwards decreases more rapidly at the
equator than in the middle of the North Atlantic; but, if I rightly
understand the theory, it does not explain (and this is the point at
issue) why at a given depth the temperature of the water in the North
Atlantic should be higher than the temperature at a corresponding depth
at the equator. Were there no horizontal circulation the greatest
thickness of warm water would certainly be found at the equator and
the least at the poles. The isothermals would in such a case gradually
slope downwards from the poles to the equator. The slope might not be
uniform, but still it would be a continuous downward slope.
V.
ON THE CAUSE OF THE COOLING EFFECT PRODUCED ON SOLIDS BY
TENSION.[330]
From the _Philosophical Magazine_ for May, 1864.
From a series of experiments made by Dr. Joule with his usual accuracy,
he found that when bodies are subjected to tension, a cooling effect
takes place. “The quantity of cold,” he says, “produced by the
application of tension was sensibly equal to the heat evolved by its
removal; and further, that the thermal effects were proportional to
the weight employed.”[331] He found that when a weight was applied to
compress a body, a certain amount of heat was evolved; but the same
weight, if applied to stretch the body, produced a corresponding amount
of cold.
This, although it does not appear to have been remarked, is a most
singular result. If we employ a force to compress a body, and then ask
what has become of the force applied, it is quite a satisfactory answer
to be told that the force is converted into heat, and reappears in the
molecules of the body as such; but if the same force be employed to
stretch the body, it will be no answer to be told that the force is
converted into cold. Cold cannot be the force under another form, for
cold is a privation of force. If a body, for example, is compressed by
a weight, the _vis viva_ of the descending weight is transmitted to the
molecules of the body and reappears under that form of force called
heat; but if the same weight is applied so as to stretch or expand the
body, not only does the force of the weight disappear without producing
heat, but the molecules which receive the force lose part of that
which they already possessed. Not only does the force of the weight
disappear, but along with it a portion of the force previously existing
in the molecules under the form of heat. We have therefore to inquire,
not merely into what becomes of the force imparted by the weight, but
also what becomes of the force in the form of heat which disappears
from the molecules of the body itself. That the _vis viva_ of the
descending weight should disappear without increasing the heat of the
molecules is not so surprising, because it may be transformed into some
other form of force different from that of heat. For it is by no means
evident _à priori_ that heat should be the only form under which it
may exist. But it is somewhat strange that it should cause the force
previously existing in the molecules in the form of heat also to change
into some other form.
When a weight, for example, is employed to stretch a solid body, it
is evident that the force exerted by the weight is consumed in work
against the cohesion of the particles, for the entire force is exerted
so as to pull them separate from each other. But the cooling effect
which takes place shows that more force disappears than simply what
is exerted by the weight; for the cooling effect is caused by the
disappearance of force in the shape of heat from the body itself. The
force exerted by the weight disappears in performing work against the
cohesion of the particles of the body stretched. But what becomes
of the energy in the form of heat which disappears from the body at
the same time? It must be consumed in performing work of some kind
or other. The force exerted by the weight cannot be the cause of the
cooling effect. The transferrence of force from the weight to the body
may be the cause of a heating effect—an increase of force in the body;
but this transferrence of force to the body cannot be the cause of a
decrease of force in the body. If a decrease of force actually follows
the application of tension, the weight can only be the occasion, not
the cause of the decrease.
In what manner, then, does the stretching of the body by the weight
become the occasion of its losing energy in the shape of heat? Or, in
other words, what is the cause of the cooling effects which result
from tension? The probable explanation of the phenomenon seems to
be this: if the molecules of a body are held together by any force,
of whatever nature it may be, which prevents any further separation
taking place, then the entire heat applied to such a body will appear
as temperature; but if this binding force becomes lessened so as to
allow further expansion, then a portion of the heat applied will be
lost in producing expansion. All solids at any given temperature expand
until the expansive force of their heat exactly balances the cohesive
force of their molecules, after which no further expansion at the
same temperature can possibly take place while the cohesive force of
the molecules remains unchanged. But if, by some means or other, the
cohesive force of the molecules become reduced, then instantly the
body will expand under the heat which it possesses, and of course a
portion of the heat will be consumed in expansion, and a cooling effect
will result. Now tension, although it does not actually lessen the
cohesive force of the molecules of the stretched body, yet produces, by
counteracting this force, the same effect; for it allows the molecules
an opportunity of performing work of expansion, and a cooling effect
is the consequence. If the piston of a steam-engine, for example, be
loaded to such an extent that the steam is unable to move it, the steam
in the interior of the cylinder will not lose any of its heat; but if
the piston be raised by some external force, the molecules of the steam
will assist this force, and consequently will suffer loss of heat in
proportion to the amount of work which they perform. The very same
occurs when tension is applied to a solid. Previous to the application
of tension, the heat existing in the molecules is unable to produce
any expansion against the force of cohesion. But when the influence of
cohesion is partly counteracted by the tension applied, the heat then
becomes enabled to perform work of expansion, and a cooling effect is
the result.
VI.
THE CAUSE OF REGELATION.[332]
There are two theories which have been advanced to explain Regelation,
the one by Professor Faraday, and the other by Professor James Thomson.
According to Professor James Thomson, pressure is the cause of
regelation. Pressure applied to ice tends to lower the melting-point,
and thus to produce liquefaction; but the water which results is
colder than the ice, and refreezes the moment it is relieved from
pressure. When two pieces of ice are pressed together, a melting takes
place at the points in contact, resulting from the lowering of the
melting-point; the water formed, re-freezing, joins the two pieces
together.
The objection which has been urged against this theory is that
regelation will take place under circumstances where it is difficult to
conceive how pressure can be regarded as the cause. Two pieces of ice,
for example, suspended by silken threads in an atmosphere above the
melting-point, if but simply allowed to touch each other, will freeze
together. Professor J. Thomson, however, attributes the freezing to
the pressure resulting from the capillary attraction of the two moist
surfaces in contact. But when we reflect that it requires the pressure
of a mile of ice—135 tons on the square foot—to lower the melting-point
one degree, it must be obvious that the lowering effect resulting
from capillary attraction in the case under consideration must be
infinitesimal indeed.
The following clear and concise account of Faraday’s theory, I quote
from Professor Tyndall’s “Forms of Water:”—
“Faraday concluded that _in the interior_ of any body, whether solid
or liquid, where every particle is grasped, so to speak, by the
surrounding particles, and grasps them in turn, the bond of cohesion
is so strong as to require a higher temperature to change the state
of aggregation than is necessary _at the surface_. At the surface of
a piece of ice, for example, the molecules are free on one side from
the control of other molecules; and they therefore yield to heat more
readily than in the interior. The bubble of air or steam in overheated
water also frees the molecules on one side; hence the ebullition
consequent upon its introduction. Practically speaking, then, the
point of liquefaction of the interior ice is higher than that of the
superficial ice....
“When the surfaces of two pieces of ice, covered with a film of the
water of liquefaction, are brought together, the covering film is
transferred from the surface to the centre of the ice, where the point
of liquefaction, as before shown, is higher than at the surface.
The special solidifying power of ice upon water is now brought
into play _on both sides of the film_. Under these circumstances,
Faraday held that the film would congeal, and freeze the two surfaces
together.”—_The Forms of Water_, p. 173.
The following appears to be a more simple explanation of the phenomena
than either of the preceding:—
The freezing-point of water, and the melting-point of ice, as Professor
Tyndall remarks, touch each other as it were at this temperature. At
a hair’s-breadth lower water freezes; at a hair’s-breadth higher ice
melts. Now if we wish, for example, to freeze water, already just about
the freezing-point, or to melt a piece of ice already just about the
melting-point, we can do this either by a change of temperature or
by a change of the melting-point. But it will be always much easier
to effect this by the former than by the latter means. Take the
case already referred to, of the two pieces of ice suspended in an
atmosphere above the melting-point. The pieces at their surfaces are
in a melting condition, and are surrounded by a thin film of water
just an infinitesimal degree above the freezing-point. The film has on
the one side solid ice at the freezing-point, and on the other a warm
atmosphere considerably above the freezing-point. The tendency of the
ice is to lower the temperature of the film, while that of the air is
to raise its temperature. When the two pieces are brought into contact
the two films unite and form one film separating the two pieces of ice.
This film is not like the former in contact with ice on the one side
and warm air on the other. It is surrounded on both sides by solid ice.
The tendency of the ice, of course, is to lower the film to the same
temperature as the ice itself, and thus to produce solidification.
It is evident that the film must either melt the ice or the ice must
freeze the film, if the two are to assume the same temperature. But the
power of the ice to produce solidification, owing to its greater mass,
is enormously greater than the power of the film to produce fluidity,
consequently regelation is the result.
VII.
LIST OF PAPERS WHICH HAVE APPEARED IN DR. A. PETERMANN’S
_GEOGRAPHISCHE MITTHEILUNGEN_ RELATING TO THE GULF-STREAM AND
THERMAL CONDITION OF THE ARCTIC REGIONS.
The most important memoir which we have on the Gulf-stream and its
influence on the climate of the arctic regions is the one by Dr. A.
Petermann, entitled “Der Golfstrom und Standpunkt der thermometrischen
Kenntniss des nord-atlantischen Oceans und Landgebiets im Jahre 1870.”
_Geographische Mittheilungen_, Band XVI. 1870.
Dr. Petermann has, in this memoir, by a different line of argument
from that which I have pursued in this volume, shown in the most clear
and convincing manner that the abnormally high temperature of the
north-western shores of Europe and the seas around Spitzbergen is owing
entirely to the Gulf-stream, and not to any general circulation such as
that advocated by Dr. Carpenter. From a series of no fewer than 100,000
observations of temperature in the North Atlantic and in the arctic
seas, he has been enabled to trace with accuracy on his charts the very
footsteps of the heat in its passage from the Gulf of Mexico up to the
shores of Spitzbergen.
The following is a list of the more important papers bearing on the
subject which have recently appeared in Dr. Petermann’s _Geogr.
Mittheilungen_:—
An English translation of Dr. Petermann’s Memoir, and of a few more in
the subjoined list, has been published in a volume, with supplements,
by the Hydrographic Department of the United States, under the
superintendence of Commodore R. H. Wyman.
The papers whose titles are in English have appeared in the American
volume. In that volume the principal English papers on the subject,
in as far as they relate to the north-eastern extension of the
Gulf-stream, have also been reprinted.
The System of Oceanic Currents in the Circumpolar Basin of the Northern
Hemisphere. By Dr. A. Mühry. Vol. XIII., Part II. 1867.
The Scientific Results of the first German North Polar Expedition. By
Dr. W. von Freeden. Vol. XV., Part VI. 1869.
The Gulf-stream, and the Knowledge of the Thermal Properties of the
North Atlantic Ocean and its Continental Borders, up to 1870. By Dr. A.
Petermann. _Geographische Mittheilungen_, Vol. XVI., Part VI. 1870.
The Temperature of the North Atlantic Ocean and the Gulf-stream. By
Rear-Admiral C. Irminger. Vol. XVI., Part VI. 1870.
Meteorological Observations during a Winter Stay on Bear Island,
1865−1866. By Sievert Tobilson. Vol. XVI., Part VII. 1870.
Die Temperatur-verhältnisse in den arktischen Regionen. Von Dr.
Petermann. Band XVI., Heft VII. 1870.
Preliminary Reports of the Second German North Polar Expedition, and of
minor Expeditions, in 1870. Vol. XVII.
Preliminary Report of the Expedition for the Exploration of the
Nova-Zembla Sea (the sea between Spitzbergen and Nova Zembla), by
Lieutenants Weyprecht and Payer, June to September, 1871. By Dr. A.
Petermann. Vol. XVII. 1871.
Der Golfstrom ostwärts vom Nordkap. Von A. Middendorff. Band XVII.,
Heft I. 1871.
Kapitän E. H. Johannesen’s Umfahrung von Nowaja Semlä im Sommer 1870,
und norwegischer Finwalfang östlich vom Nordkap. Von Th. v. Heuglin.
Band XVII., Heft I. 1871.
Die Nordpol-Expeditionen, das sagenhafte Gillis-land und der Golfstrom
im Polarmeere. Von Dr. A. Petermann. 5 Nov. 1870.
Th. v. Heuglin’s Aufnahmen in Ost-Spitzbergen. Begleitworte zur neuen
Karte dieses Gebiets. Tafel 9. 1870. Band XVII., Heft V. 1871.
Die zweite deutsche Nordpolar-Expedition, 1869−70. Schlittenreise
an der Küste Grönlands nach Norden, 8 März−27 April, 1870. Von
Ober-Lieutenant Julius Payer. Band XVII., Heft V. 1871.
Die Entdeckung des Kaiser Franz Josef-Fjordes in Ost-Grönland, August,
1870. Von Ober-Lieutenant Julius Payer. Band XVII., Heft V. 1871.
Die Erschliessung eines Theiles des nördlichen Eismeeres durch die
Fahrten und Beobachtungen der norwegischen Seefahrer Torkildsen,
Ulve, Mack Qvale, und Nedrevaag im karischen Meere, 1870. Von Dr. A.
Petermann. Band XVII., Heft III. 1871.
Die zweite deutsche Nordpolar-Expedition, 1869−70. Schlittenreise nach
Ardencaple Inlet, 8−29 Mai, 1870. Von Ober-Lieutenant Julius Payer.
Band XVII., Heft XI. 1871.
Ein Winter unter dem Polarkreise. Von Ober-Lieutenant Julius Payer.
Band XVII., Heft XI. 1871.
Die Entdeckung eines offenen Polarmeeres durch Payer und Weyprecht im
September, 1871. Von Dr. A. Petermann. Band XVII., Heft XI. 1871.
James Lamont’s Nordfahrt, Mai-August, 1871. Die Entdeckungen von
Weyprecht, Payer, Tobiesen, Mack, Carlsen, Ulve, und Smyth im Sommer,
1871.
Stand der Nordpolarfrage zu Ende des Jahres 1871. Von Dr. A. Petermann.
Band XVII., Heft XII. 1871.
Das Innere von Grönland. Von Dr. Robert Brown. Band XVII., Heft X. 1871.
Captain T. Torkildsen’s Cruise from Tromsö to Spitzbergen, July 26 to
September 26, 1871. Vol. XVIII. 1872.
The Sea north of Spitzbergen, and the most northern Meteorological
Observations. Vol. XVIII. 1872.
Results of the Observations of the Deep-sea Temperature in the Sea
between Greenland, Northern Europe, and Spitzbergen. By Professor H.
Möhn. Vol. XVIII. 1872.
The Norwegian Cruises to Nova Zembla and the Kara Sea in 1871. Vol.
XVIII. 1872.
The Cruises in the Polar Sea in 1872. Vol. XVIII. 1872.
The Cruise of Smyth and Ulve, June 19 to September 27, 1871. Vol.
XVIII. 1872.
Die fünfmonatliche Schiffbarkeit des sibirischen Eismeeres um Nowaja
Semlja, erwiesen durch die norwegischen Seefahrer in 1869 und 1870,
ganz besonders aber in 1871. Von Dr. A. Petermann. Band XVIII., Heft X.
1872.
Die neuen norwegischen Aufnahmen des nordöstlichen Theiles von Nowaja
Semlja durch Mack, Dörma, Carlsen, u. A., 1871. Von Dr. Petermann. Band
XVIII., Heft X. 1872.
Nachrichten über die sieben zurückgekehrten Expeditionen unter Graf
Wiltschek, Altmann, Johnsen, Nilsen, Smith, Gray, Whymper; die
drei Überwinterungs-Expeditionen; die Amerikanische, Schwedische,
Österreichisch-Ungarische; und die zwei neuen: die norwegische
Winter-Expedition und diejenige unter Kapitän Mack. Von Dr. A.
Petermann. Band XVIII., Heft XII. 1872.
Konig Karl-Land im Osten von Spitzbergen und seine Erreichung und
Aufnahme durch norwegische Schiffer im Sommer 1872. Von Professor H.
Möhn. Band XIX., Heft IV. 1873.
Resultate der Beobachtungen angestellt auf der Fahrt des Dampfers
“Albert” nach Spitzbergen im November und Dezember, 1872. Von Professor
Möhn. Band XIX., Heft VII. 1873.
Die amerikanische Nordpolar-Expedition unter C. F. Hall, 1871−3. Von
Dr. A. Petermann. Band XIX., Heft VIII. 1873.
Die Trift der Hall’schen Nordpolar-Expedition, 16 August bis 15
Oktober, 1872, und die Schollenfahrt der 20 bis zum 30 April, 1873. Von
Dr. A. Petermann. Band XIX., Heft X. 1873.
Das offene Polarmeer bestätigt durch das Treibholz an der Nordwestküste
von Grönland. Von Dr. A. Petermann. Band XX., Heft V. 1874.
Das arktische Festland und Polarmeer. Von Dr. Joseph Chavanne. Band
XX., Heft VII. 1874.
Die Umkehr der Hall’schen Polar-Expedition nach den Aussagen der
Offiziere. Von Dr. A. Petermann. Band XX., Heft VII. 1874.
Die zweite österreichisch-ungarische Nordpolar-Expedition unter
Weyprecht und Payer, 1872−4. Von Dr. A. Petermann. Band XX., Heft X.
1874.
Beiträge zur Klimatologie und Meteorologie des Ost-polar-Meeres. Von
Professor Möhn. Band XX., Heft V. 1874.
Kapitän David Gray’s Reise und Beobachtungen im ost-grönländischen
Meere, 1874, und seine Ansichten über den besten Weg zum Nordpol.
Original-Mittheilungen an A. Petermann, d.D., Peterhead, Dezember,
1874. Band XXI., Heft III. 1875.
VIII.
LIST OF PAPERS BY THE AUTHOR TO WHICH REFERENCE IS MADE
IN THIS VOLUME.
On the Influence of the Tidal Wave on the Earth’s Rotation and on the
Acceleration of the Moon’s Mean Motion.—_Phil. Mag._, April, 1864.
On the Nature of Heat-vibrations.—_Phil. Mag._, May, 1864.
On the Cause of the Cooling Effect produced on Solids by
Tension.—_Phil. Mag._, May, 1864.
On the Physical Cause of the Change of Climate during Geological
Epochs.—_Phil. Mag._, August, 1864.
On the Physical Cause of the Submergence of the Land during the Glacial
Epoch.—The _Reader_, September 2nd and October 14th, 1865.
On Glacial Submergence.—The _Reader_, December 2nd and 9th, 1865.
On the Eccentricity of the Earth’s Orbit.—_Phil. Mag._, January, 1866.
Glacial Submergence on the Supposition that the Interior of the Globe
is in a Fluid Condition.—The _Reader_, January 13th, 1866.
On the Physical Cause of the Submergence and Emergence of the Land
during the Glacial Epoch, with a Note by Professor Sir William
Thomson.—_Phil. Mag._, April, 1866.
On the Influence of the Tidal Wave on the Motion of the Moon.—_Phil.
Mag._, August and November, 1866.
On the Reason why the Change of Climate in Canada since the Glacial
Epoch has been less complete than in Scotland.—_Trans. Geol. Soc. of
Glasgow_, 1866.
On the Eccentricity of the Earth’s Orbit, and its Physical Relations to
the Glacial Epoch.—_Phil. Mag._, February, 1867.
On the Reason why the Difference of Reading between a Thermometer
exposed to direct Sunshine and one shaded diminishes as we ascend in
the Atmosphere.—_Phil. Mag._, March, 1867.
On the Change in the Obliquity of the Ecliptic; its Influence on the
Climate of the Polar Regions and Level of the Sea.—_Trans. Geol. Soc.
of Glasgow_, vol. ii., p. 177. _Phil. Mag._, June, 1867.
Remarks on the Change in the Obliquity of the Ecliptic, and its
Influence on Climate.—_Phil. Mag._, August, 1867.
On certain Hypothetical Elements in the Theory of Gravitation
and generally received Conceptions regarding the Constitution of
Matter.—_Phil. Mag._, December, 1867.
On Geological Time, and the probable Date of the Glacial and the Upper
Miocene Period.—_Phil. Mag._, May, August, and November, 1868.
On the Physical Cause of the Motions of Glaciers.—_Phil. Mag._, March,
1869. _Scientific Opinion_, April 14th, 1869.
On the Influence of the Gulf-stream.—_Geol. Mag._, April, 1869.
_Scientific Opinion_, April 21st and 28th, 1869.
On Mr. Murphy’s Theory of the Cause of the Glacial Climate.—_Geol.
Mag._, August, 1869. _Scientific Opinion_, September 1st, 1869.
On the Opinion that the Southern Hemisphere loses by Radiation more
Heat than the Northern, and the supposed Influence that this has on
Climate.—_Phil. Mag._, September, 1869. _Scientific Opinion_, September
29th and October 6th, 1869.
On Two River Channels buried under Drift belonging to a Period when the
Land stood several hundred feet higher than at present.—_Trans. Geol.
Soc. of Edinburgh_, vol. i., p. 330.
On Ocean-currents: Ocean-currents in Relation to the Distribution of
Heat over the Globe.—_Phil. Mag._, February, 1870.
On Ocean-currents: Ocean-currents in Relation to the Physical Theory of
Secular Changes of Climate.—_Phil. Mag._, March, 1870.
The Boulder Clay of Caithness a Product of Land-ice.—_Geol. Mag._, May
and June, 1870.
On the Cause of the Motion of Glaciers.—_Phil. Mag._, September, 1870.
On Ocean-currents: On the Physical Cause of Ocean-currents. Examination
of Lieutenant Maury’s Theory.—_Phil. Mag._, October, 1870.
On the Transport of the Wastdale Granite Boulders.—_Geol. Mag._,
January, 1871.
On a Method of determining the Mean Thickness of the Sedimentary Rocks
of the Globe.—_Geol. Mag._, March, 1871.
Mean Thickness of the Sedimentary Rocks.—_Geol. Mag._, June, 1871.
On the Age of the Earth as determined from Tidal Retardation.—_Nature_,
August 24th, 1871.
Ocean-currents: On the Physical Cause of Ocean-currents. Examination of
Dr. Carpenter’s Theory.—_Phil. Mag._, October, 1871.
Ocean-currents: Further Examination of the Gravitation Theory.—_Phil.
Mag._, February, 1874.
Ocean-currents: The Wind Theory of Oceanic Circulation.—_Phil. Mag._,
March, 1874.
Ocean-currents.—_Nature_, May 21st, 1874.
The Physical Cause of Ocean-currents.—_Phil. Mag._, June, 1874.
_American Journal of Science and Art_, September, 1874.
On the Physical Cause of the Submergence and Emergence of the Land
during the Glacial Epoch.—_Geol. Mag._, July and August, 1874.
INDEX.
Absolute heating-power of ocean-currents, 23
〃 amount of heat received from the sun per day, 26
Adhémar, M., theory founded upon a mistake in regard to
radiation, 81, 85
〃 on submergence, 368
〃 on influence of eccentricity on climate, 542
Aërial currents increased in action by formation of snow and ice, 76
〃 function of, stated, 51
〃 heat conveyed by, 27
Africa, South, glacial and inter-glacial periods of, 242
〃 boulder clay of Permian age, 300
Age and origin of the sun, 346
Air, on absorption of rays by, 59
〃 when humid, absorbs rays which agree with it in period, 59
〃 when perfectly dry incapable of absorbing radiant heat, 59
Airy, Professor, earth’s axis of rotation permanent, 7
Aitken’s, Mr., experiment on density of polar water, 129
Aland islands, striation of, 447
Alternate cold and warm periods, 236
Allermuir, striations on summit of, 441
America, low temperature in January, 72
〃 thickness of ice-sheet of North, 381
Anderson, Captain Sir James, never observed a stone on an iceberg, 282
Antarctic regions, mean summer temperature of, below
freezing-point, 63
Antarctic ice-cap, probable thickness of, 375
〃 diagram representing thickness of, 377
〃 thickness of, estimated from icebergs, 384
Antarctic snowfall, estimates of, 382
Aphelion, glacial conditions at maximum when winter solstice is at, 77
Arago, M., on influence of eccentricity on climate, 536
Arctic climate, influence of ocean-currents on, during glacial
period, 260
Arctic regions, influence of Gulf-stream on climate of, 45
〃 mean summer temperature of, 63
Arctic regions, amount of heat received by, per unit surface, 195
〃 warm periods best marked in, 258
〃 warm inter-glacial periods in, 258−265
〃 state of, during glacial period, 260
〃 evidence of warm periods in, 261
〃 occurrence of recent trees in, 261, 265
〃 evidence of warm inter-glacial periods, 293
〃 warm climate during Old Red Sandstone period in, 295
〃 glacial period during Carboniferous age in, 297
〃 warm climate during Permian period in, 301
〃 list of papers relating to, 556
Arctic Ocean, area of, 195
〃 according to gravitation theory ought to be warmer than Atlantic
in torrid zone, 195
〃 heat conveyed into, by currents, compared with that received by it
from the sun, 195
〃 blocked up with polar ice, 444
Armagh, boulder beds of, 299
Arran, Island of, glacial conglomerate of Permian age in, 299
Astronomical causes of change of climate, 10
Astronomy and geology, supposed analogy between, 355
Atlantic, atmospheric pressure on middle of, 33
〃 inability of, to heat the south-west winds without the
Gulf-stream, 34
〃 mean annual temperature of, 36
〃 mean temperature of, raised by Gulf-stream, 36, 40
〃 isothermal lines of, compared with those of the Pacific, 46
〃 area of, from equator to Tropic of Cancer, 194
〃 inquiry whether the area of, is sufficient to supply heat
according to Dr. Carpenter’s theory, 194
Atlantic, North, heat received by, from torrid zone by currents, 194
〃 according to Dr. Carpenter’s theory ought to be warmer in
temperate regions than in the torrid zone, 195
〃 great depth of warm water in, 198
〃 North, an immense whirlpool, 216
〃 above the level of equator, 221
〃 probable antiquity of, 367
〃 from Scandinavia to Greenland probably filled with ice, 451
Atmosphere-pressure in Atlantic a cause of south-west winds, 33
Atmosphere, on difference between black-bulbed and shaded thermometer
in upper strata of, 547
Australia, evidence of ice-action in conglomerate of, 295
Ayrshire, ice-action during Silurian period in, 293
Bakewell, Mr. R., on influence of eccentricity on climate, 540
Banks’s Land, discovery of ancient forest in, 261
〃 Professor Heer, on fossilized wood of, 309
Ball, Mr., objection to Canon Moseley’s results, 501
Baltic current, 171
Baltic, glaciation of islands in, 448
Baltic glacier, passage of, over Denmark, 449
Bath, grooved rock surfaces of, 464
Bay-ice grinds but does not striate rocks, 277
Belcher, Sir E., tree dug up by, in latitude 75° N., 263
〃 carboniferous fossils found in arctic regions by, 298
Belle-Isle, Strait of, observations on action of icebergs in, 276
Bell, Mr. A., on Mediterranean forms in glacial bed at Greenock, 254
Belt, Mr. Thomas, theory of the cause of glacial epochs, 415
Bennie, Mr. James, on surface geology, 468
〃 on deposits filling buried channel, 486
Blanford, Mr., on ice-action during Carboniferous age in India, 297
Borings, evidence of inter-glacial beds from, 254
〃 examination of drift by, 467
〃 journals of, 483, 484
Boulder clays of former glacial epochs, why so rare, 269
〃 a product of land-ice, 284
〃 if formed from icebergs must be stratified, 284
〃 scarcity of fossils in, 285
〃 formed chiefly from rock on which it lies, 285
〃 of Caithness a product of land-ice, 435
〃 on summit of Allermuir, 441
Boulders, how carried from a lower to a higher level, 527
Boussingault on absorption of carbon by vegetation, 428
Britain, climate of, affected most by south-eastern portion of
Gulf-stream, 33
Brown, Dr. R., cited on Greenland ice-sheet, 378, 380
〃 on inland ice of Greenland, 284
〃 on cretaceous formation of Greenland, 305
〃 on Miocene beds of the Disco district, 310
Brown, Mr. Robert, on growth of coal plants, 421
Brown and Dickeson, on sediment of Mississippi, 330
Buchan, Mr., on atmosphere-pressure in the Atlantic, 33
〃 on force of the wind, 220
Buchanan, Mr. J. Y., on vertical distribution of heat of the
ocean, 550
Buckland, Dr., observations by, on occurrence of red chalk on
Cotteswold hills, 459
Buff, Professor, on oceanic circulation, 145
Buried river channels, 466
〃 channel from Kilsyth to Grangemouth, 468
〃 section at Grangemouth, 474
〃 from Kilsyth to Clyde, 481
〃 not excavated by sea nor by ice, 469
〃 other examples of, 488−494
Caithness, difficulty of accounting for
the origin of the boulder clay of, 435
Caithness, boulder clay of, a product of land-ice, 435
〃 boulder clay not formed by icebergs, 437
〃 theories regarding the origin of the boulder clay of, 437
〃 why the ice was forced over it, 444
〃 Professor Geikie and B. N. Peach on path of ice over, 453
Cambrian conglomerate of Islay, 292
Campbell, Mr., observations of, on icebergs, 276
〃 on supposed striation of rocks by large icebergs, 278
〃 evidence that river-ice does not striate rocks, 279
Canada, change of climate less complete than in Scotland, 71
Carboniferous period of arctic regions, 298
〃 evidence of glacial epoch during, 296−298
〃 temperate climate of, 422
Carboniferous limestone, mode of formation, 433
Carpenter’s, Dr., objections examined, 141
〃 theory, mechanics of, 145
〃 idea of a 〃vertical circulation〃 stated, 153
Carpenter’s, Dr., radical error in theory of, 155
〃 on difference of density between waters of Atlantic and
Mediterranean, 168
〃 theory, inadequacy of, 191
〃 estimate of thermal work of Gulf-stream, 199
Charpentier’s, M., theory of glacier-motion, 513
Carse clays, date of, 405
Cattegat, ice-markings on shore of, 446
Cave and river deposits, 251
Chalk, erratic blocks found in, 304
〃 _débris_, conclusion of Mr. Searles Wood, 460
_Challenger’s_ temperature-soundings at equator, 119
〃 crucial test of the wind and gravitation theories, 220
Chambers, Dr. Robert, on striated pavements, 255
〃 observations on glaciation of Gothland, 446
Champlain Lake, inter-glacial bed of, 241
Chapelhall, ancient buried channel at, 491
〃 inter-glacial sand-bed, 244
Chart showing the agreement between system of currents and system
of winds, 212
Christianstadt, crossed by Baltic glacier, 450
Circulation without difference of level, 176
Climate, Secular changes of, intensified by reaction of physical
causes, 75, 76
〃 affected most by temperature of the surface of ground, 88
〃 ocean-currents in relation to, 226
〃 cold conditions of, inferred from absence of fossils, 288
〃 cold condition of, difficulty of determining, from fossil
remains, 289
〃 warm, of arctic regions during Old Red Sandstone period, 295
〃 rough sketch of the history of, during the last 60,000 years, 409
〃 of Coal period inter-glacial in character, 420
〃 alternate changes of, during Coal period, 426
Climates, Mr. J. Geikie on difficulty of detecting evidence of ancient
glacial conditions, 289
〃 evidence of, from ancient sea-bottoms, 289
Coal an inter-glacial formation, 420
Coal beds, alternate submergence and emergence during formation
of, 424
〃 preservation of, by submergence, 426
Coal period, flatness of the land during, 430
Coal plants, conditions necessary for, preservation of, 423
Coal seams, thickness of, indicative of length of inter-glacial
periods, 428
Coal seams, time occupied in formation of, 429
Coal strata, on absence of ice-action in, 429
Coal measures, oscillations of sea-level during formation of, 425
Cold periods best marked in temperate regions, 258
Colding, Dr., oceanic circulation, 95
Confusion of ideas in reference to the agency of polar cold, 179
Continental ice, inadequate conceptions of, 385
〃 absence of, during glacial epochs of Coal period, 432
Contorted drift near Musselburgh, 465
Cook, Captain, description of Sandwich Land by, 60
〃 on South Georgia, 60
Cornwall, striated rocks of, 464
Cotteswold hills, red chalk from Yorkshire found on, 459
Couthony, Mr., on action of icebergs, 275
Coutts, Mr. J., on buried channel, 493
Craig, Mr. Robert, on inter-glacial beds at Overton Hillhead and
Crofthead, 247
Craiglockhart hill, inter-glacial bed of, 245
“Crawling” theory considered, 507
“Crevasses,” origin of, according to molecular theory, 521
Cretaceous period, evidence of ice-action during, 303−305
Cretaceous age, evidence of warm periods during, 304
Cretaceous formation of Greenland, 305
Crofthead, inter-glacial bed at, 248
Cromer forest bed, 250
Crosskey, Rev. Mr., comparison of Clyde and Canada shell beds, 71
〃 on southern shells in Clyde beds, 253
Croydon, block of granite found in chalk at, 303
Crucial test of the wind and gravitation theories, 220
Crystallization, force of, a cause of glacier-motion, 523
Currents, effects of their stoppage on temperatures of equator and
poles, 42
〃 produced by saltness neutralize those produced by temperature, 106
Dalager, excursion in Greenland by, 378
Dana, Professor, on action of icebergs, 275
〃 on striations by icebergs, 275
〃 on thickness of ice-sheet of North America, 381
Darwin, Mr., on alternate cold and warm periods, 231
〃 on migration of plants and animals during glacial epoch, 395
〃 on peat of Falkland Islands, 422
Date of the 40-foot beach, 409
Date when conditions were favourable to formations of the Carse
clay, 409
Davis’ Straits, current of, 132
Dawkins, Mr. Boyd, on the animals of cave and river deposits, 251
Dawson, Principal, on esker of Carboniferous age, 296
〃 on habitats of coal plants, 424
Deflection of ocean-currents chief cause of change of climate, 68
De la Beche, Sir H. T., on influence of eccentricity on climate, 539
De Mairan, on influence of eccentricity on climate, 528
Denmark, crossed by Baltic glacier, 449−452
Denudation, method of measuring rate of, 329
〃 as a measure of geological time, 329
〃 measured by sediment of Mississippi, 330
〃 subaërial rate of, 331
〃 law which determines rate of, 333
〃 marine, trifling, 337
Deposition, rates of, generally adopted, quite arbitrary, 360
〃 rate of, determined by rate of denudation, 362
〃 range of, restricted to a narrow fringe surrounding the
continents, 364
〃 area of, 365
〃 during glacial epoch probably less than present, 366
Deposits from icebergs cannot be wholly unstratified, 437
Despretz, tables by, of temperature of maximum density of
sea-water, 117
Desor, M., on tropical fauna of the Eocene formation in
Switzerland, 306
Derbyshire, breaks in limestone of, marks of cold periods, 434
Derbyshire limestone a product of inter-glacial periods, 434
Devonshire, boulder clay discovered in, 463
Diagram illustrating descent of water from equator to poles, 155
〃 showing variations of eccentricity, 313
〃 illustrative of fluidity of interior of the earth, 396
〃 showing formation of coal beds, 426
Dick, Mr., chalk flints in boulder clay, 454
Dick, Mr. R., on buried channel, 491
Difference of level essential to gravitation theory, 176
Dilatation of sea-water by increase of temperature calculated by Sir
John Herschel, 116
Disco district, Dr. R. Brown cited on Miocene beds of, 310
Disco Island, Upper Miocene period of, 307−308
Distribution, how effected by ocean-currents, 231
Dove, Professor, method of constructing normal temperature tables
by, 40
〃 on mean annual temperature, 401
Dover, mass of coal imbedded in chalk found at, 303
Drayson, Lieutenant-Colonel, on obliquity of ecliptic, 410
Drayson, Lieutenant-Colonel, theory of the cause of the glacial
epoch, 410
Drift, examination by borings, 467
Drumry, deep surface deposits at, 482
Dubuat’s, M., experiments, 182
〃 experiments by, on water flowing down an incline, 120
Duncan, Captain, on under current in Davis’ Strait, 134
Dürnten lignite beds, 240
Dürnten beds an example of inter-glacial coal formation, 433
Durham, buried river channel at, 488
Earth’s axis of rotation permanent, 7
Earth, mean temperature of, increased by water at equator, 30
〃 not habitable without ocean-currents, 54
〃 mean temperature of, greatest in aphelion, 77, 78
〃 centre of gravity of, effects of ice-cap on, 370, 371
Eccentricity of the earth’s orbit, Mr. Stockwell’s researches
regarding, 54
〃 primary cause of change of climate, 54
〃 primary cause of glacial epochs, 77
〃 how it affects the winds, 228
〃 tables of, 314−321
〃 its influence on temperature, 323
〃 explanation of tables of, 324
〃 De Marian, on influence of, on climate, 528
〃 Sir J. F. Herschel, on influence of, on climate, 529
〃 Œpinus, on influence of, on climate, 529
〃 R. Kirwan, on influence of, on climate, 529
〃 of planetary orbits, superior limits as determined by Lagrange,
Leverrier, and Mr. Stockwell, 531
〃 Sir Charles Lyell, on influence of, on climate, 529, 535
〃 M. Arago, on influence of, on climate, 536
〃 Baron Humboldt, on influence of, on climate, 538
〃 Sir H. T. de la Beche, on influence of, on climate, 539
〃 Professor Phillips, on influence of, on climate, 539
〃 Mrs. Somerville, on influence of, on climate, 540
〃 L. W. Meech, on influence of, on climate, 540
〃 Mr. R. Bakewell, on influence of, on climate, 540
〃 M. Jean Reynaud, on influence of, on climate, 541
〃 M. Adhémar, on influence of, on climate, 542
Equator, reduction of level by denudation, 336
Ecliptic, supposed effect of a change of obliquity of, 8
〃 changes of, effects on climate, 398−417
〃 obliquity of, Lieutenant-Colonel Drayson on, 410
Emergence, physical cause of, 368
England, inter-glacial beds of, 249
〃 glacial origin of Old Red Sandstone of, 294
〃 ice-action during Permian period in, 298
〃 North of, ice-sheet of, 456
〃 ice-sheet of South of, 463
Eocene period, total absence of fossils in flysch, 286
〃 glacial epoch of, 305
Eocene and Miocene periods, date of, 357
Equator, heat received per square mile at, 26
〃 temperature of earth increased by water at, 30
〃 and poles, effects of stoppage of currents on temperature of, 42
〃 surface-currents warmer than the under currents, 92
〃 heat transferred by currents from southern hemisphere compared
with that received by land at, 93
〃 temperature soundings at, 119
〃 temperature of sea at, decreases most rapidly at the surface, 119
〃 heat received by the three zones compared with that received by
the, 194
〃 migration across, 234
〃 glaciation of, 234
Equatorial current, displacement of, 229
Erratic blocks in stratified rocks, evidence of former land-ice, 269
〃 in chalk, 304
〃 why not found in coal strata, 432
Erratics extend further south in America than in Europe, 72
Etheridge, R., jun., on glacial conglomerate in Australia of Old Red
Sandstone age, 295
Europe, influence of Gulf-stream on climate of, 31
〃 effect of deflection of Gulf-stream on condition of, 68
〃 glacial condition of, if Gulf-stream was stopped, 71
〃 river systems of, unaltered since glacial period, 393
Faraday, Professor, on cause of regelation, 554
Faroe Islands glaciated by land-ice from Scandinavia, 450
Ferrel, Mr., on Dr. Carpenter’s theory, 126
〃 argument from the tides, 184
Findlay, Mr. A. G., objection by, considered, 31, 203
〃 estimate of heat conveyed by Gulf-stream, 206
Fisher, Rev. O., on the 〃trail〃 of Norwich, 251
〃 on glacial submergence, 387
Fitzroy, Admiral, on temperature of Atlantic, 36
Fluid molecules crystallize in interstices, 523
Fluvio-marine beds of Norwich, 250
“Flysch” of Eocene period, absence of fossils in, 286
〃 of Switzerland of glacial origin, 306
Fogs prevent the sun’s heat from melting ice and snow in arctic
regions, 60
Forbes, Professor J. D., method adopted by, of ascertaining
temperatures, 48
〃 on temperature of equator and poles, 48
〃 on the conductivity of different kinds of rock, 86
〃 on underground temperature, 86
〃 experiments by, on the power of different rocks to store up
heat, 86
Forest bed of Cromer, 250
Former glacial periods, 266−310
〃 why so little known of, 266
〃 geological evidence of, 292
France, evidence of ice-action during Carboniferous period in, 296
Fraserburgh, glaciation of, 450
〃 crossed by North Sea ice, 454
Fundamental problem of geology, 1
Ganges, amount of sediment conveyed by, 331
Gases, radiation of, 38
Gastaldi, M., on the Miocene glacial epoch of Italy, 306
Geikie, Professor, on geological agencies, 1
〃 on inter-glacial beds of Scotland, 243
〃 remarks on inter-glacial beds, 245
〃 on striated pavements, 256
〃 on ice-markings on Scandinavian coast, 281
〃 striated stones found in carboniferous conglomerate by, 296
〃 on sediment of European rivers, 332
〃 on modern denudation, 332
〃 suggestion regarding the loess, 452
〃 on striation of Caithness, 453
〃 on buried channel at Chapelhall, 491
〃 and Mr. James, on glacial conglomerate of Lower Carboniferous
age, 296
Geikie, Mr. James, on Crofthead inter-glacial bed, 248
〃 on the gravels of Switzerland, 268
〃 on difficulty of recognising former glacial periods, 289
〃 on Cambrian conglomerate of north-west of Scotland, 293
〃 on ice-action in Ayrshire during Silurian period, 293
〃 on boulder conglomerate of Sutherland, 301
〃 on buried channels, 492
Geogr. Mittheilungen, list of papers in, relating to arctic
regions, 556
Geological agencies climatic, 2
Geological principle, nature of, 4
Geological climates, theories of, 6
Geological time, 311−359
〃 measurable from astronomical data, 311
〃 why it has been over-estimated, 325
〃 method of measuring, 328, 329
〃 Professor Ramsay on, 343
Geology, fundamental problem of, 1
〃 a dynamical science, 5
〃 and astronomy, supposed analogy between, 355
German Polar Expedition on density of polar water, 151
〃 list of papers relating to, 556
German Ocean once dry land, 479
Germany, Professor Ramsay on Permian breccia of, 300
Gibraltar current, Dr. Carpenter’s theory of, 167
〃 cause of, 215
Glacial conditions increased by reaction of various physical
causes, 75
〃 reach maximum when winter solstice arrives at aphelion, 77
Glacial epoch, date of, 327
〃 circumstances which show recent date of, 341
〃 Mr. Belt’s theory of cause of, 415
Glacial epochs dependent upon deflection of ocean-currents, 68
〃 caused primarily by eccentricity, 77
〃 why so little known of, formerly, 266
〃 boulder clays of former, why so rare, 269
〃 geological evidence of former, 292
Glacial period in America more severe than in Western Europe, 73
〃 mean temperature of the earth greatest at aphelion during, 78
〃 records of, fast disappearing, 270
〃 of the Eocene formation, 305
Glacial periods, indirect evidence of, in Eocene and Miocene
formations, 287
〃 difficulty of determining, from fossil remains, 289
Glacial submergence resulting from displacement of the earth’s centre
of gravity, 389
Glaciation a cause of submergence, 390
〃 remains of, found chiefly on land surfaces, 267
〃 of Scandinavia inexplicable by theory of local glaciers, 448
Glacier des Bois, 497
Glacier-motion, Canon Moseley’s theory of, 507
〃 Professor James Thomson’s theory of, 512
〃 M. Charpentier’s theory of, 513
〃 molecular, 516
Glacier-motion, present state of the question, 514
〃 molecular theory of, 514−527
〃 heat necessary to, 515
〃 due to force of crystallization, 523
〃 due chiefly to internal molecular pressure, 523
Glaciers, pressure exerted by, 274
〃 physical cause of the motion of, 495−527
〃 difficulties in accounting for motion of, 495
Glasgow, actual January temperature of, 28° above normal, 72
Godwin-Austen, Mr., on ice-action during the Carboniferous period in
France, 296
〃 on evidence of ice-action during Cretaceous period, 303
〃 on mass of coal found in chalk at Dover, 304
〃 on the flatness of the land during Coal period, 430
Gothland, glaciation of, 446
Grangemouth, buried river channel at, 468
〃 surface-drift of, 484
Gravitation, the whole work of, performed by descent of water down the
slope, 154
〃 of sun’s mass, 348
〃 insufficient to account for sun’s heat, 349, 350
Gravitation theory, its relation to the theory of Secular changes of
climate, 97
〃 three modes of determining it, 115
〃 mechanics of, 145
〃 of the Gibraltar current, 167
〃 inadequacy of, 191
〃 _crucial_ test of, 220
〃 of the sun’s heat, 346−355
Gravity, force of, impelling water from equator to poles, 119, 120
〃 force of, insensible at a short distance below the surface, 120
〃 work performed by, 150
〃 diagram illustrating the action of, in producing currents, 155
〃 amount of work performed by, due solely to _difference_ of
temperature between equatorial and polar waters, 164
〃 specific difference in, between water of Atlantic and
Mediterranean insufficient to produce currents, 169
〃 centre of, displacement, by polar ice-cap, 368
Greenland, summer warm if free from ice, 59
〃 receives as much heat in summer as England, 66
〃 continental ice free from clay or mud, 284
〃 North, warm climate during Oolitic period in, 302
〃 Cretaceous formation of, 305
Greenland, evidence of warm conditions during Miocene period in, 307
〃 Professor Heer cited on Miocene flora of, 308, 309
〃 state of, during glacial period, 259
〃 effect of removal of ice from, 260
Greenland ice-sheet, probable thickness of, 378
〃 invaded the American continent, 445
Greenland inland ice, 379
Gulf-stream, estimate of its volume, 24
〃 United States’ coast survey of, 24
〃 absolute amount of heat conveyed by, 25, 26
〃 heat conveyed by, compared with that carried by aërial
currents, 27
〃 heat conveyed by, compared with that received by the frigid zone
from the sun, 27
〃 influence on climate of Europe, 31
〃 efficiency of, due to the slowness of its motion, 32
〃 climate of Britain influenced by south-eastern portion of, 33
〃 heat conveyed by, compared with that derived by temperate regions
from the sun, 34
〃 heat of, expressed in foot-pounds of energy, 35
〃 mean temperature of Atlantic increased one-fourth by, 36
〃 the only current that can heat arctic regions, 45
〃 influence of, on climate of arctic regions, 45
〃 the compensating warm current, 46
〃 palæontological objections to influence of, 53
〃 agencies which deflect the, in glacial periods, 69
〃 result, if stopped, 71
〃 large portion of the heat derived from southern hemisphere, 94
〃 Lieut. Maury on propulsion of, by specific gravity, 102
〃 contradictory nature of, the causes supposed by Lieut. Maury for
the, 110
〃 higher temperature of, considered by Lieut. Maury as the real
cause of its motion, 111
〃 amount of heat conveyed by, not over-estimated, 197
〃 amount of heat conveyed by, 192
〃 amount of heat conveyed by, compared with that by general oceanic
circulation, 194
〃 heat conveyed by, compared with that received by torrid zone from
the sun, 194
〃 heat conveyed by, into Arctic Ocean compared with that received by
it from the sun, 195
〃 Capt. Nares’s observations of, 198
〃 Dr. Carpenter’s estimate of the thermal work of, 199
Gulf-stream, volume and temperature of, according to Mr. A. G.
Findlay, 203, 206
〃 erroneous notion regarding depth of, 207
〃 list of papers relating to, 556
Haughton, Professor, on recent trees in arctic regions, 263
〃 on fragments of granite in carboniferous limestone, 296
〃 on coal beds of arctic regions, 298
〃 on _Ammonites_ of Oolitic period in arctic regions, 303
Hayes, Dr., on Greenland ice-sheet, 379
Heat received from the sun per day, 26
〃 received by temperate regions from the sun, 34
〃 radiant, absorbed by ice remains insensible, 60
〃 sun’s, amount of, stored up in ground, 87
〃 transferred from southern to northern hemisphere, 93
〃 internal, supposed influence of, 176
〃 received by the three zones compared with that received by the
equator, 194
〃 amount radiated from the sun, 346
〃 received by polar regions 11,700 years ago, 403
〃 necessary to glacier-motion, 515
〃 how transmitted through ice, 517
Heat-vibrations, nature of, 544
Heath, Mr. D. D., on glacial submergence, 387
Heer, Professor, on Dürnten lignite beds, 241
〃 on Miocene flora of Greenland, 308−310
〃 on Miocene flora of Spitzbergen, 309
Hills, ice-markings on summits of, as evidence of continental ice, 458
Helmholtz’s gravitation theory of sun’s heat, 348
Henderson, Mr. John, on inter-glacial bed at Redhall quarry, 247
Herschel, Sir John, on influence of eccentricity, 11
〃 estimate of the Gulf-stream by, 25
〃 on the amount of the sun’s heat, 26
〃 on inadequacy of specific gravity to produce ocean-currents, 116
〃 his objections to specific gravity not accepted, 117
〃 on influence of eccentricity on climate, 529
Home, Mr. Milne, on buried river channels, 478
Hooker, Sir W., on tree dug up by Capt. Belcher, 264
Hooker, Dr., on preponderance of ferns among coal plants, 421
Horne, Mr. J., on conglomerates of Isle of Man, 295
Hoxne, inter-glacial bed of, 241
Hudson’s Bay, low mean temperature of, in June, 62
Hull, Professor, on ice-action during Permian age in Ireland, 299
〃 on equable temperature of Coal period, 421
〃 on estuarine origin of coal measures, 424
Hull, buried channel at, 489
Humboldt, Baron, on loss of heat from radiation, 82
〃 on rate of growth of coal, 429
〃 on influence of eccentricity on climate, 538
Humphreys and Abbot on sediment of Mississippi, 330
Ice, latent heat of, 60
Ice, effects of removal of, from polar regions, 64
〃 heat absorbed by, employed wholly in mechanical work, 60
〃 slope necessary for motion of continental, 375
〃 does not shear in the solid state, 516
〃 how heat is transmitted through, 517
〃 how it can ascend a slope, 525
〃 how it can excavate a rock basin, 525
Icebergs do not striate sea-bottom, 272
〃 markings made by, are soon effaced, 273
〃 exerting little pressure perform little work, 273
〃 behaviour of, when stranded, 274
〃 action of, on sea-bottoms, 274
〃 rocks ground smooth, but not striated by, 276
〃 stones seldom seen on, 281
〃 evidence of, in Miocene formation of Italy, 307
〃 comparative thickness of arctic and antarctic, 381
〃 great thickness of antarctic, 382
Ice-cap, effects of, on the earth’s centre of gravity, 369
〃 probable thickness of antarctic, 375
〃 evidence from icebergs as to thickness of antarctic, 383−385
Ice-markings, modern, observed by Sir Charles Lyell, 280
Ice-sheet, probable thickness of in Greenland, 380
〃 of north of England, 456
Ice-worn pebbles found on summit of Allermuir, 441
Iceland, lignite of Miocene age in, 308
〃 probably glaciated by land-ice from North Greenland, 451
India, evidences of glacial action of Carboniferous age in, 297
Indian Ocean, low temperature at bottom, 123
Internal heat, no influence on climate, 6
〃 supposed influence of, 176
Inter-tropical regions, greater portion of moisture falls as rain, 29
Inter-glacial bed at Slitrig, 243
〃 at Chapelhall, 244
〃 of Craiglockhart hill, 245
〃 at Kilmaurs, 248
Inter-glacial beds, Professor Geikie on, 243
〃 of Dürnten, 240
〃 of Scotland, 243
〃 of England, 249
〃 at Norwich, 250
〃 evidence of, from borings, 254
Inter-glacial character of cave and river deposits, 251
Inter-glacial climate during Old Red Sandstone period in arctic
regions, 295
Inter-glacial periods, 236
〃 reason why overlooked, 237
〃 of Switzerland, 239
〃 evidence of, from shell-beds, 252
〃 evidence from striated pavements of, 255
〃 reasons why so few vestiges remain of, 257
〃 in arctic regions, 258−265
〃 of Silurian age in arctic regions, 293
〃 of Carboniferous age in arctic regions, 297
〃 of Eocene formation in Switzerland, 306
〃 formation of coal during, 420
〃 length of, indicated by thickness of coal-seams, 428
Inglefield, Captain, erect trees found in Greenland by, 309
Ireland, on ice-action during Permian age in, 299
Isbister, Mr., on carboniferous limestone of arctic regions, 297
Islay, Cambrian conglomerate of, 292
Italy, glacial epoch of Miocene period in, 306
Jack, Mr. R. L., on deflection of ice across England, 461
Jamieson, Mr. T. F., on boulder clay of Caithness, 435
〃 opinion that Caithness was glaciated by floating ice, 437
〃 on thickness of ice in the north Highlands, 439
〃 glaciation of headland of Fraserburgh, 450, 455
January temperature of Glasgow and Cumberland, difference between, 72
Jeffreys, Mr. Gwyn, on Swedish glacial shell beds, 253
Johnston, Dr. A. Keith, on coast-line of the globe, 337
Joule’s, Dr., experiments on the thermal effect of tension, 552
Judd, Mr., on boulders of Jurassic age in the Highlands, 302
Jukes, Mr., on warm climate of North Greenland during Oolitic period, 302
July, why hotter than June, 89
Kane, Dr., on mean temperature of Von Rensselaer Harbour, 62
Karoo beds, glacial character of, 301
〃 evidence of subtropical during deposition of, 301
Kelvin, ancient bed of, 481
Kielsen, Mr., excursion upon Greenland ice-sheet, by, 378
Kilmours, inter-glacial bed at, 248
Kirwan, Richard, on influence of eccentricity on climate, 529
Kyles of Bute, southern shell bed in, 253
Labrador, mean temperature of, for January, 72
〃 Mr. Packard on glacial phenomena of, 282
Lagrange, M., on eccentricity of the earth’s orbit, 54
〃 table of superior limits of eccentricity, 531
Land at equator would retain the heat at equator, 30
〃 radiates heat faster than water, 91
〃 elevation of, will not explain glacial epoch, 391
〃 submergence and emergence during glacial epoch, 368−397
〃 successive upheavals and depressions of, 391
Land-ice necessarily exerts enormous pressure, 274
〃 evidence of former, from erratic blocks on stratified
deposits, 269
Land-surfaces, remains of glaciation found chiefly on, 267
〃 (ancient) scarcity of, 268
Laplace, M., on obliquity of ecliptic, 398
Laughton, Mr., on cause of Gibraltar current, 215
Leith Walk, inter-glacial bed at, 246
Leverrier, M., on superior limit of eccentricity, 54
〃 on obliquity of ecliptic, 398
〃 table, by, of superior limits of eccentricity, 531
〃 formulæ, of, 312
Lignite beds of Dürnten, 240
Loess, origin of, 452
London, temperature of, raised 40° degrees by Gulf-stream, 43
Lomonds, ice-worn pebbles found on, 439
Lubbock, Sir J., on cave and river deposits, 252
Lucy, Mr. W. C., on glaciation of West Somerset, 463
〃 on northern derivation of drift on Cotteswold hills, 460
Lyell’s, Sir C., theory of the effect of distribution of land and
water, 8
〃 on action of river-ice, 280
〃 on tropical character of the fauna of the Cretaceous
formation, 305
〃 on warm conditions during Miocene period in Greenland, 307
〃 on influence of eccentricity, 324
〃 on sediment of Mississippi, 331
〃 on comparison of existing rocks with those removed, 362
〃 on submerged areas during Tertiary period, 392
〃 on change of obliquity of ecliptic, 418
〃 on climate best adapted for coal plants, 420
〃 on influence of eccentricity on climate, 529, 535
Mackintosh, Mr., observations on the glaciation of Wastdale Crag, 457
Magellan, Straits of, temperature at midsummer, 61
Mahony, Mr. J. A., on Crofthead inter-glacial bed, 248
Mälar Lake crossed by ice, 447
Man, Isle of, Mr. Cumming on glacial origin of Old Red Sandstone
of, 294
Mars, uncertainty as to its climatic condition, 80
〃 objection from present condition of, 79
Marine denudation trifling, 337
Markham, Clements, on density of Gulf-stream water, 129
〃 on motion of icebergs in Davis’ Straits, 133
Martins’s, Professor Charles, objections, 79
Mathews, Mr., on Canon Moseley’s experiment, 499
Maury, Lieutenant, his estimate of the Gulf-stream, 25
〃 his theory examined, 95
〃 on temperature as a cause of difference of specific gravity, 102
〃 on difference of saltness as a cause of ocean-currents, 103
〃 discussion of his views of the causes of ocean-currents, 104
〃 his objection to wind theory of ocean-currents, 211
McClure, Captain, discovery of ancient forest in Banks’s Land, 261
Mecham, Lieutenant, discovery of recent trees in Prince Patrick’s
Island, 261
Mechanics of gravitation theory, 145
Mediterranean shells in glacial shell bed of Udevalla, 253
〃 shells in glacial beds at Greenock, 254
Meech, Mr., on amount of sun’s rays cut off by the atmosphere, 26
〃 on influence of eccentricity on climate, 540
Melville Island, summer temperature of, 65
〃 discovery of recent trees in, 262
〃 plants found in coal of, 298
Mer de Glace, Professor Tyndall’s observations on, 498
Meteoric theory of sun’s heat, 347
Method of measuring rate of denudation, 329
Miller, Hugh, on absence of hills in the land of the Coal period, 431
Migration of plants and animals, how influenced by ocean-currents, 231
〃 across equator, 234
Millichen, remarkable section of drift at, 483
Miocene glacial period, 286
Miocene period, glacial epoch of, in Italy, 306
Miocene, warm period of, in Greenland, 307
Miocene and Eocene periods, date of, 357
Mississippi, amount of sediment in, 330
〃 volume of, 330
Mitchell, Mr., on cause of Gulf-stream, 131
Molecular theory of origin of 〃Crevasses,” 521
〃 modification of, 523
Moore, Mr. J. Carrick, on ice-action of Silurian age in
Wigtownshire, 293
Moore, Mr. Charles, on grooved rocks in Bath district, 464
Morlot, M., on inter-glacial periods of Switzerland, 240
Moseley, Canon, experiment to determine unit of shear, 498
〃 on motion of glaciers, 498
〃 unit of shear uncertain, 504
〃 his theory examined, 507
Motion of the sea, how communicated to a great depth, 136
Motion in space, origin of sun’s heat, 353
Mühry, M., on circumpolar basin, 133, 556
Mundsley, freshwater beds of, 250
Muncke on the expansion of sea-water, 118
Murchison, Sir R., on southern shells at Worcester, 253
〃 on trees in arctic regions, 262
〃 on striation of islands in the Baltic, 448
Murphy’s, Mr., theory, 66
Musselburgh, section of contorted drift near, 465
Nares, Captain, on low temperature of antarctic regions, 64
〃 discovery of great depth of warm water in North Atlantic, 198
〃 estimate of volume and temperature of Gulf-stream, 198
〃 temperature soundings by, 119, 222
〃 thermal condition of Southern Ocean, 225
Natal, boulder clay of, 300
Newberry, Professor, on inter-glacial peat-bed of Ohio, 249
〃 on boulder of quartzite found in seam of coal, 296
Nicholson, Dr., on Wastdale Crag, 457
Nicol, Professor, on inter-glacial buried channel, 244
Nordenskjöld, Professor, on inland ice of Greenland, 379
North Sea rendered shallow by drift deposits, 443
Northern seas probably filled with land-ice during glacial period, 438
Northern hemisphere, condition of, when deprived of heat from
ocean-current, 68
Norway, southern species in glacial shell beds, 253
Norwich Crag, its glacial character, 249
Norwich fluvio-marine beds, 250
Norwich inter-glacial beds, 250
Obliquity of ecliptic, its effects on climate, 398−419
〃 change of, influence on sea-level, 403
〃 Lieutenant-Colonel Drayson on, 410
〃 Mr. Belt on change of, 415
〃 Sir Charles Lyell on change of, 418
Ocean, imperfect conception of its area, 135
〃 condition of, inconsistent with the gravitation theory, 136
〃 low temperature at bottom a result of under currents, 142
〃 circulation, pressure as a cause of, 187
〃 antiquity of, 367
Ocean-currents, absolute heating power of, 23
〃 influence of, on normal temperatures overlooked, 40
〃 maximum effects of, reached at equator and poles, 49
〃 compensatory at only one point, 49
〃 heating effects of, greatest at the poles, 50
〃 cooling effects of, greatest at equator, 50
〃 earth not habitable without, 51
〃 result of deflection into Southern Ocean, 68
〃 palæontological objections against influence of, 53
〃 deflection of, the chief cause of changes of climate, 68
〃 how deflected by eccentricity, 69
〃 deflected by trade-winds, 70
〃 temperature of southern hemisphere lowered by transference of
heat to northern hemisphere by, 92
〃 take their rise in the Southern Ocean, 92
〃 cause of, never specially examined by physicists, 95
〃 if due to specific gravity, strongest on cold hemisphere, 97
〃 if due to eccentricity, strongest on warm hemisphere, 97
〃 if due to specific gravity, act only by descent, 99
〃 mode by which specific gravity causes, 100, 101
〃 the true method of estimating the amount of heat conveyed by, 207
〃 due to system of winds, 212
〃 system of, agrees with the system of the winds, 213
〃 how they mutually intersect, 219
〃 in relation to climate, 226
〃 direction of, depends on direction of winds, 227
〃 causes which deflect, affect climate, 228
〃 in relation to distribution of plants and animals, 231
〃 effects of, on Greenland during glacial period, 260
Œpinus on influence of eccentricity on climate, 529
Ohio inter-glacial beds, 249
Old Red Sandstone, evidence of ice-action in conglomerate of, 294, 295
Oolite of Sutherlandshire, 454
Oolitic period, evidence of ice-action during, 301−303
〃 warm climate in North Greenland during, 302
Organic remains, absence of, in glacial conglomerate of Upper Miocene
period, 286
Organic life, paucity of, a characteristic of glacial periods, 287
Orkney Islands, glaciated by land-ice, 444
Osborne, Captain, remarks on recent forest trees in arctic
regions, 262, 263
Oudemans, Dr., on planet Mars, 80
Overton Quarry, inter-glacial bed in, 247
Pacific Ocean, depth of, 147
Packard, Mr., on glacial phenomena of Labrador, 282
Page, Professor, on temperate climate of Coal period, 422
〃 on character of coal plants, 421
〃 on old watercourse at Hailes quarry, 490
Palæontological objections against influence of ocean-currents, 53
Palæontological evidence of last glacial period, 285
Parry, Captain, discovery of recent trees in Melville Island by, 262
Peach, Mr. C. W., on inter-glacial bed at Leith Walk, 246
〃 on boulder clay of Caithness, 436
〃 on striated rock surfaces in Cornwall, 464
Peach, Mr. B. N., on striation of Caithness, 453
Pengelly, Mr. W., on raised beaches, 407
Perigee, nearness of sun in, cause of snow and ice, 74
Perihelion, warm conditions at maximum when winter solstice is at, 77
Permian period, evidence of ice-action in, 298−303
Perthshire hills, ice-worn surfaces at elevations of 2,200 feet on
the, 440
Petermann, Dr. A., on Dr. Carpenter’s theory, 138
〃 on thermal condition of the sea, 138
〃 chart of Gulf-stream and Polar current, 219
〃 _Geogr. Mittheilungen_ of, list of papers in relation to arctic
regions, 556
Phillips, Professor, on influence of eccentricity on climate, 539
Poisson’s theory of hot and cold parts of space, 7
Polar regions, effect of removal of ice from, 64
〃 influence of ice on climate, 64
〃 low summer temperature of, 66
Polar cold considered by Dr. Carpenter the _primum mobile_ of
ocean-currents, 173
〃 confusion of ideas regarding its influence, 180
〃 influence of, according to Dr. Carpenter, 180
Polar ice-cap, displacement of the earth’s centre of gravity by, 368
Port Bowen, mean temperature of, 63
Portobello, striated pavements near, 255, 256
Post-tertiary formations, hypothetical thickness of, 366
Pouillet, M., on the amount of the sun’s heat, 26
〃 on amount of sun’s rays cut off by the atmosphere, 26
Pratt, Archdeacon, on glacial submergence, 387
Prestwich, Professor, on Hoxne inter-glacial bed, 241
Pressure as a cause of circulation, 187
Principles of geology, nature of, 4
Prince Patrick’s Island, discovery of recent tree in, 261
Radiation, rate of, increases with increase of temperature, 37
〃 of gases, 38
〃 the way by which the earth loses heat, 39
〃 how affected by snow covering the ground, 58
〃 how affected by humid air, 59
〃 accelerated by increased formation of snow and ice, 75
Raised beaches, date of, 407
〃 Mr. Pengelly on, 407
Ramsay, Professor, on glacial origin of Old Red Sandstone of North
of England, 294
〃 on Old Red Sandstone, 367
〃 on geological time, 343
〃 on ice-action during Permian period, 298
〃 on boulders of Permian age in Natal, 301
〃 on thickness of stratified rocks of Britain, 267, 361
Redhall Quarry, inter-glacial bed in, 247
Red Sea, why almost rainless, 30
Regelation, _rationale_ of, 520, 554
〃 Professor James Thomson on cause of, 554
〃 Professor Faraday on cause of, 554
Regnault, M., on specific heat of sandstone, 86
Reynaud, Jean, on influence of eccentricity on climate, 541
Rhine, ancient, bed in German Ocean, 480
Ridge between Capes Trafalgar and Spartel, influence of, 167
Rink, Dr., on inland ice of Greenland, 380
River-ice, effect of, 279
River-ice does not produce striations, 279
River systems, carrying-power measure of denudation, 336
River valleys, how striated across, 525
Robertson, Mr. David, on Crofthead and Hillhead inter-glacial
beds, 247, 248
〃 on foraminifera in red clay, 485
Rock-basins, how excavated by ice, 525
Rocks removed by denudation, 361
Ross, Capt. Sir James, on South Shetland, 61
〃 on temperature of antarctic regions in summer, 63
Sandwich Land, description by Capt. Cook, 60
〃 cold summers of, not due to latitude, 64
Salter, Mr., on carboniferous fossils of arctic regions, 298
〃 on warm climate of North Greenland during Oolitic period, 302
Saltness of the ocean, difference of, as a cause of motion, 103
〃 in direct opposition to temperature in producing
ocean-currents, 104
Scandinavian ice, track of, 447
Scandinavian ice-sheet in the North Sea, 444
Scoresby, Dr., on condition of arctic regions in summer, 58, 62
〃 on density of Gulf-stream water, 129
Scotland, inter-glacial beds of, 243−249
〃 evidence of ice-action in carboniferous conglomerate of, 296
〃 buried under ice, 439
〃 ice-sheet of, in North Sea, 442
〃 why ice-sheet was so thick, 452
Sea, height of, at equator above poles, 119
〃 rise of, due to combined effect of eccentricity and obliquity, 403
〃 bottoms not striated by icebergs, 272
Sea and land, present arrangement indispensable to life, 52
Sea-level, oscillations of, in relation to distribution, 394
〃 oscillations of, during formation of coal measures, 424
〃 raised, by melting of antarctic ice-cap, 388
〃 influence of obliquity of ecliptic on, 403
Section of Mid-Atlantic, 222
Section across antarctic ice-cap, 377
Sedimentary rocks existing fragmentary, 361
〃 of the globe, mean thickness of, hitherto unknown, 361
〃 how mean thickness might be determined, 362
〃 mean thickness of, over-estimated, 364
Shearing-force of ice, 496
〃 momentary loss of, 518
Shetland islands glaciated by land-ice from Scandinavia, 450
Shetland, South, glacial condition of, 61
Shell-beds, evidence of warm inter-glacial periods from, 252
Shells of the boulder clay of Caithness, 450
Shore-ice, striations produced by, in Bay of Fundy, 280
Silurian period, ice-action in Ayrshire during, 293
〃 evidence in Wigtownshire of ice-action during, 293
Slitrig, inter-glacial bed of, 243
Slope of surface of maximum density has no power to produce
motion, 120
〃 from equator to pole, erroneous view regarding, 120
Smith, Mr. Leigh, temperature soundings, 129
Smith, Mr., of Jordanhill, on striated pavements, 256
Snow, how radiation is affected by, 58
〃 common in summer in arctic regions, 62
〃 rate of accumulation of, increased by sun’s rays being cut off by
fogs, 75
〃 formation increased by radiation, 75
Somerset, West, glaciation of, 463
Somerville, Mrs., on influence of eccentricity on climate, 540
South Africa, glaciation of, 242
〃 boulder clay of Permian age in, 300
South of England ice-sheet, 463
South Shetland, glacial condition of, at mid summer, 61
South-west winds, heat conveyed by, not derived from equatorial
regions, 28
〃 heat conveyed by, derived from Gulf-stream, 28
Southern hemisphere, present extension of ice on, due partly to
eccentricity, 78
〃 why colder than northern, 81−92
〃 absorbs more heat than the northern, 90
〃 lower temperature of, due to ocean-currents, 92
〃 surface currents from, warmer than under currents to, 92
〃 glacial and inter-glacial periods of, 242
Southern Ocean, thermal condition of, 225
Specific gravity can act only by causing water to descend a slope, 99
〃 mode of action in causing ocean-currents, 100
〃 inadequacy of, to produce ocean-currents demonstrated by Sir John
Herschel, 116
Spitzbergen, Gulf-stream and under current at, 134
〃 Miocene flora of, 309
Stellar space, temperature of, 35
〃 received temperature of, probably too high, 39
Stewart, Professor Balfour, experiment on radiation, 37
〃 on cause of glacial cold, 79
Stirling, Mr., on old watercourse near Grangemouth, 481
St. John’s River, action of ice on banks of, 279
St. Lawrence, action of ice on bank of river, 279
Stockwell, Mr., on eccentricity of earth’s orbit, 54
〃 on obliquity of ecliptic, 399
〃 table of superior limits of eccentricity, 531
Stone, Mr., on eccentricity of the earth’s orbit, 322
Stow, G. W., on glacial beds of South Africa, 242
〃 on Karoo beds, 301
Striæ, direction of, show the clay of Caithness came from the sea, 436
Striations obliterated rather than produced by icebergs, 274
Striated pavements why so seldom observed, 256
〃 evidence of inter-glacial periods from, 255
Striated stones found in conglomerate of Lower Carboniferous age by
Professor Geikie, 296
〃 in Permian breccias, 299
〃 in the glacial conglomerate of the Superga, Turin, 306
Stratified rocks may be formed at all possible rates, 360
〃 rate of formation of, as estimated by Professor Huxley, 363
Struve, M., formula of obliquity of ecliptic, 404
Subaërial denudation, rate of, 331
Submarine forests, 409
〃 (ancient), coal seams the remains of, 428
Submergence, physical causes of, 368
〃 coincident with glaciation, 389
〃 of land resulting from melting of antarctic ice-cap, 389
〃 how affected by fluidity of interior of the earth, 395
〃 necessary for preservation of coal plants, 423
〃 frequent during formation of coal beds, 426
Subsidence insufficient to account for general submergence, 390
〃 necessary to accumulation of coal seams, 427
Sun supposed by some to be a variable star, 8
〃 maximum and minimum distance of, 55
〃 rays of, cut off by fogs in ice-covered regions, 60
〃 nearness in perigee a cause of snow and ice, 74
〃 total amount of heat radiated from, 346
〃 age and origin of, 346
〃 source of its energy, 347
〃 heat of, origin and chief source of, 349
〃 originally an incandescent mass, 350
〃 energy of, may have originally been derived from motion in
space, 355
Surface currents which cross the equator warmer than the compensatory
under currents, 92
Surface currents from poles to equator, according to Maury, produced
by saltness, 108
Sutherland, Dr., observations by, on stranding of icebergs, 275
〃 testimony, that icebergs do not striate rocks, 278
〃 on the boulder clay of Natal, 300
Sutherland, boulder conglomerate of Oolitic period of, 302
Sweden, Southern, shells in glacial shell beds of, 253
Switzerland, inter-glacial period of, 239
〃 M. Morlat on inter-glacial periods of, 240
〃 gravels of, by Mr. James Geikie, 268
〃 Eocene glacial epoch in, 305
Table of June temperatures in different latitudes, 65
〃 soundings in temperate regions, 222
Tables of eccentricity, 314−321
〃 of eccentricity, explanation of, 322
Tay, valley of, striated across, 526
〃 ancient buried channel of, 490
Temperate regions, cold periods best marked in, 258
Temperature of space, 532
〃 reasons why it should be reconsidered, 39
Temperature (mean) of equator and poles compared, 41
〃 why so low in polar regions during summer, 66
〃 how difference of specific gravity is caused by, 102
〃 higher, of the waters of Gulf-stream considered by Lieutenant
Maury as the real causes of its motion, 111
〃 of sea at equator decreases most rapidly at the surface, 119
〃 of Greenland in Miocene period, 310
〃 of poles when obliquity was at its superior limit, 402
Tension, effect of, on ice, 522
〃 the cause of the cooling effect produced by, 552
Tertiary period, climate of, error in regard to, 288
Thermal condition of Southern Ocean, 225
Thibet, table-land of, 418
Thomson, Professor James, on cause of regelation, 554
〃 theory of glacier-motion, 512
Thomson, Mr. James, on glacial conglomerate in Arran, 299
〃 on ice-action in Cambrian conglomerate of Islay, 292
Thomson, Professor Wyville, on Dr. Carpenter’s theory, 129
〃 cited, 130
〃 thermal condition of the sea, 138
Thomson, Sir W., amount of internal heat passing through earth’s
crust, 142
〃 on limit to age of the globe, 343
〃 on influence of ice-cap on sea-level, 372
〃 climate not affected by internal heat, 6
〃 earth’s axis of rotation permanent, 7
〃 on volume and mass of the sun, 347
Tidal wave, effect of friction, 336
Tides, supposed argument from, 184
Time, geological, 311−359
〃 as represented by geological phenomena, 326
〃 represented by existing rocks, 361
Torrid zone, annual quantity of heat received by, per unit of
surface, 194
Towncroft farm, section of channel at, 474
Towson, Mr., on icebergs of Southern Ocean, 383
Trade-winds (anti), heat conveyed by, over-estimated, 28
〃 (anti) derive their heat from the Gulf-stream, 32
〃 of warm hemisphere overborne by those of cold hemisphere, 70
〃 causes which determine the strength of, 70
〃 strongest on glaciated hemisphere, 70
〃 reaction upon trade-winds by formation of snow and ice, 76
〃 influence of, in turning ocean-currents on warm hemisphere, 97
〃 do not explain the antarctic current, 211
Tiddeman on North of England ice-sheet, 458
〃 displacement of, 230
Transport of boulders and rubbish the proper function of icebergs, 281
Trafalgar, effect of ridge between Capes Spartel and on Gibraltar
current, 167
Turner, Professor, on arctic seal found at Grangemouth, 485
Tylor, Alfred, on denudation of Mississippi basin, 333
Tyndall, Professor, on heat in aqueous vapour, 29
〃 on sifted rays, 47
〃 on diathermancy of air, 59
〃 on glacial epoch, 78
Udevalla, Mediterranean shell in glacial shells, bed of, 253
Under currents to southern hemisphere colder than surface currents
from, 92
〃 produced by saltness, flow from equator to poles, 106
〃 account for cold water at equator, 124, 142
〃 in Davis’ Strait, 134
〃 take path of least resistance, 130
〃 why considered improbable, 135
〃 difficulty regarding, obviated, 217
〃 theory of, 217
Underground temperature, Professor J. D. Forbes on, 86
Underground temperature exerts no influence on the climate, 88
〃 absolute amount of heat derived from, 142
〃 supposed influence of, 176
Uniformity, modern doctrine of, 325
United States’ coast survey of Gulf-stream, 24
〃 hydrographic department, papers published by, 556
Unstratified boulder clay must be the product of land-ice, 437
Upsala and Stockholm striated by Baltic glacier, 447
Vertical circulation, Lieutenant Maury’s theory of, 108
〃 according to Dr. Carpenter, 153
Vertical descent of polar column caused by extra pressure of water
upon it, 154
〃 effects of, and slope, the same, whether performed simultaneously
or alternately, 159
〃 of polar column illustrated by diagram, 160
Vertical distribution of heat in the ocean, Mr. Buchanan’s theory, 550
Vogt, Professor, on Dürnten lignite bed, 241
Warm hemisphere made warmer by increased reaction of physical
causes, 76
Warm periods best marked in arctic regions, 258
〃 in arctic regions, evidence of, 261
〃 better represented by fossils than cold periods, 288
〃 evidence of, during Cretaceous age, 304
Warm inter-glacial periods in arctic regions, 258−265
Water at equator the best means of distributing heat derived from the
sun, 30
Water, a worse radiator than land, 91
Wastdale granite boulders, difficulty of accounting for transport
of, 456
Wastdale Crag glaciated by continental ice, 457
Weibye, M., striation observed by, 280
Wilkes, Lieutenant, on cold experienced in antarctic regions in
summer, 63
Wellington Sound, ancient trees found at, 265
Winter-drift of ice on coast of Labrador, 276
West winds, moisture of, derived from Gulf-stream, 29
Wind, work in impelling currents, 219
Winds, ocean-currents produced by, 212
〃 system of, agrees with the system of ocean-currents, 213
Wind theory of oceanic circulation, 210
〃 crucial test of, 220
Wigtownshire, ice-action during Silurian age, 293
Work performed by descent of polar column, 157
Wood, Mr. Nicholas, on buried channel, 488
Wood, Jun., Mr. Searles, middle drift, 250
〃 on occurrence of chalk _débris_ in south-west of England, 460
Woodward, Mr. H. B., on boulder clay in Devonshire, 463
Wunsch, Mr. E. A., on glacial conglomerate in Arran, 299
Yare, ancient buried channel of, 489
Young, Mr. J., objection considered, 482
Yorkshire drift common in south of England, 460
Zenger, Professor, on the moon’s influence on climate, 324
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+----------------------------------------------------------------------+
| FOOTNOTES: |
| |
| [1] Trans. of Edin. Geol. Soc., vol. ii. p. 252. |
| |
| [2] Phil. Mag., January, 1863. |
| |
| [3] _Athenæum_, September 22, 1860. |
| |
| [4] Trans. Glasgow Geol. Soc., vol. iv., p. 313. |
| |
| [5] See Mr. Hopkin’s remarks on this theory, Quart. Journ. Geol. |
| Soc., vol. viii. |
| |
| [6] See Chap. xxv. |
| |
| [7] See Chap. iv. |
| |
| [8] “Treatise on Astronomy,” § 315; “Outlines,” § 368. |
| |
| [9] _Annuaire_ for 1834, p. 199. Edin. New Phil. Journ., April, |
| 1834, p. 224. |
| |
| [10] “Cosmos,” vol. iv. p. 459 (Bohn’s Edition). “Physical |
| Description of the Heavens,” p. 336. |
| |
| [11] Phil. Mag. for February, 1867, p. 127. |
| |
| [12] The Gulf-stream at the narrowest place examined by the Coast |
| Survey, and where also its velocity was greatest, was found to be |
| over 30 statute miles broad and 1,950 feet deep. But we must not |
| suppose that this represents all the warm water which is received |
| by the Atlantic from the equator; a great mass flows into the |
| Atlantic without passing through the Straits of Florida. |
| |
| [13] It is probable that a large proportion of the water |
| constituting the south-eastern branch of the Gulf-stream is never |
| cooled down to 40°; but, on the other hand, the north-eastern |
| branch, which passes into the arctic regions, will be cooled far |
| below 40°, probably below 30°. Hence I cannot be over-estimating |
| the extent to which the water of the Gulf-stream is cooled down in |
| fixing upon 40° as the average minimum temperature. |
| |
| [14] “Physical Geography of the Sea,” § 24, 6th edition. |
| |
| [15] “Physical Geography,” § 54. |
| |
| [16] Trans. of Roy. Soc. of Edin., vol. xxi., p. 57. Phil. Mag., § |
| 4, vol. ix., p. 36. |
| |
| [17] “Smithsonian Contributions to Knowledge,” vol. ix. |
| |
| [18] “Heat as a Mode of Motion,” art. 240. |
| |
| [19] Trans. Roy. Soc. of Edin., vol. xxv., part 2. |
| |
| [20] See “Smithsonian Contributions to Knowledge,” vol. ix. |
| |
| [21] “Meteorology,” section 36. |
| |
| [22] _Comptes-Rendus_, July 9, 1838. Taylor’s “Scientific Memoirs,” |
| vol. iv., p. 44 (1846). |
| |
| [23] The mean temperature of the Atlantic between the tropics and |
| the arctic circle, according to Admiral Fitzroy’s chart, is about |
| 60°. But he assigns far too high a temperature for latitudes above |
| 50°. It is probable that 56° is not far from the truth. |
| |
| [24] The probable physical cause of this will be considered in the |
| Appendix. |
| |
| [25] The mean temperature of the equator, according to Dove, is |
| 79°·7, and that of the north pole 2°·3. But as there is, of course, |
| some uncertainty regarding the actual mean temperature of the |
| poles, we may take the difference in round numbers at 80°. |
| |
| [26] Trans. of Roy. Soc. Edin., vol. xxii., p. 75. |
| |
| [27] _Connaissance des Temps_ for 1863 (Additions). Lagrange’s |
| determination makes the superior limit 0·07641 (Memoirs of the |
| Berlin Academy for 1782, p. 273). Recently the laborious task of |
| re-investigating the whole subject of the secular variations of the |
| elements of the planetary orbits was undertaken by Mr. Stockwell, |
| of the United States. He has taken into account the disturbing |
| influence of the planet Neptune, the existence of which was not |
| known when Leverrier’s computations were made; and he finds that |
| the eccentricity of the earth’s orbit will always be included |
| within the limits of 0 and 0·0693888. Mr. Stockwell’s elaborate |
| Memoir, extending over no fewer than two hundred pages, will be |
| found in the eighteenth volume of the “Smithsonian Contributions to |
| Knowledge.” |
| |
| [28] When the eccentricity is at its superior limit, the absolute |
| quantity of heat received by the earth during the year is, however, |
| about one three-hundredth part greater than at present. But this |
| does not affect the question at issue. |
| |
| [29] Scoresby’s “Arctic Regions,” vol. ii., p. 379. Daniell’s |
| “Meteorology,” vol. ii., p. 123. |
| |
| [30] Tyndall, “On Heat,” article 364. |
| |
| [31] Tyndall, “On Heat,” article 364. |
| |
| [32] See Phil. Mag., March, 1870, p. |
| |
| [33] Captain Cook’s “Second Voyage,” vol. ii., pp. 232, 235. |
| |
| [34] “Antarctic Regions,” vol. ii., pp. 345−349. |
| |
| [35] Ibid., vol. i., p. 167. |
| |
| [36] Ibid., vol. ii., p. 362. |
| |
| [37] Edinburgh Philosophical Journal, vol. iv., p. 266. |
| |
| [38] Scoresby’s “Arctic Regions,” vol. i., p. 378. |
| |
| [39] Ibid., p. 425. |
| |
| [40] See Meech’s memoir “On the Intensity of the Sun’s Heat and |
| Light,” “Smithsonian Contributions,” vol. ix. |
| |
| [41] “Antarctic Regions,” vol. i., p. 240. |
| |
| [42] _Challenger_ Reports, No. 2, p. 10. |
| |
| [43] See “Smithsonian Contributions,” vol. ix. |
| |
| [44] Quart. Journ. Geol. Soc., vol. xxv., p. 350. |
| |
| [45] Trans. of Glasgow Geol. Soc. for 1866. |
| |
| [46] _Revue des Deux Mondes_ for 1867. |
| |
| [47] Letter to the author, February, 1870. |
| |
| [48] “Révolutions de la Mer,” p. 37 (second edition). |
| |
| [49] Edin. Phil. Journ., vol. iv., p. 262 (1821). |
| |
| [50] Phil. Mag., § 4, vol. xxviii., p. 131. _Reader_, December 2nd, |
| 1865. |
| |
| [51] This point will be found discussed at considerable length in |
| the Phil. Mag. for September, 1869. |
| |
| [52] See Phil. Mag. for October, 1870, p. 259. |
| |
| [53] Proceedings of the Royal Society, No. 138, p. 596, foot-note. |
| |
| [54] The edition from which I quote, unless the contrary is stated, |
| is the one published by Messrs. T. Nelson and Sons, 1870, which is |
| a reprint of the new edition published in 1859 by Messrs. Sampson |
| Low and Co. |
| |
| [55] “Physical Geography,” article 57. |
| |
| [56] Philosophical Magazine, vol. xii. p. 1 (1838). |
| |
| [57] “Mémoires par divers Savans,” tom. i., p. 318, St. |
| Petersburgh, 1831. See also twelfth number of Meteorological |
| Papers, published by the Board of Trade, 1865, p. 16. |
| |
| [58] Dubuat’s “Hydraulique,” tom. i., p. 64 (1816). See also |
| British Association Report for 1834, pp. 422, 451. |
| |
| [59] See Proceedings of the Royal Society for December, 1868, |
| November, 1869. Lecture delivered at the Royal Institute, _Nature_, |
| vol. i., p. 490. Proceedings of the Royal Geographical Society, |
| vol. xv. |
| |
| [60] Trans. of Glasgow Geol. Soc. for April, 1867. Phil. Mag. for |
| February, 1867, and June, 1867 (Supplement). |
| |
| [61] Phil. Mag. for February, 1870. |
| |
| [62] “The Depths of the Sea,” pp. 376 and 377. |
| |
| [63] “The Threshold of the Unknown Region,” p. 95. |
| |
| [64] See “Physical Geography of the Sea,” chap. ix., new edition, |
| and Dr. A. Mühry “On Ocean-currents in the Circumpolar Basin of the |
| North Hemisphere.” |
| |
| [65] “Depths of the Sea,” _Nature_ for July 28, 1870. |
| |
| [66] “Memoir on the Gulf-stream,” _Geographische Mittheilungen_, |
| vol. xvi. (1870). |
| |
| [67] Dr. Carpenter “On the Gulf-stream,” Proceedings of Royal |
| Geographical Society for January 9, 1871, § 29. |
| |
| [68] Dr. Petermann’s _Mittheilungen_ for 1872, p. 315. |
| |
| [69] Proceedings of the Royal Society, vol. xvii., p. 187, xviii., |
| p. 463. |
| |
| [70] The average depth of the Pacific Ocean, as found by the |
| soundings of Captain Belknap, of the U.S. steamer _Tuscarora_, made |
| during January and February, 1874, is about 2,400 fathoms. The |
| depth of the Atlantic is somewhat less. |
| |
| [71] Proceedings of Royal Geographical Society, vol. xv., § 22. |
| |
| [72] It is a well-established fact that in polar regions the |
| temperature of the sea decreases from the surface downwards; |
| and the German Polar Expedition found that the water in very |
| high latitudes is actually less dense at the surface than at |
| considerable depths, thus proving that the surface-water could not |
| sink in consequence of its greater density. |
| |
| [73] Proceedings of the Royal Society, vol. xix., p. 215. |
| |
| [74] _Nature_ for July 6, 1871. |
| |
| [75] Since the above objection to the Gravitation Theory of the |
| Gibraltar Current was advanced three years ago, Dr. Carpenter |
| appears to have abandoned the theory to a great extent. He now |
| admits (Proceedings of Royal Geographical Society, vol. xviii., |
| pp. 319−334, 1874) that the current is almost wholly due not to |
| difference of specific gravity, but to an excess of evaporation in |
| the Mediterranean over the return by rain and rivers. |
| |
| [76] Proceedings of Royal Society, No. 138, § 26. |
| |
| [77] Proceedings of Royal Geographical Society, January 9, 1871. |
| |
| [78] Ibid. |
| |
| [79] See §§ 20, 34; also Brit. Assoc. Report for 1872, p. 49, and |
| other places. |
| |
| [80] See also to the same effect Brit. Assoc. Report, 1872, p. 50. |
| |
| [81] Phil. Mag. for Oct. 1871. |
| |
| [82] The actual slope, however, does not amount to more than 1 in |
| 7,000,000. |
| |
| [83] Proc. of Roy. Geog. Soc., January 9, 1871, § 29. |
| |
| [84] Trans. of Geol. Soc. of Glasgow for April, 1867; Phil. Mag. |
| for June, 1867. |
| |
| [85] _Nature_, vol. i., p. 541. Proc. Roy. Soc., vol. xviii., p. |
| 473. |
| |
| [86] Chapter II. |
| |
| [87] Chapter II. |
| |
| [88] Chapter II. |
| |
| [89] Mr. Findlay considers that the daily discharge does not exceed |
| 333 cubic miles (Brit. Assoc. Rep., 1869, p. 160). My estimate |
| makes it 378 cubic miles. Mr. Laughton’s estimate is 630 cubic |
| miles (Paper “On Ocean-currents,” Journal of Royal United-Service |
| Institution, vol. xv.). |
| |
| [90] Proceedings of the Royal Geographical Society, vol. xviii., p. |
| 393. |
| |
| [91] Phil. Mag. for October, 1871, p. 274. |
| |
| [92] Proceedings of the Royal Geographical Society, vol. xv. |
| |
| [93] Phil. Mag., February, 1870. |
| |
| [94] Brit. Assoc. Report, 1869, Sections, p. 160. |
| |
| [95] Journal of Royal United-Service Institute, vol. xv. |
| |
| [96] Dr. Carpenter (Proc. of Roy. Geog. Soc., vol. xviii., p. |
| 334) misapprehends me in supposing that I attribute the Gibraltar |
| current wholly to the Gulf-stream. In the very page from which he |
| derives or could derive his opinion as to my views on the subject |
| (Phil. Mag. for March, 1874, p. 182), I distinctly state that |
| “the excess of evaporation over that of precipitation within the |
| Mediterranean area would of itself produce a considerable current |
| through the Strait.” That the Gibraltar current is due to two |
| causes, (1) the pressure of the Gulf-stream, and (2) excess of |
| evaporation over precipitation in the Mediterranean, has always |
| appeared to me so perfectly obvious, that I never held nor could |
| have held any other opinion on the subject. |
| |
| [97] Paper read to the Edinburgh Botanical Society on January 8, |
| 1874. |
| |
| [98] Proc. Roy. Geog. Soc., vol. xviii., p. 362. A more |
| advantageous section might have been chosen, but this will suffice. |
| The section referred to is shown in Plate III. The peculiarity of |
| this section, as will be observed, is the thinness of the warm |
| strata at the equator, as compared with that of the heated water in |
| the North Atlantic. |
| |
| [99] The temperature of column C in Dr. Carpenter’s section is |
| somewhat less than that given in the foregoing table; so that, |
| according to that section, the difference of level between column C |
| and columns A and B would be greater than my estimate. |
| |
| [100] Captain Nares’s Report, July 30, 1874. |
| |
| [101] See Chapter IV. |
| |
| [102] Phil. Mag. for August, 1864, February, 1867, March, 1870; see |
| Chap. IV. |
| |
| [103] Quarterly Journal of Science for October, 1874. |
| |
| [104] See a paper by M. Morlot, on “The Post-Tertiary and |
| Quaternary Formations of Switzerland.” Edin. New Phil. Journal, New |
| Series, vol. ii., 1855. |
| |
| [105] Edin. New Phil. Journ., New Series, vol. ii., p. 28. |
| |
| [106] Vogt’s “Lectures on Man,” pp. 318−321. |
| |
| [107] See Mr. Prestwich on Flint Implements, Phil. Trans. for 1860 |
| and 1864. Lyell’s “Antiquity of Man,” Second Edition, p. 168. |
| |
| [108] Edin. New Phil. Journ., New Series, vol. ii., p. 28. |
| Silliman’s Journ., vol. xlvii., p. 259 (1844). |
| |
| [109] Quart. Journ. Geol. Soc., vol. xxvii., p. 534. |
| |
| [110] Ibid., vol. xxviii., p. 17. |
| |
| [111] “Glacial Drift of Scotland,” p. 54. |
| |
| [112] “Glacial Drift of Scotland,” p. 58. |
| |
| [113] Quart. Journ. Geol. Soc., vol. v., p. 22. |
| |
| [114] “Glacial Drift of Scotland,” p. 64. |
| |
| [115] Trans. Edin. Geol. Soc., vol. ii., p. 391. |
| |
| [116] Trans. of Geol. Soc. of Glasgow, vol. iv., p. 146. |
| |
| [117] Geol. Mag., vi., p. 391. |
| |
| [118] See “Memoirs of Geological Survey of Scotland,” Explanation |
| of sheet 22, p. 29. See also Trans. Glasgow Geol. Soc., iv., p. 150. |
| |
| [119] “Great Ice Age,” p. 374. |
| |
| [120] “Great Ice Age,” p. 384. |
| |
| [121] “Geological Survey of Ohio, 1869,” p. 165. See also “Great |
| Ice Age,” chap. xxviii. |
| |
| [122] Quart. Journ. Geol. Soc., xxviii., p. 435. |
| |
| [123] Brit. Assoc. Report, 1863. |
| |
| [124] Trans. Glasgow Nat. Hist. Soc., vol. i., p. 115. |
| |
| [125] Trans. of the Geol. Soc. of Glasgow, vol. iii., p. 133. See |
| also “Great Ice Age,” chaps. xii. and xiii. |
| |
| [126] Chap. XXIX. |
| |
| [127] Edin. New Phil. Journ., vol. liv., p. 272. |
| |
| [128] “Newer Pliocene Geology,” p. 129. John Gray & Co., Glasgow. |
| |
| [129] “Glacial Drift of Scotland,” p. 67. |
| |
| [130] “Glacial Drift of Scotland,” p. 12. |
| |
| [131] See Chapter IV. |
| |
| [132] “Discovery of the North-West Passage,” p. 213. |
| |
| [133] “Voyage of the _Resolute_,” p. 294. |
| |
| [134] Quart. Journ. Geol. Soc., vol. xi., p. 540. |
| |
| [135] “McClure’s North-West Passage,” p. 214. Second Edition. |
| |
| [136] “British Association Report for 1855,” p. 381. “The Last of |
| the Arctic Voyages,” vol. i., p. 381. |
| |
| [137] Mr. James Geikie informs me that the great accumulations of |
| gravel which occur so abundantly in the low grounds of Switzerland, |
| and which are, undoubtedly, merely the re-arranged materials |
| originally brought down from the Alps as till and as moraines |
| by the glaciers during the glacial epoch, rarely or never yield |
| a single scratched or glaciated stone. The action of the rivers |
| escaping from the melting ice has succeeded in obliterating all |
| trace of striæ. It is the same, he says, with the heaps of gravel |
| and sand in the lower grounds of Sweden and Norway, Scotland and |
| Ireland. These deposits are evidently in the first place merely the |
| materials carried down by the swollen rivers that issued from the |
| gradually melting ice-fields and glaciers. The stones of the gravel |
| derived from the demolition of moraines and till, have lost all |
| their striæ and become in most cases well water-worn and rounded. |
| |
| [138] Report on Icebergs, read before the Association of American |
| Geologists, _Silliman’s Journal_, vol. xliii., p. 163 (1842). |
| |
| [139] “Manual of Geology,” p. 677. |
| |
| [140] Quart. Journ. Geol. Soc., vol. ix., p. 306. |
| |
| [141] Dana’s “Manual of Geology,” p. 677. |
| |
| [142] Quart. Journ. Geol. Soc., vol. ix., p. 306. |
| |
| [143] “Journal,” vol. i., p. 38. |
| |
| [144] “Short American Tramp,” pp. 168, 174. |
| |
| [145] “Short American Tramp,” pp. 239−241. |
| |
| [146] “Travels in North America,” vol. ii., p. 137. |
| |
| [147] Ibid., vol. ii., p. 174. |
| |
| [148] Proceedings of the Royal Society of Edinburgh, Session |
| 1865−66, p. 537. |
| |
| [149] “Short American Tramp,” pp. 77, 81, 111. |
| |
| [150] “Second Visit,” vol. ii., p. 367. |
| |
| [151] “Memoirs of Boston Society of Natural History,” vol. i. |
| (1867), p. 228. |
| |
| [152] “Antiquity of Man,” p. 268. Third Edition. |
| |
| [153] “Great Ice Age,” p. 512. |
| |
| [154] Brit. Assoc., 1870, p. 88. |
| |
| [155] Quart. Journ. Geol. Soc., vol. v., p. 10. Phil. Mag. for |
| April, 1865, p. 289. |
| |
| [156] “Great Ice Age,” p. 512. |
| |
| [157] Jukes’ “Manual of Geology,” p. 421. |
| |
| [158] See also Quarterly Journal Geological Society, vol. xi., p. |
| 510. |
| |
| [159] The _Reader_ for August 12, 1865. |
| |
| [160] “History of the Isle of Man,” p. 86. My colleague, Mr. John |
| Horne, in his “Sketch of the Geology of the Isle of Man,” Trans. of |
| Edin. Geol. Soc., vol. ii., part iii., considers this conglomerate |
| to be of Lower Carboniferous age. |
| |
| [161] See Selwyn, “Phys. Geography and Geology of Victoria.” 1866. |
| pp. 15−16; Taylor and Etheridge, _Geol. Survey Vict., Quarter Sheet |
| 13, N.E._ |
| |
| [162] Report on the Geology of the District of Ballan, Victoria. |
| 1866. p. 11. |
| |
| [163] _Atrypa reticularis._ |
| |
| [164] Quart. Journ. Geol. Soc., vol. xii., p. 58. |
| |
| [165] “Great Ice Age,” p. 513. |
| |
| [166] “Great Ice Age,” p. 513. |
| |
| [167] Brit. Assoc. Report for 1873. |
| |
| [168] Quart. Journ. Geol. Soc., vol. xi., p. 519. |
| |
| [169] _Orthis resupinata._ |
| |
| [170] _Prod. semireticulatus_ var. _Martini_. Sow. |
| |
| [171] “Belcher’s Voyage,” vol. ii., p. 377. |
| |
| [172] “Journal of a Boat Voyage through Rupert-Land,” vol. ii., p. |
| 208. |
| |
| [173] Quart. Journ. Geol. Soc., vol. xi., p. 197. |
| |
| [174] Explanation Memoir to Sheet 47, “Geological Survey of |
| Ireland.” |
| |
| [175] Phil. Mag., vol. xxix., p. 290. |
| |
| [176] “Memoirs of the Geological Survey of India,” vol. i., part i. |
| |
| [177] Quart. Journ. Geol. Soc., vol. xxvi., p. 514. |
| |
| [178] Ibid., vol. xxvii., p. 544. |
| |
| [179] Phil. Mag., vol. xxix., p. 290. |
| |
| [180] Journal of the Royal Dublin Society for February, 1857. |
| |
| [181] Quart. Journ. Geol. Soc., vol. xi., p. 519. |
| |
| [182] “The Last of the Arctic Voyages,” by Captain Sir E. Belcher, |
| vol. ii., p. 389. Appendix Brit. Assoc. Report for 1855, p. 79. |
| |
| [183] Ibid., vol. ii., p. 379. Appendix. |
| |
| [184] “Manual of Geology,” pp. 395, 493. |
| |
| [185] Appendix to McClintock’s “Arctic Discoveries.” |
| |
| [186] Quart. Journ. Geol. Soc., vol. xiv., p. 262. Brit. Assoc. |
| Report for 1857, p. 62. |
| |
| [187] Quart. Journ. Geol. Soc., vol. xvi., p. 327. _Geologist_, |
| 1860, p. 38. |
| |
| [188] Phil. Mag., vol. xxix., p. 290. |
| |
| [189] Trans. Geol. Soc. of Glasgow, vol. v., p. 64. |
| |
| [190] “Principles,” vol. i., p. 209. Eleventh Edition. |
| |
| [191] “Memoirs of the Royal Academy of Science of Turin,” Second |
| Series, vol. xx. I am indebted for the above particulars to |
| Professor Ramsay, who visited the spot along with M. Gastaldi. |
| |
| [192] “Antiquity of Man,” Second Edition, p. 237. |
| |
| [193] Dr. Robert Brown, in a recent Memoir on the Miocene Beds of |
| the Disco District (Trans. Geol. Soc. Glasg., vol. v., p. 55), |
| has added considerably to our knowledge of these deposits. He |
| describes the strata in detail, and gives lists of the plant and |
| animal remains discovered by himself and others, and described by |
| Professor Heer. Professor Nordenskjöld has likewise increased the |
| data at our command (Transactions of the Swedish Academy, 1873); |
| and still further evidence in favour of a warm climate having |
| prevailed in Greenland during Miocene times has been obtained by |
| the recent second German polar expedition. |
| |
| [194] The following are M. Leverrier’s formulæ for computing the |
| eccentricity of the earth’s orbit, given in his “Memoir” in the |
| _Connaissance des Temps_ for 1843:— |
| |
| Eccentricity in (_t_) years after January 1, 1800 |
| _____________ |
| = √_h_^2 + _l_^2 where |
| |
| _h_ = 0·000526 Sin (_gt_ + _ß_) + 0·016611 Sin (_g_{1}t_ + _ß_{1}_) |
| + 0·002366 Sin (_g_{2}t_ + _ß_{2}_) |
| + 0·010622 Sin (_g_{3}t_ + _ß_{3}_) |
| - 0·018925 Sin (_g_{4}t_ + _ß_{4}_) |
| + 0·011782 Sin (_g_{5}t_ + _ß_{5}_) |
| - 0·016913 Sin (_g_{6}t_ + _ß_{6}_) |
| and |
| |
| _l_ = 0·000526 Cos (_gt_ + _ß_) + 0·016611 Cos (_g_{1}t_ + _ß_{1}_) |
| + 0·002366 Cos (_g_{2}t_ + _ß_{2}_) |
| + 0·010622 Cos (_g_{3}t_ + _ß_{3}_) |
| - 0·018925 Cos (_g_{4}t_ + _ß_{4}_) |
| + 0·011782 Cos (_g_{5}t_ + _ß_{5}_) |
| - 0·016913 Cos (_g_{6}t_ + _ß_{6}_) |
| |
| _g_ = 2″·25842 _ß_ = 126° 43′ 15″ |
| _g_{1}_ = 3″·71364 _ß_{1}_ = 27 21 26 |
| _g_{2}_ = 22″·4273 _ß_{2}_ = 126 44 8 |
| _g_{3}_ = 5″·2989 _ß_{3}_ = 85 47 45 |
| _g_{4}_ = 7″·5747 _ß_{4}_ = 35 38 43 |
| _g_{5}_ = 17″·1527 _ß_{5}_ = −25 11 33 |
| _g_{6}_ = 17″·8633 _ß_{6}_ = −45 28 59 |
| |
| [195] See Professor C. V. Zenger’s paper “On the Periodic Change cf |
| Climate caused by the Moon,” Phil. Mag. for June, 1868. |
| |
| [196] Phil. Mag. for February, 1867. |
| |
| [197] Phil. Mag. for May, 1868. |
| |
| [198] Student’s “Elements of Geology,” p. 91. Second Edition. |
| |
| [199] In an interesting memoir, published in the Phil. Mag. for |
| 1850, Mr. Alfred Tylor estimated that the basin of the Mississippi |
| is being lowered at the rate of one foot in 10,000 years by the |
| removal of the sediment; and he proceeds further, and reasons that |
| one foot removed off the general surface of the land during that |
| period would raise the sea-level three inches. Had it not been that |
| Mr. Tylor’s attention was directed to the effects produced by the |
| removal of sediment in raising the level of the ocean rather than |
| in lowering the level of the land, he could not have failed to |
| perceive that he was in possession of a key to unfold the mystery |
| of geological time. |
| |
| [200] Proc. Roy. Soc., No. 152, 1874. |
| |
| [201] I have taken for the volume and mass of the sun the values |
| given in Professor Sir William Thomson’s memoir, Phil. Mag., vol. |
| viii. (1854). |
| |
| [202] Phil. Mag., § 4, vol. xi., p. 516 (1856). |
| |
| [203] Phil. Mag. for July, 1872, p. 1. |
| |
| [204] “Principles,” p. 210. Eleventh Edition. |
| |
| [205] “Principles,” vol. i., p. 107. Tenth Edition. |
| |
| [206] The conception of submergence resulting from displacement of |
| the earth’s centre of gravity, caused by a heaping up of ice at |
| one of the poles, was first advanced by M. Adhémar, in his work |
| “_Révolutions de la Mer_,” 1842. When the views stated in this |
| chapter appeared in the _Reader_, I was not aware that M. Adhémar |
| had written on the subject. An account of his mode of viewing the |
| question is given in the Appendix. |
| |
| [207] Petermann’s _Geog. Mittheilungen_, 1871, Heft. x., p. 377. |
| |
| [208] Geol. Mag., 1872, vol. ix., p. 360. |
| |
| [209] “Open Polar Sea,” p. 134. |
| |
| [210] Journal of the Royal Geographical Society, 1853, vol. xxiii. |
| |
| [211] “Physics of Arctic Ice,” Quart. Journ. Geol. Soc. for |
| February, 1871. |
| |
| [212] Some writers have objected to the conclusion that the |
| antarctic ice-cap is thickest at the pole, on the ground that the |
| snowfall there is probably less than at lower latitudes. The fact |
| is, however, overlooked, that the greater thickness of an ice-cap |
| at its centre is a physical necessity not depending on the rate of |
| snowfall. Supposing the snowfall to be greater at, say, lat. 70° |
| than at 80°, and greater at 80° than at the pole; nevertheless, the |
| ice will continue to accumulate till it is thicker at 80° than at |
| 70°, and at the pole than it is at 80°. |
| |
| [213] It is a pity that at present no record is kept, either by |
| the Board of Trade or by the Admiralty, of remarkable icebergs |
| which may from time to time be met with. Such a record might be of |
| little importance to navigation, but it would certainly be of great |
| service to science. |
| |
| [214] See Chapter XXVII., and also Geol. Mag. for May and June, |
| 1870, and January, 1871. |
| |
| [215] Phil. Mag. for April, 1866, p. 323. |
| |
| [216] Ibid., for March, 1866, p. 172. |
| |
| [217] _Reader_, February 10, 1866. |
| |
| [218] In a former paper I considered the effects of another cause, |
| viz., the melting of polar ice resulting from an increase of the |
| Obliquity of the Earth’s Orbit.—Trans. Glasgow Geol. Soc., vol. |
| ii., p. 177. Phil. Mag., June, 1867. See also Chapter XXV. |
| |
| [219] Phil. Mag. for November, 1868, p. 376. |
| |
| [220] Phil. Mag., November, 1868. |
| |
| [221] “Origin of Species,” chap. xi. Fifth Edition. |
| |
| [222] Lieutenant-Colonel Drayson (“Last Glacial Epoch of Geology”) |
| and also Mr. Belt (Quart. Journ. of Science, October, 1874) state |
| that Leverrier has lately investigated the question as to the |
| extent of the variation of the plane of the ecliptic, and has |
| arrived at results differing considerably from those of Laplace; |
| viz., that the variation may amount to 4° 52′, whereas, according |
| to Laplace, it amounts to only 1° 21′. I fear they are comparing |
| things that are totally different; viz., the variation of the |
| plane of the ecliptic in relation to its mean position with its |
| variation in relation to the equator. Laplace estimated that the |
| plane of the ecliptic would oscillate to the extent of 4° 53′ 33″ |
| on each side of its mean position, a result almost identical with |
| that of Leverrier, who makes it 4° 51′ 42″. But neither of these |
| geometricians ever imagined that the ecliptic could change in |
| relation to the equator to even one-third of that amount. |
| |
| Laplace demonstrated that the change in the plane of the ecliptic |
| affected the position of the equator, causing it to vary along with |
| it, so that the equator could never possibly recede further than |
| 1° 22′ 34″ from its mean position in relation to the ecliptic |
| (“_Mécanique Céleste_,” vol. ii., p. 856, Bowditch’s Translation; |
| see also Laplace’s memoir, “Sur les Variations de l’Obliquité de |
| l’Écliptique,” _Connaissance des Temps_ for 1827, p. 234), and I am |
| not aware that Leverrier has arrived at a different conclusion. |
| |
| [223] Memoir on the Secular Variations of the Elements of the |
| Orbits of the Planets, “Smithsonian Contributions to Knowledge,” |
| vol. xvii. |
| |
| [224] “Smithsonian Contributions to Knowledge,” vol. ix. |
| |
| [225] “Distribution of Heat on the Surface of the Globe,” p. 14. |
| |
| [226] Chapter IV. |
| |
| [227] Quart. Journ. Geol. Soc., June, 1866, p. 564. |
| |
| [228] Quart. Journ. Geol. Soc., vol. xxi., p. 186. |
| |
| [229] “Geological Observer,” p. 446. See also Mr. James Geikie’s |
| valuable Memoir, “On the Buried Forests and Peat Mosses of |
| Scotland.” Trans. of the Royal Society of Edinburgh, vol. xxiv., |
| and Chambers’ “Ancient Sea-Margins.” |
| |
| [230] See Lyell’s “Antiquity of Man,” Second Edition, p. 282; |
| “Elements,” Sixth Edition, p. 162. |
| |
| [231] In order to determine the position of the solstice-point |
| in relation to the aphelion, it will not do to assume, as is |
| commonly done, that the point makes a revolution from aphelion to |
| aphelion in any regular given period, such as 21,000 years; for |
| it is perfectly evident that owing to the great irregularity in |
| the motion of the aphelion, no two revolutions will probably be |
| performed in the same length of period. For example, the winter |
| solstice was in the aphelion about the following dates: 11,700, |
| 33,300, and 61,300 years ago. Here are two consecutive revolutions, |
| the one performed in 21,600 years and the other in 28,000 years; |
| the difference in the length of the two periods amounting to no |
| fewer than 6,400 years. |
| |
| [232] Quart. Journ. Geol. Soc., vol. xxvii., p. 232. See also “The |
| Last Glacial Epoch of Geology,” by the same author. |
| |
| [233] Quart. Journ. of Science, October, 1874. |
| |
| [234] The longer diameter passes from long. 14° 23′ E. to long. |
| 165° 37′ W. |
| |
| [235] “Principles,” vol. i., p. 294. Eleventh Edition. |
| |
| [236] Phil. Mag. for August, 1864. |
| |
| [237] “Elementary Geology,” p. 399. |
| |
| [238] “The Past and Present Life of the Globe,” p. 102. |
| |
| [239] “Memoirs of the Geological Survey,” vol. ii., Part 2, p. 404. |
| |
| [240] “Coal Fields of Great Britain,” p. 45. Third Edition. |
| |
| [241] “Journal of Researches,” chap. xiii. |
| |
| [242] “Coal Fields of Great Britain,” p. 67. |
| |
| [243] See “Smithsonian Report for 1857,” p. 138. |
| |
| [244] Quart. Journ. Geol. Soc., May, 1865, p. civ. |
| |
| [245] “Geology of Fife and the Lothians,” p. 116. |
| |
| [246] “Life on the Earth,” p. 133. |
| |
| [247] Quart. Journ. Geol. Soc., vol. xi., p. 535. |
| |
| [248] Ibid., vol. xii., p. 39. |
| |
| [249] Miller’s “Sketch Book of Practical Geology,” p. 192. |
| |
| [250] From Geological Magazine, May and June, 1870; with a few |
| verbal corrections, and a slight re-arrangement of the paragraphs. |
| |
| [251] See Phil. Mag. for November, 1868, p. 374. |
| |
| [252] See Phil. Mag. for November, 1868, pp. 366−374. |
| |
| [253] Journ. Geol. Soc., vol. xxi., p. 165. |
| |
| [254] Specimens of the striated summit and boulder clay stones are |
| to be seen in the Edinburgh Museum of Science and Art. |
| |
| [255] Phil. Mag. for April, 1866. |
| |
| [256] “Tracings of the North of Europe,” 1850, pp. 48−51. |
| |
| [257] Quart. Journ. Geol. Soc., vol. ii., p. 364. |
| |
| [258] “Tracings of the North of Europe,” by Robert Chambers, pp. |
| 259, 285. “Observations sur les Phénomènes d’Erosion en Norvège,” |
| by M. Hörbye, 1857. See also Professor Erdmann’s “Formations |
| Quaternaires de la Suède.” |
| |
| [259] “Glacial Drift of Scotland,” p. 29. |
| |
| [260] Geological Magazine, vol. ii., p. 343. Brit. Assoc. Rep., |
| 1864 (sections), p. 59. |
| |
| [261] Trans. Roy. Soc. Edin., vol. vii., p. 265. |
| |
| [262] “Tracings of Iceland and the Faroe Islands,” p. 49. |
| |
| [263] See Chap. XXIII. |
| |
| [264] Mr. Thomas Belt has subsequently advanced (Quart. Jour. Geol. |
| Soc., vol. xxx., p. 490), a similar explanation of the steppes of |
| Siberia. He supposes that an overflow of ice from the polar basin |
| dammed back all the rivers flowing northward, and formed an immense |
| lake which extended over the lowlands of Siberia, and deposited the |
| great beds of sand and silt with occasional freshwater shells and |
| elephant remains, of which the steppes consist. |
| |
| [265] Proc. Roy. Phys. Soc., Edin., vols. ii. and iii. |
| |
| [266] From Geol. Mag. for January, 1871. |
| |
| [267] Quart. Journ. Geol. Soc., xxvi., p. 517. |
| |
| [268] British Assoc. Report for 1864 (sections), p. 65. |
| |
| [269] Quart. Journ. Geol. Soc., xxvi., p. 90. |
| |
| [270] Geol. Mag., vii., p. 349. |
| |
| [271] Trans. Edin. Geol. Soc., vol. i., p. 136. |
| |
| [272] Geol. Mag. for June, 1870. See Chap. XXVII. |
| |
| [273] This was done by Mr. R. H. Tiddeman of the Geological Survey |
| of England (Quart. Journ. Geol. Soc. for November, 1872), and the |
| result established the correctness of the above opinion as to the |
| existence of a North of England ice-sheet. Additional confirmation |
| has been derived from the important observations of Mr. D. |
| Mackintosh, and also of Mr. Goodchild, of the Geological Survey of |
| England. |
| |
| [274] Trans. Geol. Soc., vol. v., p. 516 (first series). |
| |
| [275] Quart. Journ. Geol. Soc., vol. xi., p. 492. “Memoir of the |
| Country around Cheltenham,” 1857. “Geology of the Country around |
| Woodstock,” 1859. |
| |
| [276] Geol. Mag., vol. vii., p. 497. |
| |
| [277] Quart. Journ. Geol. Soc., vol. xxvi., p. 90. |
| |
| [278] My colleague, Mr. R. L. Jack. |
| |
| [279] The greater portion of this chapter is from the Trans. of |
| Geol. Soc. of Edinburgh, for 1869. |
| |
| [280] Chapter XV., p. 253. |
| |
| [281] Trans. of the Geol. Soc. of Glasgow, vol. iii., part i., page |
| 133. |
| |
| [282] Mr. Milne Home has advanced, in his “Estuary of the Firth of |
| Forth,” p. 91, the theory that this trough had been scooped out |
| during the glacial epoch by icebergs floating through the Midland |
| valley from west to east when it was submerged. The bottom of the |
| trough, be it observed, at the watershed at Kilsyth, is 300 feet |
| above the level of its bottom at Grangemouth; and this Mr. Milne |
| Home freely admits. But he has not explained how an iceberg, which |
| could float across the shallow water at Kilsyth, say, 100 feet |
| deep, could manage to grind the rocky bottom at Grangemouth, where |
| it was not less than 400 feet deep. “The impetus acquired in the |
| Kyle at Kilsyth,” says Mr. Milne Home, “would keep them moving on, |
| and the prevailing westerly winds would also aid, so that when |
| _grating_ on the subjacent carboniferous rocks they would not have |
| much difficulty in scooping out a channel both wider and deeper |
| than at Kilsyth.” But how could they “_grate_ on the subjacent |
| carboniferous rocks” at Grangemouth, if they managed to _float_ |
| at Kilsyth? Surely an iceberg that could “_grate_” at Grangemouth |
| would “_ground_” at Kilsyth. |
| |
| [283] Trans. of the Geol. Soc. of Glasgow, vol. iii., p. 141. |
| |
| [284] Mr. John Young and Mr. Milne Home advanced the objection, |
| that several trap dykes cross the valley of the Clyde near Bowling, |
| and come to so near the present surface of the land, that the |
| Clyde at present flows across them with a depth not exceeding |
| 20 feet. I fear that Mr. Young and Mr. Milne Home have been |
| misinformed in regard to the existence of these dykes. About a mile |
| _above_ Bowling there are one or two dykes which approach to the |
| river-bank, and may probably cross, but these could not possibly |
| cut off a channel entering the Clyde at Bowling. In none of the |
| borings or excavations which have been made by the Clyde Trustees |
| has the rock been reached from Bowling downwards. I may also state |
| that the whole Midland valley, from the Forth of Clyde to the Firth |
| of Forth, has been surveyed by the officers of the Geological |
| Survey, and only a single dyke has been found to cross the buried |
| channels, viz., one (Basalt rock) running eastward from Kilsyth to |
| the canal bridge near Dullatur. But as this is not far from the |
| watershed between the two channels it cannot affect the question at |
| issue. See sheet 31 of Geological Survey Map of Scotland. |
| |
| [285] Trans. Geol. Soc. Glasgow, vol. iv., p. 166. |
| |
| [286] “Great Ice Age,” chap. xiii. |
| |
| [287] See further particulars in Mr. Bennie’s paper on the Surface |
| Geology of the district around Glasgow, Trans. Geol. Soc. of |
| Glasgow, vol. iii. |
| |
| [288] See also Smith’s “Newer Pliocene Geology,” p. 139. |
| |
| [289] British Association Report for 1863, p. 89. _Geologist_ for |
| 1863, p. 384. |
| |
| [290] See Geological Magazine, vol. ii., p. 38. |
| |
| [291] Proc. Geol. Soc., vol. iii., 1840, p. 342. |
| |
| [292] “Antiquity of Man” (Third Edition), p. 249. |
| |
| [293] “Glacial Drift of Scotland,” p. 65. Trans. Geol. Soc. Glas., |
| vol. i., part 2. |
| |
| [294] “Memoir, Geological Survey of Scotland,” Sheet 23, p. 42. |
| |
| [295] Mr. Robert Dick had previously described, in the Trans. Geol. |
| Soc. Edinburgh, vol. i., p. 345, portions of these buried channels. |
| He seems, however, to have thought that they formed part of one and |
| the same channel. |
| |
| [296] A description of this channel was read to the Natural History |
| Society of Glasgow by Mr. James Coutts, the particulars of which |
| will appear in the Transactions of the Society. |
| |
| [297] “Occasional Papers,” pp. 166, 223. |
| |
| [298] Memoir read before the Royal Society, January 7, 1869. |
| |
| [299] “Alpine Journal,” February, 1870. |
| |
| [300] Phil. Mag., January, 1872. |
| |
| [301] Phil. Mag., July, 1870; February, 1871. |
| |
| [302] Philosophical Magazine for January, 1870, p. 8; Proceedings |
| of the Royal Society for January, 1869. |
| |
| [303] Philosophical Magazine for March, 1869. |
| |
| [304] Proceedings of Bristol Naturalists’ Society, p. 37 (1869). |
| |
| [305] Ibid., vol. iv., p. 37 (new series). |
| |
| [306] Phil. Mag., S. 4, vol. x., p. 303. |
| |
| [307] Proceedings of the Bristol Naturalists’ Society, vol. iv., p. |
| 39 (new series). |
| |
| [308] See Philosophical Transactions, December, 1857. |
| |
| [309] There is one circumstance tending slightly to prevent the |
| rupture of the glacier, when under tension, which I do not remember |
| to have seen noticed; that is, the cooling effect which is produced |
| in solids, such as ice, when subjected to tension. Tension would |
| tend to lower the temperature of the ice-molecules, and this |
| lowering of temperature would have the tendency of freezing them |
| more firmly together. The cause of this cooling effect will be |
| explained in the Appendix. |
| |
| [310] Phil. Mag., March, 1869; September, 1870. |
| |
| [311] “Forms of Water,” p. 127. |
| |
| [312] See text, p. 10. |
| |
| [313] Mathematical and Physical Series, vol. xxxvi. (1765). |
| |
| [314] “Memoirs of St. Petersburg Academy,” 1761. |
| |
| [315] The calculations here referred to were made by Lagrange |
| nearly half a century previous to the appearance of this paper, and |
| published in the “Mémoires de l’Académie de Berlin,” for 1782, p. |
| 273. Lagrange’s results differ but slightly from those afterwards |
| obtained by Leverrier, as will be seen from the following table; |
| but as he had assigned erroneous values to the masses of the |
| smaller planets, particularly that of Venus, the mass of which he |
| estimated at one-half more than its true value, full confidence |
| could not be placed in his results. |
| |
| Superior limits of eccentricity as determined by Lagrange, |
| Leverrier, and Mr. Stockwell:— |
| |
| By Lagrange. By Leverrier. By Mr. Stockwell. |
| |
| Mercury 0·22208 0·225646 0·2317185 |
| Venus 0·08271 0·086716 0·0706329 |
| Earth 0·07641 0·077747 0·0693888 |
| Mars 0·14726 0·142243 0·139655 |
| Jupiter 0·06036 0·061548 0·0608274 |
| Saturn 0·08408 0·084919 0·0843289 |
| Uranus — 0·064666 0·0779652 |
| Neptune — — 0·0145066 |
| |
| [J. C.] |
| |
| [316] “Mém. de l’Acad. royale des Sciences.” 1827. Tom. vii., p. |
| 598. |
| |
| [317] Absolute zero is now considered to be only 493° Fah. below |
| the freezing-point, and Herschel himself has lately determined |
| 271° below the freezing-point to be the temperature of space. |
| Consequently, a decrease, or an increase of one per cent. in the |
| mean annual amount of radiation would not produce anything like the |
| effect which is here supposed. But the mean annual amount of heat |
| received cannot vary much more than one-tenth part of one per cent. |
| In short, the effect of eccentricity on the mean annual supply of |
| heat received from the sun, in so far as geological climate is |
| concerned, may be practically disregarded.—[J. C.] |
| |
| [318] “Principles of Geology,” p. 110. “Mr. Lyell, however, in |
| stating the actual excess of eight days in the duration of the |
| sun’s presence in the northern hemisphere over that in the southern |
| as productive of an excess of light and heat annually received by |
| the one over the other hemisphere, appears to have misconceived the |
| effect of elliptic motion in the passage here cited, since it is |
| demonstrable that whatever be the ellipticity of the earth’s orbit |
| the two hemispheres must receive equal absolute quantities of light |
| and heat per annum, the proximity of the sun in perigee exactly |
| compensating the effect of its swifter motion. This follows from a |
| very simple theorem, which may be thus stated: ‘The amount of heat |
| received by the earth from the sun while describing any part of |
| its orbit is proportional to the angle described round the sun’s |
| centre,’ so that if the orbit be divided into two portions by a |
| line drawn _in any direction_ through the sun’s centre, the heats |
| received in describing the two unequal segments of the ellipse so |
| produced will be equal.” |
| |
| [319] When the eccentricity of the earth’s orbit is at its superior |
| limit, the absolute quantity of heat received by the globe during |
| one year will be increased by only 1/300th part; an amount which |
| could produce no sensible influence on climate.—[J. C.] |
| |
| [320] Sir Charles has recently, to a certain extent, adopted the |
| views advocated in the present volume, viz., that the cold of the |
| glacial epoch was brought about not by a _decrease_, but by an |
| _increase_ of eccentricity. (See vol. i. of “Principles,” tenth |
| and eleventh editions.) The decrease in the mean annual quantity |
| of heat received from the sun, resulting from the decrease in |
| the eccentricity of the earth’s orbit—the astronomical cause to |
| which he here refers—could have produced no sensible effect on |
| climate.—[J. C.] |
| |
| [321] It is singular that both Arago and Humboldt should appear to |
| have been unaware of the researches of Lagrange on this subject. |
| |
| [322] “Révolutions de la Mer,” p. 37. Second Edition. |
| |
| [323] See text, p. 37. |
| |
| [324] See _Philosophical Magazine_ for December, 1867, p. 457. |
| |
| [325] _Silliman’s American Journal_ for July, 1864. _Philosophical |
| Magazine_ for September, 1864, pp. 193, 196. |
| |
| [326] _Philosophical Magazine_ for August, 1865, p. 95. |
| |
| [327] See text, p. 80. |
| |
| [328] See text, p. 222. |
| |
| [329] Proc. Roy. Soc., No. 157, 1875. |
| |
| [330] See text, p. 522. |
| |
| [331] Phil. Trans. for 1859, p. 91. |
| |
| [332] See text, p. 527. |
| |
+----------------------------------------------------------------------+
Transcriber’s Notes:
- Text enclosed by underscores is in italics (_italics_).
- Blank pages have been removed.
- Obvious typographical errors have been silently corrected.
- Some spelling and hyphenation variations have been made consistent.
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