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Title: Volcanoes: Past and Present
Author: Hull, Edward
Language: English
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*** Start of this LibraryBlog Digital Book "Volcanoes: Past and Present" ***
The Contemporary Science Series.
Edited by Havelock Ellis.
VOLCANOES: PAST AND PRESENT.
[Illustration: Fig. 1.--Eruption of Vesuvius, 1872-1873]
VOLCANOES: PAST AND PRESENT.
by
EDWARD HULL, M.A., LL.D., F.R.S.
Examiner in Geology to the University of London.
With 41 Illustrations and 4 Plates of Rock-Sections.
London:
Walter Scott, Limited,
24, Warwick Lane, Paternoster Row.
1892.
_BY THE SAME AUTHOR._
The Coal-fields of Great Britain: their History, Structure, and
Resources. 4th edit. (1881.) E. Stanford.
The Physical History of the British Isles. With a Dissertation on
the Origin of Western Europe and of the Atlantic Ocean. (1882.) E.
Stanford.
The Physical Geology and Geography of Ireland. 2nd edit. (1891.)
E. Stanford.
Treatise on the Building and Ornamental Stones of Great Britain
and Foreign Countries. (1872.) Macmillan and Co.
Memoir on the Physical Geology and Geography of Arabia-Petræa,
Palestine, and adjoining Districts. (1886.) Committee of the
Palestine Exploration Fund.
Mount Seir, Sinai, and Western Palestine. Being a Narrative of a
Scientific Expedition, 1883-84. (1885.) Committee of the Palestine
Exploration Fund.
Text-book of Physiography. (1888.) C. W. Deacon and Co.
Sketch of Geological History. (1887.) C. W. Deacon and Co.
PREFACE.
It has not been my object to present in the following pages even an
approximately complete description of the volcanic and seismic phenomena
of the globe; such an undertaking would involve an amount of labour
which few would be bold enough to attempt; nor would it be compatible
with the aims of the _Contemporary Science Series_.
I have rather chosen to illustrate the most recent conclusions regarding
the phenomena and origin of volcanic action, by the selection of
examples drawn from the districts where these phenomena have been most
carefully observed and recorded under the light of modern geological
science. I have also endeavoured to show, by illustrations carried back
into later geological epochs, how the volcanic phenomena of the present
day do not differ in kind, though they may in degree, from those of the
past history of our globe. For not only do the modes of eruption of
volcanic materials in past geological times resemble those of the
present or human epoch, but the materials themselves are so similar in
character that it is only in consequence of alterations in structure or
composition which the original materials have undergone, since their
extrusion, that any important distinctions can be recognised between the
volcanic products of recent times and those of earlier periods.
I have, finally, endeavoured to find an answer to two interesting and
important questions: (1) Are we now living in an epoch of extraordinary
volcanic energy?--a question which such terrible outbursts as we have
recently witnessed in Japan, the Malay Archipelago, and even in Italy,
naturally suggest; and (2) What is the ultimate cause of volcanic
action? On this latter point I am gratified to find that my conclusions
are in accordance with those expounded by one who has been appropriately
designated "the Nestor of Modern Geology," Professor Prestwich.
Within the last few years the study of the structure and composition of
volcanic rocks, by means of the microscope brought to bear on their
translucent sections, has added wonderfully to our knowledge of such
rocks, and has become a special branch of petrological investigation.
Commenced by Sorby, and carried on by Allport, Zirkel, Rosenbusch, Von
Lasaulx, Teall, and many more enthusiastic students, it has thrown a
flood of light upon our knowledge of the mutual relations of the
component minerals of igneous masses, the alteration these minerals have
undergone in some cases, and the conditions under which they have been
erupted and consolidated. But nothing that has been observed has tended
materially to alter conclusions arrived at by other processes of
reasoning regarding volcanic phenomena, and for these we have to fall
back upon observations conducted in the field on a more or less large
scale, and carried on before, during, and after eruptions. Macroscopic
and microscopic observations have to go hand in hand in the study of
volcanic phenomena.
E. H.
CONTENTS.
PART I.
_INTRODUCTION._
PAGE
Chap. I. Historic Notices of Volcanic Action 1-9
" II. Form, Structure, and Composition of Volcanic
Mountains 10-19
" III. Lines and Groups of Active Volcanic Vents 20-29
" IV. Mid-ocean Volcanic Islands 30-40
PART II.
_EUROPEAN VOLCANOES._
Chap. I. Vesuvius 41-60
" II. Etna 61-68
" III. The Lipari Islands, Stromboli 69-75
" IV. The Santorin Group 76-83
" V. European Extinct or Dormant Volcanoes 84-91
" VI. Extinct Volcanoes of Central France 92-112
" VII. The Volcanic District of the Rhine Valley 13-125
PART III.
_DORMANT OR MORIBUND VOLCANOES OF OTHER PARTS OF THE WORLD._
Chap. I. Dormant Volcanoes of Palestine and Arabia 126-135
" II. The Volcanic Regions of North America 136-145
" III. Volcanoes of New Zealand 146-153
PART IV.
_TERTIARY VOLCANIC DISTRICTS OF THE BRITISH ISLES._
Chap. I. Antrim 154-159
" II. Succession of Volcanic Eruptions 160-171
" III. Island of Mull and Adjoining Coast 172-176
" IV. Isle of Skye 177-179
" V. The Scuir of Eigg 180-184
" VI. Isle of Staffa 185-186
PART V.
_PRE-TERTIARY VOLCANIC ROCKS._
Chap. I. The Deccan Trap-series of India 187-189
" II. Abyssinian Table-lands 190-193
" III. Cape Colony 194-195
" IV. Volcanic Rocks of Past Geological
Periods of the British Isles 196-199
PART VI.
_SPECIAL VOLCANIC AND SEISMIC PHENOMENA._
Chap. I. The Eruption of Krakatoa in 1883 201-216
" II. Earthquakes 217-224
PART VII.
_VOLCANIC AND SEISMIC PROBLEMS._
Chap. I. The Ultimate Cause of Volcanic Action 225-235
" II. Lunar Volcanoes 236-252
" III. Are we Living in an Epoch of Special
Volcanic Activity? 253-257
APPENDIX.
A Brief Account of the Principal Varieties of
Volcanic Rocks 259-265
Index 268
ILLUSTRATIONS.
Fig. 1. Eruption of Vesuvius, 1872-73 _Frontispiece_
" 2. Cotopaxi _Page_ 16
" 3. Volcanic Cone of Orizaba " 21
Map of the World, showing Active and
Extinct Volcanoes " 23
" 4. Teneriffe, seen from the Ocean " 31
" 5. View of the Summit of Teneriffe " 35
" 6. Probable Aspect of Vesuvius at Beginning
of Christian Era " 43
" 7. View of Vesuvius before 1767 " 50
" 8. Map of District bordering Bay of Naples " 52
" 9. View of Vesuvius in 1872 " 53
" 10. Ideal Section through Etna " 63
" 11. Map of the Lipari Islands " 70
" 12. The Island of Vulcano in Eruption " 71
" 13. Ideal Section through Gulf of Santorin " 76
" 14. Bird's-eye View of Gulf of Santorin " 79
" 15. Ground Plan of Rocca Monfina " 80
" 16. Geological Section of Tiber Valley at
Rome " 88
" 17. Generalised Section Through the Vale
of Clermont " 93
Fig. 18. View of Puy de Dôme and Neighbouring
Volcanoes _Page_ 95
" 19. Mont Demise, seen from the S.E. " 103
" 20. Sketch Map of Rhenish Area in the
Miocene Epoch " 114
" 21. The Volcanic Range of the Siebengebirge " 117
" 22. Section of Extinct Crater of the Roderberg " 120
" 23. Plan and Section of the Laacher See " 122
" 24. Extinct Craters in the Jaulân " 130
" 25. Mount Shasta " 139
" 26. Forms of Volcanic Tuff-Cones, Auckland " 148
" 27. "The White Rocks," Portrush, Co.
Antrim " 157
" 28. Section across the Volcanic Plateau
of Antrim " 159
" 29. Section at Templepatrick " 161
" 30. Cliff above the Giant's Causeway " 163
" 31. The Giant's Causeway, Co. Antrim " 165
" 32. "The Chimneys," North Coast of
Antrim " 166
" 33. Section at Alt na Searmoin, Mull " 175
" 34. View of the Scuir of Eigg from the East " 181
Map of Volcanic Band of the Moluccas " 200
" 35. Map of the Krakatoa Group of Islands " 203
" 36. Section from Verlaten Island through
Krakatoa " 204
Fig. 37. Isoseismals of the Charleston Earthquake _Page_ 223
" 38. Photograph of the Moon's Surface " 241
" 39. Portion of the Moon's Surface " 243
_PLATES._
I. & II. Magnified Sections of Vesuvian Lavas.
III. & IV. Magnified Sections of Volcanic Rocks.
Volcanoes: Past and Present.
PART I.
INTRODUCTION.
CHAPTER I.
HISTORIC NOTICES OF VOLCANIC ACTION.
There are no manifestations of the forces of Nature more calculated to
inspire us with feelings of awe and admiration than volcanic eruptions
preceded or accompanied, as they generally are, by earthquake shocks.
Few agents have been so destructive in their effects; and to the real
dangers which follow such terrestrial convulsions are to be added the
feelings of uncertainty and revulsion which arise from the fact that the
earth upon which we tread, and which we have been accustomed to regard
as the emblem of stability, may become at any moment the agent of our
destruction. It is, therefore, not surprising that the ancient Greeks,
who, as well as the Romans, were close observers of the phenomena of
Nature, should have investigated the causes of terrestrial disturbances,
and should have come to some conclusions upon them in accordance with
the light they possessed. These terrible forces presented to the Greeks,
who clothed all the operations of Nature in poetic imagery and deified
her forces, their poetical and mystical side; and as there was a deity
for every natural force, so there was one for earthquakes and volcanoes.
Vulcan, the deformed son of Juno (whose name bears so strange a
resemblance to that of "the first artificer in iron" of the Bible, Tubal
Cain), is condemned to pass his days under Mount Etna, fabricating the
thunderbolts of Jove, and arms for the gods and great heroes of
antiquity.
The Pythagoreans appear to have held the doctrine of a central fire
(meson pyr) as the source of volcanic phenomena; and in the Dialogues of
Plato allusion is made to a subterranean reservoir of lava, which,
according to Simplicius, was in accordance with the doctrine of the
Pythagoreans which Plato was recounting.[1] Thucydides clearly describes
the effect of earthquakes upon coast-lines of the Grecian Archipelago,
similar to that which took place in the case of the earthquake of
Lisbon, the sea first retiring and afterwards inundating the shore.
Pliny supposed that it was by earthquake avulsion that islands were
naturally formed. Thus Sicily was torn from Italy, Cyprus from Syria,
Euboea from Boeotia, and the rest; but this view was previously
enunciated by Aristotle in his "Peri kosmou," where he states that
earthquakes have torn to pieces many parts of the earth, while lands
have been converted into sea, and that tracts once covered by the sea
have been converted into dry land.
But the most philosophical views regarding terrestrial phenomena are
those given by Ovid as having been held by Pythagoras (about B.C. 580).
In the _Metamorphoses_ his views regarding the interchange of land and
sea, the effects of running water in eroding valleys, the growth of
deltas, the effect of earthquakes in burying cities and diverting
streams from their sources, are remarkable anticipations of doctrines
now generally held.[2] But what most concerns us at present are his
views regarding the changes which have come over volcanic mountains. In
his day Vesuvius was dormant, but Etna was active; so his illustrations
are drawn from the latter mountain; and in this connection he observes
that volcanic vents shift their position. There was a time, he says,
when Etna was not a burning mountain, and the time will come when it
will cease to burn; whether it be that some caverns become closed up by
the movements of the earth, or others opened, or whether the fuel is
finally exhausted.[3] Strabo may be regarded as having originated the
view, now generally held, that active volcanoes are safety-valves to the
regions in which they are situated. Referring to the tradition recorded
by Pliny, that Sicily was torn from Italy by an earthquake, he observes
that the land near the sea in those parts was rarely shaken by
earthquakes, since there are now orifices whereby fire and ignited
matters and waters escape; but formerly, when the volcanoes of Etna, the
Lipari Islands, Ischia, and others were closed up, the imprisoned fire
and wind might have produced far more violent movements.[4]
The account of the first recorded eruption of Vesuvius has been
graphically related by the younger Pliny in his two letters to Tacitus,
to which I shall have occasion to refer further on.[5] These bring down
the references to volcanic phenomena amongst ancient authors to the
commencement of the Christian era; from all of which we may infer that
the more enlightened philosophers of antiquity had a general idea that
eruptions had their origin in a central fire within the interior of the
earth, that volcanic mountains were liable to become dormant for long
periods, and afterwards to break out into renewed activity, that there
existed a connection between volcanic action and earthquakes, and that
volcanoes are safety-valves for the regions around.
It is unnecessary that I should pursue the historical sketch further.
Those who wish to know the views of writers of the Middle Ages will find
them recorded by Sir Charles Lyell.[6] The long controversy carried on
during the latter part of the eighteenth century between "Neptunists,"
led by Werner on the one side, and "Vulcanists," led by Hutton and
Playfair on the other, regarding the origin of such rocks as granite and
basalt, was finally brought to a close by the triumph of the
"Vulcanists," who demonstrated that such rocks are the result of igneous
fusion; and that in the cases of basalt and its congeners, they are
being extruded from volcanic vents at the present day. The general
principles for the classification of rocks as recognised in modern
science may be regarded as having been finally established by James
Hutton, of Edinburgh, in his _Theory of the Earth_,[7] while they were
illustrated and defended by Professor Playfair in his work entitled,
_Illustrations of the Huttonian Theory of the Earth_,[8] although other
observers, such as Desmarest, Collini, and Guettard, had in other
countries come to very clear views on this subject.
The following are some of the more important works on the phenomena of
volcanoes and earthquakes published during the present century:--[9]
1. Poulett Scrope, F.R.S., _Considerations on Volcanoes_ (1825). This
work is dedicated to Lyell, his fellow-worker in the same department of
science, and was undertaken, as he says, "in order to help to dispel
that signal delusion as to the mode of action of the subtelluric forces
with which the Elevation-Crater theory had mystified the geological
world." The second edition was published in 1872.
2. This was followed by the admirable work, _On the Extinct Volcanoes
of Central France_, published in 1826 (2nd edition, 1858), and is one of
the most complete monographs on a special volcanic district ever
written.
3. Dr. Samuel Hibbert, _History of the Extinct Volcanoes of the Basin of
Neuwied on the Lower Rhine_ (1832). Dr. Hibbert's work is one of
remarkable merit, if we consider the time at which it was written. For
not only does it give a clear and detailed account of the volcanic
phenomena of the Eifel and the Lower Rhine, but it anticipates the
principles upon which modern writers account for the formation of river
valleys and other physical features; and in working out the physical
history of the Rhine valley below Mainz, and its connection with the
extinct volcanoes which are found on both banks of that river, he has
taken very much the same line of reasoning which was some years
afterwards adopted by Sir A. Ramsay when dealing with the same subject.
It does not appear that the latter writer was aware of Dr. Hibbert's
treatise.
4. Leopold von Buch, _Description Physique des Iles Canaries_ (1825),
translated from the original by C. Boulanger (1836); _Geognostische
Reise_ (Berlin, 1809), 2 vols.; and _Reise durch Italien_ (1809). From a
large number of writings on volcanoes by this distinguished traveller,
whom Alexander von Humboldt calls "dem geistreichen Forscher der Natur,"
the above are selected as being the most important. That on the Canaries
is accompanied by a large atlas, in which the volcanoes of Teneriffe,
Palma, and Lancerote, with some others, are elaborately represented, and
are considered to bear out the author's views regarding the formation
of volcanic cones by elevation or upheaval. The works dealing with the
volcanic phenomena of Central and Southern Italy are also written with
the object, in part at least, of illustrating and supporting the same
theoretical views; with these we have to deal in the next chapter.
5. Dr. Charles Daubeny, F.R.S., _Description of Active and Extinct
Volcanoes, of Earthquakes, and of Thermal Springs, with remarks on the
causes of these phenomena, the character of their respective products,
and their influence on the past and present condition of the globe_ (2nd
edition, 1848). In this work the author gives detailed descriptions of
almost all the known volcanic districts of the globe, and defends what
is called "the chemical theory of volcanic action"--a theory at one time
held by Sir Humphrey Davy.
6. Wolfgang Sartorius von Waltershausen, _Der Ætna_. This work possesses
a melancholy interest from the fact that its distinguished author did
not live to see its publication. Von Waltershausen, having spent several
years in making an elaborate survey of Etna, produced an atlas
containing numerous detailed maps, views, and drawings of this mountain
and its surroundings, which were published at Weimar by Engelmann in
1858. A description in MS. to accompany the atlas was also prepared, but
before it was printed, the author died, on the 16th October 1876. The
MS. having been put into the hands of the late Professor Arnold von
Lasaulx by the publisher of the atlas, it was subsequently brought out
under the care of this distinguished petrologist, who was so fully
fitted for an undertaking of this kind.
7. Sir Charles Lyell in his _Principles of Geology_[10] devotes several
chapters to the consideration of volcanic phenomena, in which, being in
harmony with the views of his friend, Poulett Scrope, he combats the
"elevation theory" of Von Buch, as applied to the formation of volcanic
mountains, holding that they are built up of ashes, stones, and scoriæ
blown out of the throat of the volcano and piled around the orifice in a
conical form. Together with these materials are sheets of lava extruded
in a molten condition from the sides or throat of the crater itself.
8. Professor J. W. Judd, F.R.S., in his able work entitled, _Volcanoes:
What they are, and what they teach_,[11] has furnished the student of
vulcanicity with a very complete manual of a general character on the
subject. The author, having extensive personal acquaintance with the
volcanoes of the south of Europe and the volcanic rocks of the British
Isles, was well equipped for undertaking a work of the kind; and in it
he supports the views of Lyell and Scrope regarding the mode of
formation of volcanic mountains.
9. Sir Archibald Geikie, F.R.S., in his elaborate monograph[12] on the
Tertiary Volcanic Rocks of the British Isles, has recorded his views
regarding the origin and succession of the plateau basalts and
associated rocks over the region extending from the north of Ireland to
the Inner Hebrides; and in dealing with these districts in the following
pages I have made extensive use of his observations and conclusions.
10. _Report published by the Royal Society on the Eruption of
Krakatoa_--drawn up by several authors (1885)--and the work on the same
subject by Chev. Verbeek, and published by the Government of the
Netherlands (1886). In these works all the phenomena connected with the
extraordinary eruptions of Krakatoa in 1883 are carefully noted and
scientifically discussed, and illustrated by maps and drawings.
11. _The Charleston Earthquake of August 31, 1886_, by Captain Clarence
Edward Dutton, U.S. Ordnance Corps. Ninth Annual Report of the United
States Geological Survey, 1887-88, with maps and illustrations.
12. Amongst other works which may be consulted with advantage is that of
Mr. T. Mellard Reade on _The Origin of Mountain Ranges_; the Rev. Osmond
Fisher's _Physics of the Earth_; Professor G. H. Darwin and Mr. C.
Davison on "The Internal Tension of the Earth's Crust," _Philosophical
Transactions of the Royal Society_, vol. 178; Mr. R. Mallet, "On the
Dynamics of Earthquakes," _Trans. Roy. Irish Academy_, vol. xxi.;
Professor O'Reilly's "Catalogues of Earthquakes," _Trans. Roy. Irish
Academy_, vol. xxviii. (1884 and 1888); and Mr. A. Ent. Gooch _On the
Causes of Volcanic Action_ (London, 1890). These and other authorities
will be referred to in the text.
[1] See Julius Schwarez _On the Failure of Geological Attempts made by
the Greeks_. (Edition 1888.)
[2]
"Vidi ego, quod fuerat quondam solidissima tellus,
Esse fretum. Vidi factas ex æquore terras:
Et procul à pelago conchæ jacuere marinæ;
Et vetus inventa est in montibus anchora sumnis.
Quodque fuit campus, vallem de cursus aquarum
Fecit; et eluvie mons est deductus in æquor:
Eque paludosa siccis humus aret arenis;
Quæque sitim tulerant, stagnata paludibus hument.
Hic fontes Natura novos emissit, at illuc
Clausit: et antiquis concussa tremoribus orbis
Fulmina prosiliunt...."
--Lib. xv. 262.
[3]
"Nec, quæ sulfureis ardet fornacibus, Ætne
Ignea semper erit; neque enim fuit ignea semper.
Nam, sive est animal tellus, et vivit, habetque
Spiramenta locis flammam exhalantia multis;
Spirandi mutare vias, quotiesque movetur,
Has finire potest, illas aperire cavernas:
Sive leves imis venti cohibentur in antris;
Saxaque cum saxis...."
--_Ibid._, 340.
[4] Strabo, lib. vi.
[5] Tacitus, lib. vi. 16, 20.
[6] _Principles of Geology_, 11th edition, vol. i., ch. 3.
[7] 2 vols., Edin. (1795).
[8] Edin. (1802).
[9] A more extended list of early works will be found in Daubeny's
_Volcanoes_ (1848).
[10] 11th edition (1872).
[11] 4th edition (1888).
[12] "The History of Volcanic Action during the Tertiary Period in the
British Isles," _Trans. Roy. Soc., Edin._ Vol. xxxv, (1888).
CHAPTER II.
FORM, STRUCTURE, AND COMPOSITION OF VOLCANIC MOUNTAINS.
The conical form of a volcanic mountain is so generally recognised, that
many persons who have no intelligent acquaintance with geological
phenomena are in the habit of attributing to all mountains having a
conical form, and especially if accompanied by a truncated apex, a
volcanic origin. Yet this is very far from being the fact, as some
varieties of rock, such as quartzite, not unfrequently assume this
shape. Of such we have an example in the case of Errigal, a quartzite
mountain in Donegal, nearly 3000 feet high, which bears a very near
approach in form to a perfect cone or pyramid, and yet is in no way
connected, as regards its origin or structure, with volcanic phenomena.
Another remarkable instance is that of Schehallion in Scotland, also
composed of quartz-rock; and others may be found amongst the ranges of
Islay and Jura, described by Sir A. Geikie.[1]
Notwithstanding, however, such exceptions, which might be greatly
multiplied, the majority of cone-shaped mountains over the globe have a
volcanic origin.[2] The origin of this form in each case is entirely
distinct. In the case of quartzite mountains, the conical form is due to
atmospheric influences acting on a rock of uniform composition,
traversed by numerous joints and fissures crossing each other at obtuse
angles, along which the rock breaks up and falls away, so that the sides
are always covered by angular shingle forming slopes corresponding to
the angle of friction of the rock in question. In the case of a volcanic
mountain, however, the same form is due either to accumulation of
fragmental material piled around the cup-shaped hollow, or crater, which
is usually placed at the apex of the cone, and owing to which it is
bluntly terminated, or else to the welling up from beneath of viscous
matter in the manner presently to be described.
_Views of Sir Humphrey Davy and L. von Buch._--The question how a
volcanic cone came to be formed was not settled without a long
controversy carried on by several naturalists of eminence. Some of the
earlier writers of modern times on the subject of vulcanicity--such as
Sir Humphrey Davy and Leopold von Buch--maintained that the conical form
was due to upheaval by a force acting from below at a central focus,
whereby the materials of which the mountain is formed were forced to
assume a _quâ-quâ versal_ position--that is, a position in which the
materials dip away from the central focus in every direction. But this
view, originally contested by Scrope and Lyell, has now been generally
abandoned. It will be seen on reflection that if a series of strata of
ashes, tuff, and lava, originally horizontal, or nearly so, were to be
forced upwards into a conical form by a central force, the result would
be the formation of a series of radiating fissures ever widening from
the circumference towards the focus. In the case of a large mountain
such fissures, whether filled with lava or otherwise, would be of great
breadth towards the focus, or central crater, and could not fail to make
manifest beyond dispute their mechanical origin. But no fissures of the
kind here referred to are, as a matter of fact, to be observed. Those
which do exist are too insignificant and too irregular in direction to
be ascribed to such an origin; so that the views of Von Buch and Davy
must be dismissed, as being unsupported by observation, and as untenable
on dynamical grounds. As a matter of fact, the "elevatory theory," or
the "elevation-crater theory," as it is called by Scrope, has been
almost universally abandoned by writers on vulcanicity.
_Principal Varieties of Volcanic Mountains as regards Form._--But whilst
rejecting the "elevatory theory," it is necessary to bear in mind that
volcanic cones and dome-shaped elevations have been formed in several
distinct ways, giving rise to varieties of structure essentially
different. Two of the more general of these varieties of form, the
crater-cone and the dome, are found in some districts, as in Auvergne,
side by side. The crater-cone consists of beds or sheets of ashes,
lapilli, and slag piled up in a conical form, with a central crater (or
cup) containing the principal pipe through which these materials have
been erupted; the dome, of a variety of trachytic lava, which has been
extruded in a molten, or viscous, condition from a central pipe, and in
such cases there is no distinct crater. There are other forms of
volcanic mountains, such as those built up of basaltic matter, of which
I shall have to speak hereafter, but the two former varieties are the
most prevalent; and we may now proceed to consider the conditions under
which the crater-cone volcanoes have been formed.
_Crateriform Volcanic Cones._--Of this class nearly all the active
volcanoes of the Mediterranean region--Etna, Vesuvius, Stromboli, and
the Lipari Islands--may be considered as representatives. They consist
essentially of masses of fragmental material, which have from time to
time been blown out of an orifice and piled up around with more or less
regularity (according to the force exerted, and direction of the
prevalent winds), alternating with sheets of lava. In this way mountains
several thousand feet in height and of vast horizontal extent are
formed. The fragmental materials thus accumulated are of all sizes, from
the finest dust up to blocks many tons in weight, the latter being
naturally piled around nearest to the orifice. The fine dust, blown high
into the air by the explosive force of the gases and vapours, is often
carried to great distances by the prevalent winds. Thus during the
eruption of Vesuvius in A.D. 472 showers of ashes, carried high into the
air by the westerly wind, fell over Constantinople at a distance of 750
miles.[3]
These loose, or partially consolidated, fragmental materials are rudely
stratified, and slope downwards and outwards from the edge of the
crater, so as to present the appearance of what is known as "the dip" of
stratified deposits which have been upraised from the horizontal
position by terrestrial forces. It was this excentrical arrangement
which gave rise to the supposition that such volcanic ash-beds had been
tilted up by a force acting in the direction of the volcanic throat, or
orifice of eruption. The interior wall of Monte di Somma, the original
crater of Vesuvius, presents a good illustration of such fragmental
beds. I shall have occasion further on to describe more fully the
structure of this remarkable mountain; so that it will suffice to say
here that this old prehistoric crater, the walls of which enclose the
modern cone of Vesuvius, is seen to be formed of irregular beds of ash,
scoriæ, and fragmental masses, traversed by numerous dykes of lava, and
sloping away outwards towards the surrounding plains.
Of similar materials are the flanks of Etna composed, even at great
distances from the central crater; the beds of ash and agglomerate
sometimes alternating with sheets of solidified lava and traversed by
dykes of similar material of later date, injected from below through
fissures formed during periods of eruptive energy. Numerous similar
examples are to be observed in the Auvergne region of Central France and
the Eifel. And here we find remarkable cases of "breached cones," or
craters, which will require some special description. Standing on the
summit of the Puy de Dôme, and looking northwards or southwards, the eye
wanders over a tract formed of dome-shaped hills and of extinct
crater-cones rising from a granitic platform. But what is most peculiar
in the scene is the ruptured condition of a large number of the cones
with craters. In such cases the wall of the crater has been broken down
on one side, and we observe that a stream of lava has been poured out
through the breach and overflowed the plain below. The cause of this
breached form is sufficiently obvious. In such cases there has been an
explosion of ashes, stones, and scoriæ from the volcanic throat, by
which a cone-shaped hill with a crater has been built up. This has been
followed by molten lava welling up through the throat, and gradually
filling the crater. But, as the lava is much more dense than the
material of which the crater wall is composed, the pressure of the lava
outwards has become too great for the resistance of the wall, which
consequently has given way at its weakest part and, a breach being
formed, the molten matter has flowed out in a stream which has inundated
the country lying at the base of the cone. In one instance mentioned by
Scrope, the original upper limit of the lake of molten lava has left its
mark in the form of a ring of slag on the inside of the breached
crater.[4]
_Craterless Domes._--These differ essentially both in form and
composition from those just described, and have their typical
representatives in the Auvergne district, though not without their
analogues elsewhere, as in the case of Chimborazo, in South America, one
of the loftiest volcanic mountains in the world.
[Illustration: Fig. 2.--Cotopaxi, a volcano of the Cordilleras of
Quito, still active, and covered by snow down to a level of 14,800 feet.
Below this is a zone of naked rock, succeeded by another of forest
vegetation. Owing to the continuous extrusion of lava from the crater,
the cone is being gradually built up of fresh material, and the crater
is comparatively small in consequence.--(A diagrammatic view after A.
von Humboldt.)]
Taking the Puy de Dôme, Petit Suchet, Cliersou, Grand Sarcoui in
Auvergne, and the Mamelon in the Isle of Bourbon as illustrations, we
have in all these cases a group of volcanic hills, dome-shaped and
destitute of craters, the summits being rounded or slightly flattened.
We also observe that the flanks rise more abruptly from their bases, and
contrast in outline with the graceful curve of the crater cones. The
dome-shaped volcanoes are generally composed of felsitic matter, whether
domite, trachyte, or andesite, which has been extruded in a molten or
viscous condition from some orifice or fissure in the earth's crust, and
being piled up and spreading outwards, necessarily assumes such a form
as that of a dome, as has been shown by experiment on a small scale by
Dr. E. Reyer, of Grätz.[5] The contrast between the two forms (those of
the dome and the crater-cone) is exemplified in the case of the Grand
Sarcoui and its neighbours. The former is composed of a species of
trachyte; the latter of ashes and fragmental matter which have been
blown out of their respective vents of eruption into the air, and piled
up and around in a crateriform manner with sides of gradually
diminishing slope outwards, thus giving rise to the characteristic
volcanic curve. The two varieties here referred to, contrasting in form,
composition, and colour of material, can be clearly recognised from the
summit of the Puy de Dôme, which rises by a head and shoulders above its
fellows, and thus affords an advantageous standpoint from which to
compare the various forms of this remarkable group of volcanic
mountains.
Cotopaxi (Fig. 2) has been generally supposed to be a dome; but Whymper,
who ascended the mountain in 1880, shows that it is a cone with a
crater, 2,300 feet in largest diameter. He determined the height to be
19,613 feet above the ocean. Its real elevation above the sea is
somewhat masked, owing to the fact that it rises from the high plain of
Tapia, which is itself 8,900 feet above the sea surface. The smaller
peak on the right (Fig. 2) is that of Carihuairazo, which reaches an
elevation of over 16,000 feet.
Chimborazo, in Columbia, province of Quito, is one of the loftiest of
the chain of the Andes, and is situated in lat. 1° 30' S., long. 78° 58'
W. Though not in a state of activity, it is wholly composed of volcanic
material, and reaches an elevation of over 20,000 feet above the ocean;
its sides being covered by a sheet of permanent snow to a level of 2,600
feet below the summit.[6] Seen from the shores of the Pacific, after the
long rains of winter, it presents a magnificent spectacle, "when the
transparency of the air is increased, and its enormous circular summit
is seen projected upon the deep azure blue of the equatorial sky. The
great rarity of the air through which the tops of the Andes are seen
adds much to the splendour of the snow, and aids the magical effect of
its reflection."
Chimborazo was ascended by Humboldt and Bonpland in 1802 almost to the
summit; but at a height of 19,300 feet by barometrical measurement,
their further ascent was arrested by a wide chasm. Boussingault, in
company with Colonel Hall, accomplished the ascent as far as the foot of
the mass of columnar "trachyte," the upper surface of which, covered by
a dome of snow, forms the summit of the mountain. The whole mass of the
mountain consists of volcanic rock, varieties of andesite; there is no
trace of a crater, nor of any fragmental materials, such as are usually
ejected from a volcanic vent of eruption.[7]
_Lava Crater-Cones._--A third form of volcanic mountain is that which
has been built up by successive eruptions of basic lava, such as basalt
or dolerite, when in a molten condition. These are very rare, and the
slope of the sides depends on the amount of original viscosity. Where
the lava is highly fused its slope will be slight, but if in a viscous
condition, successive outpourings from the orifice, unable to reach the
base of the mountain, will tend to form a cone with increasing slope
upwards. Mauna Loa and Kilauea, in the Hawaiian Group, according to
Professor J. D. Dana, are basalt volcanoes in a normal state. They have
distinct craters, and the material of which the mountain is formed is
basalt or dolerite. The volcano of Rangitoto in Auckland, New Zealand,
appears to belong to this class.
Basalt is the most fusible of volcanic rocks, owing to the augite and
magnetite it contains, so that it spreads out with a very slight slope
when highly fused. Trachyte, on the other hand, is the least fusible
owing to the presence of orthoclase felspar, or quartz; so that the
volcanic domes formed of this material stand at a higher angle from the
horizon than those of basaltic cones.
[1] _Scenery and Geology of Scotland_ (1865), p. 214.
[2] Humboldt says: "The form of isolated conical mountains, as those of
Vesuvius, Etna, the Peak of Teneriffe, Tunguagua, and Cotopaxi, is
certainly the shape most commonly observed in volcanoes all over the
globe."--_Views of Nature_, translated by E. C. Otté and H. G. Bohn
(1850).
[3] It is supposed that after the disastrous explosion of Krakatoa in
1883 the fine dust carried into the higher regions of the atmosphere was
carried round almost the entire globe, and remained suspended for a
lengthened period, as described in a future page.
[4] Another remarkable case is mentioned and figured by Judd, where one
of the Lipari Isles, composed of pumice and rising out of the
Mediterranean, has been breached by a lava-stream of obsidian.--_Loc.
cit._, p. 123.
[5] Reyer has produced such dome-shaped masses by forcing a quantity of
plaster of Paris in a pasty condition up through an orifice in a board;
referred to by Judd, _loc. cit._, p. 125.
[6] Whymper determined the height to be 20,498 feet; Reiss and Stübel
make it 20,703 feet. Whymper thinks there may be a crater concealed
beneath the dome of snow.--_Travels amongst the Great Andes of the
Equator_, by Edward Whymper (1892).
[7] Whymper states that there is a prevalent idea that Cotopaxi and a
volcano called Sangai act as safety-valves to each other. Sangai reaches
an elevation (according to Reiss and Stübel) of 17,464 feet, and sends
intermittent jets of steam high into the air, spreading out into vast
cumulus clouds, which float away southwards, and ultimately
disappear.--_Ibid._, p. 73.
CHAPTER III.
LINES AND GROUPS OF ACTIVE VOLCANIC VENTS.
The globe is girdled by a chain of volcanic mountains in a state of
greater or less activity, which may perhaps be considered a girdle of
safety for the whole world, through which the masses of molten matter in
a state of high pressure beneath the crust find a way of escape; and
thus the structure of the globe is preserved from even greater
convulsions than those which from time to time take place at various
points on its surface. This girdle is partly terrestrial, partly
submarine; and commencing at Mount Erebus, near the Antarctic Pole,
ranging through South Shetland Isle, Cape Horn, the Andes of South
America, the Isthmus of Panama, then through Central America and Mexico,
and the Rocky Mountains to Kamtschatka, the Aleutian Islands, the
Kuriles, the Japanese, the Philippines, New Guinea, and New Zealand,
reaches the Antarctic Circle by the Balleny Islands. This girdle sends
off branches at several points. (See Map, p. 23.)
[Illustration: Fig. 3.--Volcanic cone of Orizaba (Cittaltepeth), in
Mexico, now extinct; the upper part snow-clad, and at its base forest
vegetation; it reaches a height of 16,302 Parisian feet above the
sea.--(After A. von Humboldt.)]
(_a._) The linear arrangement of active or dormant volcanic vents has
been pointed out by Humboldt, Von Buch, Daubeny, and other writers. The
great range of burning mountains of the Andes of Chili, Peru, Bolivia,
and Mexico, that of the Aleutian Islands, of Kamtschatka and the Kurile
Islands, extending southwards into the Philippines, and the branching
range of the Sunda Islands are well-known examples. That of the West
Indian Islands, ranging from Grenada through St. Vincent, St. Lucia,
Martinique, Dominica, Guadeloupe, Montserrat, Nevis, and St. Eustace,[1]
is also a remarkable example of the linear arrangement of volcanic
mountains. On tracing these ranges on a map of the world[2] (Map, p.
23), it will be observed that they are either strings of islands, or lie
in proximity to the ocean; and hence the view was naturally entertained
by some writers that oceanic water, or at any rate that of a large lake
or sea, was a necessary agent in the production of volcanic eruptions.
This view seems to receive further corroboration from the fact that the
interior portions of the continents and large islands such as Australia
are destitute of volcanoes in action, with the remarkable exceptions of
Mounts Kenia and Kilimanjaro in Central Africa, and a few others. It is
also very significant in this connection that many of the volcanoes now
extinct, or at least dormant, both in Europe and Asia, appear to have
been in proximity to sheets of water during the period of activity. Thus
the old volcanoes of the Haurân, east of the Jordan, appear to have been
active at the period when the present Jordan valley was filled with
water to such an extent as to constitute a lake two hundred miles in
length, but which has now shrunk back to within the present limits of
the Dead Sea.[3] Again, at the period when the extinct volcanoes of
Central France were in active operation, an extensive lake overspread
the tract lying to the east of the granitic plateau on which the craters
and domes are planted, now constituting the rich and fertile plain of
Clermont.
[Illustration: Map Of The World Showing Active And Extinct Volcanoes
(Large Dots)]
Such instances are too significant to allow us to doubt that water in
some form is very generally connected with volcanic operations; but it
does not follow that it was necessary to the original formation of
volcanic vents, whether linear or sporadic. If this were so, the extinct
volcanoes of the British Isles would still be active, as they are close
to the sea-margin, and no volcano would now be active which is not near
to some large sheet of water. But Jorullo, one of the great active
volcanoes of Mexico, lies no less than 120 miles from the ocean, and
Cotopaxi, in Ecuador, is nearly equally distant. Kilimanjaro, 18,881
feet high, and Kenia, in the equatorial regions of Central Africa, are
about 150 miles from the Victoria Nyanza, and a still greater distance
from the ocean; and Mount Demavend, in Persia, which rises to an
elevation of 18,464 feet near the southern shore of the Caspian Sea, a
volcanic mountain of the first magnitude, is now extinct or dormant.[4]
Such facts as these all tend to show that although water may be an
accessory of volcanic eruptions, it is not in all cases essential; and
we are obliged, therefore, to have recourse to some other theory of
volcanic action differing from that which would attribute it to the
access of water to highly heated or molten matter within the crust of
the earth.
(_b._) _Leopold von Buch on Rents and Fissures in the Earth's
Crust._--The view of Leopold von Buch, who considered that the great
lines of volcanic mountains above referred to rise along the borders of
rents, or fissures, in the earth's crust, is one which is inherently
probable, and is in keeping with observation. That the crust of the
globe is to a remarkable extent fissured and torn in all directions is a
phenomenon familiar to all field geologists. Such rents and fissures are
often accompanied by displacement of the strata, owing to which the
crust has been vertically elevated on one side or lowered on the other,
and such displacements (or "faults") sometimes amount to thousands of
feet. It is only occasionally, however, that such fractures are
accompanied by the extrusion of molten matter; and in the North of
England and Scotland dykes of igneous rock, such as basalt, which run
across the country for many miles in nearly straight lines, often cut
across the faults, and are only rarely coincident with them.
Nevertheless, it can scarcely be a question that the grand chain of
volcanic mountains which stretches almost continuously along the Andes
of South America, and northwards through Mexico, has been piled up along
the line of a system of fissures in the fundamental rocks parallel to
the coast, though not actually coincident therewith.
(_c._) _The Cordilleras of Quito._--The structure and arrangement of the
Cordilleras of Quito, for example, are eminently suggestive of
arrangement along lines of fissure. As shown by Alexander von
Humboldt,[5] the volcanic mountains are disposed in two parallel chains,
which run side by side for a distance of over 500 miles northwards into
the State of Columbia, and enclose between them the high plains of Quito
and Lacunga. Along the eastern chain are the great cones of El Altar,
rising to an elevation of 16,383 feet above the ocean, and having an
enormous crater apparently dormant or extinct, and covered with snow;
then Cotopaxi (Fig. 2), its sides covered with snow, and sending forth
from its crater several columns of smoke; then Guamani and Cayambe
(19,000 feet), huge truncated cones apparently extinct; these constitute
the eastern chain of volcanic heights. The western chain contains even
loftier mountains. Here we find the gigantic Chimborazo, an extinct
volcano whose summit is white with snow; Carihuairazo[6] and Illiniza, a
lofty pointed peak like the Matterhorn; Corazon, a snow-clad dome,
reaching a height of 15,871 feet; Atacazo and Pichincha, the latter an
extinct volcano reaching an elevation of 15,920 feet; such is the
western chain, remarkable for its straightness, the volcanic cones being
planted in one grand procession from south to north. This rectilinear
arrangement of the western chain, only a little less conspicuous in the
eastern, is very suggestive of a line of fracture in the crust beneath.
And when we contemplate the prodigious quantity of matter included
within the limits of these colossal domes and their environments, all of
which has been extruded from the internal reservoirs, we gain some idea
of the manner in which the contracting crust disposes of the matter it
can no longer contain.[7]
Between the volcanoes of Quito and those of Peru there is an intervening
space of fourteen degrees of latitude. This is occupied by the Andes,
regarding the structure of which we have not much information except
that at this part of its course it is not volcanic. But from Arequipa in
Peru (lat. 16° S.), an active volcano, we find a new series of volcanic
mountains continued southwards through Tacora (19,740 feet), then
further south the more or less active vents of Sajama (22,915 feet),
Coquina, Tutupaca, Calama, Atacama, Toconado, and others, forming an
almost continuous range with that part of the desert of Atacama
pertaining to Chili. Through this country we find the volcanic range
appearing at intervals; and still more to the southwards it is doubtless
connected with the volcanoes of Patagonia, north of the Magellan
Straits, and of Tierra del Fuego. Mr. David Forbes considers that this
great range of volcanic mountains, lying nearly north and south,
corresponds to a line of fracture lying somewhat to the east of the
range.[8]
(_d._) _Other Volcanic Chains._--A similar statement in all probability
applies to the systems of volcanic mountains of the Aleutian Isles,
Kamtschatka, the Kuriles, the Philippines, and Sunda Isles. Nor can
it be reasonably doubted that the western American coast-line has
to a great extent been determined, or marked out, by such lines of
displacement; for, as Darwin has shown, the whole western coast of South
America, for a distance of between 2000 and 3000 miles south of the
Equator, has undergone an upward movement in very recent times--that is,
within the period of living marine shells--during which period the
volcanoes have been in activity.[9]
(_e._) _The Kurile Islands._--This chain may also be cited in evidence
of volcanic action along fissure lines. It connects the volcanoes of
Kamtschatka with those of Japan, and the linear arrangement is apparent.
In the former peninsula Erman counted no fewer than thirteen active
volcanic mountains rising to heights of 12,000 to 15,000 feet above the
sea.[10] In the chain of the Kuriles Professor John Milne counted
fifty-two well-defined volcanoes, of which nine, perhaps more, are
certainly active.[11] They are not so high as those of Kamtschatka; but,
on the other hand, they rise from very deep oceanic waters, and have
been probably built up from the sea bottom by successive eruptions of
tuff, lava, and ash. According to the view of Professor Milne, the
volcanoes of the Kurile chain are fast becoming extinct.
(_f._) _Volcanic Groups._--Besides the volcanic vents arranged in lines,
of which we have treated above, there are a large number, both active
and extinct, which appear to be disposed in groups, or sporadically
distributed, over various portions of the earth's surface. I say _appear
to be_, because this sporadic distribution may really be resolvable (at
least in some cases) into linear distribution for short distances. Thus
the Neapolitan Group, which might at first sight seem to be arranged
round Vesuvius as a centre, really resolves itself into a line of active
and extinct vents of eruption, ranging across Italy from the Tyrrhenian
Sea to the Adriatic, through Ischia, Procida, Monte Nuovo and the
Phlegræan Fields, Vesuvius, and Mount Vultur.[12] Again, the extinct
volcanoes of Central France, which appear to form an isolated group,
indicate, when viewed in detail, a linear arrangement ranging from north
to south.[13] Another region over which extinct craters are distributed
lies along the banks of the Rhine, above Bonn and the Moselle; a fourth
in Hungary; a fifth in Asia Minor and Northern Palestine; and a sixth in
Central Asia around Lake Balkash. These are all continental, and the
linear distribution is not apparent.
[1] For an interesting account of this range of volcanic islands see
Kingsley's _At Last_. The grandest volcanic peak is that of Guadeloupe,
rising to a height of 5000 feet above the ocean, amidst a group of
fourteen extinct craters. But the most active vent of the range is the
Souffrière of St. Vincent. In the eruption of 1812 this mountain sent
forth clouds of pumice, scoriæ and ashes, some of which were carried by
an upper counter current to Barbados, one hundred miles to the eastward,
covering the surface with volcanic dust to a depth of several inches.
[2] An excellent, and perhaps the most recent, map of this kind is that
given by Professor Prestwich in his _Geology_, vol. i. p. 216. One on a
larger scale is that by Keith Johnston in his _Physical Atlas_.
[3] _Memoir on the Physical Geology and Geography of Arabia Petræa,
Palestine_, etc., published for the Committee of the Palestine
Exploration Fund (1886), p. 113, etc.
[4] This mountain was ascended in 1837 by Mr. Taylor Thomson, who found
the summit covered with sulphur, and from a cone fumes at a high
temperature issued forth, but there was no eruption.--_Journ. Roy.
Geographical Soc._, vol. viii. p. 109.
[5] Humboldt, _Atlas der Kleineren Schriften_ (1853).
[6] Ascended by Whymper June 29, 1880. He found the elevation to be
16,515 feet.
[7] The arrangement of the volcanoes of Peru and Bolivia is also
suggestive of a double line of fissure, while those of Chili suggest one
single line. The volcanoes of Arequipa, in the southern part of Peru,
are dealt with by Dr. F. H. Hatch, in his inaugural dissertation, _Ueber
die Gesteine der Vulcan-Gruppe von Arequipa_ (Wien, 1886). The volcanoes
rise to great elevations, having their summits capped by snow. The
volcano of Charchani, lying to the north of Arequipa, reaches an
elevation of 18,382 Parisian feet. That of Pichupichu reaches a height
of 17,355 Par. feet. The central cone of Misti has been variously
estimated to range from 17,240 to 19,000 Par. feet. The rocks of which
the mountains are composed consist of varieties of andesite.
[8] D. Forbes, "On the Geology of Bolivia and Southern Peru," _Quarterly
Journal of the Geological Society_, vol. xvii. p. 22 (1861).
[9] Darwin, _Structure and Distribution of Coral Reefs_, second edition,
p. 186.
[10] Erman, _Reise um die Welt_.
[11] Milne, "Cruise amongst the Kurile Islands," _Geol. Mag._, New Ser.
(August 1879).
[12] See Daubeny, _Volcanoes_, Map I.
[13] Sir A. Geikie has connected as a line of volcanic vents those of
Sicily, Italy, Central France, the N. E. of Ireland, the Inner Hebrides
and Iceland, of which the central vents are extinct or dormant, the
extremities active.
CHAPTER IV.
MID-OCEAN VOLCANIC ISLANDS.
_Oceanic Islands._--By far the most extensive regions with sporadically
distributed volcanic vents, both active and extinct, are those which are
overspread by the waters of the ocean, where the vents emerge in the
form of islands. These are to be found in all the great oceans, the
Atlantic, the Pacific, and the Indian; but are especially numerous over
the central Pacific region. As Kotzebue and subsequently Darwin have
pointed out, all the islands of the Pacific are either coral-reefs or of
volcanic origin; and many of these rise from great depths; that is to
say, from depths of 1000 to 2000 fathoms. It is unnecessary here to
attempt to enumerate all these islands which rise in solitary grandeur
from the surface of the ocean, and are the scenes of volcanic
operations; a few may, however, be enumerated.
[Illustration: Fig. 4.--The Peak of Teneriffe (Pic de Teyde) as seen
from the ocean.--(From a photograph.)]
(_a._) _Iceland._--In the Atlantic, Iceland first claims notice, owing
to the magnitude and number of its active vents and the variety of the
accompanying phenomena, especially the geysers. As Lyell has
observed,[1] with the exception of Etna and Vesuvius, the most complete
chronological records of a series of eruptions in existence are those
of Iceland, which come down from the ninth century of our era, and which
go to show that since the twelfth century there has never been an
interval of more than forty years without either an eruption or a great
earthquake. So intense is the volcanic energy in this island that some
of the eruptions of Hecla have lasted six years without cessation.
Earthquakes have often shaken the whole island at once, causing great
changes in the interior, such as the sinking down of hills, the rending
of mountains, the desertion by rivers of their channels, and the
appearance of new lakes. New islands have often been thrown up near the
coast, while others have disappeared. In the intervals between the
eruptions, innumerable hot springs afford vent to the subterranean heat,
and solfataras discharge copious streams of inflammable matter. The
volcanoes in different parts of the island are observed, like those of
the Phlegræan Fields, to be in activity by turns, one vent serving for a
time as a safety-valve for the others. The most memorable eruption of
recent years was that of Skaptár Jokul in 1783, when a new island was
thrown up, and two torrents of lava issued forth, one 45 and the other
50 miles in length, and which, according to the estimate of Professor
Bischoff, contained matter surpassing in magnitude the bulk of Mont
Blanc. One of these streams filled up a large lake, and, entering the
channel of the Skaptâ, completely dried up the river. The volcanoes of
Iceland may be considered as safety-valves to the region in which lie
the British Isles.
(_b._) _The Azores, Canary, and Cape de Verde Groups._--This group of
volcanic isles rises from deep Atlantic waters north of the Equator,
and the vents of eruption are partially active, partially dormant, or
extinct. It must be supposed, however, that at a former period volcanic
action was vastly more energetic than at present; for, except at the
Grand Canary, Gomera, Forta Ventura, and Lancerote, where various
non-volcanic rocks are found, these islands appear to have been built up
from their foundations of eruptive materials. The highest point in the
Azores is the Peak of Pico, which rises to a height of 7016 feet above
the ocean. But this great elevation is surpassed by that of the Peak of
Teneriffe (or Pic de Teyde) in the Canaries, which attains to an
elevation of 12,225 feet, as determined by Professor Piazzi Smyth.[2]
This great volcanic cone, rising from the ocean, its summit shrouded in
snow, and often protruding above the clouds, must be an object of
uncommon beauty and interest when seen from the deck of a ship. (Fig.
4.) The central cone, formed of trachyte, pumice, obsidian, and ashes,
rises out of a vast caldron of older basaltic rocks with precipitous
inner walls--much as the cone of Vesuvius rises from within the
partially encircling walls of Somma. (Fig. 5.) From the summit issue
forth sulphurous vapours, but no flame.
Piazzi Smyth, who during a prolonged visit to this mountain in 1856 made
a careful survey of its form and structure, shows that the great cone is
surrounded by an outer ring of basalt enclosing two _foci_ of eruption,
the lavas from which have broken through the ring of the outer crater
on the western side, and have poured down the mountain. At the top of
the peak its once active crater is filled up, and we find a convex
surface ("The Plain of Rambleta") surmounted towards its eastern end by
a diminutive cone, 500 feet high, called "Humboldt's Ash Cone." The
slope of the great cone of Teneriffe ranges from 28° to 38°; and below a
level of 7000 feet the general slope of the whole mountain down to the
water's edge varies from 10° to 12° from the horizontal. The great cone
is penetrated by numerous basaltic dykes.
The Cape de Verde Islands, which contain beds of limestone along with
volcanic matter, possess in the island of Fuego an active volcano,
rising to a height of 7000 feet above the surface of the ocean. The
central cone, like that of Teneriffe, rises from within an outer crater,
formed of basalt alternating with beds of agglomerate, and traversed by
numerous dykes of lava. This has been broken down on one side like that
of Somma; and over its flanks are scattered numerous cones of scoriæ,
the most recent dating from the years 1785 and 1799.[3]
[Illustration: Fig. 5.--View of the summit of the Peak of Teneriffe
(12,225 feet) and of the secondary crater, or outer ring of basaltic
sheets which surrounds its base; seen from the east.--(After Leopold von
Buch.)]
The volcanoes of Lancerote have a remarkably linear arrangement from
west to east across the island. They are not yet extinct; for an
eruption in 1730 destroyed a large number of villages, and covered with
lava the most fertile tracts in the island, which at the time of Leopold
von Buch's visit lay waste and destitute of herbage.[4] In the island of
Palma there is one large central crater, the Caldera de Palma, three
leagues in diameter, the walls of which conform closely to the margin
of the coast. Von Buch calls this crater "une merveille de la nature,"
for it distinguishes this isle from all the others, and renders it one
of the most interesting and remarkable amongst the volcanic islands of
the ocean. The outer walls are formed of basaltic sheets, and towards
the south this great natural theatre is connected with the ocean by a
long straight valley, called the "Barranco de los Dolores," along whose
sides the structure of the mountain is deeply laid open to view. The
outer flanks of the crater are furrowed by a great number of smaller
barrancos radiating outward from the rim of the caldera. Von Buch
regards the barrancos as having been formed during the upheaval of the
island, according to his theory of the formation of such mountains (the
elevation-theory); but unfortunately for his views, these ravines widen
outwards from the centre, or at least do not become narrower in that
direction, as would be the case were the elevation-theory sound. The
maps which accompany Von Buch's work are remarkably good, and were
partly constructed by himself.
(_c._) _Volcanic Islands in the Atlantic south of the Equator._--The
island of Ascension, formed entirely of volcanic matter, rises from a
depth of 2000 fathoms in the very centre of the Atlantic. As described
by Darwin, the central and more elevated portions are formed of
trachytic matter, with obsidian and laminated ash beds. Amongst these
are found ejected masses of unchanged granite, fragments of which have
been torn from the central pipe during periods of activity, and would
seem to indicate a granitic floor, or at least an original floor upon
which more recent deposits may have been superimposed. In St. Helena we
seem, according to Daubeny, to have the mere wreck of one great crater,
no one stream of lava being traceable to its source, while dykes of lava
are scattered in profusion throughout the whole substance of the
basaltic masses which compose the island. Tristan da Cunha, in the
centre of the South Atlantic, rises abruptly from a depth of 12,150
feet, at a distance of 1500 miles from any land; and one of its summits
reaches an elevation of 7000 feet, being a truncated cone composed of
alternating strata of tuff and augitic lava, surrounding a crater in
which is a lake of pure water. The volcano is extinct or dormant.
Were the waters of the ocean to be drawn off, these volcanic islands
would appear like stupendous conical mountains, far loftier, and with
sides more precipitous, than any to be found on our continental lands,
all of which rise from platforms of considerable elevation. The enormous
pressure of the water on their sides enables these mid-oceanic islands
to stand with slopes varying from the perpendicular to a smaller extent
than if they were sub-aerial; and it is on this account that we find them
rising with such extraordinary abruptness from the "vasty deep."
(_d._) _Volcanic Islands of the Pacific._--The volcanic islands of this
great ocean are scattered over a wide tract on both sides of the
equator. Those to the north of this line include the Sandwich Islands,
the Mariana or Ladrone Islands, South Island, and Bonin Sima; south of
the equator, the Galapagos, New Britain, Salomon, Santa Cruz, New
Hebrides, the Friendly and Society Isles. While the coral reefs and
islands of the Pacific may be recognised by their slight elevation
above the surface of the waters, those of volcanic origin and containing
active or extinct craters of eruption generally rise into lofty
elevations, so that the two kinds are called the _Low_ Islands and
_High_ Islands respectively. Amongst the group are trachytic domes such
as the Mountain of Tobreonu in the Society Islands, rising to a height
probably not inferior to that of Etna, with extremely steep sides, and
holding a lake on its summit.[5] The linear arrangement of some of the
volcanic islands of the Pacific is illustrated by those of the Tonga, or
Friendly, Group, lying to the north of New Zealand. They consist of
three divisions--(1) the volcanic; (2) those formed of stratified
volcanic tuff, sometimes entirely or partially covered by coralline
limestone; and (3) those which are purely coralline. The first form a
chain of lofty cones and craters, lying in a E.N.E. and W.S.W.
direction, and rising from depths of over 1000 fathoms. Mr. J. J.
Lister, who has described the physical characters of these islands, has
shown very clearly that they lie along a line--probably that of a great
fissure--stretching from the volcanic island of Amargura on the north
(lat. 18° S.), through Lette, Metis, Kao (3030 feet), Tofua, Falcon,
Honga Tonga, and the Kermadec Group into the New Zealand chain on the
south. Some of these volcanoes are in a state of intermittent activity,
as in the case of Tofua (lat. 20° 30' S.), Metis Island, and Amargura;
the others are dormant or extinct. The whole group appears to have been
elevated at a recent period, as some of the beds of coral have been
raised 1272 feet and upward above the sea-level, as in the case of Eua
Island.[6] The greater number of the Pacific volcanoes appear to be
basaltic; such as those of the Hawaiian Group, which have been so fully
described by Professor J. D. Dana.[7] Here fifteen volcanoes of the
first class have been in brilliant action; all of which, except three,
are now extinct, and these are in Hawaii the largest and most eastern of
the group. This island contains five volcanic mountains, of which Kea,
13,805 feet, is the highest; next to that, Loa, 13,675 feet; after
these, Hualalai, rising 8273 feet; Kilauea, 4158 feet; and Kohala, 5505
feet above the sea; this last is largely buried beneath the lavas of
Mauna Kea. The group contains a double line of volcanoes, one lying to
the north and west of the other; and as the highest of the Hawaiian
Group rises from a depth in the ocean of over 2000 fathoms, the total
elevation of this mountain from its base on the bed of the ocean is not
far from 26,000 feet, an elevation about that of the Himalayas. Mauna
Kea has long been extinct, Hualalai has been dormant since 1801; but
Mauna Loa is terribly active, there having been several eruptions,
accompanied by earthquakes, within recent years, the most memorable
being those of 1852 and 1868. In the former case the lava rose from the
deep crater into "a lofty mountain," as described by Mr. Coan,[8] and
then flowed away eastward for a distance of twenty miles. The interior
of the crater consists of a vast caldron, surrounded by a precipice 200
to 400 feet in depth, with a circumference of about fifteen miles, and
containing within it a second crater, bounded by a black ledge with a
steep wall of basalt--a crater within a crater; and from the floor of
the inner crater, formed of molten basalt, in a seething and boiling
state, arise a large number of small cones and pyramids of lava, some
emitting columns of grey smoke, others brilliant flames and streams of
molten lava, presenting a wonderful spectacle, the effect of which is
heightened by the constant roaring of the vast furnaces below.[9]
[1] _Principles of Geology_, 11th edition, vol. ii. p. 48.
[2] Smyth, _Report on the Teneriffe Astronomical Experiment of 1856_.
Humboldt makes the elevation 12,090 feet. A beautiful model of the Peak
was constructed by Mr. J. Nasmyth from Piazzi Smyth's plans, of which
photographs are given by the latter.
[3] Daubeny, _loc. cit._, p. 460.
[4] _Iles Canaries_, p. 37.
[5] Daubeny, _loc. cit._, p. 426.
[6] Lister, "Notes on the Geology of the Tonga Islands," _Quart. Jour.
Geol. Soc._, No. 188, p. 590 (1891).
[7] Dana, _Characteristics of Volcanoes, with Contributions of Facts and
Principles from the Hawaiian Islands_. London, 1890.--Also, _Geology of
the American Exploring Expedition--Volcanoes of the Sandwich Islands_.
[8] Coan, _Amer. Jour. of Science_, 1853.
[9] W. Ellis, the missionary, has given a vivid description of this
volcano in his _Tour of Hawaii_. London, 1826.--Plans of the crater will
be found in Professor Dana's work above quoted.
PART II.
EUROPEAN VOLCANOES.
CHAPTER I.
VESUVIUS.
Having now dealt in a necessarily cursory manner with volcanoes of
distant parts of the globe, we may proceed to the consideration of the
group of active volcanoes which still survive in Europe, as they possess
a special interest, not only from their proximity and facility of
access, at least to residents in Europe and the British Isles, but from
their historic incidents; and in this respect Vesuvius, though not by
any means the largest of the group, stands the first, and demands more
special notice. The whole group rises from the shores of the
Mediterranean, and consists of Vesuvius, Etna, Stromboli, one of the
Lipari Islands, and Vulcano, a mountain which has given the name to all
mountains of similar origin with itself.[1] Along with these are
innumerable cones and craters of extinct or dormant volcanoes, of which
a large number have been thrown out on the flanks of Etna.
(_a._) _Prehistoric Ideas regarding the Nature of this Mountain._--Down
to the commencement of the Christian era this mountain had given no
ostensible indication that it contained within itself a powerful focus
of volcanic energy. True, that some vague tradition that the mountain
once gave forth fire hovered around its borders; and several ancient
writers, amongst them Diodorus Siculus and Strabo, inferred from the
appearances of the higher parts of the mountain and the character of the
rocks, which were "cindery and as if eaten by fire," that the country
was once in a burning state, "being full of fiery abysses, though now
extinct from want of fuel." Seneca (B.C. 1 to A.D. 64) had detected the
true character of Vesuvius, as "having been a channel for the internal
fire, but not its food;" nevertheless, at this period the flanks of the
mountain were covered by fields and vineyards, while the summit,
partially enclosed with precipitous walls of the long extinct volcano,
Somma, was formed of slaggy and scoriaceous material, with probably a
covering of scrub. Here it was that the gladiator Spartacus (B.C. 72),
stung by the intolerable evils of the Roman Government, retreated to the
very summit of the mountain with some trusty followers. Clodius the
Prætor, according to the narration of Plutarch, with a party of three
thousand men, was sent against them, and besieged them in a mountain
(meaning Vesuvius or Somma) having but one narrow and difficult passage,
which Clodius kept guarded; all the rest was encompassed with broken and
slippery precipices, but upon the top grew a great many wild vines; the
besieged cut down as many as they had need of, and twisted them into
ladders long enough to reach from thence to the bottom, by which,
without any danger, all got down except one, who stayed behind to throw
them their arms, after which he saved himself with the rest.[2] "On the
top" must (as Professor Phillips observes) be interpreted the summit of
the exterior slope or crater edge, which would appear from the narrative
to have broken down on one side, affording an entrance and mode of
egress by which Spartacus fell upon, and surprised, the negligent
Clodius Glabrus.
[Illustration: Fig. 6.--Probable aspect of Vesuvius as it appeared at
the beginning of the Christian era; seen from the Bay of Naples.]
In fancied security, villas, temples, and cities had been erected on
the slopes of the mountain. Herculaneum, Pompeii, and Stabiæ, the abodes
of art, luxury, and vice, had sprung up in happy ignorance that they
"stood on a volcano," and that their prosperity was to have a sudden and
disastrous close.[3]
(_b._) _Premonitory Earthquake Shocks._--The first monitions of the
impending catastrophe occurred in the 63rd year after Christ, when the
whole Campagna was shaken by an earthquake, which did much damage to the
towns and villas surrounding the mountain even beyond Naples. This was
followed by other shocks; and in Pompeii the temple of Isis was so much
damaged as to require reconstruction, which was undertaken and carried
out by a citizen at his own expense.[4] These earthquake shakings
continued for sixteen years. At length, on the night of August 24th,
A.D. 79, they became so violent that the whole region seemed to reel and
totter, and all things appeared to be threatened with destruction. The
next day, about one in the afternoon, there was seen to rise in the
direction of Vesuvius a dense cloud, which, after ascending from the
summit of the mountain into the air for a certain height in one narrow,
vertical trunk, spread itself out laterally in such a form that the
upper part might be compared to the cluster of branches, and the lower
to the stem of the pine which forms so common a feature in the Italian
landscape.[5]
(_c._) _Pliny's Letters to Tacitus._--For an account of what followed we
are indebted to the admirable letters of the younger Pliny, addressed to
the historian Tacitus, recounting the events which caused, or
accompanied, the death of his uncle, the elder Pliny, who at the time of
this first eruption of Vesuvius was in command of the Roman fleet at the
entrance to the Bay of Naples. These letters, which are models of style
and of accurate description, are too long to be inserted here; but he
recounts how the dense cloud which hung over the mountain spread over
the whole surrounding region, sometimes illuminated by flashes of light
more vivid than lightning; how showers of cinders, stones, and ashes
fell in such quantity that his uncle had to flee from Stabiæ, and that
even at so great a distance as Misenum they encumbered the surface of
the ground; how the ground heaved and the bed of the sea was upraised;
how the cloud descended on Misenum, and even the island of Capreæ was
concealed from view; and finally, how, urged by a friend who had
arrived from Spain, he, with filial affection, supported the steps of
his mother in flying from the city of destruction. Such being the
condition of the atmosphere and the effects of the eruption at a point
so distant as Cape Misenum, some sixteen geographical miles from the
focus of eruption, it is only to be expected that places not half the
distance, such as Herculaneum, Pompeii, and even Stabiæ, with many
villages and dwellings, should have shared a worse fate. The first of
these cities, situated on the coast of the Bay of Naples, appears to
have been overwhelmed by volcanic mud; Pompeii was buried in ashes and
lapilli, and Stabiæ probably shared a similar fate.[6]
(_d._) _Appearance of the Mountain at the Commencement of the Christian
Era._--At the time of the first recorded eruption Vesuvius appears to
have consisted of only a single cone with a crater, now known as Monte
di Somma, the central cone of eruption which now rises from within this
outer ruptured casing not having been formed. (Fig. 6.) The first effect
of the eruption of the year 79 was to blow out the solidified covering
of slag and scoriæ forming the floor of the caldron. Doubtless at the
close of the eruption a cone of fragmental matter and lava of some
slight elevation was built up, and, if so, was subsequently destroyed;
for, as we shall presently see by the testimony of the Abate Guilio
Cesare Braccini, who examined the mountain not long before the great
eruption of A.D. 1631, there was no central cone to the mountain at that
time; and the mountain had assumed pretty much the appearance it had at
the time that Spartacus took refuge within the walls of the great
crater.
(_e._) _Destruction of Pompeii._--Pompeii was overwhelmed with dry ashes
and lapilli. Sir W. Hamilton found some of the stones to weigh eight
pounds. At the time of the author's visit, early in April 1872, the
excavations had laid open a section about ten feet deep, chiefly
composed of alternating layers of small pumice stones (lapilli) and
volcanic dust. It was during the sinking of a well in 1713 upon the
theatre containing the statues of Hercules and Cleopatra that the
existence of the ancient city was accidentally discovered.
(_f._) _More recent eruptions._--Since the first recorded eruption in
A.D. 79 down to the present day, Vesuvius has been the scene of numerous
intermittent eruptions, of which some have been recorded; but many,
doubtless, are forgotten.
In A.D. 203, during the reign of Severus, an eruption of extraordinary
violence took place, which is related by Dion Cassius, from whose
narrative we may gather that at this time there was only one large
crater, and that the central cone of Vesuvius had not as yet been
upraised. In A.D. 472 an eruption occurred of such magnitude as to cover
all Europe with fine dust, and spread alarm even at Constantinople.
(_g._) _Eruption of 1631._--In December 1631 occurred the great
convulsion whose memorials are written widely on the western face of
Vesuvius in ruined villages. This eruption left layers of ashes over
hundreds of miles of country, or heaps of mud swept down by hot water
floods from the crater; the crater itself having been dissipated in the
convulsion. Braccini, who examined the mountain not long before this
eruption, found apparently no cone (or mount) like that of the modern
Vesuvius. He states that the crater was five miles in circumference,
about a thousand paces deep (or in sloping descent), and its sides
covered with forest trees and brushwood, while at the bottom there was a
plain on which cattle grazed.[7] It would seem that the mountain had at
this time enjoyed a long interval of rest, and that it had reverted to
very much the same state in which it was at the period of the first
eruption, when the flanks were peopled by inhabitants living in fancied
security. But six months of violent earthquakes, which grew more violent
towards the close of 1631, heralded the eruption which took place in
December, accompanied by terrific noises from within the interior of the
mountain. The inhabitants of the coast were thus warned of the
approaching danger, and had several days to arrange for their safety;
but in the end, a great part of Torre del Greco was destroyed, and a
like fate overtook Resina and Granatello, with a loss of life reported
at 18,000 persons. During the eruption clouds condensed into tempests of
rain, and hot water from the mountain, forming deluges of mud, swept
down the sides, and reached even to Nola and the Apennines. Nor was the
sea unmoved. It retired during the violent earthquakes, and then
returned full thirty paces beyond its former limits.
Not indeed until near the close of the seventeenth century is there any
evidence that the central cone of Vesuvius was in existence; but in
October 1685 an eruption occurred which is recorded by Sorrentino,
during which was erected "a new mountain within, and higher than the old
one, and visible from Naples," a statement evidently referable to the
existing cone--so that it is little more than two centuries since this
famous volcanic mountain assumed its present form.
(_h._) _Eruptions between the years 1500 and 1800._--Since A.D. 1500
there have been fifty-six recorded eruptions of Vesuvius; one of these
in 1767 was of terrific violence and destructiveness, and is represented
by Sir William Hamilton in views taken both before and during the
eruption. A pen-and-ink drawing of the appearance of the crater before
the eruption is here reproduced from Hamilton's picture, from which it
will be seen that the central crater contained within itself a second
crater-cone, from whence steam, lava, and stones were being erupted
(Fig. 7). Thus it will be seen that Vesuvius at this epoch consisted of
three crater-cones within each other. The first, Monte di Somma; the
second, the cone of Vesuvius; and the third, the little crater-cone
within the second. During this eruption, vast lava-sheets invaded the
fields and vineyards on the flanks of the mountain. A vivid account of
this eruption, as witnessed by Padre Torre, is given by Professor
Phillips.[8] We shall pass over others without further reference until
we come down to our own times, in which Vesuvius has resumed its old
character, and in one grand exhibition of volcanic energy, which took
place in 1872, has evinced to the world that it still contains within
its deep-seated laboratory all the elements of destructive force which
it exhibited at the commencement of our era.
[Illustration: Fig. 7.--View of the crater of Vesuvius before the
eruption of 1767, showing an interior crater-cone rising from the centre
of the exterior crater.--(After Sir W. Hamilton.)]
(_i._) _Structure of the Neapolitan Campagna._--But before giving a
description of this terrific outburst of volcanic energy, it may be
desirable to give some account of the physical position and structure of
this mountain, by which the phenomena of the eruption will be better
understood. Vesuvius and the Neapolitan Campagna are formed of volcanic
materials bounded on the west by the Gulf of Naples, and on the east and
south by ranges of Jurassic limestone, a prolongation of the Apennines,
which send out a spur bounding the bay on the south, and forming the
promontory of Sorrento. The little island of Capri is also formed of
limestone, and is dissevered from the promontory by a narrow channel.
The northern side of the bay is, however, formed of volcanic materials;
it includes the Phlegræan Fields (Campi Phlegræi), and terminates in the
promontory of Miseno. Lying in the same direction are the islands of
Procida and Ischia, also volcanic. Hence it will be seen that the two
horns of the bay are formed of entirely different materials, that of
Miseno on the north being volcanic, that of Sorrento on the south being
composed of Jurassic limestone, of an age vastly more ancient than the
volcanic rocks on the opposite shore. (Map, p. 52.)
The general composition of the Neapolitan Campagna, from which the
mountain rises, has been revealed by means of the Artesian well sunk to
a depth of about 500 metres (1640 feet) at the Royal Palace of Naples,
and may be generalised as follows:--
{ Recent beds of volcanic tuff
(1) From surface to depth of { with marine shells, and containing
715 feet { fragments of trachytic
{ lava, etc. (_Volcanic Beds_).
{ Bituminous sands and marls
(2) From 715 to 1420 { with marine shells of recent
{ species(?) (_Pre-Volcanic Beds_).
(3) From 1420 to 1574 { EOCENE BEDS. Micaceous sandstone
{ and marl (_Macigno_).
(4) From 1574 to bottom { JURASSIC BEDS. Apennine
{ Limestone.
[Illustration: Fig. 8.--Map of the district bordering the Bay of
Naples, with the islands of Capri, Ischia, and Procida.]
From the above section, for which we are indebted to Mr. Johnston-Lavis,
the most recent writer on Vesuvius, it would appear that the first
volcanic explosions by which the mountain was ultimately to be built up
took place after the deposition of the sands and marls (No. 2), while
the whole Campagna was submerged under the waters of the Mediterranean.
By the accumulation of the stratified tuff (No. 1), the sea-bed was
gradually filled up during a period of volcanic activity, and afterwards
elevated into dry land.[9]
[Illustration: Fig. 9.--View of Vesuvius from the Harbour of Naples at
the commencement of the eruption of 1872.--(From a sketch by the
author.)]
(_j._) _Present Form and Structure of Vesuvius and Somma._--The outer
cone of Vesuvius, or Monte di Somma, rises from a circular platform by a
moderately gentle ascent, and along the north and east terminates in a
craggy crest, with a precipitous cliff descending into the Atria del
Cavallo, forming the wall of the ancient crater throughout half its
circumference; this wall is formed of scoriæ, ashes, and lapilli, and is
traversed by numerous dykes of lava. Along the west and south this old
crater has been broken down; but near the centre there remains a
round-backed ridge of similar materials, once doubtless a part of the
original crater of Somma, rising above the slopes of lava on either
hand. On this has been erected the Royal Observatory, under the
superintendence of Professor Luigi Palmieri, where continuous
observations are being made, by means of delicate seismometers, of the
earth-tremors which precede or accompany eruptions; for it is only
justice to say that Vesuvius gives fair warning of impending mischief,
and the instruments are quick to notify any premonitory symptoms of a
coming catastrophe. The elevation of the Observatory is 2080 feet above
the sea.
On either side of the Observatory ridge are wide channels filled to a
certain height with lavas of the nineteenth and preceding centuries, the
most recent presenting an aspect which can only be compared to a
confused multitude of black serpents and pachyderms writhing and
interlocked in some frightful death-struggle. Some of this lava, ten
years old, as we cross its rugged and black surface presents gaping
fissures, showing the mass to be red-hot a few feet from the surface, so
slow is the process of cooling. These lava-streams--some of them
reaching to the sea-coast--have issued forth from the Atria at
successive periods of eruption.
From the midst of the Atria rises the central cone, formed of cinders,
scoriæ, and lava-streams, and fissured along lines radiating from the
axis. This cone is very steep, the angle being about 40°-45° from the
horizontal, and is formed of loose cindery matter which gives way at
every step, and is rather difficult to climb. But on reaching the summit
we look down into the crater, displaying a scene of ever-varying
characters, rather oval in form, and about 1100 yards in diameter. From
the map of Professor Guiscardi, published in 1855, there are seen two
minor craters within the central one, formed in 1850, and an outflow of
lava from the N.W. down the cone. At the time of the author's visit the
crater was giving indications, by the great quantity of sulphurous gas
and vapour rising from its surface, and small jets of molten lava
beginning to flow down the outer side, of the grand outburst of internal
forces which was presently to follow.
(_k._) _Eruption of 1872._--The grand eruption of 1872, of which a
detailed account is given by Professor Palmieri,[10] commenced with a
slight discharge of incandescent projectiles from the crater; and on the
13th January an aperture appeared on the upper edge of the cone from
which at first a little lava issued forth, followed by the uprising of a
cone which threw out projectiles accompanied by smoke, whilst the
central crater continued to detonate more loudly and frequently. This
little cone ultimately increased in size, until in April it filled the
whole crater and rose four or five metres above the brim. At this time
abundant lavas poured down from the base of the cone into the Atria del
Cavallo, thence turned into the Fossa della Vetraria in the direction of
the Observatory and towards the Crocella, where they accumulated to
such an extent as to cover the hillside for a distance of about 300
metres; then turning below the Canteroni, formed a hillock without
spreading much farther.
In October another small crater was formed by the falling in of the
lava, which after a few days gave vent to smoke and several jets of
lava; and towards the end of October the detonations increased, the
smoke from the central crater issued forth more densely mixed with
ashes, and the seismographical apparatus was much disturbed. On the 3rd
and 4th November copious and splendid lava-streams coursed down the
principal cone on its western side, but were soon exhausted; and in the
beginning of 1872 the little cone, regaining vigour, began to discharge
lava from the summit instead of the base as heretofore.
In the month of March 1873, with the full moon, the cone opened on the
north-west side--the cleavage being indicated by a line of
fumaroles--and lava issued from the base and poured down into the Atria
as far as the precipices of Monte di Somma. On the 23rd April (another
full moon) the activity of the craters increased, and on the evening of
the 24th splendid lava-streams descended the cone in various directions,
attracting on the same night the visits of a great many strangers. A
lamentable event followed on the 26th. A party of visitors, accompanied
by inexperienced guides, and contrary to the advice of Professor
Palmieri, insisted on ascending to the place from which the lava issued.
At half-past three on the morning of the 26th they were in the Atria del
Cavallo, when the Vesuvian cone was rent in a north-west direction and a
copious torrent of lava issued forth. Two large craters formed at the
summit of the mountain, discharging incandescent projectiles and ashes.
A cloud of smoke enveloped the unhappy visitors, who were under a
hail-storm of burning projectiles. Eight were buried beneath it, or in
the lava, while eleven were grievously injured.[11] The lava-stream,
flowing over that of 1871 in the Atria, divided into two branches, the
smaller one flowing towards Resina, but stopping before reaching the
town; the larger precipitated itself into the Fossa della Vetraria,
occupying the whole width of 800 metres, and traversing the entire
length of 1300 metres in three hours. It dashed into the Fossa di
Farone, and reached the villages of Massa and St. Sebastiano, covering a
portion of the houses, and, continuing its course through an artificial
foss, or trench, invaded cultivated ground and several villages. If it
had not greatly slackened after midnight, from failure of supply at its
source, it would have reached Naples by Ponticelli and flowed into the
sea. The eruption towards the end of April had reached its height. The
Observatory ridge was bounded on either side by two fiery streams, which
rendered the heat intolerable. Simultaneously with the opening of the
great fissure two large craters opened at the summit, discharging with a
dreadful noise an immense cloud of smoke and ashes, with bombs which
rose to a height of 1300 metres above the brim of the volcano.[12] The
torrents of fire which threatened Resina, Bosco, and Torre Annunziata,
and which devastated the fertile country of Novelle, Massa, St.
Sebastiano, and Cerole, and two partially buried cities, the continual
thunderings and growling of the craters, caused such terror, that
numbers abandoned their dwellings, flying for refuge into Naples, while
many Neapolitans went to Rome or other places. Fortunately, the paroxysm
had now passed, the lava-streams stopped in their course, and the great
torrent which passed the shoulders of the Observatory through the Fossa
della Vetraria lowered the level of its surface below that of its sides,
which appeared like two parallel ramparts above it. Had these streams
continued to flow on the 27th of April as they had done on the previous
night, they would have reached the sea, bringing destruction to the very
walls of Naples. During this eruption Torre del Greco was upraised to
the extent of two metres, and nearly all the houses were knocked down.
The igneous period of eruption having terminated, the ashes, lapilli,
and projectiles became more abundant, accompanied by thunder and
lightning. On the 28th they darkened the air, and the terrific noise of
the mountain continuing or increasing, the terror at Resina, Portici,
and Naples became universal. It seemed as though the tragic calamities
of the eruption of A.D. 79 were about to be repeated. But gradually the
force of the explosions decreased, and the noise from the crater became
discontinuous, so that on the 30th the detonations were very few, and by
the 1st May the eruption was completely over.
Such is a condensed account of one of the most formidable eruptions of
our era. In the frontispiece of this volume a representation, taken (by
permission) from a photograph by Negretti & Zambra, is given, showing
the appearance of Vesuvius during the final stage of the eruption, when
prodigious masses of smoke, steam, and illuminated gas issued forth from
the summit and overspread the whole country around with a canopy which
the light of the sun could scarcely penetrate.
It will be noticed in the above account that, concurrently with the full
moon, there were two distinct and special outbreaks of activity; one
occurring in March, the other in the month following. That the
conditions of lunar and solar attraction should have a marked effect on
a part of the earth's crust, while under the tension of eruptive forces,
is only what might be expected. At full moon the earth is between the
sun and the moon, and at new moon the moon is between the sun and the
earth; under these conditions (the two bodies acting in concert) we have
spring tides in the ocean, and a maximum of attraction on the mass of
the earth. Hence the crust, which at the time referred to was under
tremendous strain, only required the addition of that caused by the
lunar and solar attractions to produce rupture in both cases, giving
rise to increased activity, and the extrusion of lava and volatile
matter. It may, in general, be safely affirmed that low barometric
pressure on the one hand, and the occurrence of the syzygies (when the
attractions of the sun and moon are in the same line) on the other, have
had great influence in determining the crises of eruptions of volcanic
mountains when in a state of unrest.
_Contrast between the Northern and Southern Slopes._--Before leaving
Vesuvius it may be observed that throughout all the eruptions of modern
times the northern side of the mountain, that is the old crater and
flank of Somma, has been secure from the lava-flows, and has enjoyed an
immunity which does not belong to the southern and western side. If we
look at a map of the mountain showing the direction of the streams
during the last three centuries,[13] we observe that all the streams of
that period flowed down on the side overlooking the Bay of Naples; on
the opposite side the wall of Monte di Somma presents an unbroken front
to the lava-streams. From this it may be inferred that one side, the
west, is weaker than the other; and consequently, when the lava and
vapours are being forced upwards, under enormous pressure from beneath,
the western side gives way under the strain, as in the case of the
fissure of 1872, and the lava and vapours find means of escape. From
what has happened in the past it is clear that no place on the western
side of the mountain is entirely safe from devastation by floods of
lava; while the prevalent winds tend to carry the ashes and lapilli,
which are hurled into the air, in the same westerly direction.
[1] For an excellent view of this remarkable volcanic group see Judd's
_Volcanoes_, 4th edition, p. 43.
[2] Plutarch, _Life of Cassius_; _ed. Reiske_, vol. iii. p. 240.
[3] Strabo gives the following account of the appearance and condition
of Vesuvius in his day:--"Supra hæc loca situs est Vesuvius mons, agris
cinctus optimis; dempto vertice, qui magna sui parte planus, totus
sterilis est, adspectu sinereus, cavernasque ostendens fistularum plenas
et lapidum colore fuliginoso, utpote ab igni exesorum. Ut conjectarum
facere possis, ista loca quondam arsisse et crateras ignis habuisse,
deinde materia deficiente restricta fuisse."--_Rer. Geog._, lib. v.
[4] A tablet over the entrance records this act of pious liberality, and
is given by Phillips, _loc. cit._, p. 12.
[5] The stone pine, _Pinus pinea_, which Turner knew how to use with so
much effect in his Italian landscapes.
[6] Bulwer Lytton's _Last Days of Pompeii_ presents to the reader a
graphic picture of the terrible event here referred to:--"The eyes of
the crowd followed the gesture of the Egyptian, and beheld with
ineffable dismay a vast vapour shooting from the summit of Vesuvius, in
the form of a gigantic pine tree; the trunk--blackness, the
branches--fire! A fire that shifted and wavered in its hues with every
moment--now fiercely luminous, now of a dull and dying red that again
blazed terrifically forth with intolerable glare!... Then there arose on
high the shrieks of women; the men stared at each other, but were
speechless. At that moment they felt the earth shake beneath their feet;
the walls of the theatre trembled; and beyond, in the distance, they
heard the crash of falling roofs; an instant more and the mountain-cloud
seemed to roll towards them, dark and rapid; at the same time it cast
forth from its bosom a shower of ashes mixed with vast fragments of
burning stone. Over the crushing vines--over the desolate streets--over
the amphitheatre itself--far and wide, with many a mighty splash in the
agitated sea, fell that awful shower." A visit to the disinterred city
will probably produce on the mind a still more lasting and vivid
impression of the swift destruction which overtook this city.
[7] Quoted by Phillips, _loc. cit._, p. 45.
[8] _Vesuvius_, p. 72 _et seq._
[9] Johnston-Lavis, "On the Geology of Monti Somma and Vesuvius,"
_Quart. Jour. Geol. Soc._, vol. 40 (1884).
[10] Palmieri, _Eruption of Vesuvius in 1872_, with notes, etc., by
Robert Mallet, F.R.S. London, 1873.
[11] Those who lost their lives were medical students, and an Assistant
Professor in the University, Antonio Giannone by name.
[12] Involving, as Mr. Mallet calculates, an initial velocity of
projection of above 600 feet per second.
[13] Such as that given by Professor Phillips in his _Vesuvius_.
CHAPTER II.
ETNA.
(_a._) _Structure of the Mountain._--Etna, unlike Vesuvius, has ever
been a burning mountain; hence it was well known as such to classic
writers before the Christian era. The structure and features of this
magnificent mountain have been abundantly illustrated by Elie de
Beaumont,[1] Daubeny,[2] Baron von Waltershausen,[3] and Lyell,[4] of
whose writings I shall freely avail myself in the following account, not
having had the advantage of a personal examination of this region.
_Structure of Etna._--So large is Etna that it would enclose within its
ample skirts several cones of the size of Vesuvius. It rises to a height
of nearly 11,000 feet above the waters of the Mediterranean,[5] and is
planted on a floor consisting of stratified marine volcanic matter, with
clays, sands, and limestones of newer Pliocene age. Its base is nearly
circular, and has a circumference of 87 English miles. In ascending its
flanks we pass successively over three well-defined physical zones: the
lowest, or fertile zone, comprising the tract around the skirts of the
mountain up to a level of about 2500 feet, being well cultivated and
covered by dwellings surrounded by olive groves, fields, vineyards, and
fruit-trees; the second, or forest zone, extending to a level of about
6270 feet, clothed with chestnut, oak, beech, and cork trees, giving
place to pines; and the third, extending to the summit and called "the
desert region," a waste of black lava and scoriæ with mighty crags and
precipices, terminating in a snow-clad tableland, from which rises the
central cone, 1100 feet high, emitting continually steam and sulphurous
vapours, and in the course of almost every century sending forth streams
of molten lava.
The forest zone is remarkable for the great number of minor craters
which rise up from the midst of the foliage, and are themselves clothed
with trees. Sartorius von Waltershausen has laid down on his map of Etna
about 200 of these cones and craters, some of which, like those of
Auvergne, have been broken down on one side. Many of these volcanoes of
second or third magnitude lie outside the forest zone, both above and
below it; such as the double hill of Monti Rossi, near Nicolosi, formed
in 1659, which is 450 feet in height, and two miles in circumference at
its base. Sir C. Lyell observes that these minor crater-cones present us
with one of the most delightful and characteristic scenes in Europe.
They occur of every variety of height and size, and are arranged in
picturesque groups. However uniform they may appear when seen from the
sea or the plains below, nothing can be more diversified than their
shape when we look from above into their ruptured craters. The cones
situated in the higher parts of the forest zone are chiefly clothed with
lofty pines; while those at a lower elevation are adorned with
chestnuts, oaks, and beech trees. These cones have from time to time
been buried amidst fresh lava-streams descending from the great crater,
and thus often become obliterated.
[Illustration: Fig. 10.--Ideal Section through Etna. (After Lyell.)--A.
Axis of present cone of eruption; B. Axis of extinct cone of eruption;
_a._ Older lavas, chiefly trachytic; _b._ Newer lavas, erupted (with
_a_) before origin of the Val del Bove; _c._ Scoria and lava of recent
age; T. Tertiary strata forming the foundation to the volcanic rocks.
The position of the Val del Bove before its formation is shown by the
lightly-shaded portion above B.]
(_b._) _Val del Bove._--The most wonderful feature of Mount Etna is the
celebrated Val del Bove (Valle del Bue), of which S. von Waltershausen
has furnished a very beautiful plate[6]--a vast amphitheatre hewn out of
the eastern flank of the mountain, just below the snow-mantled platform.
It is a physical feature somewhat after the fashion of Monte Somma in
Vesuvius, but exceeds it in magnitude as Etna exceeds Vesuvius. The Val
del Bove is about five miles in diameter, bounded throughout
three-fourths of its circumference by precipitous walls of ashes,
scoriæ, and lava, traversed by innumerable dykes, and rising inwards to
a height of between 3000 and 4000 feet. Towards the east the cliffs
gradually fall to a height of about 500 feet, and at this side the vast
chasm opens out upon the slope of the mountain. At the head of the Val
del Bove rises the platform, surmounted by the great cone and crater. It
will thus be seen that by means of this hollow we have access almost to
the very heart of the mountain.
What is very remarkable about the structure of this valley is that the
beds exhibit "the _quâ-quâ_ versal dip"--in other words, they dip away
on all sides from the centre--which has led to the conclusion that in
the centre is a focus of eruption which had become closed up
antecedently to the formation of the valley itself. Lyell has explained
this point very clearly by showing that this focus had ceased to eject
matter at some distant period, and that the existing crater at the
summit of the mountain had poured out its lavas over those of the
extinct orifice. This was prior to the formation of the Val del Bove
itself; and the question remains for consideration how this vast natural
amphitheatre came to be hollowed out; for its structure shows
unquestionably that it owes its form to some process of excavation.
In the first place, it is certainly not the work of running water, as in
the case of the cañons of Colorado; the porous matter of which the
mountain is formed is quite incapable of originating and supporting a
stream of sufficient volume to excavate and carry away such enormous
masses of matter within the period required for the purpose. We must
therefore have recourse to some other agency. Numerous illustrations are
to be found of the explosive action of volcanoes in blowing off either
the summits of mountains, or portions of their sides. For example, there
is reason for believing that the first result of the renewed energy of
Vesuvius was to blow into the air the upper surface of the mountain.
Again, so late as 1822, during a violent earthquake in Java, a country
which has been repeatedly devastated by earthquakes and volcanic
eruptions, the mountain of Galongoon, which was covered by a dense
forest, and situated in a fertile and thickly-peopled region, and had
never within the period of tradition been in activity, was thus ruptured
by internal forces. In the month of July 1822, after a terrible
earthquake, an explosion was heard, and immense columns of boiling
water, mixed with mud and stones, were projected from the mountain like
a water-spout, and in falling filled up the valleys, and covered the
country with a thick deposit for many miles, burying villages and their
inhabitants. During a subsequent eruption great blocks of basalt were
thrown to a distance of seven miles; the result of all being that an
enormous semicircular gulf was formed between the summit and the plain,
bounded by steep cliffs, and bearing considerable resemblance to the Val
del Bove. Other examples of the power of volcanic explosions might be
cited; but the above are sufficient to show that great hollows may thus
be formed either on the summits or flanks of volcanic mountains. Chasms
may also be formed by the falling in of the solidified crust, owing to
the extrusion of molten matter from some neighbouring vent of eruption;
and it is conceivable that by one or other of these processes the vast
chasm of the Val del Bove on the flanks of Etna may have been produced.
(_c._) _The Physical History of Etna._--The physical history of Etna
seems to be somewhat as follows:--
_First Stage._--Somewhere towards the close of the Tertiary
period--perhaps early Pliocene or late Miocene--a vent of eruption
opened on the floor of the Mediterranean Sea, from which sheets of lava
were poured forth, and ashes mingled with clays and sands, brought down
from the neighbouring lands, were strewn over the sea-bed. During a
pause in volcanic activity, beds of limestone with marine shells were
deposited.
_Second Stage._--This sea-bed was gradually upraised into the air, while
fresh sheets of lava and other _ejecta_ were accumulated round the vents
of eruption, of which there were two principal ones--the older under the
present Val del Bove, the newer under the summit of the principal cone.
Thus was the mountain gradually piled up.
_Third Stage._--The vent under the Val del Bove ceased to extrude more
matter, and became extinct. Meanwhile the second vent continued active,
and, piling up more and more matter round the central crater, surmounted
the former vent, and covered its _ejecta_ with newer sheets of lava,
ashes, and lapilli, while numerous smaller vents, scattered all over the
sides of the mountain, gave rise to smaller cones and craters.
_Fourth Stage._--This stage is signalised by the formation of the Val
del Bove through some grand explosion, or series of explosions, by which
this vast chasm was opened in the side of the mountain, as already
explained.
_Fifth Stage._--This represents the present condition of the mountain,
whose height above the sea is due, not only to accumulation of volcanic
materials round the central cone, but to elevation of the whole island,
as evinced by numerous raised beaches of gravel and sand, containing
shells and other forms of marine species now living in the waters of the
Mediterranean.[7] Since then the condition and form of the mountain has
remained very much the same, varied only by the results of occasional
eruptions.
(_d._) _Dissimilarity in the Constitution of the Lavas of Etna and
Vesuvius._--Before leaving the subject we have been considering, it is
necessary that I should mention one remarkable fact connected with the
origin of the lavas of Etna and Vesuvius respectively; I refer to their
essential differences in mineral composition. It might at first sight
have been supposed that the lavas of these two volcanic
mountains--situated at such a short distance from each other, and
evidently along the same line of fracture in the crust--would be of the
same general composition; but such is not the case. In the lava of
Vesuvius leucite is an essential, and perhaps the most abundant mineral.
It is called by Zirkel _Sanidin-Leucitgestein_. (See Plate IV.) But in
that of Etna this mineral is (as far as I am aware) altogether absent.
We have fortunately abundant means of comparison, as the lavas of these
two mountains have been submitted to close examination by petrologists.
In the case of the Vesuvian lavas, an elaborate series of chemical
analyses and microscopical observations have been made by the Rev.
Professor Haughton, of Dublin University, and the author,[8] from
specimens collected by Professor Guiscardi from the lava-flows extending
from 1631 to 1868, in every one of which leucite occurs, generally as
the most abundant mineral, always as an essential constituent. On the
other hand, the composition of the lavas of Etna, determined by
Professor A. von Lasaulx, from specimens taken from the oldest
(vorätnäischen) sheets of lava down to those of the present day,
indicates a rock of remarkable uniformity of composition, in which the
components are plagioclase felspar, augite, olivine, magnetite, and
sometimes apatite; but of leucite we have no trace.[9] In fact, the
lavas of Etna are very much the same in composition as the ordinary
basalts of the British Isles, while those of Vesuvius are of a different
type. This seems to suggest an origin of the two sets of lavas from a
different deep-seated magma; the presence of leucite in such large
quantity requiring a magma in which soda is in excess, as compared with
that from which the lavas of Etna have been derived.[10]
[1] _Mémoires pour Servir_, etc., vol. ii.
[2] Daubeny, _Volcanoes_, p. 270.
[3] Von Waltershausen, _Der Ætna_, edited by A. von Lasaulx.
[4] Lyell, _Principles of Geology_, vol. ii., edition 1872.
[5] Its height, as determined by Captain Smyth in 1875
trigonometrically, was 10,874 feet, and afterwards by Sir J. Herschel
barometrically, 10,872 feet.
[6] _Atlas des Ætna_ (Weimar, 1858), in which the different lava-streams
of 1688, 1802, 1809, 1811, 1819, 1824, and 1838 are delineated.
[7] Sir William Hamilton observes that history is silent regarding the
first eruptions of Etna. It was in activity before the Trojan War, and
even before the arrival of the "Sizilien" settlers. Diodorus and
Thucydides notice the earliest recorded eruptions, those from 772 to 388
B.C., during which time the mountain was thrice in eruption. Later
eruptions took place in the year 140, 135, 125, 122 B.C. In the year 44
B.C., in the reign of Julius Cæsar, there was a very violent outburst of
volcanic activity.--_Neuere Beobachtungen über die Vulkane Italiens und
am Rhein_, p. 173, Frankfurt (1784).
[8] "Report on the Chemical and Mineralogical Characters of the Lavas of
Vesuvius from 1631 to 1868," _Transactions of the Royal Irish Academy_,
vol. xxvi. (1876). In the lava of 1848 leucite was found to reach 44.9
per cent. of the whole mass. In that of Granatello, 1631, it reaches its
lowest proportion--viz., 3.37 per cent.
[9] A. von Lasaulx, in Von Waltershausen's _Der Ætna_, Book II., x. 423.
[10] The view of Professor Judd, that leucite easily changes into
felspar, and that some ancient igneous rocks which now contain felspar
were originally leucitic, does not seem to be borne out by the above
facts. In such cases the felspar crystals ought to retain the forms of
leucite. See _Volcanoes_, 4th edition, p. 268.
CHAPTER III.
THE LIPARI ISLANDS, STROMBOLI.
(_a._) A brief account of this remarkable group of volcanic islands must
here be given, inasmuch as they seem to be representatives of a stage of
volcanic action in which the igneous forces are gradually losing their
energy. According to Daubeny, the volcanic action in these islands seems
to be developed along two lines, nearly at right angles to each other,
one parallel to that of the Apennines, beginning with Stromboli,
intersecting Panaria, Lipari, and Vulcano; the other extending from
Panaria to Salina, Alicudi, and Felicudi, and again visible in the
volcanic products which make their appearance at Ustica. (See Map, Fig.
11.) The islands lie between the north coast of Sicily and that of
Italy, and from their position seem to connect Etna with Vesuvius; but
this is very problematical, as would appear from the difference of their
lavas. The principal islands are those of Stromboli, Panaria, Lipari,
Vulcano, Salina, Felicudi, and Alicudi. These three last are extinct or
dormant, but Salina contains a crater, rising, according to Daubeny, not
less than 3500 feet above the sea.[1] Vulcano (referred to by Strabo
under the name of Hiera) consists of a crater which constantly emits
large quantities of sulphurous vapours, but was in a state of activity
in the year 1786, when, after frequent earthquake shocks and
subterranean noises, it vomited forth during fifteen days showers of
sand, together with clouds of smoke and flame, altering materially the
shape of the crater from which they proceeded.
[Illustration: LIPARI ISLANDS.
Fig. 11.--Map to show the position of these islands, showing the
branching lines of volcanic action--one parallel to that of the
Apennines, the other stretching westwards at right angles thereto.]
The islands of Lipari are formed of beds of tuff, penetrated by numerous
dykes of lava, from which uprise two or three craters, formed of pumice
and obsidian passing into trachyte. Volcanic operations might have here
been said to be extinct, were it not that their continuance is
manifested by the existence of hot springs and "stufes," or vapour
baths, at St. Calogero, about four miles from the town of Lipari.
Daubeny considers it not improbable that this island may have had an
active volcano even within the historical period, a view which is borne
out by the statement of Strabo.[2]
[Illustration: Fig. 12.--Island of Vulcano, one of the Lipari Group, in
eruption.--(After Sir W. Hamilton.)]
(_b._) But by far the most remarkable island of the group, as regards
its present volcanic condition, is Stromboli, which has ever been in
active eruption from the commencement of history down to the present
day. Professor Judd, who visited this island in 1874, and has produced a
striking representation of its aspect,[3] gives an account of which I
shall here avail myself.[4] The island is of rudely circular outline,
and rises into a cone, the summit of which is 3090 feet above the level
of the Mediterranean. From a point on the side of the mountain masses of
vapour are seen to issue, and these unite to form a cloud over the
summit; the outline of this vapour-cloud varying continually according
to the hygrometric state of the atmosphere, and the direction and force
of the wind. At the time of Professor Judd's visit, the vapour-cloud
was spread in a great horizontal stratum overshadowing the whole island;
but it was clearly seen to be made up of a number of globular masses,
each of which is a product of a distinct outburst of volcanic forces.
Viewed at night-time, Stromboli presents a far more striking and
singular spectacle. When watched from the deck of a vessel, a glow of
red light is seen to make its appearance from time to time above the
summit of the mountain; it may be observed to increase gradually in
intensity, and then as gradually to die away. After a short interval the
same appearances are repeated, and this goes on till the increasing
light of dawn causes the phenomenon to be no longer visible. The
resemblance presented by Stromboli to a "flashing light" on a most
gigantic scale is very striking, and the mountain has long been known as
"the lighthouse of the Mediterranean."
The mountain is built up of ashes, slag, and scoriæ, to a height of (as
already stated) over 3000 feet above the surface of the sea; but, as
Professor Judd observes, this by no means gives a just idea of its vast
bulk. Soundings in the sea surrounding the island show that the bottom
gradually shelves around the shores to a depth of nearly 600 fathoms, so
that Stromboli is a great conical mass of cinders and slaggy materials,
having a height above its floor of about 6600 feet, and a base the
diameter of which exceeds four miles.
The crater of Stromboli is situated, not at the apex of the cone, but at
a distance of 1000 feet below it. The explosions of steam, accompanied
by the roaring as of a smelting furnace, or of a railway engine when
blowing off its steam, are said by Judd to take place at very irregular
intervals of time, "varying from less than one minute to twenty minutes,
or even more." On the other hand, Hoffmann describes them as occurring
at "perfectly regular intervals," so that, perhaps, some variation has
taken place within the interval of about forty years between each
observation. Both observers agree in stating that lava is to be seen
welling up from some of the apertures within the crater, and pouring
down the slope towards the sea, which it seldom or never reaches.[5] The
intermittent character of these eruptions appears to be due, as Mr.
Scrope has suggested, to the exact proportion between the expansive and
repressive forces; the expansive force arising from the generation of a
certain amount of aqueous vapour and of elastic gas; the repressive,
from the pressure of the atmosphere and from the weight of the
superincumbent volcanic products. Steam is here, as in a steam-engine,
not the originating agent in the phenomena recorded; but the result of
water coming in contact with molten lava constantly welling up from the
interior, by which it is converted into steam, which from time to time
acquires sufficient elastic force to produce the eruptions; the water
being obviously derived from the surrounding sea, which finds its way by
filtration through fissures, or through the porous mass of which the
mountain is formed. Were it not for the access of water this volcano
would probably appear as a fissure-cone extruding a small and continuous
stream of molten lava. The adventitious access of the sea water gives
rise to the phenomena of intermittent explosions. The vitality of the
volcano is therefore due, not to the presence of water, but to the
welling up of matter from the internal reservoir through the throat of
the volcano.
_Pantelleria._--This island, lying between the coast of Sicily and Cape
Bon in Africa, is wholly volcanic. It has a circumference of thirty
miles, and from its centre rises an extinct crater-cone to a height of
about 3000 feet. The flanks of this volcano are diversified by several
fresh craters and lava-streams, while hot springs burst out with a
hissing noise on its southern flank, showing that molten matter lies
below at no very great depth.
This island probably lies along the dividing line between the
non-volcanic and volcanic region of the Mediterranean, and is
consequently liable to intermittent eruptions. It was at a short
distance from this island that the remarkable submarine outburst of
volcanic forces took place on October 17th, 1891, for an account of
which we are indebted to Colonel J. C. Mackowen.[6] On that day, after a
succession of earthquake shocks, the inhabitants were startled by
observing a column of "smoke" rising out of the sea at a distance of
three miles, in a north-westerly direction. The Governor, Francesco
Valenza, having manned a boat, rowed out towards the fiery column, and
on arriving found it to consist of black scoriaceous bombs, which were
being hurled into the air to a height of nearly thirty yards; some of
them burst in the air, others, discharging steam, ran hissing over the
water; many of them were very hot, some even red-hot. One of these
bombs, measuring two feet in diameter, was captured and brought to
shore. It was observed that after the eruption the earthquake shocks
ceased. A vast amount of material was cast out of the submarine crater,
forming an island 500 yards in length and rising up to nine feet above
the surface, but after a few days it was broken up and dispersed over
the sea-bed by the action of the waves.
[1] _Volcanoes_, p. 262. These islands are described by Hoffmann,
_Poggendorf Annal._, vol. xxvi. (1832); also by Lyell, _Principles of
Geology_, vol. ii., and by Judd, who personally visited them, and gives
a very vivid account of their appearance and structure.
[2] Strabo, lib. vi.
[3] Judd, _Volcanoes_, p. 8.
[4] Stromboli has also been described by Spallanzani, Hoffmann, Daubeny,
and others. The account of Judd is the most recent. Of this island
Strabo says, "Strongyle a rotundate figuræ sic dicta, ignita ipsa
quoque, violentia flammarum minor, fulgore excellens; ibi habitasse
Æcolum ajunt."--Lib. vi.
[5] _Poggend. Annal._, vol. xxvi., quoted by Daubeny.
[6] Communicated by Captain Petrie to the Victoria Institute, 1st
February 1892. See also a detailed and illustrated account of the
eruption communicated by A. Ricco to the _Annali dell' Ufficio centrale
Meteorologico e Geodonamico_, Ser. ii., Parte 3, vol. xi. Summarised by
Mr. Butler in _Nature_, April 21, 1892.
CHAPTER IV.
THE SANTORIN GROUP.
[Illustration: Fig. 13.--Ideal Section through the Gulf of Santorin, to
show the structure of the submerged volcano.--_a._ Island of Aspronisi;
_b._ Island of Thera; 1. Old Kaimeni Island; 2. New Kaimeni Island; 3.
Little Kaimeni Island.]
(_a._) Before leaving the subject of European active volcanoes, it is
necessary to give some account of the remarkable volcanic island of
Santorin, in the Grecian archipelago. This island for 2000 years has
been the scene of active volcanic operations, and in its outline and
configuration, both below and above the surface of the Mediterranean,
presents the aspect of a partially submerged volcanic mountain. (See
Section, Fig. 13.) If, for example, we can imagine the waters of the sea
to rise around the flanks of Vesuvius until they have entered and
overflowed to some depth the interior caldron of Somma, thus converting
the old crater into a crescent-shaped island, and the cone of Vesuvius
into an island--or group of islands--within the caldron, then we shall
form some idea of the appearance and structure of the Santorin group.
_Form of the Group._--The principal island, Thera, has somewhat the
shape of a crescent, breaking off in a precipitous cliff on the inner
side, but on the outer side sloping at an angle of about fifteen degrees
into deep water. Continuing the curvature of the crescent, but separated
by a channel, is the island of Therasia; and between this and the
southern promontory of Thera is another island called Aspronisi. All
these islands, if united, would form the rim of a crater, in which the
volcanic matter slopes outward into deep water, descending at a short
distance to a depth of 200 fathoms and upwards. In the centre of the
gulf thus formed rise three islands, called the Old, New, and Little
Kaimenis. These may be regarded as cones of eruption, which history
records as having been thrown up at successive intervals. According to
Pliny, the year 186 B.C. gave birth to Old Kaimeni, also called Hiera,
or the Sacred Isle; and in the first year of our era Thera (the Divine)
made its appearance above the water, and was soon joined to the older
island by subsequent eruptions. Old Kaimeni also increased in size by
the eruptions of 726 and 1427. A century and a half later, in 1573,
another eruption produced the cone and crater called Micra-Kaimeni. Thus
were formed, or rather were rendered visible above the water, the
central craters of eruption; and between these and the inner cliff of
Thera and Therasia is a ring of deep water, descending to a depth of
over 200 fathoms. So that, were these islands raised out of the sea, we
should have presented to our view a magnificent circular crater about
six miles in diameter, bounded by nearly vertical walls of rock from
1000 to 1500 feet in height, and ruptured at one point, from the centre
of which would rise two volcanic cones--namely, the Kaimenis--one with a
double crater, still foci of eruption, and from time to time bursting
forth in paroxysms of volcanic energy, of which those of 1650, 1707, and
1866 were the most violent and destructive.[1] Of this last I give a
bird's-eye view (Fig. 14).
The only rock of non-volcanic origin in these islands consists of
granular limestone and clay slate forming the ridge of Mount St. Elias,
which rises to a height of 1887 feet at the south-eastern side of the
island of Thera, crossing the island from its outer margin nearly to the
interior cliff, so that the volcanic materials have been piled up along
its sides. The rocks of St. Elias are much more ancient than any of the
volcanic materials around; and, as Bory St. Vincent has shown, have been
subjected to the same flexures, dip and strike, as those sedimentary
rocks which go to form the non-volcanic islands of the Grecian
archipelago.
[Illustration: Fig. 14.--Bird's-eye View of the Gulf of Santorin during
the volcanic eruption of February 1866.--(After Lyell.)]
[Illustration: _Ground Plan of Rocca Monfina_
Fig. 15.--Rocca Monfina, in Southern Italy, showing a crater-ring of
trachytic tuffs, from the midst of which, according to Judd, an andesite
lava-cone has been built up. Compare with the Santorin Group.]
(_b._) _Origin of the Santorin Group._--In reference to the origin of
the Santorin group, Lyell regards it as a remnant of a great volcanic
mountain which possessed a focus of eruption rising in the position of
the present foci, but afterwards partially destroyed and the whole
submerged to a depth of over 1000 feet. But another explanation is open
to us, and one not inconsistent with what we now know of the physical
changes to which the Mediterranean has been subjected since early
Tertiary times. To my mind it is difficult to conceive how such a
volcanic mountain as that of Santorin could have been formed under
water; while, on the other hand, its physical structure and contour bear
so striking a resemblance (as already observed) to those of Vesuvius and
Rocca Monfina that we are much tempted to infer that it had a somewhat
similar origin. Now we know that Vesuvius was built up by means of
successive eruptions taking place under the air; and the question arises
whether it could be possible that Santorin had a similar origin owing to
the waters of the Mediterranean having been temporally lowered at a
later Tertiary epoch. It has been stated by M. Fouqué that the age of
the more ancient volcanic beds of Santorin belong, as shown by the
included fossils, to the newer Pliocene epoch. These are of course the
unsubmerged, and therefore more recent strata, and may have been
recently upheaved during one or more of the outbursts of volcanic
energy. But it seems an impossibility that the Gulf of Santorin, with
its precipitous walls and deep circular interior channel, as shown by
the Ideal Section (Fig. 13), could have been formed otherwise than under
the air. We are led, therefore, to inquire whether there was a time in
the history of the Mediterranean, since the Eocene period, when the
waters were lower than at present. That this was the case we have clear
evidence. The remains of elephants, hippopotami, and other animals,
which have been discovered in great numbers in the Maltese caves, show
that this island was united to Sicily, and this again to Europe, during
the later Pliocene epoch, so as to have become the abode of an
Europasian fauna. According to Dr. Wallace, a causeway of dry land
existed, stretching from Italy to Tunis in North Africa through the
Maltese Islands--an inference involving the lowering of the waters of
the Mediterranean by several hundred feet.[2] There is every reason for
supposing that the old volcano of Santorin was in active eruption at
this period; and its history may be considered to be similar to that of
Vesuvius until, at the rising of the waters during the Pluvial (or
Post-Pliocene) epoch, during which they rose higher than at present,
Santorin was converted into a group of islands, slightly differing in
form from those of the present day. This view seems to meet the
difficulties regarding the origin of this group, difficulties which
Lyell had long since clearly recognised.
(_c._) _Limit of the Mediterranean Volcanic Region._--With the Santorin
group we conclude our account of the active European volcanoes. It may
be observed, however, that from some cause not ascertained the volcanic
districts of the Mediterranean and its shores are confined to the north
side of that great inland sea; so that as regards vulcanicity the
African coast presents a striking contrast to that of the opposite side.
If we draw a line from the shores of the Levant to the Straits of
Gibraltar, by Candia, Malta, and to the south of Pantelleria and
Sardinia, we shall find that the volcanic islands and districts of the
mainland lie to the north of it.[3] This has doubtless some connection
with the internal geological structure. The immunity of the Libyan
desert from volcanic irruptions is in keeping with the remarkably
undisturbed condition of the Secondary strata, which seldom depart much
from the horizontal position; while the igneous rocks of the Atlas
mountains are probably of great geological antiquity. On the other hand,
the Secondary and Tertiary formations of the northern shores and islands
of the Mediterranean are generally characterised by the highly-inclined,
flexured, and folded position of the strata. Hence we may suppose that
the crust over the region lying to the north of the volcanic line, owing
to its broken and ruptured condition, was less able to resist the
pressure of the internal forces of eruption than that lying to the south
of it; and that, in consequence, vents and fissures of eruption were
established over the former of these regions, while they are absent in
the latter.
[1] Fuller details will be found in Daubeny's _Volcanoes_, chap. xviii.,
and Lyell's _Principles of Geology_, vol. ii. p. 65 (edition 1872). The
bird's-eye view is taken from this latter work by kind permission of the
publisher, Mr. J. Murray, as also the accompanying Ideal Section, Fig.
13.
[2] Wallace, _Geographical Distribution of Animals_ (1876). The author's
_Sketch of Geological History_, p. 130 (Deacon & Co., 1887).
[3] The _volcanic area_ lying to the north of this line will include
Sardinia, Sicily, Pantelleria, the Grecian Archipelago, Asia Minor, and
Syria; the _non-volcanic area_ lying to the south of this line will
include the African coast, Malta, Isles of Crete and Cyprus. The Isle of
Pantelleria is apparently just on the line, which, continued eastward,
probably follows the north coast of Cyprus, parallel to the strike of
the strata and of the central axis of that island.--See "Carte
Géologique de l'île de Chypre, par MM. Albert Gaudry et Amedée Damour"
(1860).
CHAPTER V.
EUROPEAN EXTINCT OR DORMANT VOLCANOES.
We are naturally led on from a consideration of the active volcanoes of
Europe to that of volcanoes which are either dormant or extinct in the
same region. Such are to be found in Italy, Central France, both banks
of the Rhine and Moselle, the Westerwald, Vogelsgebirge, and other
districts of Germany; in Hungary, Styria, and the borders of the Grecian
archipelago. But the subject is too large to be treated here in detail;
and I propose to confine my observations to some selected cases which
are to be found in Southern Italy, Central France, and the Rhenish
districts, where the volcanic features are of so recent an age as to
preserve their outward form and structure almost intact.
(_a._) _Southern Italy._--Extinct volcanoes and volcanic rocks occupy
considerable tracts between the western flanks of the Apennines and the
Mediterranean coast in the Neapolitan and Roman States, forming the
remarkable group of the Phlegræan fields (Campi Phlegræi), with the
adjoining islands of Ischia, Procida, Nisida, Vandolena, Ponza, and
Palmarola; at Melfi and Avellino. All the region around Rome extending
along the western slopes of the Apennines from Velletri to Orvieto,
together with Mount Annato in Tuscany, is formed of volcanic material,
and the same may be said of a large part of the island of Sardinia. From
these districts I shall select some points which seem to be of special
interest.
_Monte Nuovo and the Phlegræan Fields._--The tract of which this
celebrated district forms a part lies as it were in a bay of the
Apennine limestone of Jurassic age. The floor of this bay is composed of
puzzolana, a name given to beds of volcanic tuff of great thickness, and
rising into considerable hills in the vicinity of the city of Naples,
such as that of St. Elmo. Its composition is peculiar, as it is chiefly
formed of small pieces of pumice, obsidian, and trachyte, in beds
alternating with loam, ferriferous sand, and fragments of limestone. It
is evidently of marine formation, as Sir William Hamilton, Professor
Pilla, and others have detected sea-shells therein, of the genera
_Ostræa_, _Cardium_, _Pecten_ and _Pectunculus_, _Buccinum_, etc. It is
generally of a greyish colour, and sometimes sufficiently firm to be
used as a building stone. The Roman Campagna is largely formed of
similar materials, which were deposited at a time when the districts in
question were submerged, and matter was being erupted from volcanic
vents at various points around, and spread over the sea-bed.
Such is the character of the general floor on which the more recent
crater-cones of this district have been built. These are numerous, and
all extinct with the exception of the Solfatara, near Puzzuoli, from
which gases mixed with aqueous vapour are continually being exhaled. The
gases consist of sulphuretted hydrogen mixed with a minute quantity of
muriatic acid.[1] This district is also remarkable for containing
several lakes occupying the interiors of extinct craters; amongst
others, Lake Avernus, which, owing to its surface having been darkened
by forests, and in consequence of the effluvia arising from its stagnant
waters, has had imparted to it a character of gloom and terror, so that
Homer in the _Odyssey_ makes it the entrance to hell, and describes the
visit of Ulysses to it. Virgil follows in his steps. Another lake of
similar origin is Lake Agnano. Here also is the Grotto del Cane, a
cavern from which are constantly issuing volumes of carbonic acid gas
combined with much aqueous vapour, which is condensed by the coldness of
the external air, thus proving the high temperature of the ground from
which the gaseous vapour issues. This whole volcanic region, so replete
with objects of interest,[2] may be considered, as regards its volcanic
character, in a moribund condition; but that it is still capable of
spasmodic movement is evinced by the origin of Monte Nuovo, the most
recent of the crater-cones of the district. This mountain, rising from
the shore of the Bay of Baiæ, was suddenly formed in September 29th,
1538, and rises to a height of 440 feet above the sea-level. It is a
crater-cone, and the depth of the crater has been determined by the
Italian mineralogist Pini to be 421 English feet; its bottom is thus
only 19 feet above the sea-level. A portion of the base of the cone is
considered partly to occupy the site of the Lucrine Lake, which was
itself nothing more than the crater of a pre-existent volcano, and was
almost entirely filled up during the explosion of 1538. Monte Nuovo is
composed of ashes, lapilli, and pumice-stones; and its sudden formation,
heralded by earthquakes, and accompanied by the ejection of volcanic
matter mixed with fire and water, is recorded by Falconi, who vividly
depicts the terror and consternation of the inhabitants of the
surrounding country produced by this sudden and terrible outburst of
volcanic forces.[3]
(_b._) _Central Italy and the Roman States._--The tract bordering the
western slopes of the Apennines northward from Naples into Tuscany, and
including the Roman States, is characterised by volcanic rocks and
physical features of remarkable interest and variety. These occur in the
form of extinct craters, sometimes filled with water, and thus converted
into circular lakes; or of extensive sheets and conical hills of tuff;
or, finally, of old necks and masses of trachyte and basalt, sometimes
exhibiting the columnar structure. The Eternal City itself is built on
hills of volcanic material which some observers have supposed to be the
crater of a great volcano; but Ponzi, Brocchi, and Daubeny all concur in
the opinion that this is not the case, as will clearly appear from the
following account.
The geological structure of the valley of the Tiber at Rome is very
clearly described by Professor Ponzi in a memoir published in 1850, from
which the accompanying section is taken.[4] (Fig. 16.) From this it will
be seen that "the Seven-hilled City" is built upon promontories of
stratified volcanic tuff, of which the Campagna is formed, breaking off
along the banks of the Tiber, the hills being the result of the erosion,
or denudation, of the strata along the side of the river valley. As the
strata dip from west to east across the course of the river, it follows
that those on the western banks are below those on the opposite side;
and thus the marine sands and marls which underlie the volcanic tuff,
and are concealed by it along the eastern side of the valley, emerge on
the west, and form the range of hills on that side. Such being the
structure of the formations under Rome, it is evident that it is not
"built on a volcano."
[Illustration: Fig. 16.--Geological Section across the Valley of the
Tiber at Rome. 1. Alluvium of the Tiber; 2. Diluvium; 3. Volcanic tuff
(recent deposits); 4. Sands, etc.; 5. Blue marl (sub-Apennine
deposits).]
The tuff contains fragments of lava and pebbles of Apennine limestone,
and was deposited under the waters of an extensive lake at a time when
volcanic action was rife amidst the Alban Hills. This lacustrine
formation rests in turn on deposits of marine origin, containing
oysters, patellæ, and other sea-shells, of which the chain of hills on
the right bank of the Tiber is chiefly formed.
The district around Albano lying to the south of Rome is of peculiar
interest from the assemblage of old crater-lakes which it contains; as,
for instance, those of Albano, Vallariccia, Nemi, Juturna, and the lake
of Gabii. The lake of Albano, one of the most beautiful sheets of water
in the world, is about six miles in circumference, and surrounded by
beds of peperino, a variety of tuff presenting a bright, undecomposed
aspect when newly broken. The level of this lake was lowered by the
Romans during the siege of Veii by means of a tunnel, so that the waters
are 200 feet lower than the level at which they originally stood. In the
same district is the lake of Nemi, very regular in its circular outline;
that of Juturna lying near the foot of the Alban Hills, and that of
Ariccia lying in a deep hollow eight miles in circumference;--all may be
supposed to have been the craters of extinct volcanoes, both by reason
of their shape and of the materials of which they are formed. All these
old craters are, however, according to Daubeny, "only the dependencies
and offshoots, as it were, of the great extinct volcano, the traces of
which still remain upon the summit of the Alban Hills, and which is
comparable in its form to that of Vesuvius, as it is surrounded by an
outer circle of volcanic rock comparable to that of Somma."[5]
To the north of the city of Rome are several crateriform lakes, some of
which are of great size, such as that of Bolsena, over twenty miles in
circumference, and the Lago di Bracciano, almost as large, and lying
about twelve miles from the city. These extensive sheets of water are
surrounded by banks of tuff and volcanic sand, in which fragments of
augite, leucite, and crystals of titanite are distributed. The town of
Viterbo is built up at the foot of a steep hill called Monte Cimini, the
lower part of which is composed of trachyte; this is surmounted by
tuff, which appears to have been ejected from an extinct crater
occupying the summit of the mountain, and now converted into a lake
called the Lake of Vico. This crater is perfectly circular, and from its
centre rises a little conical hill covered by trees.
(_c._) _Physical History._--Space does not permit of a fuller
description of the remarkable volcanic features of the tract lying along
the western slope of the Apennines; but from what has been stated it
will be clear that volcanic forces have been in operation at one time on
a grand scale in the Roman States and the South of Tuscany, over a tract
extending from Mount Annato to Velletri and Segni.
This tract was separated from that of the Neapolitan volcanic region by
a range of limestone hills of Jurassic age between Segni and Gaeta, a
protrusion of the Alban Hills westward; but the general structure and
physical history of both regions are probably very similar, with the
exception that the igneous forces still retain their vitality in the
more southerly region. In the case of the Roman volcanic district, a bay
seems to have been formed about the close of the Miocene period, bounded
on all sides but the west by hills of limestone, over whose bed strata
of marl, sandstone, and conglomerate were deposited. This tract was
converted by subsequent movements into a fresh-water lake, and
contemporaneously volcanic operations commenced over the whole region,
and beds of tuff, often containing blocks of rock ejected from
neighbouring craters, were deposited over those of marine origin.
Meanwhile numerous crater-cones were thrown up; and, as the land
gradually rose, the waters of the lake were drained off, leaving dry
the Campagna and plain of the Tiber. Ultimately the volcanic fires
smouldered down and died out, whether within the historic epoch or not
is uncertain; lakes were formed within the now dormant craters, and the
face of nature gradually assumed a more placid and less forbidding
aspect over this memorable region, destined to be the site of Rome, the
Mistress of the World.
[1] As determined by Daubeny in 1825.
[2] Including the ruins of the Temple of Serapis, whose pillars are
perforated by marine boring shells up to a height of about 16 feet from
their base; indicating that the land had sunk down beneath the sea, and
afterwards been elevated to its present level.
[3] The account of Falconi, and another by Pietro Giacomo di Toledo, are
given by Sir W. Hamilton, _op. cit._, p. 198, and also reproduced by Sir
C. Lyell, _Principles_, vol. i. p. 608.
[4] Guiseppe Ponzi, "Sulla storia fisica del Bacino di Roma," _Annali di
Scienze Fisiche_ (Roma, 1850).
[5] Daubeny, _Volcanoes_, p. 171.
CHAPTER VI.
EXTINCT VOLCANOES OF CENTRAL FRANCE.
(_a._) _General Structure of the Auvergne District._--From a granitic
and gneissose platform situated near the centre of France, and separated
from the western spurs of the Alps by the wide valley of the Rhone,
there rises a group of volcanic mountains surpassing in variety of form
and structure any similar mountain group in Europe, and belonging to an
epoch ranging from the Middle Tertiary down almost to the present day.
This volcanic group of mountains gives rise to several important rivers,
such as the Loire, the Allier, the Soule (a branch of the Loire), the
Creuse, the Dordogne, and the Lot; and in the Plomb du Cantal attains an
elevation of 6130 feet above the sea. Its southern section, that of Mont
Dore, the Cantal, and the Haute Loire, is characterised by magnificent
valleys, traversing plateaux of volcanic lava, and exhibiting the
results of river erosion on a grand scale; while its northern section,
that of the Puy de Dôme, presents to us a varied succession of volcanic
crater-cones and domes, with their extruded lava-streams, almost as
fresh and unchanged in form as if they had only yesterday become
extinct. A somewhat similar, but less important, chain of extinct
volcanoes also occurs in the Velay and Vivarais, between the upper
waters of the Loire and the Allier, in the vicinity of the town of Le
Puy.[1] The principal city in this region is Clermont-Ferrand, lying
near the base of the Puy de Dôme, and ever memorable as the birthplace
of Blaise Pascal.[2]
[Illustration: Fig. 17.--Generalised Section through the Puy de Dôme and
Vale of Clermont, distance about ten miles. The general floor formed of
granite and gneiss (G); D. Domite-lava of the Puy de Dôme; Sc. Cones of
ashes and scoriæ; L. Lava-sheets; A. Alluvium of the Vale of Clermont
and Lake deposits.]
The physical structure of this region is on the whole very simple. The
fundamental rocks consist of granite and gneiss passing into schist, all
of extreme geological antiquity, forming a vast platform gradually
rising in a southerly direction towards the head waters of the Loire
and the Allier in the Departments of Haute Loire, Lozère, and Ardèche.
On this platform are planted the whole of the volcanic mountains. (See
Fig. 17.)
The granitic plateau is bounded on the east, throughout a distance of
about 50 miles, by the wide and fertile plain of Clermont, watered by
the Allier and its numerous branches descending from the volcanic
mountains, and is about 25 miles in width from east to west in the
parallel of Clermont, but gradually narrowing in a southerly direction,
till at Brioude it becomes an ordinary mountain ravine. The eastern
margin of the plain is formed by another granitic ridge expanding into a
plateau towards the south, and joining in with that already described;
but towards the north and directly east of Clermont it forms a high
ridge traversed by the railway to St. Étienne and Lyons, and descending
towards the east into the valley of the Loire. No more impressive view
is to be obtained of the volcanic region than that from the summit of
this second ridge, on arriving there towards evening from the city of
Lyons. At your feet lies the richly-cultivated plain of Clermont, dotted
with towns, villages, and hamlets, and decorated with pastures,
orchards, vineyards, and numerous trees; while beyond rises the granitic
plateau, breaking off abruptly along the margin of the plain, and deeply
indented by the valleys and gorges along which the streams descend to
join the Allier. But the chief point of interest is the chain of
volcanic crater-cones and dome-shaped eminences which rise from the
plateau, amongst which the Puy de Dôme towers supreme. Their individual
forms stand out in clear and sharp relief against the western sky, and
gradually fade away towards the south into the serried masses of Mont
Dore and Cantal, around whose summits the evening mists are gathering.
Except the first view of the Mont Blanc range from the crest of the
Jura, there is no scene perhaps which is calculated to impress itself
more vividly on the memory than that here faintly described.[3]
[Illustration: Fig. 18.--Transverse view of the Puy de Dôme and
neighbouring volcanoes from the Puy de Chopine.--(After Scrope.)]
(_b._) _The Vale of Clermont._--The plain upon which we look down was
once the floor of an extensive lake, for it is composed of various
strata of sand, clay, marl, and limestone, containing various genera and
species of fresh-water shells. These strata are of great thickness,
perhaps a thousand feet in some places; and along with such shells as
_Paludina_, _Planorbis_, and _Limnæa_ are also found remains of various
other animals, such as fish, serpents, batrachians, crocodiles,
ruminants, and those of huge pachyderms, as _Rhinoceros_, _Dinotherium_,
and _Cænotherium_. This great lake, occupying a hollow in the old
granitic platform of Central France, must have been in existence for an
extensive period, which MM. Pomel, Aymard, and Lyell all unite in
referring to that of the Lower Miocene. But what is to us of special
interest is the fact that, in the deposits of this lake of the Haute
Loire, with the exception of the very latest, there is no intermixture
of volcanic products such as might have been expected to occur if the
neighbouring volcanoes had been in activity during its existence. Hence
it may be supposed that, as Scrope suggested, the waters of the lake
were drained off owing to the disturbance in the levels of the country
caused by the first explosions of the Auvergne volcanoes.[4] If this be
so, then we possess a key by which to determine the period of the first
formation of volcanoes in Central France; for, as the animal remains
enclosed in the lacustrine deposits of the Vale of Clermont belong to
the early Miocene stage, and the earliest traces of contemporaneous
volcanic _ejecta_ are found only in the uppermost deposits, we may
conclude that the first outburst of volcanic action occurred towards the
close of the Miocene period--a period remarkable for similar exhibitions
of internal igneous action in other parts of the world.
(_c._) _Successive Stages of Volcanic Action in Auvergne._--The volcanic
region here described, which has an area of about one hundred square
miles, does not appear to have been at one and the same period of time
the theatre of volcanic action over its whole extent. On the contrary,
this action appears to have commenced at the southern border of the
region in the Cantal, and travelling northwards, to have broken out in
the Mont Dore region; finally terminating its outward manifestations
among the craters and domes of the Puy de Dôme. In a similar manner the
volcanic eruptions of the Haute Loire and Ardèche, lying to the
eastward, and separated from those of the Cantal by the granitoid ridge
of the Montagnes de Margeride, belong to two successive periods
referable very closely to those of the Mont Dore and the Puy de Dôme
groups.[5] The evidence in support of this view is very clear and
conclusive; for, while the volcanic craters formed of ash, lapilli, and
scoriæ, together with the rounded domes of trachytic rock of which the
Puy de Dôme group is composed, preserve the form and surface indications
of recently extinguished volcanoes, those which we may assume to have
been piled up in the region of Mont Dore and Cantal have been entirely
swept away by prolonged rain and river action, and the sites of the
ancient craters and cones of eruption are only to be determined by
tracing the great sheets of lava up the sides of the valleys to their
sources, generally situated at the culminating points of their
respective groups. Other points of evidence of the great antiquity of
the latter groups might be adduced from the extent of the erosion which
has taken place in the sheets of lava having their sources in the vents
of the Plomb du Cantal and of Mont Dore, owing to which, magnificent
valleys, many miles in length and hundreds of feet in depth, have been
cut out of these sheets of lava and their supporting rocks, whether
granitic or lacustrine, and the materials carried away by the streams
which flow along their beds. These points will be better understood when
I come to give an account of the several groups; and in doing so I will
commence with that of the Cantal.[6]
(_d._) _The Volcanoes of the Cantal._--The original crater-cones of
this group have entirely disappeared throughout the long ages which have
elapsed since the lava-streams issued forth from their internal
reservoirs. The general figure of this group of volcanic mountains is
that of a depressed cone, whose sides slope away in all directions from
the central heights, which are deeply eroded by streams rising near the
apex and flowing downwards in all directions towards the circumference
of the mountain, where they enter the Lot, the Dordogne, and the Allier.
The orifice of eruption was situated at the Plomb du Cantal, formed of
solid masses of trachyte, which, owing, as Mr. Scrope supposes, to a
high degree of fluidity, were able to extend to great distances in
extensive sheets, and were afterwards covered by repeated and
widely-spread flows of basalt; so that the trachyte towards the margin
of the volcanic area becomes less conspicuous than the basalt by which
it is more or less concealed from view, or overlapped. Extensive beds of
tuff and breccia accompany the trachytic masses.
Magnificent sections of the rocks are laid open to view along the sides
of the valleys, which are steep and rock-bound. Except towards the
south-west, about Aurillac, where lacustrine strata overlie the granite,
the platform from which rises the volcanic dome is composed of granitic
or gneissose rocks. Accompanying the lava-streams are great beds of
volcanic agglomerate, which Mr. Scrope considers to have been formed
contemporaneously with the lava which they envelop, and to be due to
torrents of water tumultuously descending the sides of the volcano at
periods of eruption, and bearing down immense volumes of its fragmental
_ejecta_ in company with its lava-streams.[7] Nowhere throughout this
region do beds of trachyte and basalt alternate with one another; in all
cases the basalt is the newer of the two varieties of rock, and this is
generally the case throughout the region here described.
(_e._) _Volcanoes of Mont Dore._--This mountain lies to the north of
that of Cantal, and somewhat resembles it in general structure and
configuration. Like Cantal, it is destitute of any distinct crater; all
that is left of the central focus of eruption being the solidified
matter which filled the throat of the original volcano, and which forms
a rocky mass of lava, rising in its highest point, the Pic de Saucy, to
an elevation (as given by Ramond) of 6258 feet above the level of the
sea, thus exceeding that of the Plomb du Cantal by 128 feet. Its figure
will be best understood by supposing seven or eight rocky summits
grouped together within a circle of about a mile in diameter, from
whence, as from the apex of an irregular and flattened cone, all the
sides slope more or less rapidly downwards, until their inclination is
gradually lost in the plain around. This dome-shaped mass has been
deeply eroded on opposite sides by the valleys of the Dordogne and
Chambon; while it is further furrowed by numerous minor streams.[8]
The great beds of volcanic rock, disposed as above stated, consist of
prodigious layers of scoriæ, pumice-stones, and detritus, alternating
with beds of trachyte and basalt, which often descend in uninterrupted
currents till they reach the granite platform, and then spread
themselves for miles around. The sheets of basalt are found to stretch
to greater distances than those of trachyte, and have flowed as far as
15 or 20 miles from their orifices of eruption; while in some cases, on
the east and north sides, they have extended as far as 25 or 30 miles
from the central height. On the other hand, a radius of about ten miles
from the centre would probably include all the streams of trachyte;--so
much greater has been the viscosity of the basalt over the latter rock.
Some portions of these great sheets of lava, cut off by river valleys or
eroded areas from the main mass of which they once formed a part, are
found forming isolated terraces and plateaux either on the granitic
platform, or resting on the fresh-water strata of the valley of the
Allier, while in a northern direction they overspread a large portion of
the granitic plateau from which rise the Puy de Dôme and associated
volcanic mountains. Still more remarkable are the cases in which these
lava-streams have descended into the old river channels which drained
the granitic plateau. Thus the current which took its origin in the Puy
Gros descended into the valley of the Dordogne, while another stream
invaded the gorge of Champeix on the eastern side.
The more ancient lava-streams just described are invaded by currents and
surmounted by cones of eruption of more recent date, similar to those of
the Puy de Dôme group lying to the northward. Such cones and currents,
amongst which are the Puy de Tartaret and that of Montenard, present
exactly the same characters as those of this group, to which we shall
return further on.
(_f._) _Volcanoes of the Haute Loire and Ardèche._--Separated by the
valley of the Allier and the granitic ridge of La Margeride from the
volcanic regions of Cantal and Mont Dore is another volcanic region of
great extent, which reaches its highest elevation in the central points
of Mont Mezen, attaining an elevation (according to Cordier) of 5820
feet, and formed of "clinkstone." The volcanic products of Mezen have
been erupted from one central orifice of vast size, and consist mainly
of extensive sheets of "clinkstone," a variety of trachytic lava, which
have taken courses mainly towards the north-west and south-east. These
great sheets, one of which appears to have covered a space more than 26
miles in length with an average breadth of 6 miles, thus overspreading
an estimated area of 156 square miles, has been deeply eroded by streams
draining into the Loire, along whose banks the rocks tower in lofty
cliffs; while it has also suffered enormous denudation, by which
outlying fragments are disconnected from the main mass, and form
flat-topped hills and plateaux as far distant as Roche en Reigner and
Beauzac, at the extreme distance (as stated above) of 26 miles from the
source of eruption.
But even more remarkable than the above are the vast basaltic sheets
which stretch away for a distance of 30 miles by Privas almost to the
banks of the Rhône, opposite Montlimart. These have their origin amongst
the clinkstone heights of Mont Mezen, and taking their course along the
granitic plateau in a south-easterly direction, ultimately pass over on
to the Jurassic and Cretaceous formations composing the plateau of the
Coiron, which break off in vertical cliffs from 300 to 400 feet in
height, surmounting the slopes that rise from the banks of the Ardèche
and Escourtais rivers near Villeneuve de Bere. This is probably one of
the most extensive sheets of basalt with which we are acquainted in the
European area, and it is only a remnant of a vastly greater original
sheet.[9]
[Illustration: Fig. 19.--Mont Demise, near Le Puy, seen from the S.E.
(After Scrope.)--1. Building standing on old breccia, rocks of the Col;
2. Road to Brioude; 3. Croix de la Paille; 4. Orgue d'Expailly (basalt);
5. Spot where human bones were found.]
(_g._) _Newer Volcanoes of the Haute Loire (the Velay and
Vivarais)._--Subsequently to the formation of the lava-streams above
described, and probably after the lapse of a lengthened period, the
region of the Haute Loire and Ardèche became the scene of a fresh
outburst of volcanic action, during which the surface of the older
lavas, or of the fundamental granite, was covered by numerous
crater-cones and lava-streams strewn along the banks of the Allier and
of the Loire for many miles. These cones and craters are not quite so
fresh as those of the Mont Dôme group; those of the Haute Loire being
slightly earlier in point of time, and, as Daubeny shows, belonging to a
different system. So numerous are these more recent cones and craters
that Scrope counted more than 150 of them, and probably omitted many.
The volcanic phenomena now described have a special interest as bearing
on the question whether man was an inhabitant of this region at the time
of these later eruptions. The question seems to be answered in the
affirmative by the discovery of a human skull and several bones in the
volcanic breccia of Mont Demise, in company with remains of the elephant
(_E. primigenius_), rhinoceros (_R. tichorhinus_), stag, and other large
mammifers. The discovery of these remains was made in the year 1844, and
the circumstances were fully investigated and reported upon by M.
Aymard, and afterwards by Mr. Poulett Scrope, upon whose mind no
possible doubt of the fact remained. From what we now know of the
occurrence of human remains and works of art in other parts of France
and Europe, no surprise need be felt at the occurrence of human remains
in company with some extinct mammalia in these volcanic tuffs, which
belong to the Post-Pliocene or superficial alluvia antecedent to the
historic period.[10]
(_h._) _Mont Dôme Chain._--We now come to the consideration of the most
recent of all the volcanic mountain groups of the region of Central
France, that of the Puy de Dôme, lying to the north of Mont Dore and
Cantal. We have seen that there is almost conclusive evidence that man
was a witness to the later volcanic outbursts of the Vivarais, and as
these craters seem to be of somewhat earlier date than those of the Puy
de Dôme group, we cannot doubt that they were in active eruption when
human beings inhabited the country, and not improbably within what is
known as the _Historic Period_. No mention, however, is made either by
Cæsar, Pliny, or other Roman writers of the existence of active
volcanoes in this region. Cæsar, who was a close observer, and who
carried the Roman arms into Auvergne, makes no mention of such; nor yet
does the elder Pliny, who enumerated the known burning mountains of his
day all over the Roman Empire. It is not till we come down to the fifth
century of our era that we find any notices which might lead us to infer
the existence of volcanic action in Central France. This is the
well-known letter written by Sidonius Apollonarius, bishop of Auvergne,
to Alcinus Avitus, bishop of Vienne, in which the former refers to
certain terrific terrestrial manifestations which had occurred in the
diocese of the latter. But, as Dr. Daubeny observes, this is no evidence
of volcanic action in Auvergne, where Sidonius himself resided; the
terrestrial disturbances above referred to may have been earthquake
shocks of extreme severity.[11]
But although we have no reliably historical record of volcanic action
amongst the mountains of the Mont Dôme group, the fact that these are,
comparatively, extremely recent will be evident to an observer visiting
this district, and this conclusion is based on three principal grounds:
first, because of the well-preserved forms of the original craters,
though generally composed of very loose material, such as ashes,
lapilli, and slag; secondly, because of the freshness of the
lava-streams over whose rugged surfaces even a scanty herbage has in
some places scarcely found a footing;[12] and thirdly, because the lava
from the crater-cones has invaded channels previously occupied by the
earlier lavas, or those which had been eroded since the overflow of the
great basaltic sheets of Mont Dore. Still, as in the case of the valleys
of Lake Aidot, of Channonat, and of Royat, these streams are
sufficiently ancient to have given time for the existing rivers to have
worn out in them channels of some depth, but bearing no comparison to
the great valleys which had been eroded out of the more ancient lavas,
such as those of the Coiron, of the Ardèche, and of the Dordogne and
Chambon in the district of Mont Dore.
(_i._) _Dome-shaped Volcanic Hills._--I have previously (page 15)
referred to the two classes of volcanic eminences to be found in the
chain of the Puy de Dôme; one indicated by the name itself, formed of a
variety of trachytic lava called "domite," and of the form of a dome;
the other, composed of fragmental matter piled up in the form of a
crater or cup, often ruptured on one side by a stream of lava which has
burst through the side, owing to its superior density. Of the former
class the Puy de Dôme and the Grand Sarcoui (see Fig. 18) are the most
striking examples out of the five enumerated by Scrope, while there is a
large number, altogether sixty-one, belonging to the latter class. These
domes and crater-cones, as already stated, rise from a platform of
granite, either directly or from one formed of the lava-sheets of the
Mont Dore region, which in turn overlies the granitic platform. Of the
nearly perfect craters there are the Petit Puy de Dôme, lying partially
against the northern flank of the greater eminence; the Puy de Cone,
remarkable for the symmetry of its conical form, rising to a height of
900 feet from the plain; and the Puys de Chaumont and Thiolet lying to
the north of the Puy de Dôme. Of those to the south of this mount, two
out of the three craters of the Puy de Barme and the Puy de Vichatel are
perfect; but most of the crater-cones south of the Puy de Dôme are
breached. Some of the lava streams by which these craters were broken
down flowed for long distances. That the lava followed the showers of
ashes and lapilli forming the walls of the craters is rendered very
evident in the case of the Puy de la Vache, whose lava-stream coalescing
with those from the Puy de la Solas and Puy Noir, deluged the
surrounding tracts and flowed down the Channonat Valley as far as La
Roche Blanc in the Vale of Clermont. In the interior of the upper part
of the crater still remaining may be seen the level (so to speak) to
which the molten lava rose before it burst its barrier. This level is
marked by a projecting platform of reddish or yellow material, rich in
specular iron, apparently part of the frothy scum which formed on the
surface of the lava and adhered to the side of the basin at the moment
of its being emptied.
Space does not permit a fuller description of this remarkable assemblage
of extinct volcanoes, and the reader must be referred for further
details to the work of Mr. Scrope. I shall content myself with some
further reference to the central figure in this grand chain, the Puy de
Dôme itself.
_Ascent of the Puy de Dôme._--On ascending by the winding path up the
steep side of the mount, and on reaching the somewhat flattened summit,
the first objects which strike the eye are the massive foundations of
the Roman temple of Mercury; they are hewn out of solid grey lava,
altogether different from the rock of the Puy de Dôme itself, which must
have been obtained from one of the lava-sheets of the Mont Dore group.
To have carried these large blocks to their present resting-place must
have cost no little labour and effort. The temple is supposed to have
been surmounted by a colossal statue of the winged deity, visible from
all parts of the surrounding country which was dedicated to his honour,
and the foundations were only discovered a few years ago when excavating
for the foundation of the observatory, which stands a little further on
under the charge of Professor Janssen. On proceeding to the northern
crest of the platform a wonderful view of the extinct craters and
domes--about forty in number, and terminating in the Puy de Beauny, the
most northerly member of the chain--is presented to the spectator. To
the right is the Vale of Clermont and the rich valley of the Allier
merging into the great plain of Central France. On the south side of the
platform a no less remarkable spectacle meets the eye. The chain of Puys
and broken craters stretches away southwards for a distance of nearly
ten miles, while the horizon is bounded in that direction by the lofty
masses of the Mont Dore, Cantal, and Le Puy ranges. Nor does it require
much effort of the imagination to restore the character of the region
when these now dormant volcanoes were in full activity, projecting
showers of ashes and stones high into the air amidst flames of fire and
vast clouds of incandescent gas and steam.
The material of which the Puy de Dôme is formed consists of a light
grey, nearly white, soft felsitic lava, containing crystals of mica,
hornblende, and specular iron-ore. It is highly vesicular, and was
probably extruded in a pasty condition from a throat piercing the
granitic plateau and the overlying sheet of ancient lava of Mont Dore.
It has been suggested that such highly felsitic and acid lavas as that
of which the Puy de Dôme, the Grand Sarcoui, and Cliersou are composed,
may have had their origin in the granite itself, melted and rendered
viscous by intense heat. Dr. E. Gordon Hull has suggested that the
domite hills (owing to their low specific gravity) may have filled up
pre-existing craters of ashes and scoriæ without rupturing them, as in
the case of the heavier basaltic lavas, and then still continuing to be
extruded, may have entirely enveloped them in its mass; so that each
domite hill encloses within its interior a crater formed of ashes,
stones, and scoriæ. In the case of the Puy de Dôme there is some
evidence that the domite matter rests on a basis of ashes and scoriæ,
which may be seen in a few places around the base of the cone. It is
difficult without some such theory as this to explain how a viscous mass
was able to raise mountains some 2000 or 3000 feet above the surrounding
plain.[13]
(_j._) _Sketch of the Volcanic History of Central France._--It now only
remains to give a brief _resumé_ of the volcanic history of this region
as it may be gathered from the relations of the rocks and strata to the
volcanic products, and of these latter to each other.
_1st Stage._--It would appear that at the close of the Eocene period
great terrestrial changes occurred. The bed of the sea was converted
into dry land, the strata were flexured and denuded, and a depression
was formed in the granitic floor of Central France, which, in the
succeeding Miocene period, was converted into an extensive lake peopled
by molluscs, fishes, reptiles, and pachyderms of the period.
_2nd Stage._--Towards the close of the Miocene epoch volcanic eruptions
commenced on a grand scale over the granitic platform in the districts
now called Mont Dore, Cantal, and the Vivarais. Vast sheets of trachytic
and basaltic lavas successively invaded the tracts surrounding the
central orifices of eruption, now constituting the more ancient of the
lava-sheets of the Auvergne region, and, invading the waters of the
neighbouring lake, overspread the lacustrine deposits which were being
accumulated therein. These volcanic eruptions probably continued
throughout the Pliocene period, interrupted by occasional intervals of
inactivity, and ultimately altogether ceased.
_3rd Stage._--Towards the close of the Pliocene period terrestrial
movements took place, owing to which the waters of the lake began to
fall away, and the sheets of lava were subjected to great denudation.
This process, probably accelerated by excessive rainfall during the
succeeding Post-Pliocene and Pluvial periods, was continued until plains
and extensive river-valleys were eroded out of the sheets of lava and
their supporting granitic rocks and the adjoining lacustrine strata.
_4th Stage._--A new outburst of volcanic forces marks this stage, during
which the chain of the Puy de Dôme was thrown up on the west, and that
of the newer cones of the Vivarais on the south-east of the lacustrine
tract. The waters of the lake were now completely drained away through
the channel of the Allier, and denudation, extending down to the present
day, began over the area now forming the Vale of Clermont and adjoining
districts. The volcanic action ultimately spent its force; and somewhere
about the time of the appearance of man, the mammoth, rhinoceros, stag,
and reindeer on the scene, eruptions entirely ceased, and gradually the
region assumed those conditions of repose by which it is now physically
characterised.
[1] The literature referring to this region is very extensive. Guettard
in 1775, afterwards Faujas, published descriptions of the rocks of the
Vivarais and Velay; and Desmarest's geological map, published in 1779,
is a work of great merit. The district was afterwards described by
Daubeny, Lyell, Von Buch, and others; but by far the most complete work
is that of Scrope, entitled _Volcanoes of Central France_, containing
maps and numerous illustrations, published in 1826, and republished in a
more extended form in 1858; to this I am largely indebted.
[2] A monument to Pascal, erected by the citizens, occupies the centre
of the square in Clermont. It will be remembered that Pascal verified
the conclusions arrived at by Torricelli regarding the pressure of the
atmosphere, by carrying a Torricellian tube to the summit of the Puy de
Dôme, and recording how the mercury continually fell during the ascent,
and rose as he descended. This experiment was made in 1645.
[3] In this visit to Auvergne in the summer of 1880, the author was
accompanied by his son, Dr. E. Gordon Hull, and Sir Robert S. Ball. On
reaching the station at the summit of the ridge it seemed as if the
volcanic fires had again been lighted, for the whole sky was aglow with
the rays of the western sun.
[4] On the other hand, certain beds of ash and other volcanic _ejecta_
occur in _the uppermost_ strata of lake deposits of Limagne, so that
these may indicate the commencement of the period of eruption, as
suggested further on.
[5] Only very closely; for Mr. Scrope considers that the crater-cones of
the chain of the Haute Loire give evidence of a somewhat earlier epoch
of activity than those of the Puy de Dôme, as they have undergone a
greater amount of subaerial erosion.
[6] The extent of this river erosion has been clearly brought out by
Scrope, and is admirably illustrated by several of his panoramic views,
such as that in Plate IX. of his work.
[7] Scrope, _loc. cit._, p. 147.
[8] Scrope, _loc. cit._, p. 144.
[9] Scrope gives a view of these remarkable basaltic cliffs in Plate
XII. of his work, from which the above account is taken. At one spot
near the village of Le Gua there is a break in the continuity of the
sheet.
[10] See Scrope, _loc. cit._, p. 181; also Appendix, p. 228. While there
is no _primâ facie_ reason for questioning the origin of the Demise
skull, yet from what Lyell states in his _Antiquity of Man_, p. 196, it
will be seen that he found it impossible to identify its position, or to
determine beyond question that its interment was due to natural causes.
But assuming this to be the case, he shows how the individual to whom it
belonged might have been enveloped in volcanic tuff or mud showered down
during the final eruption of the volcano of Demise. MM. Hébert and
Lartet, on visiting the locality, also failed to find _in situ_ any
exact counterpart of the stone now in the museum of Le Puy.
[11] See Daubeny, _Volcanoes_, p. 31.
[12] That is to say, the surfaces of the lava-streams are not at all, or
only slightly, decomposed into soil suitable for the growth of plants,
except in rare instances.
[13] E. G. Hull, "On the Domite Mountains of Central France," _Scien.
Proc. Roy. Dublin Society_, July 1881, p. 145. Dr. Hull determined the
density of the domite of the Puy de Dôme to be 2.5, while that of lava
is about 3.0.
CHAPTER VII.
THE VOLCANIC DISTRICT OF THE RHINE VALLEY.
The region bordering the Rhine along both its banks above Bonn, and
extending thence along the valley of the Moselle and into the Eifel, has
been the theatre of active volcanic phenomena down into recent times,
but at the present day the volcanoes are dormant or extinct.
(_a._) _Geological Structure._--The fundamental rocks of this region
belong to the Silurian, Devonian, and Carboniferous systems, consisting
of schists, grits, and limestones, with occasional horizontal beds of
Miocene sandstone and shale with lignite, resting on the upturned edges
of the older rocks. Scattered over the greater part of the district here
referred to are a number of conical eminences, often with craters, the
bottoms of which are usually sunk much below the present level of the
country, and thus receiving the surface drainage, have been converted
into little lakes called "maars," differing from ordinary lakes by their
circular form and the absence of any _apparent_ outlet for their
waters.[1]
But before entering into details, it may be desirable to present the
reader with a short outline of the physical history of the region (which
has been ably done by Dr. Hibbert in his treatise, to which I have
already referred), so as to enable him better to understand the
succession of physical events in its volcanic history.
[Illustration: Fig. 20.--Sketch Map to show the physical condition of
the Rhenish area in the Miocene epoch.--(After Hibbert.)]
(_b._) _Physical History._--From the wide distribution of stratified
deposits of sand and clay at high levels on both banks of the Rhine
north of the Moselle, it would appear that an extensive fresh-water
basin, which Dr. Hibbert calls "The Basin of Neuwied," occupied a
considerable tract on both banks, in the centre of which the present
city of Neuwied stands. This basin was bounded towards the south by the
slopes of the Hündsruck and Taunus, which at the time here referred to
formed a continuous chain of mountains. (Fig. 20.) To the south of this
chain lay the Tertiary basin of Mayence, which was connected at an early
period--that of the Miocene--with the waters of the ocean, as shown by
the fact that the lower strata contain marine shells; these afterwards
gave place to fresh-water conditions. The basin of Neuwied was bounded
towards the north by a ridge of Devonian strata which extended across
the present gorge of the Rhine between Andernach and Linz, and to the
north of this barrier lay another more extensive fresh-water basin, that
of Cologne. From this it will be seen that the Rhine, as we now find it,
had then only an infantile existence; in fact, its waters to the south
of the Hündsruck ridge drained away towards the south. But towards the
commencement of the Pliocene period the barriers of the Hündsruck and
Taunus, as also that of the Linz, were broken through, and the course of
the waters was changed; and thus gradually, as the river deepened its
bed, the waters were drained off from the great lakes.[2] This rupture
of the barriers may have been due, in the first instance, to the
terrestrial disturbances accompanying the volcanic eruptions of the
Eifel and Siebengebirge, though the erosion of the gorges at Bingen and
at Linz to their present depth and dimensions is of course due to
prolonged river action. It was about the epoch we have now arrived
at--viz., the close of the Miocene--that volcanic action burst forth in
the region of the Lower Rhine. It is probable that this action commenced
in the district of the Siebengebirge, and afterwards extended into that
of the Moselle and the Eifel, the volcanoes of which bear evidence of
recent date. Layers of trachytic tuff are interstratified with the
deposits of sand, clay, and lignite of the formation known as that of
the Brown Coal--of Miocene age--which underlies nearly the whole of the
volcanic district on both sides of the Rhine near Bonn,[3] thus showing
that volcanic action had already commenced in that part to some extent;
but it does not appear from Dr. Hibbert's statement that any such
fragments of eruptive rock are to be found in the strata which were
deposited over the floor of the Neuwied basin.[4] It will be recollected
that the epoch assigned for the earliest volcanic eruptions of Auvergne
was that here inferred for those of the Lower Rhine--viz., the close of
the Miocene stage--and from evidence subsequently to be adduced from
other European districts, it will be found that there was a very widely
spread outburst of volcanic action at this epoch.
(_c._) _The Range of the Siebengebirge._--This range of hills--formed of
the older volcanic rocks of the Lower Rhine--rises along the right bank
of this noble river opposite Bonn, where it leaves the narrow gorge
which it traverses all the way from Bingen, and opens out on the broad
plain of Northern Germany. The range consists of a succession of conical
hills sometimes flat-topped--as in the case of Petersberg; and at the
Drachenfels, near the centre of the range it presents to the river a
bold front of precipitous cliffs of trachyte porphyry. The sketch (Fig.
21) here presented was taken by the author in 1857 from the old extinct
volcano of Roderberg, and will convey, perhaps, a better idea of the
character of this picturesque range than a description. The
Siebengebirge, although appearing as an isolated group of hills, is in
reality an offshoot from the range of the Westerwald, which is connected
with another volcanic district of Central Germany known as the
Vogelsgebirge. The highest point in the range is attained in the
Lohrberg, which rises 1355 feet above the sea; the next, the Great
Tränkeberg, 1330 feet; and the next, Great Oelberg, 1296 feet.
[Illustration: Fig. 21.--The Volcanic Range of the Siebengebirge, seen
from the left bank of the Rhine, above Bonn.--(Original.)]
The range consists mainly of trachytic rocks--namely,
trachyte-conglomerate, and solid trachyte, of which H. von Dechen makes
two varieties--that of the Drachenfels, and that of the Wolkenburg. But
associated with these highly-silicated varieties of lava--and generally,
if not always, of later date--are basaltic rocks which cap the hills of
Petersberg, Nonnenstrom, Gr. and Ll. Oelberg, Gr. Weilberg, and Ober
Dollendorfer Hardt. The question whether there is a transition from the
one variety of volcanic rock into the other, or whether each belongs to
a distinct and separate epoch of eruption, does not seem to be very
clearly determined. Mr. Leonard Horner states that it would be easy to
form a suite of specimens showing a gradation from a white trachyte to a
black basalt;[5] but we must recollect that when Mr. Horner wrote, the
microscopic examination of rocks by means of thin sections was not known
or practised, and an examination by this process might have proved that
this apparent transition is unreal. According to H. von Dechen, there
are sheets of basalt older than the greater mass of the brown coal
formation, and others newer than the trachyte;[6] while dykes of basalt
traversing the trachytic lavas are not uncommon.[7]
The trachyte-conglomerate--which seems to be associated with the upper
beds of the brown coal strata--is traversed by dykes of trachyte of
later date; and though it is difficult to trace the line between the two
varieties of this rock on the ground, Dr. von Rath has recognised the
general distinction between them, which consists in the greater
abundance of hornblende and mica in the trachyte of the Wolkenburg than
in that of the Drachenfels.
The trachyte of the Drachenfels was probably the neck of a volcano which
burst through the fundamental schists of the Devonian period. It is
remarkable for the large crystals of sanidine (glassy felspar) which it
contains, and has a rude columnar structure.
The absence of any clearly-defined craters of eruption, such as are to
be found in the Eifel district and on the left bank of the Rhine--as,
for example, in the case of the Roderberg--may be regarded as sufficient
evidence that this range is of comparatively high antiquity. It seems to
bear the same relation to the more modern craters of the Eifel and
Moselle that the Mont Dore and Cantal volcanoes do to those of the Puy
de Dôme. In both cases, denudation carried on throughout perhaps the
Pliocene and Post-Pliocene periods down to the present day has had the
effect of demolishing the original craters; so that what we now observe
as forming these ranges are the consolidated columns of original molten
matter which filled the throats of the old volcanoes, or the sheets of
lava which were extruded from them, but are now probably much reduced in
size and extent.
Having thus given a description of the older volcanic range on the right
bank of the Rhine, we shall cross the river in search of some details
regarding the more recent group of Rhenish volcanoes, commencing with
that of the Roderberg, a remarkable hill a few miles south of Bonn, from
which the view of the Seven Mountains was taken.
[Illustration: Fig. 22.--Section of the extinct crater of the Roderberg
on the bank of the Rhine, above Bonn.--(Original.)]
(_d._) _The Roderberg._--This crater, which was visited by the author in
1857, is about one-fourth of a mile in diameter, and is in the form of a
cup with gentle slopes on all sides. In its centre is a farmhouse
surrounded by corn-fields. The general section through the hill is
represented above (Fig. 22).
The flanks on the north side are composed of loose quartzose gravel
(gerolle), a remnant of the deposits formed around the margin of the
"Basin of Neuwied" described above (p. 114). This gravel is found
covering the terraces of the brown coal formation several hundred feet
above the Rhine. Besides quartz-pebbles, the deposit contains others of
slate, grit, and volcanic rock. On reaching the edge of the crater we
find the gravel covered over by black and purple scoria or slag the
superposition of the scoria on the gravel being visible in several
places, showing that the former is of more recent origin. On the
opposite side of the crater, overlooking the Rhine, we find the cliff of
Rolandsec composed of hard vesicular lava, rudely prismatic, and
extending from the summit of the hill to its base, about 250 feet below.
This is the most northerly of the group of the Eifel volcanoes.
(_e._) _District of the Rivers Brühl and Nette._--The volcanic region of
the Lower Eifel, drained by these two principal streams which flow into
the Rhine, will amply repay exploration by the student of volcanic
phenomena, owing to the variety of forms and conditions under which
these present themselves within a small space. The fundamental rock is
slate or grit of Devonian age, furrowed by numerous valleys, often
richly wooded, and diversified by conical hills of trachyte; or by
crater-cones, formed of basalt or ashes, sometimes ruptured on one side,
and occasionally sending forth streams of lava, as in the cases of the
Perlinkopf, the Bausenberg, and the Engelerkopf. The district attains
its greatest altitude in the High Acht (Der Hohe Acht), an isolated cone
of slate capped by basalt with olivine, and reaching a level of 2434
Rhenish feet.[8]
(_f._) _The Laacher See._--It would be impossible in a work of this kind
to attempt a detailed description of the Eifel volcanoes, often of a
very complex character and obscure physical history, as in the case of
the basin of Rieden, where tufaceous deposits, trachytic and basaltic
lavas and crater-cones, are confusedly intermingled, so that I shall
confine my remarks to the deservedly famous district of the Laacher
See, which I had an opportunity of personally visiting some years
since.[9]
[Illustration: Fig. 23.--Plan and Section of the Laacher See, a lake on
the borders of the Eifel, occupying the crater of an old volcano.--G.
Gravel and volcanic sand forming banks of the lake and rim of old
crater; L. Sheet of trachytic lava with columnar structure; B. Basaltic
dyke; S. Devonian slate, etc.]
The Laacher See is a lake of an oval form, over an English mile in the
shorter diameter, and surrounded by high banks of volcanic sand, gravel,
and scoriæ, except on the east side, where cliffs of clay-slate, in a
nearly vertical position, and striking nearly E.W., may be observed. Its
depth from the surface of the water is 214 feet.[10] The ashes of the
encircling banks contain blocks of slate and lava which have been torn
from the sides of the orifice or neck of the volcano and blown into the
air; and there can be no doubt that the ashes and volcanic gravel is the
result of very recent eruptions.
At the east side of the lake we find a stream of scoriaceous lava of a
purple or reddish colour, highly vesicular, and containing crystals of
mica; but the most important lava-stream is that which has taken a
southerly direction from the crater of the Laacher See towards Nieder
Mendig and Mayen, for a distance of about six miles. This great stream
is covered throughout half its distance by beds of volcanic ash and
lapilli, but emerges into the air at a distance of about two miles from
the edge of the crater (see Fig. 23), and was formerly extensively
quarried in underground caverns for millstones. Here the rock is a
vesicular trachyte, of a greyish colour, solidified in vertical columns
of hexagonal form, about four feet in diameter, and traversed by
transverse joint planes. These quarries have been worked from the time
of the Roman occupation of the country; and, before the introduction of
iron or steel rollers for grinding corn, millstones were exported to all
parts of Europe and the British Isles from this quarry.[11]
The district around the Laacher See is covered by laminated _ejecta_ of
the old volcano, probably of subaërial origin, through which bosses of
the fundamental slate peer up at intervals, while the surface is
diversified by several truncated cones.
(_g._) _Trass of the Brühl Valley._--The Brühl Valley, which unites
with that of the Rhine at the town of that name, and drains the northern
side of the volcanic region, has always been regarded with much interest
by travellers for the presence of a deposit of "trass" with which it is
partially filled. The origin of this valley was pre-volcanic, as it is
hewn out of the slaty rocks of the district. But at a later period it
became filled with volcanic mud (tuffstein), out of which the stream has
made for itself a fresh channel. The source of this mud is considered by
Hibbert[12] to have been the old volcano of the Lummerfeld, which, after
becoming dormant, was filled with water, and thus became a lake. At a
subsequent period, however, a fresh eruption took place near the edge of
the lake, resulting in the remarkable ruptured crater known as the
Kunksköpfe, which rises about four miles to the north of the Laacher
See. The eruptions of this volcano appear to have displaced the mud of
the Lummerfeld, causing it to flow down into the deep gorge of the
Brühl, which it completely filled, as stated above.
On walking down the valley one may sometimes see the junction of the
tuff with the slate-rock which enfolds it. The tuff consists of white
felspathic mud, with fragments of slate and lava, reaching a depth in
some places of 150 feet. After it has been quarried it is ground in
mills, and used for cement stone under the name of _trass_. It is said
to resemble the volcanic mud by which Herculaneum was overwhelmed during
the first eruption of Vesuvius, and which was produced by the torrents
of rain mixing with the ashes as they were blown out of the volcano.
Sufficient has probably now been written regarding the dormant, or
recently extinct, volcanic districts of Europe to give the reader a
clear idea regarding their nature and physical structure. Other
districts might be added, such as those of Central Germany, Hungary,
Transylvania, and Styria; but to do so would be to exceed the proposed
limits of this work; and we may therefore pass on to the consideration
of the volcanic region of Syria and Palestine, which adjoins the
Mediterranean district we have considered in a former page.
[1] Daubeny, _loc. cit._, p. 71. The geology of this region has had many
investigators, of whom the chief are Steininger, _Erloschenen Vulkane in
der Eifel_ (1820); Hibbert, _Extinct Volcanoes of the Basin of Neuwied_,
1832; Nöggerath, _Das Gebirge im Rheinland_, etc., 4 vols.; Horner, "On
the Geology of Bonn," _Transactions of the Geological Society, London_,
vol. iv.
[2] The views of Dr. Hibbert are not inconsistent with those of the late
Sir A. Ramsay, on "The Physical History of the Valley of the Rhine,"
_Quart. Jour. Geol. Soc._, vol. xxx. (1874).
[3] Von Dechen, _Geog. Beschreib. des Siebengebirges am Rhein_ (Bonn,
1852).
[4] Hibbert, _loc. cit._, p. 18.
[5] Horner, "Geology of Environs of Bonn," _Transactions of the
Geological Society_, vol. iv., new series.
[6] H. von Dechen, _Geog. Führer in das Siebengebirge am Rhein_ (Bonn,
1861).
[7] _Ibid._, p. 191.
[8] Dr. Hibbert's work is illustrated by very carefully drawn and
accurate views of some of the old cones and craters of this district,
accompanied by detailed descriptions.
[9] The lava of Schorenberg, near Rieden, is interesting from the fact,
stated by Zirkel, that it contains leucite, nosean, and nephelin.--_Die
Mikros. Beschaf. d. Miner. u. Gesteine_, p. 154 (1873).
[10] Hibbert, _loc. cit._, p. 23.
[11] At the time of the author's visit the underground caverns, which
are deliciously cool in summer, were used for the storage of the
celebrated beer brewed by the Moravians of Neuwied.
[12] Hibbert, _loc. cit._, p. 129.
PART III.
DORMANT OR MORIBUND VOLCANOES OF OTHER PARTS OF THE WORLD.
CHAPTER I.
DORMANT VOLCANOES OF PALESTINE AND ARABIA.
(_a._) _Region east of the Jordan and Dead Sea._--The remarkable line of
country lying along the valley of the Jordan, and extending into the
great Arabian Desert, has been the seat of extensive volcanic action in
prehistoric times. The specially volcanic region seems to be bounded by
the depression of the Jordan, the Dead Sea, and the Arabah as far south
as the Gulf of Akabah; for, although Safed, lying at the head of the Sea
of Galilee on the west of the Jordan valley, is built on a basaltic
sheet, and is in proximity to an extinct crater, its position is
exceptional to the general arrangement of the volcanic products which
may be traced at intervals from the base of Hermon into Central Arabia,
a distance of about 1000 miles.[1]
The tract referred to has been described at intervals by several
authors, of whom G. Schumacher,[2] L. Lartet,[3] Canon Tristram,[4] M.
Niebuhr,[5] and C. M. Doughty[6] may be specially mentioned in this
connection.
The most extensive manifestations of volcanic energy throughout this
long tract of country appear to be concentrated at its extreme limits.
At the northern extremity the generally wild and rugged tract of the
Jaulân and Haurân, called in the Bible _Trachonitis_, and still farther
to the eastward the plateau of the Lejah, with its row of volcanic peaks
sloping down to the vast level of Bashan, is covered throughout nearly
its whole extent by great sheets of basaltic lava, above which rise at
intervals, and in very perfect form, the old crater-cones of eruption. A
similar group of extinct craters with lava-flows has been described and
figured by a recent traveller, Mr. C. M. Doughty, in parts of Central
Arabia. The general resemblance of these Arabian volcanoes to those of
the Jaulân is unquestionable; and as they are connected with each other
by sheets of basaltic lava at intervals throughout the land of Moab, it
is tolerably certain that the volcanoes lying at either end of the chain
belong to one system, and were contemporaneously in a state of activity.
(_b._) _Geological Conditions._--Before entering any further into
particulars regarding the volcanic phenomena of this region, it may be
desirable to give a short account of its geological structure, and the
physical conditions amongst which the igneous eruptions were developed.
Down to the close of the Eocene period the whole region now under
consideration was occupied by the waters of the ocean. The mountains of
Sinai were islands in this ocean, which had a very wide range over parts
of Asia, Africa, and Europe. But at the commencement of the succeeding
Miocene stage the crust was subjected to lateral contraction, owing to
which the ocean bed was upraised. The strata were flexured, folded, and
often faulted and fissured along lines ranging north and south, the
great fault of the Jordan-Arabah valley being the most important. At
this period the mountains of the Lebanon, the table-lands of Judæa and
of Arabia, formed of limestone, previously constituting the bed of the
ocean during the Eocene and Cretaceous periods, were converted into land
surfaces. Along with this upheaval of the sea-bed there was extensive
denudation and erosion of the strata, so that valleys were eroded over
the subaërial tracts, and the Jordan-Arabah valley received its primary
form and outline.
Up to this time there does not appear to have been any outbreak of
volcanic forces; but with the succeeding Pliocene period these came into
play, and eruptions of basaltic lava took place along rents and fissures
in the strata, while craters and cones of slag, scoriæ, and ashes were
thrown up over the region lying to the east of the Sea of Galilee and
the sources of the Jordan on the one hand, and the central parts of the
great Arabian Desert on the other. These eruptions, probably
intermittent, continued into the succeeding Glacial or Pluvial period,
and only died out about the time that the earliest inhabitants appeared
on the scene.
(_c._) _The Jaulân and Haurân._--This tract is bounded by the valley of
the Jordan and the Sea of Galilee on the west, from which it rises by
steep and rocky declivities into an elevated table-land, drained by the
Yarmûk (Hieromax), the Nahr er Rukkâd, and other streams, which flow
westwards into the Jordan along deep channels in which the basaltic
sheets and underlying limestone strata are well laid open to view.
On consideration it seems improbable that the great sheets of augitic
lava, such as cover the surface of the land of Bashan, are altogether
the product of the volcanic mountains which appear to be confined to
special districts in this wide area. Some of the craters do indeed send
forth visible lava-streams, but they are insignificant as compared with
the general mass of the plateau-basalts; and the crater-cones themselves
appear in some cases to be posterior to the platforms of basalt from
which they rise. It is very probable, therefore, that the lavas of this
region have, in the main, been extruded from fissures of eruption at an
early period, and spread over the surface of the country in the same
manner as those of the Snake River region, and the borders of the
Pacific Ocean of North America, and possibly of the Antrim Plateau in
Ireland, afterwards to be described.
The volcanic hills which rise above the plateau are described in detail
by Schumacher. Of these, Tell Abû Nedîr is the largest in the Jaulân. It
reaches an elevation of 4132 feet above the Mediterranean Sea, and 1710
feet above the plain from which it rises; the circumference of its base
is three miles, and the rim of the crater itself, which is oval in form,
is 1331 yards in its larger diameter. The interior is cultivated by
Circassians, and is very fruitful; the walls descend at an angle of
about 30° on the inside, the exterior slope of the mountain being about
22°. The cone seems to be formed chiefly of scoriæ, and the lava-stream,
which issues forth from the interior, forms a frightfully stony and
lacerated district.[7]
[Illustration: Fig. 24.--Extinct Craters in the Jaulân, north-east from
the Sea of Galilee, called Tell Abû en Nedâ and Tell el Urâm, with a
central cone.--(After Schumacher.)]
Another remarkable volcano is the Tell Abû en Nedâ (Fig. 24). This is a
double crater, with a cone (probably of cinders) rising from the
interior of one of them. The highest point of the rim of one of the
craters reaches a level of 4042 feet above the sea. A lava-stream issues
forth from Abû en Nedâ, and unites with another from a neighbouring
volcano.
Tell el Ahmâr is a ruptured crater of imposing aspect, reaching an
elevation of 4060 feet, and sending forth a lava-current, which falls in
regular terraces from the outlet towards the west and north.
The ruptured crater of Tell el Akkasheh, which reaches a height of 3400
feet, has a less forbidding aspect than the greater number of the
extinct volcanoes of this region, owing to the fact that its sides are
covered by oaks, which attain to magnificent proportions along the
summit. Numerous other volcanic hills occur in this district, but the
most remarkable is that called Tell el Farras (the Hill of the Horse).
It is an isolated mountain, visible from afar, and reaches an elevation
of 3110 feet, or nearly 800 feet above the surrounding plain. The oval
crater of this volcano opens towards the north, and has a depth of 108
feet below the edge, with moderately steep sloping sides (17°-32°),
while the slope of the exterior, at first steep, gradually lessens to
20°-21°. These slopes are covered with reddish or yellowish slag. The
above examples will probably suffice to afford the reader a general idea
of the size and form of the volcanoes in this little known region.
It has been stated above that the great lava-floods have probably been
poured forth intermittently. The statement receives confirmation from
the observations of Canon Tristram, made in the valley of the Yarmûk.[8]
This impetuous torrent rushes down a gorge, sometimes having limestone
on one side and a wall of basalt on the other. This is due to the fact
that the river channel had been eroded before the volcanic eruptions had
commenced; but on the lava-stream reaching the channel, it naturally
descended towards the valley of the Jordan along its bed, displacing the
river, or converting it into clouds of steam. Subsequently the river
again hewed out its channel, sometimes in the lava, sometimes between
this rock and the chalky limestone. But, in addition to this, it has
been observed that there is a bed of river gravel interposed between two
sheets of basalt in the Yarmûk ravine; showing that after the first flow
of that molten rock the river reoccupied its channel, which was
afterwards invaded by another molten lava-stream, into which the waters
have again furrowed the channel which they now occupy. The basaltic
sheets descend under the waters of the Sea of Galilee on the east side,
and were probably connected with those of Safed, crossing the Jordan
valley north of that lake; owing to this the waters of the Lake of Merom
(Huleh) were pent up, and formerly covered an extensive tract, now
formed of alluvial deposits.
(_d._) _Land of Moab._--Proceeding southwards into the Land of Moab, the
volcanic phenomena are here of great interest. Extensive sheets of
basaltic lava, described as far back as 1807 by Seetzen, and more
recently by Lartet and Tristram, are found at intervals between the
Wâdies Mojib (Arnon) and Haidan. On either side of the Mojib, cliffs of
columnar basalt are seen capping the beds of white Cretaceous limestone,
while a large mass has descended into the W. Haidan between cliffs of
limestone and marl on either hand.
Around Jebel Attarus--a dome-shaped hill of limestone--a sheet of
basaltic lava has been poured, and has descended the deep gorge of the
Zerka Maïn, which enters the Dead Sea some 2000 feet below. This gorge
had been eroded before the basaltic eruption, so that the stream of
molten lava took its course down the bed of this stream to the water's
edge, and grand sections have been laid bare by subsequent erosion along
the banks. Pentagonal columns of black basalt form perpendicular walls,
first on one side, then on the other; while considerable masses of
scoriæ, peperino, and breccia appear at the head of the glen, probably
marking the orifice of eruption. Other eruptions of basalt occur, one at
Mountar ez Zara, to the south of Zerka Maïn, and another at Wady
Ghuweir, near the north-eastern end of the Dead Sea. There are no
lava-streams on the western side of the Ghor, or of the Dead Sea.[9]
The outburst of the celebrated thermal springs of Callirrhoë, together
with nine or ten others, along the channel of the Zerka Maïn, is a
circumstance which cannot be dissociated from the occurrence of basaltic
lava at this spot. In a reach of three miles, according to Tristram,
there are ten principal springs, of which the fifth in descent is the
largest; but the seventh and eighth, about half a mile lower down, are
the most remarkable, giving forth large supplies of sulphurous water.
The tenth and last is the hottest of all, indicating a temperature of
143° Fahr. Thus it would appear that the heat increases with the depth
from the upper surface of the table-land; a result which might be
expected, supposing the heated volcanic rocks to be themselves the
source of the high temperature. To a similar cause may be attributed the
hot-springs of Hammath, near Tiberias, and those of the Yarmûk near its
confluence with the Jordan. Some of these and other springs break out
along, or near, the line of the great Jordan-Arabah fault which ranges
throughout the whole extent of this depression, from the base of Hermon
to the Gulf of Akabah, generally keeping close to the eastern margin of
the valley.
(_e._) _The Arabian Desert._--The basaltic lava-floods occupy a very
large extent of the Arabian Desert, from El Hisma (lat. 27° 35' N.) to
the neighbourhood of Mecca on the south, a distance of about 440 miles,
with occasional intervals. The lava-sheets are called "Harras" (or
"Harrat"), one of which, Harrat Sfeina, terminates about ten miles north
of Mecca. The lava-sheets rest sometimes on the red sandstone, at other
times, on the granite and other crystalline rocks of great geological
antiquity. In addition to the sheets of basalt, numerous crater-cones
rise from the basaltic platform at a level of 5000 feet above the sea,
and two volcanic mountains, rising far to the west of the principal
range, called respectively Harrât Jeheyma and H. Rodwa, almost overlook
the coast of the Red Sea.[10]
(_f._) _Age of the Volcanic Eruptions._--It is very clear that the first
eruptions, producing the great basaltic sheets of Moab and Arabia,
occurred after the principal features of the country had been developed.
The depression of the Jordan-Arabah valley, the elevation of the eastern
side of this valley along the great fault line, and the channels of the
principal tributary streams, such as those of the Yarmûk and Zerka Maïn,
all these had been eroded out before they were invaded by the molten
streams of lava. Now, as these physical features were developed and
sculptured out during the Miocene period, as I have elsewhere shown to
be the case,[11] we may with great probability refer the volcanic
eruptions to the geological epoch following--namely, the Pliocene. How
far downwards towards the historic period the eruptions continued is not
so certain. Dr. Daubeny, quoting several passages from the Old Testament
prophets,[12] says it might be inferred that volcanoes were in activity
even so late as to admit of their being included within the limits of
authentic history. The poetic language and imagery used in these
passages by the prophets certainly lends a probability to this view, but
nothing more. On the other hand, these regions have suffered through
many centuries from the secondary effects of seismic action and
subterranean forces, and earthquake shocks have laid in ruins the great
temples and palaces of Palmyra, Baalbec, and other cities of antiquity.
The same uncertainty regarding the time at which volcanic action died
out, with reference to the appearance of man on the scene, hangs over
the region of Arabia and Syria, as we have seen to be the case in
reference to the extinct volcanoes of Auvergne, the Eifel, and the Lower
Rhine. In all these cases the commencement and close of eruptive action
appear to have been very much about the same period--namely, the Miocene
period on the one hand, and that at which man entered upon the scene on
the other; but in the case of Syria and Western Palestine, the close of
the volcanic period may have been somewhat more than 2000 B.C.
[1] Lake Phiala, near the Lake of Huleh, is also situated to the west of
the Jordan valley. Its origin, according to Tristram, is volcanic.
[2] Schumacher, "The Jaulân," _Quarterly Statement of the Palestine
Exploration Fund_, 1886 and 1888; and _Across the Jordan_, London, 1886.
[3] Lartet, _Voyage d'Exploration de la mer Morte_ (Géologie), Paris,
1880.
[4] Tristram, _Land of Moab_, London, 1873; and _Land of Israel_, 1866.
[5] Niebuhr, _Beschreibung von Arabien_, 1773.
[6] C. M. Doughty, _Arabia Deserta_, 2 vols., 1888. A generalised
account of this volcanic region by the author will be found in the
"Memoir on the Physical Geology of Arabia Petræa, and Palestine,"
_Palestine Exploration Fund_, 1887.
[7] Schumacher, _loc. cit._, p. 248.
[8] _Land of Israel_, p. 461.
[9] "Geology of Arabia Petræa, and Palestine," _Memoirs of the Palestine
Exploration Fund_, p. 95.
[10] Doughty, _loc. cit._, vol. i., plate vi., p. 416. An excellent
geological sketch map accompanies this work.
[11] "Memoir of the Geology of Arabia Petræa, and Palestine," chap. vi.
p. 67.
[12] Nahum, i. 5, 6; Micah, i. 3, 4; Isaiah, lxiv. 1-3; Jeremiah, l. 25.
CHAPTER II.
THE VOLCANIC REGIONS OF NORTH AMERICA.
(_a._) _Contrast between the Eastern and Western Regions._--In no point
is there a more remarkable contrast between the physical structure of
Eastern and Western America than in the absence of volcanic phenomena in
the former and their prodigious development in the latter. The great
valley of the Mississippi and its tributaries forms the dividing
territory between the volcanic and non-volcanic areas; so that on
crossing the high ridges in which the western tributaries of America's
greatest river have their sources, and to which the name of the "Rocky
Mountains" more properly belongs, we find ourselves in a region which,
throughout the later Tertiary times down almost to the present day, has
been the scene of volcanic operations on the grandest scale; where
lava-floods have been poured over the country through thousands of
square miles, and where volcanic cones, vying in magnitude with those of
Etna, Vesuvius, or Hecla, have established themselves. This region,
generally known as "The Great Basin," is bounded on the west by the
"Pacific Range" of mountains, and includes portions of New Mexico,
Arizona, California, Nevada, Utah, Colorado, Idaho, Oregon, Wyoming,
Montana, and Washington. To the south it passes into the mountainous
region of Mexico, also highly volcanic; and thence into the ridge of
Panama and the Andes. It cannot be questioned but that the volcanic
nature of the Great Basin is due to the same causes which have
originated the volcanic outbursts of the Andes; but, from whatever
cause, the volcanic forces have here entered upon their secondary or
moribund stage. In the Yellowstone Valley, geysers, hot springs, and
fumaroles give evidence of this condition. In other districts the
lava-streams are so fresh and unweathered as to suggest that they had
been erupted only a few hundred years ago; but no active vent or crater
is to be found over the whole of this wide region. A few special
districts only can here be selected by way of illustration of its
special features in connection with its volcanic history.
(_b._) _The Plateau Country of Utah and Arizona._--This tract, which is
drained by the Colorado River and its tributaries, is bounded on the
north by the Wahsatch range, and extends eastwards to the base of the
Sierra Nevada. Round its margin extensive volcanic tracts are to be
found, with numerous peaks and truncated cones--the ancient craters of
eruption--of which Mount San Francisco is the culminating eminence.
South of the Wahsatch, and occupying the high plateaux of Utah, enormous
masses of volcanic products have been spread over an area of 9000 square
miles, attaining a thickness of between 3000 and 4000 feet. The earlier
of these great lava-floods appear to have been trachytic, but the later
basaltic; and in the opinion of Captain Dutton, who has described them,
they range in point of time from the Middle Tertiary (Miocene) down to
comparatively recent times.
(_c._) _The Grand Cañon._--To the south of the high plateaux of Utah
are many minor volcanic mountains, now extinct; and as we descend
towards the Grand Cañon of Colorado we find numerous cinder-cones
scattered about at intervals near the cliffs.[1] Extensive lava-fields,
surmounted by cinder-cones, occupy the plateau on the western side of
the Grand Cañon; and, according to Dutton, the great sheets of basaltic
lava, of very recent age, which occupy many hundred square miles of
desert, have had their sources in these cones of eruption.[2] Crossing
to the east of the Grand Cañon, we find other lava-floods poured over
the country at intervals, surmounted by San Francisco--a volcanic
mountain of the first magnitude--which reaches an elevation, according
to Wheeler, of 12,562 feet above the ocean. It has long been extinct,
and its summit and flanks are covered with snow-fields and glaciers.
Other parts of Arizona are overspread by sheets of basaltic lava,
through which old "necks" of eruption, formed of more solid lava than
the sheets, rise occasionally above the surface, and are prominent
features in the landscape.
Further to the eastward in New Mexico, and near the margin of the
volcanic region, is another volcanic mountain little less lofty than San
Francisco, called Mount Taylor, which, according to Dutton, rises to an
elevation of 11,390 feet above the ocean, and 8200 feet above the
general level of the surrounding plateau of lava. This mountain forms
the culminating point of a wide volcanic tract, over which are
distributed numberless vents of eruption. Scores of such
vents--generally cinder-cones--are visible in every part of the plateau,
and always in a more or less dilapidated condition.[3] Mount Taylor is a
volcano, with a central pipe terminating in a large crater, the wall of
which was broken down on the east side in the later stage of its
history.
[Illustration: Fig. 25.--Mount Shasta (14,511 feet), a snow-clad
volcanic cone in California, with Mount Shastina, a secondary crater, on
the right; the valley between is filled with glacier-ice.--(After
Dutton).]
(_d._) _California._--Proceeding westwards into California, we are
again confronted with volcanic phenomena on a stupendous scale. The
coast range of mountains, which branches off from the Sierra Nevada at
Mount Pinos, on the south, is terminated near the northern extremity of
the State by a very lofty mountain of volcanic origin, called Mount
Shasta, which attains an elevation of 14,511 feet (see Fig. 25). This
mountain was first ascended by Clarence King in 1870,[4] and although
forming, as it were, a portion of the Pacific Coast Range, it really
rises from the plain in solitary grandeur, its summit covered by snow,
and originating several fine glaciers.
The summit of Mount Shasta is a nearly perfect cone, but from its
north-west side there juts out a large crater-cone just below the
snow-line, between which and the main mass of the mountain there exists
a deep depression filled with glacier ice. This secondary crater-cone
has been named Mount Shastina, and round its inner side the stream of
glacier ice winds itself, sometimes surmounting the rim of the crater,
and shooting down masses of ice into the great caldron. The length of
this glacier is about three miles, and its breadth about 4000 feet.
Another very lofty volcanic mountain is Mount Rainier, in the Washington
territory, consisting of three peaks of which the eastern possesses a
crater very perfect throughout its entire circumference. This mountain
appears to be formed mainly of trachytic matter. Proceeding further
north into British territory, several volcanic mountains near the
Pacific Coast are said to exhibit evidence of activity. Of these may be
mentioned Mount Edgecombe, in lat. 57°.3; Mount Fairweather, lat. 57°.20
which rises to a height of 14,932 feet; and Mount St. Elias, lat. 60°.5,
just within the divisional line between British and Russian territory,
and reaching an altitude of 16,860 feet. This, the loftiest of all the
volcanoes of the North American continent, except those of Mexico, may
be considered as the connecting link in the volcanic chain between the
continent and the Aleutian Islands.[5]
(_e._) _Lake Bonneville._--Returning to Utah we are brought into contact
with phenomena of special interest, owing to the inter-relations of
volcanic and lacustrine conditions which once prevailed over large
tracts of that territory. The present Great Salt Lake, and the smaller
neighbouring lakes, those called Utah and Sevier, are but remnants of an
originally far greater expanse of inland water, the boundaries of which
have been traced out by Mr. C. K. Gilbert, and described under the name
of Lake Bonneville.[6] The waters of this lake appear to have reached
their highest level at the period of maximum cold of the Post-Pliocene
period, when the glaciers descended to its margin, and large streams of
glacier water were poured into it. Eruptions of basaltic lava from
successive craters appear to have gone on before, during, and after the
lacustrine epochs; and the drying up of the waters over the greater
extent of their original area, now converted into the Sevier Desert, and
their concentration into their present comparatively narrow basins,
appears to have proceeded _pari passu_ with the gradual extinction of
the volcanic outbursts. Two successive epochs of eruption of basalt
appear to have been clearly established--an earlier one of the "Provo
Age," when the lava was extruded from the Tabernacle craters, and a
later epoch, when the eruptions took place from the Ice Spring craters.
The oldest volcanic rock appears to be rhyolite, which peers up in two
small hills almost smothered beneath the lake deposits. Its eruption was
long anterior to the lake period. On the other hand, the cessation of
the eruptions of the later basaltic sheets is evidently an event of such
recent date that Mr. Gilbert is led to look forward to their resumption
at some future, but not distant, epoch. As he truly observes, we are not
to infer that, because the outward manifestations of volcanic action
have ceased, the internal causes of those manifestations have passed
away. These are still in operation, and must make themselves felt when
the internal forces have recovered their exhausted energies; but perhaps
not to the same extent as before.
(_f._) _Region of the Snake River._--The tract of country bordering the
Snake River in Idaho and Washington is remarkable for the vast sheets of
plateau-basalt with which it is overspread, extending sometimes in one
great flood farther than the eye can reach, and what is still more
remarkable, they are often unaccompanied by any visible craters or vents
of eruption. In Oregon the plateau-basalt is at least 2,000 feet in
thickness, and where traversed by the Columbia River it reaches a
thickness of about 3,000 feet. The Snake and Columbia rivers are lined
by walls of volcanic rock, basaltic above, trachytic below, for a
distance of, in the former, one hundred, in the latter, two hundred,
miles. Captain Dutton, in describing the High Plateau of Utah, observes
that the lavas appear to have welled up in mighty floods without any of
that explosive violence generally characteristic of volcanic action.
This extravasated matter has spread over wide fields, deluging the
surrounding country like a tide in a bay, and overflowing all
inequalities. Here also we have evidence of older volcanic cones buried
beneath seas of lava subsequently extruded.
(_g._) _Fissures of Eruption._--The absence, or rarity, of volcanic
craters or cones of eruption in the neighbourhood of these great sheets
has led American geologists to the conclusion that the lavas were in
many cases extruded from fissures in the earth's crust rather than from
ordinary craters.[7] This view is also urged by Sir A. Geikie, who
visited the Utah region of the Snake River in 1880, and has vividly
described the impression produced by the sight of these vast fields of
basaltic lava. He says, "We found that the older trachytic lavas of the
hills had been deeply trenched by the lateral valleys, and that all
these valleys had a floor of black basalt that had been poured out as
the last of the molten materials from the now extinct volcanoes. There
were no visible cones or vents from which these floods of basalt could
have proceeded. We rode for hours by the margin of a vast plain of
basalt stretching southward and westward as far as the eye could
reach.... I realised the truth of an assertion made first by
Richthofen,[8] that our modern volcanoes, such as Vesuvius and Etna,
present us with by no means the grandest type of volcanic action, but
rather belong to a time of failing activity. There have been periods of
tremendous volcanic energy, when instead of escaping from a local vent,
like a Vesuvian cone, the lava has found its way to the surface by
innumerable fissures opened for it in the solid crust of the globe over
thousands of square miles."[9]
(_h._) _Volcanic History of Western America._--The general succession of
volcanic events throughout the region of Western America appears to have
been somewhat as follows:--[10]
The earliest volcanic eruptions occurred in the later Eocene epoch and
were continued into the succeeding Miocene stage. These consisted of
rocks moderately rich in silica, and are grouped under the heads of
propylite and andesite. To these succeeded during the Pliocene epoch
still more highly silicated rocks of trachytic type, consisting of
sanidine and oligoclase trachytes. Then came eruptions of rhyolite
during the later Pliocene and Pleistocene epochs; and lastly, after a
period of cessation, during which the rocks just described were greatly
eroded, came the great eruptions of basaltic lava, deluging the plains,
winding round the cones or plateaux of the older lavas, descending into
the river valleys and flooding the lake beds, issuing forth from both
vents and fissures, and continuing intermittently down almost into the
present day--certainly into the period of man's appearance on the scene.
Thus the volcanic history of Western America corresponds remarkably to
that of the European regions with which we have previously dealt, both
as regards the succession of the various lavas and the epochs of their
eruption.
(_i._) _The Yellowstone Park._--The geysers and hot springs of the
Yellowstone Park, like those in Iceland and New Zealand, are special
manifestations of volcanic action, generally in its secondary or
moribund stage. The geysers of the Yellowstone occur on a grand scale;
the eruptions are frequent, and the water is projected into the air to a
height of over 200 feet. Most of these are intermittent, like the
remarkable one known as Old Faithful, the Castle Geyser, and the
Giantess Geyser described by Dr. Hayden, which ejects the water to a
height of 250 feet. The geyser-waters hold large quantities of silica
and sulphur in solution, owing to their high temperature under great
pressure, and these minerals are precipitated upon the cooling of the
waters in the air, and form circular basins, often gorgeously tinted
with red and yellow colours.[11]
[1] J. W. Powell, _Exploration of the Cañons of the Colorado_, pp. 114,
196. Major Powell describes a fault or fissure through which floods of
lava have been forced up from beneath and have been poured over the
surface. Many cinder-cones are planted along the line of this fissure.
[2] Capt. C. E. Dutton. _Sixth Ann. Rep. U.S. Geol. Survey_, 1884-85.
[3] Dutton, _loc. cit._, chap. iv. p. 165.
[4] _Amer. Jour. Science_, vol. 3., ser. (1871). A beautiful map of this
mountain is given in the _Fifth Annual Report, U.S. Geol. Survey_,
1883-84. Plate 44.
[5] Daubeny, _loc. cit._, p. 474.
[6] Gilbert, _Monograph U.S. Geol. Survey_, vol. i. (1890).
[7] Powell, _Exploration of the Colorado River_, p. 177, etc. (1875).
Hayden, _Rep. U.S. Geol. Survey of the Colorado, etc._ (1871-80).
[8] Richthofen, _Natural System of Volcanic Rocks_, Mem. California
Acad. Sciences, vol. i. (1868).
[9] Geikie, _Geological Sketches at Home and Abroad_, p. 271 (1882).
[10] Prestwich, _Geology_, vol. i. p. 370, quoting from Richthofen.
[11] The origin of geysers is variously explained; see Prestwich,
_Geology_, vol. i. p. 170. They are probably due to heated waters
suddenly converted into steam by contact with rock at a high
temperature.
CHAPTER III.
VOLCANOES OF NEW ZEALAND.
One other region of volcanic action remains to be noticed before passing
on to the consideration of those of less recent age. New Zealand is an
island wherein seem to be concentrated all the phenomena of volcanic
action of past and present time. Though it is doubtful if the term
"active," in its full sense, can be applied to any of the existing
craters (with two or three exceptions, such as Tongariro and Whakari
Island), we find craters and cones in great numbers in perfectly fresh
condition, extensive sheets of trachytic and basaltic lavas, ashes, and
agglomerates; lava-floods descending from the ruptured craters of ashes
and scoriæ; old crater-basins converted into lakes; geysers, hot springs
and fumaroles which may be counted by hundreds, and cataracts breaking
over barriers of siliceous sinter; and, lastly, lofty volcanic mountains
vying in magnitude with Vesuvius and Etna. All these wonderful
exhibitions of moribund volcanic action seem to be concentrated in the
northern island of Auckland. The southern island, which is the larger,
also has its natural attractions, but they are of a different kind;
chief of all is the grand range of mountains called, not
inappropriately, the "Southern Alps," vying with its European
representative in the loftiness of its peaks and the splendour of its
snowfields and glaciers, but formed of more ancient and solid rocks than
those of the northern island.
(_a._) _Auckland District._--We are indebted to several naturalists for
our knowledge of the volcanic regions of New Zealand, but chiefly to
Ferdinand von Hochstetter, whose beautiful maps and graphic descriptions
leave nothing to be desired.[1] In this work Hochstetter was assisted by
Julius Haast and Sir J. Hector. From their account we learn that the
Isthmus of Auckland is one of the most remarkable volcanic districts in
the world. It is characterised by a large number of extinct
cinder-cones, in a greater or less perfect state of preservation, and
giving origin to lava-streams which have poured down the sides of the
hills on to the plains. Besides these are others formed of stratified
tuff, with interior craters, surrounding in mural cliffs eruptive cones
of scoriæ, ashes, and lapilli; these cones are scattered over the
isthmus and shores of Waitemata and Manukau. The tuff cones and craters
rise from a floor of Tertiary sandstone and shale, the horizontal strata
of which are laid open in the precipitous bluffs of Waitemata and
Manukau harbours; they sometimes contain fossil shells of the genera
_Pecten_, _Nucula_, _Cardium_, _Turbo_, and _Neritæ_. As the volcanic
tuff-beds are intermingled with the Upper Tertiary strata, it is
inferred that the first outbursts of volcanic forces occurred when the
region was still beneath the waters of the ocean. Cross-sections show
that the different layers slope both outwards (parallel to the sides)
and inwards towards the bottom of the craters. Sometimes these craters
have been converted into lakes, as in the case of those of the Eifel;
but generally they are dry or have a floor of morass. Of the
crater-lakes, those of Kohuora, five in number, are perhaps the most
remarkable; and in the case of two of these the central cones of slag
appear as islets rising from the surface of the waters. The fresh-water
lake Pupuka has a depth of twenty-eight fathoms. To the north of
Auckland Harbour rises out of the waters of the Hauraki Gulf the cone of
Rangitoto, 920 feet high, the flanks formed of rugged streams of basalt,
and the summit crowned by a circular crater of slag and ash, out of the
centre of which rises a second cone with the vent of eruption. This is
the largest and newest of the Auckland volcanoes, and appears to have
been built up by successive outpourings of basaltic lava from the
central orifice, after the general elevation of the island.
[Illustration: Fig. 26.--Forms of volcanic tuff cones, with their
cross-sections, in the Province of Auckland.--No. 1. Simple tuff cone
with central crater; No. 2. Outer tuff cone with interior cinder cone
and crater; No. 3. The same with lava-stream issuing from the interior
cone.--(After Hochstetter.)]
Before leaving the description of the tuff-cones, which are a peculiar
feature in the volcanic phenomena of New Zealand, and are of many forms
and varieties, we must refer to that of Mount Wellington (Maunga Rei).
This is a compound volcano, in which the oldest and smallest of the
group is a tuff-crater-cone, exhibiting very beautifully the outward
slope of its beds. Within this crater arise two cones of cinders, each
with small craters. It would appear that after a long interval the
larger of the two principal cones, formed of cinders and known as Mount
Wellington, burst forth from the southern margin of the older tuff-cone,
and, being built up to a height of 850 feet, gradually overspread the
sides of its older neighbour. Mount Wellington itself has three craters,
and from these large streams of basaltic lava have issued forth in a
westerly direction, while a branch entered and partially filled the old
tuff-crater to the northwards.
Southwards from Manukau Harbour, and extending a short distance from
the coast-line to Taranaki Point, there occurs a plateau of
basalt-conglomerate (_Basaltkonglomerat_), with sheets of basaltic lava
overspreading the Tertiary strata. These plateau-basalts are intersected
by eruptive masses in the form of dykes, but still there are no craters
or cones of eruption to be seen; so that we may infer that the sheets,
at least, were extruded from fissures in the manner of those of the
Colorado or Idaho regions of America. Proceeding still further south
into the interior of the island, we here find a lofty plateau of an
average elevation of 2,000 feet, interposed between the Tertiary beds of
the Upper and Middle Waikato, and formed of trachytic and pitch-stone
tuff, amongst which arise old extinct volcanic cones, such as those of
Karioi, Pirongia, Kakepuku, Maunga Tautari, Aroha, and many others.
These trachytic lavas would seem to be more ancient than the basaltic,
previously described.
(_b._) _Taupo Lake, and surrounding district._--But of all these
volcanic districts, none is more remarkable than that surrounding the
Taupo Lake, which lies amidst the Tertiary strata of the Upper Waikato
Basin. The surface of this lake is 1,250 feet above that of the ocean,
and its margin is enclosed within a border of rhyolite and
pitchstone--rising into a mass of the same material 1,800 feet high on
the eastern side. The form of the lake does not suggest that it is
itself the crater of a volcano, but rather that it was originated by
subsidence. On all sides, however, trachytic cones arise, of which the
most remarkable group lies to the south of the lake, just in front of
the two giant trachytic cones, the loftiest in New Zealand, one called
Tongariro, rising about 6,500 feet, and the other Ruapahu, which attains
an elevation of over 9,000 feet, with the summit capped by snow. These
two lofty cones, standing side by side, are supposed by the Maoris to be
the husband and wife to whom were born the group of smaller cones above
referred to as occupying the southern shore of Taupo Lake. The volcano
of Tongariro may still be considered as in a state of activity, as its
two craters (Ngauruhoe and Ketetahi) constantly emit steam, and several
solfataras break out on its flanks.[2]
(_c._) _Roto Mahana._--In a northerly direction from Tongariro, and
distant from the coast by a few miles, lies in the Bay of Plenty the
second of the active volcanoes of New Zealand, the volcanic island of
Whakari (White Island), from the crater of which are constantly erupted
vast masses of steam clouds. The distance between these two active
craters is 120 nautical miles; and along the tract joining them
steam-jets and geysers issue forth from the deep fissures through which
the lava sheets have formerly been extruded. Numerous lakes also occupy
the larger cavities in the ground; and hot-springs, steam-fumaroles and
solfataras burst out in great numbers along the banks of the Roto Mahana
Lake and the Kaiwaka River by which it is drained. Amongst such
eruptions of hot-water and steam we might expect the formation of
siliceous sinter, and the deposition of sulphur and other minerals; nor
will our expectations be disappointed. For here we have the wonderful
terraces of siliceous sinter deposited by the waters entering Roto
Mahana as they descend from the numerous hot-springs or pools near its
margin. All travellers concur in describing these terraces as the most
wonderful of all the wonders of the Lake district of New Zealand--so
great is their extent, and so rich and varied is their colouring.
The beautiful map of Roto Mahana on an enlarged scale by Hochstetter
shows no fewer than ten large sinter terraces descending towards the
margin of this lake, besides several mud-springs, fumaroles, and
solfataras. But the largest and most celebrated of all the sinter
terraces has within the last few years been buried from view beneath a
flood of volcanic trass, or mud, an event which was as unexpected as it
was unwelcome. In May, 1887, the mountain of Tarawera, which rises to
the north-east of Roto Mahana, and on the line of eruption above
described, suddenly burst forth into violent activity, covering the
country for miles around with clouds of ashes, and, pouring down
torrents of mud, completely enveloped the beautiful terrace of sinter
which had previously been one of the wonders of New Zealand. By the same
eruption several human beings were entombed, and their residences
destroyed.
The waters of Roto Mahana, together with the hot-springs and fountains
are fed from rain, and from the waters of Taupo Lake, which, sinking
through fissures in the ground, come in contact with the interior heated
matter, and thus steam at high temperature and pressure is generated.[3]
(_d._) _Moribund condition of New Zealand Volcanoes._--From what has
been said, it will be inferred that in the case of New Zealand, as in
those of Auvergne, the Eifel and Lower Rhine, Arabia, and Western
America, we have an example of a region wherein the volcanic forces are
well-nigh spent, but in which they were in a state of extraordinary
activity throughout the later Tertiary, down to the commencement of the
present epoch. In most of these cases the secondary phenomena of
vulcanicity are abundantly manifest; but the great exhibitions of
igneous action, when the plains were devastated by sheets of lava, and
cones and craters were piled up through hundreds and thousands of feet,
have for the present, at least, passed away.
[1] _Geol.-topographischer Atlas von Neu-Seeland_, von Dr. Ferd. von
Hochstetter und Dr. A. Petermann. Gotha: Justus Perthes (1863). Also
_New Zealand_, trans. by E. Sauter, Stuttgart (1867).
[2] Tongariro was visited in 1851 by Mr. H. Dyson, who describes the
eruption of steam.
[3] Mr. Froude figures and describes the two terraces, the "White" and
"Pink," in _Oceana_, 2nd edition, pp. 285-291.
PART IV.
TERTIARY VOLCANIC DISTRICTS OF THE BRITISH ISLES.
CHAPTER I.
ANTRIM.
It is an easy transition to pass from the consideration of European and
other dormant, or extinct, volcanic regions to those of the British
Isles, though the volcanic forces may have become in this latter
instance quiescent for a somewhat longer period. In all the cases we
have been considering, whether those of Central Italy, of the Rhine and
Moselle, of Auvergne, or of Syria and Arabia, the cones and craters of
eruption are generally present entire, or but slightly modified in form
and size by the effects of time. But in the case of the Tertiary
volcanic districts of the British Isles this is not so. On the contrary,
these more prominent features of vulcanicity over the surface of the
ground have been removed by the agents of denudation, and our
observations are confined to the phenomena presented by extensive sheets
of lava and beds of ash, or the stumps and necks of former vents of
eruption, together with dykes of trap by which the plateau-lavas are
everywhere traversed or intersected.
The volcanic region of the British Isles extends at intervals from the
North-east of Ireland through the Island of Mull and adjoining districts
on the mainland of Morvern and Ardnamurchan into the Isle of Skye, and
comprises several smaller islets; the whole being included in the
general name of the Inner Hebrides. It is doubtful if the volcanic lavas
of Co. Antrim were ever physically connected with those of the west of
Scotland, though they may be considered as contemporary with them; and
in all cases the existing tracts of volcanic rock are mere fragments of
those originally formed by the extrusion of lavas from vents of
eruption. In addition to these, there are large areas of volcanic rock
overspread by the waters of the ocean.
(_a._) _Geological Age._--The British volcanic eruptions now under
consideration are all later than the Cretaceous period. Throughout
Antrim, and in parts of Mull, the lavas are found resting on highly
eroded faces either of the Upper Chalk (Fig. 27), or, where it has been
altogether denuded away, on still older Mesozoic strata. From the
relations of the basaltic sheets of Antrim to the Upper Chalk, it is
clear that the latter formation, after its deposition beneath the waters
of the Cretaceous seas, was elevated into dry land and exposed to a long
period of subaërial erosion before the first sheets of lava invaded the
surface of the ground. We are, therefore, tolerably safe in considering
the first eruptions to belong to the Tertiary period; but the evidence,
derived as it is exclusively from plant remains, is somewhat conflicting
as to the precise epoch to which the lavas and beds of tuff containing
the plant-remains are to be referred. The probabilities appear to be
that they are of Miocene age; and if so, the trachytic lavas, which in
Antrim are older than those containing plants, may be referred to a
still earlier epoch--namely, that of the Eocene.[1] As plant remains are
not very distinctive, the question regarding the exact time of the first
volcanic eruptions will probably remain for ever undecided; but we are
not likely to be much in error if we consider the entire volcanic period
to range from the close of the Eocene to that of the Miocene; by far the
greater mass of the volcanic rocks being referable to the latter epoch.
In describing the British volcanic districts it will be most convenient
to deal with them in three divisions--viz., those of Antrim, Mull, and
Skye, commencing with Antrim.[2]
(_b._) _Volcanic Area._--The great sheets of basalt and other volcanic
products of the North-east of Ireland overspread almost the whole of the
County Antrim, and adjoining districts of Londonderry and Tyrone,
breaking off in a fine mural escarpment along the northern shore of
Belfast Lough and the sea coast throughout the whole of its range from
Larne Harbour to Lough Foyle; the only direction in which these features
subside into the general level of the country being around the shores of
Lough Neagh. Several outliers of the volcanic sheets are to be found at
intervals around the great central plateau; such as those of Rathlin
Island, Island Magee, and Scrabo Hill in Co. Down. The area of the
basaltic plateau may be roughly estimated at 2,000 square miles.
[Illustration: Fig. 27.--"The White Rocks," Portrush, Co. Antrim,
showing the plateau-basalt resting on an eroded surface of the Upper
Chalk, with bands of flint.--(From a photograph.)]
The truncated edges of this marginal escarpment rising to levels of
1,000 to 1,260 feet, as in the case of Benevenagh in Co. Derry, and
1,825 feet at Mullaghmore, attest an originally greatly more extended
range of the basaltic sheets; and it is not improbable that at the close
of the Miocene epoch they extended right across the present estuary of
Lough Foyle to the flanks of the mountains of Inishowen in Donegal in
one direction, and to those of Slieve Croob in the other. In the
direction of Scotland the promontories of Kintyre and Islay doubtless
formed a part of the original margin. Throughout this vast area the
volcanic lavas rest on an exceedingly varied rocky floor, both as
regards composition and geological age. (See Fig. 28.) Throughout the
central, southern, eastern, and northern parts of their extent, the
Chalk formation may be considered to form this floor; but in the
direction of Armagh and Tyrone, towards the southwestern margin, the
basaltic sheets are found resting indiscriminately on Silurian,
Carboniferous, and Triassic strata. The general relations of the
plateau-basalts to the underlying formations show, that at the close of
the Cretaceous period there had been considerable terrestrial
disturbances and great subaërial denudation, resulting in some cases in
the complete destruction of the whole of the Cretaceous strata, before
the lava floods were poured out; owing to which, these latter are found
resting on formations of older date than the Cretaceous.[3]
[Illustration: Fig. 28.--Section across the volcanic plateau of Antrim,
from the Highlands of Inishowen, Co. Donegal, on the N.W., to Belfast
Lough on the S.E., to show the relations of the volcanic rocks to the
older formations.--B. Basaltic sheets breaking off in high escarpments;
T. Trachyte porphyry of Tardree mountain rising from below the newer
plateau-basalts; C. Upper Chalk with flints; N.R. New Red marl and
sandstone (Trias); M. Metamorphic beds of quartzite, various schists and
crystalline limestone; F. Large fault.]
[1] Mr. J. Starkie Gardner, from a recent comparison of the
plant-remains of Antrim and Mull, concludes that "that they might belong
to any age between the beginning and the end of the warmer Eocene
period; and that they cannot be of earlier, and are unlikely to be of
later, date."--_Trans. Palæont. Soc._, vol. xxxvii. (1883).
[2] Having dealt with this district rather fully in _The Physical
Geology and Geography of Ireland_ (Edit. 1891, p. 81), and also in my
Presidential Address (Section C.) at the meeting of the British
Association, 1874, a brief review of the subject will be sufficient
here, the reader being referred to the former treatises for fuller
details. The following should also be consulted: Gen. Portlock, _Geology
of Londonderry and Tyrone_ (1843); Sir A. Geikie, "History of Volcanic
Action during the Tertiary Period in the British Isles," _Trans. Roy.
Soc. Edinburgh_, 1888; and the _Descriptive Memoirs_ of the Geological
Survey relating to this tract of country.
[3] Owing to the superposition of the basaltic masses on beds of chalk
throughout a long line of coast, we are presented with the curious
spectacle of the whitest rocks in nature overlain by the blackest, as
may be seen in the cliffs at Larne, Glenarm, Kinbane and Portrush. (See
Fig. 27.)
CHAPTER II.
SUCCESSION OF VOLCANIC ERUPTIONS.
(_c._) _First Stage._--The earliest eruptions of lava in the North-east
of Ireland belonged to the highly acid varieties, consisting of
quartz-trachyte with tridymite.[1] This rock rises to the surface at
Tardree and Brown Dod hills and Templepatrick. It consists of a
light-greyish felsitic paste enclosing grains of smoke-quartz, crystals
of sanidine, plagioclase and biotite, with a little magnetite and
apatite. It is a rock of peculiar interest from the fact that it is
almost unique in the British Islands, and has its petrological
counterpart rather amongst the volcanic hills of the Siebengebirge than
elsewhere. It is generally consolidated with the columnar structure.
[Illustration: Fig. 29.--Part of the section shown in the quarry at
Templepatrick, showing the superposition of the basalt (_d_) to the
trachyte (_b_), with the intervening bed of flint gravel (_c_). All
these rocks are seen to rest upon an eroded surface of the Chalk
formation (_a_).]
The trachyte appears to have been extruded from one or more vents in a
viscous condition, the principal vent being probably situated under
Tardree mountain, where the rock occurs in greatest mass, and it
probably arose as a dome-shaped mass, with a somewhat extended margin,
above the floor of Chalk which formed the surface of the ground.[2]
(Fig. 27.) At Templepatrick the columnar trachyte may be observed
resting on the Chalk, or upon a layer of flint gravel interposed between
the two rocks, and which has been thrust out of position by a later
intrusion of basalt coming in from the side.[3] It is to be observed,
however, that the trachytic lavas nowhere appear cropping out along with
the sheets of basalt around the escarpments overlooking the sea, or
inland; showing that they did not spread very far from their vents of
eruption; a fact illustrating the lower viscosity, or fluidity, of the
acid lavas as compared with those of the basic type.
(_d._) _Second Stage._--After an interval, probably of long duration, a
second eruption of volcanic matter took place over the entire area; but
now the acid lavas of the first stage are replaced by basic lavas. Now,
for the first time, vast masses of basalt and dolerite are extruded both
from vents of eruption and fissures; and, owing to their extreme
viscosity, spread themselves far and wide until they reach the margin of
some uprising ground of old Palæozoic or Metamorphic rocks by which the
volcanic plain is almost surrounded. The great lava sheets thus produced
are generally more or less amorphous, vesicular and amygdaloidal, often
exhibiting the globular concentric structure, and weathering rapidly to
a kind of ferruginous sand or clay under the influence of the
atmosphere. Successive extrusions of these lavas produce successive
beds, which are piled one over the other in some places to a depth of
600 feet; and at the close of the stage, when the volcanic forces had
for the time exhausted themselves, the whole of the North-east of
Ireland must have presented an aspect not unlike that of one of those
great tracts of similar lava in the region of Idaho and the Snake River
in Western America, described in a previous chapter.
(_e._) _Third Stage (Inter-volcanic)._--The third stage may be described
as inter-volcanic. Owing to the formation of a basin, probably not deep,
and with gently sloping sides, a large lake was formed over the centre
of the area above described. Its floor was basalt, and the streams from
the surrounding uplands carried down leaves and stems of trees, strewing
them over its bed. Occasionally eruptions of ash took place from small
vents, forming the ash-beds with plants found at Ballypallidy, Glenarm,
and along the coast as at Carrick-a-raide. The streams also brought down
sand and gravel from the uprising domes of trachyte, and deposited them
over the lake-bed along with the erupted ashes.[4] The epoch we are now
referring to was one of economic importance; as, towards its close,
there was an extensive deposition of pisolitic iron-ore over the floor
of the lake, sometimes to the depth of two or three feet. This ore has
been extensively worked in recent years.
[Illustration: Fig. 30.--Cliff section above the Giant's Causeway, coast
of Co. Antrim, showing successive tiers of basaltic lava, with
intervening bands of bole.]
(_f._) _Fourth Stage (Volcanic)._--The last stage described was brought
to a termination by a second outburst of basic lavas on a scale probably
even grander than the preceding. These lavas consisting of basalt and
dolerite, with their varieties, and extruded from vents and fissures,
spread themselves in all directions over the pre-existing lake deposits
or the older sheets of augitic lava, and probably entirely buried the
trachytic hills. These later sheets solidified into more solid masses
than those of the second stage. They form successive terraces with
columnar structure, each terrace differing from that above and below it
in the size and length of the columns, and separated by thin bands of
"bole" (decomposed lava), often reddish in colour, clearly defining the
limits of the successive lava-flows. Nowhere throughout the entire
volcanic area are these successive terraces so finely laid open to view
as along the north coast of Antrim, where the lofty mural cliffs, worn
back into successive bays with intervening headlands by the
irresistible force of the Atlantic waves, present to the spectator a
vertical section from 300 to 400 feet in height, in which the successive
tiers of columnar basalt, separated by thin bands of bole, are seen to
rise one above the other from the water's edge to the summit of the
cliff, as shown in Fig. 30. Here, also, at the western extremity of the
line of cliffs we find that remarkable group of vertical basaltic
columns, stretching from the base of the cliff into the Atlantic, and
known far and wide by the name of "The Giant's Causeway," the upper ends
of the columns forming a tolerably level surface, gently sloping
seawards, and having very much the aspect of an artificial tesselated
pavement on a huge scale. A portion of the Causeway, with the cliff in
the background, is shown in the figure (Fig. 31). The columns are
remarkable for their symmetry, being generally hexagonal, though
occasionally they are pentagons, and each column is horizontally
traversed by joints of the ball-and-socket form, thus dividing them into
distinct courses of natural masonry. These are very well shown in the
accompanying view of the remarkable basaltic pillars known as "The
Chimneys," which stand up from the margin of the headland adjoining the
Causeway, monuments of past denudation, as they originally formed
individuals amongst the group belonging to one of the terraces in the
adjoining coast.[5] (Fig. 32).
[Illustration: Fig. 31.--The Giant's Causeway, formed of basaltic
columns in a vertical position, and of pentagonal or hexagonal section;
above the Causeway is seen a portion of the cliff composed of tiers of
lava with intervening bands of bole, etc.--(From a photograph.)]
[Illustration: Fig. 32.--"The Chimneys," columns of basalt on slope of
cliff overlooking the Atlantic, north coast of Co. Antrim. The
horizontal segments, or cup-and-ball joints, of the columns are well
shown in this figure. (From a photograph.)]
(_g._) _Original Thickness of the Antrim Lavas._--It is impossible to
determine with certainty what may have been the original thickness of
the accumulated sheets of basic lavas with their associated beds of ash
and bole. The greatest known thickness of the lower zone of lavas is, as
I have already stated, about 600 feet. The intermediate beds of ash and
bole sometimes attain a thickness of 40 feet, and the upper group of
basalt about 400 feet; these together would constitute a series of over
1,000 feet in thickness. But this amount, great as it is, is undoubtedly
below the original maximum, as the uppermost sheets have been removed by
denuding agencies, we know not to what extent. Nor is it of any great
importance. Sufficient remains to enable us to form a just conception of
the magnitude both as regards thickness and extent of the erupted matter
of the Miocene period over the North-east of Ireland and adjoining
submerged tracts, and of the magnitude of the volcanic operations
necessary for the production of such masses.
(_h._) _Volcanic Necks._--As already remarked, no craters of eruption
survive throughout the volcanic region of the North-east of Ireland,
owing to the enormous extent of the denudation which this region has
undergone since the Miocene Epoch; but the old "necks" of such
craters--in other words, the pipes filled with either solid basalt, or
basalt and ashes--are still to be found at intervals over the whole
area. Owing to the greater solidity of the lava which filled up these
"necks" over the plateau-basaltic sheets which surround them, they
appear as bosses or hills rising above the general level of the ground.
One of these bosses of highly columnar basalt occurs between Portrush
and Bushmills, not far from Dunluce Castle, another at Scawt Hill, near
Glenarm, and a third at Carmoney Hill above Belfast Lough. But by far
the most prominent of these old solidified vents of eruption is that of
Sleamish, a conspicuous mountain which rises above the general level of
the plateau near Ballymena, and attains an elevation of 1,437 feet above
the sea. Seen from the west, the mountain has the appearance of a
round-topped cone; but on examination it is found to be in reality a
huge dyke, breaking off abruptly towards the north-west, in which
direction it reaches its greatest height, then sloping downwards towards
the east. This form suggests that Sleamish is in reality one of the
fissure-vents of eruption rather than the neck of an old volcano. The
rock of which it is formed consists of exceedingly massive,
coarsely-crystalline dolerite, rich in olivine, and divided into large
quadrangular blocks by parallel joint planes. Its junction with the
plateau-basalt from which it rises can nowhere be seen; but at the
nearest point where the two rocks are traceable the plateau-basalt
appears to be somewhat indurated; breaking with a splintery fracture and
a sharp ring under the hammer, suggesting that the lava of Sleamish had
been extruded through the horizontal sheets, and had considerably
indurated the portions in contact with, or in proximity to, it.[6]
Amongst the vents filled with ash and agglomerate, the most remarkable
is that of Carrick-a-raide, near Ballycastle. It forms this rocky island
and a portion of the adjoining coast, where the beds of ash are finely
displayed; consisting of fragments and bombs of basalt, with pieces of
chalk, flint, and peperino, which is irregularly bedded. These ash-beds
attain a thickness of about 120 feet just below the road to Ballycastle,
but rapidly tail out in both directions from the locality of the vent.
Just below the ash-beds, the white chalk with flints may be seen
extending down into the sea-bed. Nowhere in Antrim is there such a
display of volcanic ash and agglomerate as at this spot.[7]
(_i._) _Dykes: Conditions under which they were Erupted._--No one can
visit the geological sections in Co. Antrim and the adjoining districts
of Down, Armagh, Derry, and Tyrone, without being struck by the great
number and variety of the igneous dykes by which the rocks are
traversed. The great majority of these dykes are basaltic, and they are
found traversing all the formations, including the Cretaceous and
Tertiary basaltic sheets. The Carlingford and Mourne Mountains are
seamed with such dykes, and they are splendidly laid open to view along
the coast south of Newcastle in Co. Down, as also along the Antrim coast
from Belfast to Larne. The fine old castle of Carrickfergus has its
foundations on one of those dyke-like intrusions, but one of greater
size than ordinary. All the dykes here referred to are not, however, of
the same age, as is conclusively proved by sections amongst the Mourne
Mountains where cliffs of Lower Silurian strata, superimposed on the
intrusive granite of the district, exhibit two sets of basaltic
dykes--one (the older) abruptly terminated at the granite margin, the
other and newer penetrating the granite and Silurian rocks alike. It is
not improbable that the older dykes belong to the Carboniferous or
Permian age, while the newer are with equal probability of Tertiary age.
Sir A. Geikie has shown that the Tertiary dykes of the North of Ireland
are representatives of others occurring at intervals over the North of
England, and Central and Western Scotland, all pointing towards the
central region of volcanic activity; or in a parallel direction thereto,
approximating to the N.W. in Ireland, the Island of Islay, and East
Argyleshire, but in the centre of Scotland generally ranging from east
to west.[8] The area affected by the dykes of undoubted Tertiary age
Geikie estimates at no less than 40,000 square miles--a territory
greater than either Scotland or Ireland, and equal to more than a third
of the total land-surface of the British Isles;[9] and he regards them
as posterior "to the rest of the geological structures of the regions
which they traverse." It is clear that the dykes referred to belong to
one great system of eruption or intrusion; and they may be regarded as
the manifestation of the final effort of internal forces over this
region of the British Isles. They testify to the existence of a
continuous _magma_ (or shell) of augitic lava beneath the crust; and as
the aggregate horizontal extent of all these dykes, or of the fissures
which they fill, must be very considerable, it is clear that the crust
through which they have been extruded has received an accession of
horizontal space, and has been fissured by forces acting from beneath,
as the late Mr. Hopkins, of Cambridge, had explained on mechanical
grounds in his elaborate essay many years ago.[10] This view occurred to
myself when examining the region of the North-east of Ireland, but I was
not then aware that it had been dealt with on mathematical principles by
so eminent a mathematician. The bulging of the crust is a necessary
consequence of the absence of plication of the strata due to the
extrusion of this enormous quantity of molten lava; and the intrusion of
thousands of dykes over the North-east of Ireland, unaccompanied by
foldings of the strata, must have added a horizontal space of several
thousand feet to that region.[11]
[1] A peculiar form of crystalline quartz first recognized in this rock
by a distinguished German petrologist, the late Prof. A. von Lasaulx,
who visited the district in 1876.
[2] Sir A. Geikie has disputed the correctness of the view, which I
advocated as far back as 1874, that the trachytic lavas of Antrim are
the earliest products of volcanic action; but at the time he wrote his
paper on the volcanic history of these islands, it was not known that
pebbles of this trachyte are largely distributed amongst the ash-beds
which occur in the very midst of the overlying basaltic sheets, as I
shall have to explain later on. This discovery puts the question at rest
as regards the relations of the two sets of rocks.
[3] This remarkable section at the chalk quarries of Templepatrick the
author has figured and described in the _Physical Geology and Geography
of Ireland_, p. 99, 2nd edit. (1891), where the reader will find the
subject discussed more fully than can be done here.
[4] These pebbles were first noticed by Mr. McHenry, of the Irish
Geological Survey, in 1890.
[5] The vertical position of the columns of the Giant's Causeway is
rather enigmatical. The Causeway cannot be a dyke, as has often been
supposed, otherwise the columns would have been horizontal, _i.e._, at
right angles to the sides of the dyke. Mr. R. G. Symes, of the
Geological Survey, has suggested that the Causeway columns have been
vertically lowered between two lines of fault, and that originally they
formed a portion of the tier of beautiful columns seen in the cliff
above, and known as "The Organ."
[6] Sleamish and several other of the Antrim vents are described by Sir
A. Geikie in the monograph already referred to, _loc. cit._, p. 101, _et
seq._ Also in the _Expl. Memoirs of the Geological Survey of Ireland_.
[7] A diagrammatised section of the Carrick-a-raide volcanic neck is
given by Sir A. Geikie, _loc. cit._, p. 105.
[8] Geikie, _loc. cit._, p. 29, _et seq._
[9] P. 32. The view that the crust of the earth has been horizontally
extended by the intrusion of dykes is noticed by McCulloch in reference
to the dykes of Skye.
[10] Hopkins, _Cambridge Phil. Trans._, vol. vi. p. 1 (1836).
[11] As suggested in my Presidential Address to Section C. of the
British Association at Belfast, 1874.
CHAPTER III.
ISLAND OF MULL AND ADJOINING COAST.
The Island of Mull, with the adjoining districts of Morvern and
Ardnamurchan, forms the more southern of the two chief centres of
Tertiary volcanic eruptions in the West of Scotland, that of Skye being
the more northern. These districts have been the subject of critical and
detailed study by several geologists, from McCulloch down to the present
day; and amongst the more recent, Sir Archibald Geikie and Professor
Judd hold the chief place. Unfortunately, the interpretation of the
volcanic phenomena by these two accomplished observers has led them to
very different conclusions as regards several important points in the
volcanic history of these groups of islands; as, for example, regarding
the relative ages of the plateau-basalts and the acid rocks, such as the
trachytes and granophyres; again as regards the presence of distinct
centres of eruption; and also as regards the relations of the gabbros of
Skye to the basaltic sheets. Such being the case, it would appear the
height of rashness on the part of the writer, especially in the absence
of a detailed examination of the sections over the whole region, to
venture on a statement of opinion regarding the points at issue; and he
must, therefore, content himself with a brief account of the phenomena
as gathered from a perusal of the writings of these and other
observers,[1] guided also to some extent by the analogous phenomena
presented by the volcanic region of the North-east of Ireland.
(_a._) _General Features._--As in the case of the Antrim district, the
Island of Mull and adjoining tracts present us with the spectacle of a
vast accumulation of basaltic lava-flows, piled layer upon layer, with
intervening beds of bole and tuff, up to a thickness, according to
Geikie, of about 3,500 feet. At the grand headland of Gribon, on the
west coast, the basaltic sheets are seen to rise in one sheer sweep to a
height of 1,600 feet, and then to stretch away with a slight easterly
dip under Ben More at a distance of some eight miles. This mountain, the
upper part of which is formed of beds of ashes, reaches an elevation of
3,169 feet, so that the accumulated thickness of the beds of basalt
under the higher part of the mountain must be at least equal to the
amount stated above--that is, twice as great as the representative
masses of Antrim. The base of the volcanic series is seen at Carsaig and
Gribon to rest on Cretaceous and Jurassic rocks, like those of Antrim;
hence the Tertiary age is fully established by the evidence of
superposition. This was further confirmed by the discovery by the Duke
of Argyll,[2] some years ago (1850), of bands of flint-gravel and tuff,
with dicotyledonous leaves amongst the basalts of Ardtun Head. The
basement beds of tuff and gravel contain, besides pebbles of flint and
chalk, others of sanidine trachyte, showing that highly acid lavas had
been extruded and consolidated before the first eruption of the
plateau-basalts; another point of analogy between the volcanic
phenomenon of Antrim and the Inner Hebrides. These great sheets of
augitic lava extend over the whole of the northern tract of Mull, the
Isles of Ulva and Staffa, and for a distance of several miles inwards
from the northern shore of the Sound of Mull, covering the wild
moorlands of Morvern and Ardnamurchan, where they terminate in
escarpments and outlying masses, indicating an originally much more
extended range than at the present day. The summits of Ben More and its
neighbouring height, Ben Buy, are formed of beds of ash and tuff. The
volcanic plateau is, according to Judd, abruptly terminated along the
southern side by a large vault, bringing the basalt in contact with
Palæozoic rocks.[3]
(_b._) _Granophyres._--The greater part of the tract lying to the south
of Loch na Keal, which almost divides Mull into two islands, and
extending southwards and eastwards to the shores of the Firth of Lorn
and the Sound of Mull, is formed of a peculiar group of acid (or highly
silicated) rocks, classed under the general term of "Granophyres." These
rocks approach towards true granites in one direction, and through
quartz-porphyry and felsite to rhyolite in another--probably depending
upon the conditions of cooling and consolidation. In their mode of
weathering and general appearance on a large scale, they present a
marked contrast to the basic lavas with which they are in contact from
the coast of L. na Keal to that of L. Buy. The nature of this contact,
whether indicating the priority of the granophyres to the
plateau-basalts or otherwise, is a matter of dispute between the two
observers above named; but the circumstantial account given by Sir A.
Geikie,[4] accompanied by drawings of special sections showing this
contact, appears to prove that the granophyre is the newer of the two
masses of volcanic rock, and that it has been intruded amongst the
basaltic-lavas at a late period in the volcanic history of these
islands. A copy of one of these sketches is here given (Fig. 33),
according to which the felsite is shown to penetrate the basaltic sheets
at Alt na Searmoin in Mull; other sections seen at Cruach Torr an
Lochain, and on the south side of Beinn Fada, appear to lead to similar
conclusions. These rocks are penetrated by numerous basaltic dykes.
[Illustration: Fig. 33.--Section at Alt na Searmoin, Mull, to show the
intrusion of felsite (or granophyre) (_b_) into basalt and dolerite
(_a_) of the plateau-basalt series.--(Geikie.)]
(_c._) _Representative Rocks of Mourne and Carlingford,
Ireland._--Assuming Sir A. Geikie's view to be correct, it is possible
that we may have in the granite and quartz-porphyries of Mourne and
Carlingford representatives of the granites, granophyres, and other acid
rocks of the later period of Mull. The granite of Mourne is peculiar in
structure, and differs from the ordinary type of that rock in which the
silica forms the ground mass. In the case of the granite of the Mourne
Mountains, the rock consists of a crystalline granular aggregate of
orthoclase, albite, smoke-quartz, and mica; it is also full of drusy
cavities, in which the various minerals crystallise out in very perfect
form. As far as regards direct evidence, the age of this rock can only
be stated to be post-Carboniferous, and earlier than certain Tertiary
basaltic dykes by which it is traversed. The granophyres of Mull are
traversed by similar dykes, which are representatives of the very latest
stage of volcanic action in the British Islands. The author is therefore
inclined to concur with Sir A. Geikie in assigning to the granite of the
Mourne Mountains, and the representative felsitic rocks of the
Carlingford Mountains, a Tertiary age--in which case the analogy between
the volcanic phenomena of the Inner Hebrides and of the North-east of
Ireland would seem to be complete.[5]
[1] Geikie, _Proc. Roy. Soc. Edinburgh_ (1867); _Brit. Assoc. Rep._
(Dundee, 1867); "Tertiary Volcanic Rocks of the British Isles," _Quart.
Journ. Geol. Soc._, vol. xxvii. p. 279; also, "History of Volcanic
Action in British Isles," _Trans. Roy. Soc. Edin._ (1888); Judd, "On the
Ancient Volcanoes of the Highlands," etc., _Quart. Journ. Geol. Soc._,
vol. xxx. p. 233; and _Volcanoes_, p. 139.
[2] _Brit. Assoc. Rep._ for 1850, p. 70.
[3] Judd, _Quart. Jour. Geol. Soc._, vol. xxx. p. 242.
[4] _History of Volcanic Action, etc._, _loc. cit._ p. 153, _et seq._
The "Granophyres" of Geikie come under the head of "Felsites," passing
into "granite" in one direction and quartz-trachyte in another,
according to Judd; the proportion of silica from 69 to 75 per
cent.--_Quart. Jour. Geol. Soc._, vol. xxx. p. 235.
[5] This view the author has expressed in a recent edition of _The
Physical Geology of Ireland_, p. 177 (1891).
CHAPTER IV.
ISLE OF SKYE.
This is the largest and most important of all the Tertiary volcanic
districts, but owing to the extensive denudation to which, in common
with other Tertiary volcanic regions of the British Isles, it has been
subjected, its present limits are very restricted comparatively to its
original extent. Not only is this evident from the manner in which the
basaltic sheets terminate along the sea-coast in grand mural cliffs, as
opposite "Macleod's Maidens," and at the entrance to Lough Bracadale on
the western coast, but the evidence is, according to Sir A. Geikie,
still more striking along the eastern coast; showing that the Jurassic,
and other older rocks there visible, were originally buried deep under
the basaltic sheets which have been stripped from off that part of the
country. These great plateau-basalts occupy about three-fourths of the
entire island along the western and northern areas, rising into terraced
mountains over 2,000 feet in height, and are deeply furrowed by glens
and arms of the sea, along which the general structure of the tableland
is laid open, sometimes for leagues at a time.
It is towards the south-eastern part of the island that the most
interesting and important phenomena are centred; for here we meet with
representatives of the acid (or highly silicated) group of rocks, and
of remarkable beds of gabbro, which have long attracted the attention of
petrologists. These latter beds, throughout a considerable distance
round the flanks of the Cuillin Hills, are interposed between the acid
rocks and the plateau-basalts; but towards the north, on approaching
Lough Sligahan, the acid rocks, consisting of granophyres,
quartz-porphyries, and hornblendic-granitites, are in direct contact
with the plateau-basalts; and, according to the very circumstantial
account of Sir A. Geikie, are intrusive into them; not only sending
veins into the basaltic sheets, but also producing a marked alteration
in their structure where they approach the newer intrusive mass. Equally
circumstantial is the same author's account of the relations of the
granophyres to the gabbros,[1] as seen at Meall Dearg and the western
border of the Cuillin Hills--where the former rock may be seen to send
numerous veins into the latter. Not only is this so, but the granophyre
is frequently seen to truncate, and abruptly terminate some of the
basaltic dykes by which the basic sheets are traversed--as in the
neighbourhood of Beinn na Dubhaic. All these phenomena strongly remind
us of the conditions of similar rocks amongst the mountains of Mourne
and Carlingford in Ireland; where, at Barnaveve, the syenite (or
hornblendic quartz-felsite) is seen to break through the masses of
olivine gabbro, and send numerous veins into this latter rock.[2]
The interpretation here briefly sketched differs widely from that
arrived at by Professor Judd. The granitoid masses of the Red Mountains
(Beinn Dearg) and the neighbouring heights are, in his view, the roots
of the great volcano from which were erupted the various lavas; the
earlier eruptions producing the acid lavas, to be followed by the
gabbros, and these by the plateau-basaltic sheets, which stretch away
towards the north and west into several peninsulas. Thus he holds that
"the rocks of basic composition were ejected subsequently to those of
the acid variety," and appeals to various sections in confirmation of
this view.[3] To reconcile these views is at present impossible; but as
the controversy between these two observers is probably not yet closed,
there is room for hope that the true interpretation of the relations of
these rocks to each other will ere long be fully established.
[1] Geikie, _loc. cit._, p. 161, etc.
[2] _Physical Geology of Ireland_, 2nd edition, p. 174 (Fig. 21).
Professor Judd has also come to the conclusion that the granite of
Mourne is of Tertiary age, _Quart. Jour. Geol. Soc._, vol. xxx. p. 275.
[3] Judd, _loc. cit._, p. 254.
CHAPTER V.
THE SCUIR OF EIGG.
Amongst the more remarkable of the smaller islets are those of Eigg,
Rum, Canna, and Muck, lying between Mull on the south and Skye on the
north, and undoubtedly at one time physically connected together. The
Island of Eigg is especially remarkable for the fact, as stated by
Geikie, that here we have the one solitary case of "a true superficial
stream of acid lava--that of the Scuir of Eigg."[1] (Fig. 34.) This
forms a sinuous ridge, composed of pitchstone of several kinds, of over
two miles in length, rising from the midst of a tableland of bedded
basalt and tuff to a height of 1,289 feet above the ocean; the
plateau-basalt is traversed by basaltic dykes, ranging in a N.W.-S.E.
direction. But what is specially remarkable is the evidence afforded by
an examination of the course of the Scuir, that it follows the channel
of an ancient river-valley, which has been hollowed out in the surface
of the plateau. The course of this channel is indicated by the presence
of a deposit of river-gravel, which in some places forms a sort of
cushion between the base of the Scuir and the side of the channel. Over
this gravel-bed the viscous pitchstone-lava appears to have flowed,
taking possession of the river-channel, and also of the beds of several
small tributary streams which flowed into the channel of the Scuir. The
recent date of the pitchstone forming this remarkable mural ridge, once
occupying the bed of a river-channel, is shown by the fact that the
basaltic dykes which traverse the plateau-basalts are truncated by the
river-gravel, which is, therefore, more recent; and, as we have seen,
the pitchstone stream is more recent than the river-gravel. But at the
time when this last volcanic eruption took place, the physical geography
of the whole region must have been very different from that of the
present time. From the character and composition of the pebbles in the
old river-bed, amongst which are Cambrian sandstone, quartzite,
clay-slate, and white Jurassic limestone, Sir A. Geikie concludes that
when the river was flowing, the island must have been connected with the
mainland to the east where the parent masses of these pebbles are found.
[Illustration: Fig. 34.--View of the Scuir of Eigg from the east. The
lower portion of the mountain is formed of bedded basalt, or dolerite
with numerous dykes and veins of basalt, felstone, and pitchstone; the
upper cliff, or Scuir, is composed of pitchstone of newer age, the
remnant of a lava flow which once filled a river channel in the basaltic
sheets. A dyke, or sheet, of porphyry is seen to be interposed between
the Scuir and the basaltic sheets.--(After Geikie.)]
_Effects of Denudation._--The position of the Scuir of Eigg and its
relations to the basaltic sheets show the enormous amount of denudation
which these latter have undergone since the stream of pitchstone-lava
filled the old river channel. The walls, or banks, of the channel have
been denuded away, thus converting the pitchstone casting into a
projecting wall of rock. That it originally extended outwards into the
ocean to a far greater distance than at present is evident from the
abruptly truncated face of the cliff; and yet this remarkable volcanic
mass seems to have been, perhaps, the most recent exhibition of volcanic
action to be found in the British Isles. It is perhaps, on this account,
the most striking of the numerous examples exhibited throughout the West
of Scotland and the North-east of Ireland of the enormous amount of
denudation to which these districts have been subjected since the
extinction of the volcanic fires; and this at a period to which we
cannot assign a date more ancient than that of the Pliocene. Yet, let us
consider for a moment to what physical vicissitudes these districts have
been subjected since that epoch. Assuming, as we may with confidence,
that the volcanic eruptions were subaërial, and that the tracts covered
by the plateau-basalts were in the condition of dry land when the
eruptions commenced, in this condition they continued in the main
throughout the period of volcanic activity. But the eruptions had
scarcely ceased, and the lava floods and dykes become consolidated,
before the succeeding glacial epoch set in; when the snows and glaciers
of the Scottish Highlands gradually descending from their original
mountain heights, and spreading outwards in all directions, ultimately
enveloped the whole of the region we are now considering until it was
entirely concealed beneath a mantle of ice moving slowly, but
irresistibly, outwards towards the Atlantic, crossing the deep channels,
such as the Sound of Mull and the Minch, climbing up the sides of
opposing rocks and islands until even the Outer Hebrides and the
North-east of Ireland were covered by one vast mantle of ice and snow.
The movement of such a body of ice over the land must have been attended
with a large amount of abrasion of the rocky floor; nor have the
evidences of that abrasion entirely disappeared even at the present day.
We still detect the grooves and scorings on the rock-surfaces where they
have been protected by a coating of boulder clay; and we still find the
surface strewn with the blocks and _débris_ of that mighty ice-flood.
But whatever may have been the amount of erosion caused by the great
ice-sheet, it was chiefly confined to the more or less horizontal
surface-planes. Erosion of another kind was to succeed, and to produce
more lasting effects on the configuration of the surface. On the
disappearance of the ice-sheet, an epoch characterised by milder
conditions of climate set in. This was accompanied by subsidence and
submersion of large tracts of the land during the Interglacial stage; so
that the sea rose to heights of several hundred feet above the present
level, and has left behind stratified gravels with shells at these
elevations in protected places. During this period of depression and of
subsequent re-emergence the wave-action of the Atlantic waters must have
told severely on the coast and islands, wearing them into cliffs and
escarpments, furrowing out channels and levelling obstructions. Such
action has gone on down to the present day. The North-west of Scotland
and of Ireland has been subjected throughout a very lengthened period to
the wear and tear of the Atlantic billows. In the case of the former,
the remarkable breakwater which nature has thrown athwart the North-west
Highlands in the direction of the waves, forming the chain of islands
constituting the Outer Hebrides, and composed of very tough Archæan
gneiss and schist, has done much to retard the inroads which the waves
might otherwise have made on the Isle of Skye; while Coll and Tiree,
composed of similar materials, have acted with similar beneficent effect
for Mull and the adjoining coasts. But such is the tremendous power of
the Atlantic billows when impelled by westerly winds, that to their
agency must be mainly attributed the small size of the volcanic
land-surfaces as compared with their original extent, and the formation
of those grand headlands which are presented by the igneous masses of
Skye, Ardnamurchan, and Mull towards the west. Rain and river action,
supplemented by that of glaciers, have also had a share in eroding
channels and wearing down the upper surface of the ground, with the
result we at present behold in the wild and broken scenery of the Inner
Hebrides and adjoining coast.
[1] Geikie, _loc. cit._, p. 178; also _Quart. Jour. Geol. Soc._, vol.
xxvii. p. 303.
CHAPTER VI.
ISLE OF STAFFA.
Reference has been made to this remarkable island in a former page, but
some more extended notice is desirable before leaving the region of the
Inner Hebrides. Along with the islands of Pladda, Treshnish, and
Blackmore, Staffa is one of the outlying volcanic islands of the group,
being distant about six miles from the coast of Mull, and indicates the
minimum distance to which the plateau-basaltic sheets originally
extended in the direction of the old marginal lands of Tiree and Coll.
The island consists of successive sheets of bedded basaltic lava, with
partings of tuff, one of which of considerable thickness is shown to lie
at the base of the cliff on the south-west side of the island.[1] The
successive lava-sheets present great varieties of structure, like those
on the north coast of Antrim; some being amorphous, others columnar,
with either straight or bent columns. The lava-sheet out of which
Fingal's Cave is excavated consists of vertical prisms, beautifully
formed, and surmounted by an amorphous mass of the same material. At the
entrance of the Boat Cave we have a somewhat similar arrangement of the
columns;[2] but at the Clam-shell Cave the prisms are curved, indicating
some movement in the viscous mass before they had been fully
consolidated.
Fingal's Cave is called after the celebrated prince of Morvern (or
Morven), a province of ancient Caledonia. He is supposed to have been
the father of Ossian, the Celtic bard rendered famous by Macpherson. The
cave, one of many which pierce the coast-cliffs of Western Scotland, is
227 feet in length, 166 feet in height, and 40 feet in width. On all
sides regular columns of basalt, some entire, others broken, rise out of
the water and support the roof. The cave is only accessible in calm
weather.
[1] A drawing of this cliff is given by Geikie in the _Manual of
Geology_ (Jukes and Geikie), 3rd edition, p. 277.
[2] Prestwich, _Geology_, vol. i. p. 281, where a view of this cave is
given.
PART V.
PRE-TERTIARY VOLCANIC ROCKS.
CHAPTER I.
THE DECCAN TRAP-SERIES OF INDIA.
The great outpourings of augitic lava of Tertiary and recent times which
we have been considering appear to have been anticipated in several
parts of the world, more especially in Peninsular India and in Africa,
and it is desirable that we should devote a few pages to the description
of these remarkable volcanic formations, as they resemble, both in their
mode of occurrence and general structure, some of the great lava-floods
of a more recent period we have been considering. Of the districts to be
described, the first which claims our notice is the Deccan.
(_a._) _Extent of the Volcanic Plateau._--The volcanic plateau of the
Deccan stretches from the borders of the Western Ghats and the sea-coast
near Bombay inland to Amarantak, at the head of the Narbudda River
(long. 82° E.), and from Belgaum (lat. 15° 31' N.) to near Goona (lat.
24° 30'). The vast area thus circumscribed is far from representing the
original extent of the tract overspread by the lava-floods, as outlying
fragments of these lavas are found as far east as long. 84° E. in one
direction, and at Kattiwar and Cutch in another. The present area,
however, is estimated to be not less than 200,000 square miles.[1]
(_b._) _Nature and Thickness of the Lava-flows._--This tract is
overspread almost continuously by sheets of basaltic lava, with
occasional bands of fresh-water strata containing numerous shells,
figured and described by Hislop, and believed by him to be of Lower
Eocene age. The lava-sheets vary considerably in character, ranging from
finest compact basalt to coarsely crystalline dolerite, in which olivine
is abundant. The columnar structure is not prevalent, the rock being
either amorphous, or weathering into concentric shells. Volcanic ash, or
bole, is frequently found separating the different lava-flows; and in
the upper amygdaloidal sheets numerous secondary minerals are found,
such as quartz, agate and jasper, stilbite and chlorite. The total
thickness of the whole series, where complete, is about 6,000 feet,
divided as follows:
1. Upper trap; with ash and inter-trappean beds 1,500 feet
2. Middle trap; sheets of basalt and ash 4,000 "
3. Lower trap; basalt with inter-trappean beds 500 "
--------
6,000 "
========
Throughout the region here described these great sheets of volcanic rock
are everywhere approximately horizontal, and constitute a table-land of
3,000 to 4,000 feet in elevation, breaking off in terraced escarpments,
and penetrated by deep river-valleys, of which the Narbudda is the most
important. The foundation rock is sometimes metamorphic schist, or
gneiss, at other times sandstone referred by Hislop to Jurassic age; and
in no single instance has a volcanic crater or focus of eruption been
observed. But outside the central trappean area volcanic foci are
numerous, as in Cutch, the Rajhipla Hills and the Lower Narbudda valley.
The original excessive fluidity of the Deccan trap is proved by the
remarkable horizontality of the beds over large areas, and the extensive
regions covered by very thin sheets of basalt or dolerite.
(_c._) _Geological Age._--As regards the geological age of this great
volcanic series much uncertainty exists, owing to the absence of marine
forms in the inter-trappean beds. One single species, _Cardita
variabilis_, has been observed as occurring in these beds, and in the
limestone below the base of the trap at Dudukur. The _facies_ of the
forms in this limestone is Tertiary; but there is a remarkable absence
of characteristic genera. On the other hand, Mr. Blanford states that
the bedded traps are seen to underlie the Eocene Tertiary strata with
_Nummulites_ in Guzerat and Cutch,[2] which would appear to determine
the limit of their age in one direction. On balancing the evidence,
however, it is tolerably clear that the volcanic eruptions commenced
towards the close of the Cretaceous period, and continued into the
commencement of the Tertiary, thus bridging over the interval between
the two epochs; and since the greater sheets have been exposed
throughout the whole of the Tertiary and Quarternary periods, it is not
surprising if they have suffered enormously from denuding agencies, and
that any craters or cones of eruption that may once have existed have
disappeared.
[1] The Deccan Traps have been described by Sykes, _Geol. Trans._, 2nd
Series, vol. iv.; also Rev. S. Hislop, "On the Geology of the
Neighbourhood of Nagpur, Central India," _Quart. Journ. Geol. Soc._,
vol. x. p. 274; and _Ibid._, vol. xvi. p. 154. Also, H. B. Medlicott and
W. T. Blanford, _Manual of the Geology of India_, vol. i. (1879).
[2] Blanford, _Geology of Abyssinia_, p. 185.
CHAPTER II.
ABYSSINIAN TABLE-LANDS.
Another region in which the volcanic phenomena bear a remarkable analogy
to those of Central India, just described, is that of Abyssinia. Nor are
these tracts so widely separated that they may not be considered as
portions of one great volcanic area extending from Abyssinia, through
Southern Arabia, into Cutch and the Deccan, in the one direction, while
the great volcanic cones of Kenia and Kilimanjaro, with their
surrounding tracts of volcanic matter, may be the extreme prolongations
in the other. Along this tract volcanic operations are still active in
the Gulf of Aden; and cones quite unchanged in form, and evidently of
very recent date, abound in many places along the coast both of Arabia
and Africa. The volcanic formations of this tract are, however, much
more recent than those which occupy the high plateaux of Central and
Southern Abyssinia of which we are about to speak.
(_a._) _Physical Features._--Abyssinia forms a compact region of lofty
plateaux intersected by deep valleys, interposed between the basin of
the Nile on the west, and the low-lying tract bordering the Red Sea and
the Indian Ocean on the east. The plateaux are deeply intersected by
valleys and ravines, giving birth to streams which feed the head waters
of the Blue Nile (Bahr el Arak) and the Atbara. Several fine lakes lie
in the lap of the mountains, of which the Zana, or Dembia, is the
largest, and next Ashangi, visited by the British army on its march to
Magdala in 1868, and which, from its form and the volcanic nature of the
surrounding hills, appears to occupy the hollow of an extinct crater.
The table-land of Abyssinia reaches its highest elevation along the
eastern and southern margin, where its average height may be 8,000 to
10,000 feet; but some peaks rise to a height of 12,000 to 15,000 feet in
Shoa and Ankobar.[1]
(_b._) _Basaltic Lava Sheets._--An enormous area of this country seems
to be composed of volcanic rocks chiefly in the form of sheets of
basaltic lava, which rise into high plateaux, and break off in
steep--sometimes precipitous--mural escarpments along the sides of the
valleys. These are divisible into the following series:--
(1) _The Ashangi Volcanic Series._--The earliest forerunners of the more
recent lavas seem to have been erupted in Jurassic times, in the form of
sheets of contemporaneous basalt or dolerite amongst the Antola
limestones which are of this period. But the great mass of the volcanic
rocks are much more recent, and may be confidently referred to the late
Cretaceous or early Tertiary epochs. Their resemblance to the great
trappean series of Western India, even in minute particulars, is
referred to by Mr. Blanford, who suggests the view that they belong to
one and the same great series of lava-flows extruded over the surface of
this part of the globe. This view is inherently probable. They consist
of basalts and dolerites, generally amygdaloidal, with nodules of agate
and zeolite, and are frequently coated with green-earth (chlorite). Beds
of volcanic ash or breccia also frequently occur, and often contain
augite crystals. At Senafé, hills of trachyte passing into claystone and
basalt were observed by Mr. Blanford, but it is not clear what are their
relations to the plateau-basaltic sheets.[2]
(2) _Magdala Volcanic Series._--This is a more recent group of volcanic
lavas, chiefly distinguished from the lower, or Ashangi, group, by the
occurrence of thick beds of trachyte, usually more or less crystalline,
and containing beautiful crystals of sanidine. The beds of trachyte
break off in precipitous scarps, and being of great thickness and
perfectly horizontal, are unusually conspicuous. Mr. Blanford says, with
regard to this group, that there is a remarkable resemblance in its
physical aspect to the scenery of the Deccan and the higher valleys of
the Western Ghats of India, but the peculiarities of the landscape are
exaggerated in Abyssinia. Many of the trachytic beds are brecciated and
highly columnar; sedimentary beds are also interstratified with those of
volcanic origin. The Magdala group is unconformable to that of Ashangi
in some places. A still more recent group of volcanic rocks appears to
occur in the neighbourhood of Senafé, consisting of amorphous masses of
trachyte, often so fine-grained and compact as to pass into claystone
and to resemble sandstone. At Akub Teriki the rocks appear to be in the
immediate vicinity of an ancient vent of eruption.
From what has been said, it will be apparent that Abyssinia offers
volcanic phenomena of great interest for the observer. There is
considerable variety in the rock masses, in their mode of distribution,
and in the scenery which they produce. The extensive horizontal sheets
of lava are suggestive of fissure-eruption rather than of eruption
through volcanic craters; and although these may have once been in
existence, denudation has left no vestiges of them at the present day.
In all these respects the resemblance of the volcanic phenomena to those
of Peninsular India is remarkably striking; it suggests the view that
they are contemporaneous as regards the time of their eruption, and
similar as regards their mode of formation.
[1] W. T. Blanford, _Geology of Abyssinia_, pp. 151-2.
[2] Blanford, _loc. cit._, p. 182.
CHAPTER III.
CAPE COLONY.
_Basalt of the Plateau._--The extensive sheets of plateau-basalt forming
portions of the Neuweld range and the elevated table-land of Cape
Colony, may be regarded as forerunners of those just described, and
possibly contemporaneous with the Ashangi volcanic series of Abyssinia.
The great basaltic sheets of the Cape Colony are found capping the
highest elevations of the Camderboo and Stormberg ranges, as well as
overspreading immense areas of less elevated land, to an extent,
according to Professor A. H. Green, of at least 120,000 square miles.[1]
Amongst these sheets, innumerable dykes, and masses of solid lava which
filled the old vents of eruption, are to be observed. The floor upon
which the lava-floods have been poured out generally consists of the
"Cave Sandstone," the uppermost of a series of deposits which had
previously been laid down over the bed of an extensive lake which
occupied this part of Africa during the Mesozoic period. After the
deposition of this sandstone, the volcanic forces appear to have burst
through the crust, and from vents and fissures great floods of augitic
lava, with beds of tuff, invaded the region occupied by the waters of
the lake. The lava-sheets have since undergone extensive denudation, and
are intersected by valleys and depressions eroded down through them into
the sandstone floor beneath; and though the precise geological period at
which they were extruded must remain in doubt, it appears probable that
they may be referred to that of the Trias.[2]
[1] Green. "On the Geology of the Cape Colony," _Quart. Jour. Geol.
Soc._, vol. xliv. (1888).
[2] The district lying along the south coast of Africa is described by
Andrew G. Bain, in the _Trans. Geol. Soc._, vol. vii. (1845); but there
is little information regarding the volcanic region here referred to.
CHAPTER IV.
VOLCANIC ROCKS OF PAST GEOLOGICAL PERIODS OF THE BRITISH ISLES.
It is beyond the scope of this work to describe the volcanic rocks of
pre-Tertiary times over various parts of the globe. The subject is far
too large to be treated otherwise than in a distinct and separate essay.
I will therefore content myself with a brief enumeration of the
formations of the British Isles in which contemporaneous volcanic action
has been recognised.[1]
There is little evidence of volcanic action throughout the long lapse of
time extending backwards from the Cretaceous to the Triassic epochs,
that is to say, throughout the Mesozoic or Secondary period, and it is
not till we reach the Palæozoic strata that evidence of volcanic action
unmistakably presents itself.
_Permian Period._--In Ayrshire, and in the western parts of Devonshire,
beds of felspathic porphyry, felstone and ash are interstratified with
strata believed to be of Permian age. In Devonshire these have only
recently been recognised by Dr. Irving and the author as of Permian age,
the strata consisting of beds of breccia, lying at the base of the New
Red Sandstone. Those of Ayrshire have long been recognised as of the
same period; as they rest unconformably on the coal measures, and
consist of porphyrites, melaphyres, and tuffs of volcanic origin.
_Carboniferous Period._--Volcanic rocks occur amongst the coal-measures
of England and Scotland, while they are also found interbedded with the
Carboniferous Limestone series in Derbyshire, Scotland, and Co. Limerick
in Ireland. The rocks consist chiefly of basalt, dolerite, melaphyre and
felstone.
_Devonian Period._--Volcanic rocks of Devonian age occur in the South of
Scotland, consisting of felstone-porphyries and melaphyres; also at
Boyle, in Roscommon, and amongst the Glengariff beds near Killarney in
Ireland.
_Upper Silurian Period._--Volcanic rocks of this stage are only known in
Ireland, on the borders of Cos. Mayo and Galway, west of Lough Mask, and
at the extreme headland of the Dingle Promontory in Co. Kerry. They
consist of porphyrites, felstones and tuffs, or breccias,
contemporaneously erupted during the Wenlock and Ludlow stages. Around
the flanks of Muilrea, beds of purple quartz-felstone with tuff are
interstratified with the Upper Silurian grits and slates.
_Lower Silurian Period._--Volcanic action was developed on a grand scale
during the Arenig and Caradoc-Bala stages, both in Wales and the Lake
district, and in the Llandeilo stage in the South of Scotland. The
felspathic lavas, with their associated beds of tuff and breccia, rise
into some of the grandest mountain crests of North Wales, such as those
of Cader Idris, Aran Mowddwy, Arenig and Moel Wyn. A similar series is
also represented in Ireland, ranging from Wicklow to Waterford, forming
a double group of felstones, porphyries, breccias, and ash-beds, with
dykes of basalt and dolerite. The same series again appears amidst the
Lower Silurian beds of Co. Louth, near Drogheda.
_Metamorphic Series presumably of Lower Silurian Age._--If, as seems
highly probable, the great metamorphic series of Donegal and Derry are
the representatives in time of the Lower Silurian series, some of the
great sheets of felspathic and hornblendic trap which they contain are
referable to this epoch. These rocks have undergone a change in
structure along with the sedimentary strata of which they were
originally formed, so that the sheets of (presumably) augitic lava have
been converted into hornblende-rock and schist. Similar masses occur in
North Mayo, south of Belderg Harbour.
_Cambrian Period._--In the Pass of Llanberis, along the banks of Llyn
Padarn, masses of quartz-porphyry, felsite and agglomerate, or breccia,
indicate volcanic action during this stage. These rocks underlie beds of
conglomerate, slate and grit of the Lower Cambrian epoch, and, as Mr.
Blake has shown, are clearly of volcanic origin, and pass upwards into
the sedimentary strata of the period. A similar group, first recognised
by Professor Sedgwick, stretches southwards from Bangor along the
southern shore of the Menai Straits. Again, we find the volcanic
eruptions of this epoch at St. David's, consisting of diabasic and
felsitic lava, with beds of ash; and in the centre of England, amongst
the grits and slates of Charnwood Forest presumably of Cambrian age,
various felstones, porphyries, and volcanic breccias are found.
Thus it will be seen that every epoch, from the earliest stage of the
Cambrian to the Permian, in the British Isles, gives evidence of the
existence of volcanic action; from which we may infer that the
originating cause, whatever it may be, has been in operation throughout
all past geological time represented by living forms. The question of
the condition of our globe in Archæan times, and earlier, is one which
only can be discussed on theoretic ground, and is beyond the scope of
this work.
[1] The reader is referred to Sir A. Geikie's Presidential Address to
the Geological Society (1891) for the latest view of this subject.
[Illustration: VOLCANIC BAND OF THE MOLUCCAS.
Map showing the volcanic belt to which Krakatoa belongs. The shaded
portion is volcanic.]
PART VI.
SPECIAL VOLCANIC AND SEISMIC PHENOMENA.
CHAPTER I.
THE ERUPTION OF KRAKATOA IN 1883.
I propose to introduce here some account of one of the most terrible
outbursts of volcanic action that have taken place in modern times;
namely, the eruption of the volcano of Krakatoa (a corruption of Rakata)
in the strait of Sunda, between the islands of Sumatra and Java, in the
year 1883. The Malay Archipelago, of which this island once formed a
member, is a region where volcanic action is constant, and where the
outbursts are exceptionally violent. With the great island of Borneo as
a solid, non-volcanic central core, a line of volcanic islands extends
from Chedooba off the coast of Pegu through Sumatra, Java, Sumbawa,
Flores, and, reaching the Moluccas, stretches northwards through the
Philippines into Japan and Kamtschatka. This is probably the most active
volcanic belt in the world, and the recent terrible earthquake and
eruption in Japan (November, 1891) gives proof that the volcanic forces
are as powerful and destructive as ever.[1]
(_a._) _Dormant Condition down to 1680._--Down to the year 1680, this
island, although from its form and structure evidently volcanic, appears
to have been in a dormant state; its sides were covered with luxuriant
forests, and numerous habitations dotted its shore. But in May of that
year an eruption occurred, owing to which the aspect of Krakatoa as
described by Vogel was entirely changed; the surface of the island when
this writer passed on his voyage to Sumatra appeared burnt up and arid,
while blocks of incandescent rock were being hurled into the air from
four distinct points. After this first recorded eruption the island
relapsed into a state of repose, and except for a stream of molten lava
which issued from the northern extremity, there was no evidence of its
dangerous condition. The luxuriant vegetation of the tropics speedily
re-established itself, and the volcano was generally regarded as
"extinct."[2] History repeats itself; and the history of Vesuvius was
repeated in the case of Krakatoa.
[Illustration: Fig. 35.--Map Of The Krakatoa Group Of Islands Before
The Eruption Of August 1883 (From Admiralty Chart)]
(_b._) _Eruption of May, 1883._[3]--On the morning of May 20, 1883, the
inhabitants of Batavia, of Buitenzorg, and neighbouring localities, were
surprised by a confused noise, mingled with detonations resembling the
firing of artillery. The phenomena commenced between ten and eleven
o'clock in the morning, and soon acquired such intensity as to cause
general alarm. The detonations were accompanied by tremblings of the
ground, of buildings and various objects contained in dwellings; but it
was generally admitted that these did not proceed from earthquake
shocks, but from atmospheric vibrations. No deviation of the magnetic
needle was observed at the Meteorological Institute of Batavia; but a
vertical oscillation was apparent, and persons who listened with the ear
placed on the ground, even during the most violent detonations, could
hear no subterranean noise whatever. It became clear that the sounds
came from some volcano burst into activity; but it is strange that for
two whole days it remained uncertain what was the particular volcano to
which the phenomena were to be referred. The detonations appeared,
indeed, to come from the direction of Krakatoa; but from Serang, Anjer,
and Merak, localities situated much nearer Krakatoa than Batavia, the
telegraph announced that neither detonations nor atmospheric vibrations
had been perceived. The distance between Batavia and Krakatoa is
ninety-three English miles. The doubts thus experienced were, however,
soon put to rest by the arrival of an American vessel under the command
of A. R. Thomas, and of other ships which hailed from the straits of
Sunda. From their accounts it was ascertained that in the direction of
Krakatoa the heavens were clouded with ashes, and that a grand column of
smoke, illumined from time to time by flashes of flame, arose from above
the island. Thus after a repose of more than two hundred years, "the
peaceable isle of Krakatoa, inhabited, and covered by thick forests, was
suddenly awakened from its condition of fancied security."
[Illustration: Fig. 36.--Section from Verlaten Island through Krakatoa,
to show the outline before and after the eruption of August, 1888. The
continuous line shows the former; the dotted line and shading, the
latter; from which it will be observed that the original island has to a
large extent disappeared. The line of section is shown in Fig. 35.]
(_c._) _Form and Appearance of the Island before the Eruption of
1883._--From surveys made in 1849 and 1881, it would appear that the
island of Krakatoa consisted of three mountains or groups of mountains
(Figs. 35, 36); the southern formed by the cone of Rakata (properly so
called), rising with a scarped face above the sea to a height of over
800 mètres (2,622 feet). Adjoining this cone, and rising from the centre
of the island, came the group of Danan, composed of many summits,
probably forming part of the _enceinte annulaire_ of a crater. And near
the northern extremity of the isle, a third group of mammelated heights
could be recognised under the general name of Perboewatan, from which
issued several obsidian lava-flows, with a steep slope; these dated back
perhaps to the period of the first known eruption of 1680. This large
and mountainous island as it existed at the beginning of May, 1883, has
been entirely destroyed by the terrible eruptions of that year, with the
exception of the peripheric rim (composed of the most ancient of the
volcanic rocks, andesite), of which Verlaten Island and Rakata formed a
part, and one very small islet, which is noted on the maps as "rots"
(rock), and on the new map of the Straits of Sunda of the Dutch Navy as
that of "Bootsmansrots."[4]
As shown by the map in the Report of the Royal Society, the group of
islands which existed previous to 1883 were but the unsubmerged portions
of one vast volcanic crater, built up of a remarkable variety of lava
allied to the andesite of the Java volcanoes, but having a larger
percentage of silica, and hence falling under the head of
"enstatite-dacite."[5] That these volcanic rocks are of very recent
origin is shown by the fact, ascertained by Verbeek, that beneath them
occur deposits of Post-Tertiary age, and that these in turn rest on the
Tertiary strata which are widely distributed through Sumatra, Java, and
the adjoining islands. According to the reasoning of Professor Judd, the
Krakatoa group at an early period of its history presented the form of a
magnificent crater-cone, several miles in circumference at the base,
which subsequent eruptions shattered into fragments or blew into the
air in the form of dust, ashes, and blocks of lava, while the central
part collapsed and fell in, leaving a vast circular ring like the
ancient crater of Somma (see Fig. 6, p. 43), and he supposes the former
eruptions to have been on a scale exceeding in magnificence those which
have caused such world-wide interest within the last few years.
(_d._) _Eruption of 26th to 28th of August._--It was, as we have seen,
in the month of May that, in the language of Chev. Verbeek, "the volcano
of Krakatoa chose to announce in a high voice to the inhabitants of the
Archipelago that, although almost nothing amongst the many colossal
volcanic mountains of the Indies, it yielded to none of them in regard
to its power." These eruptions were, however, only premonitory of the
tremendous and terrible explosion which was to commence on Sunday, the
26th of August, and which continued for several days subsequently. A
little after noon of that day, a rumbling noise accompanied by short and
feeble explosions was heard at Buitenzorg, coming from the direction of
Krakatoa; and similar sounds were heard at Anjer and Batavia a little
later. Soon these detonations augmented in intensity, especially about
five o'clock in the evening; and news was afterwards received that the
sounds had been heard in the isle of Java. These sounds increased still
more during the night, so that few persons living on the west side of
the isle of Java were able to sleep. At seven in the morning there came
a crash so formidable, that those who had hoped for a little sleep at
Buitenzorg leaped from their beds. Meanwhile the sky, which had up to
this time been clear, became overcast, so that by ten o'clock it became
necessary to have recourse to lamps, and the air became charged with
vapour. Occasional shocks of earthquake were now felt. Darkness became
general all over the straits and the bordering coasts. Showers of ashes
began to fall. The repeated shocks of earthquake, and the rapid
discharges of subterranean artillery, all combined to show that an
eruption of even greater violence than that of May was in progress at
the isle of Krakatoa.
But the most interested witnesses to this terrible outburst were those
on board the ships plying through the straits. Amongst these was the
_Charles Bal_, a British vessel under the command of Captain Watson.
This ship was ten miles south of the volcano on Sunday afternoon, and
therefore well in sight of the island at the time when the volcano had
entered upon its paroxysmal state of action. Captain Watson describes
the island as being covered by a dense black cloud, while sounds like
the discharges of artillery occurred at intervals of a second of time;
and a crackling noise (probably arising from the impact of fragments of
rock ascending and descending in the atmosphere) was heard by those on
board. These appearances became so threatening towards five o'clock in
the evening, that the commander feared to continue his voyage and began
to shorten sail. From five to six o'clock a rain of pumice in large
pieces, quite warm, fell upon the ship, which was one of those that
escaped destruction during this terrible night.[6]
(_e._) _Electrical Phenomena._--During this eruption, electrical
phenomena of great splendour were observed. Captain Wooldbridge,
viewing the eruption in the afternoon of the 26th from a distance of
forty miles, speaks of a great vapour-cloud looking like an immense wall
being momentarily lighted up "by bursts of forked lightning like large
serpents rushing through the air. After sunset this dark wall resembled
a blood-red curtain, with edges of all shades of yellow, the whole of a
murky tinge, through which gleamed fierce flashes of lightning." As
Professor Judd observes, the abundant generation of atmospheric
electricity is a familiar phenomenon in all volcanic eruptions on a
grand scale. The steam-jets rushing through the orifices of the earth's
crust constitute an enormous hydro-electrical engine, and the friction
of the ejected materials striking against one another in their ascent
and descent also does much in the way of generating electricity.[7] It
has been estimated by several observers that the column of watery vapour
ascended to a height of from twelve to seventeen and even twenty-three
miles; and on reaching the upper strata of the atmosphere, it spread
itself out in a vast canopy resembling "the pine-tree" form of Vesuvian
eruptions; and throughout the long night of the 27th this canopy
continued to extend laterally, and the particles of dust which it
enclosed began to descend slowly through the air.
(_f._) _Formation of Waves._--This tremendous outburst of volcanic
forces, which to a greater or less extent influenced the entire surface
of the globe, gave rise to waves which traversed both air and ocean; and
in consequence of the large number of observatories scattered all over
the globe, and the excellence and delicacy of the instruments of
observation, we are put in possession of the remarkable results which
have been obtained from the collection of the observations in the hands
of competent specialists. The results are related _in extenso_ in the
Report of the Royal Society, illustrated by maps and diagrams, and are
worthy of careful study by those interested in terrestrial phenomena. A
brief summary is all that can be given here, but it will probably
suffice to bring home to the reader the magnitude and grandeur of the
eruption.
The vibrations or waves generated in August, 1883, at Krakatoa may be
arranged under three heads: (1) Atmospheric Waves; (2) Sound Waves; and
(3) Oceanic Waves; which I will touch upon in the order here stated.
(1) _Atmospheric Waves._--These phenomena have been ably handled by
General Strachey,[8] from a large number of observations extending all
over the globe. From these it has been clearly established that an
atmospheric wave, originating at Krakatoa as a centre, expanded outwards
in a circular form and travelled onwards till it became a great circle
at a distance of 180 degrees from its point of origin, after which it
still advanced, but now gradually contracting to a node at the antipodes
of Krakatoa; that is to say, at a point over the surface of North
America, situated in lat. 6° N. and long. 72° W. (or thereabout). Having
attained this position, the wave was reflected or reproduced, expanding
outwards for 180 degrees and travelling backwards again to Krakatoa,
from which it again started, and returning to its original form again
overspread the globe. This wonderful repetition, due to the spherical
form of the earth, was observed no fewer than seven times, though with
such diminished force as ultimately to be outside the range of
observation by the most sensitive instruments. It is one of the triumphs
of modern scientific appliances that the course of such a wave,
generated in a fluid surrounding a globe, which might be demonstrated on
mathematical principles, has been actually determined by experiments
carried on over so great an area.
(2) _Sound Waves._--If the sound-waves produced at the time of maximum
eruption were not quite as far-reaching as those of the air, they were
certainly sufficiently surprising to be almost incredible, were it not
that they rest, both as regards time and character, upon incontestible
authority. The sound of the eruption, resembling that of the discharge
of artillery, was heard not only over nearly all parts of Sumatra, Java,
and the coast of Borneo opposite the Straits of Sunda, but at places
over two thousand miles distant from the scene of the explosions.
Detailed accounts, collected with great care, are given in the Report of
the Royal Society, from which the following are selected as examples:--
1. At the port of Acheen, at the northern extremity of Sumatra,
distant 1,073 miles, it was supposed that the port was being
attacked, and the troops were put under arms.
2. At Singapore, distant 522 miles, two steamers were dispatched to
look out for the vessel which was supposed to be firing guns as
signals of distress.
3. At Bankok, in Siam, distant 1,413 miles, the report was heard on
the 27th; as also at Labuan, in Borneo, distant 1,037 miles.
4. At places in the Philippine Islands, distant about 1,450 miles,
detonations were heard on the 27th, at the time of the eruption.
The above places lie northwards of Krakatoa. In the opposite direction,
we have the following examples:--
5. At Perth, in Western Australia, distant 1,092 miles, sounds as
of guns firing at sea were heard; and at the Victorian Plains,
distant about 1,700 miles, similar sounds were heard.
6. In South Australia, at Alice's Springs, Undoolga, and other
places at distances of over 2,000 miles, the sounds of the eruption
were also heard.
7. In a westerly direction at Dutch Bay, Ceylon, distant 2,058
miles, the sounds were heard between 7 a.m. and 10 a.m. on the
morning of the 27th of August.
8. Lastly, at the Chagos Islands, distant 2,267 miles, the
detonations were audible between 10 and 11 a.m. of the same day.
Some of the above distances are so great that we may fail to realise
them; but they will be more easily appreciated, perhaps, if we change
the localities to our own side of the globe, and take two or three cases
with similar distances. Then, if the eruption had taken place amongst
the volcanoes of the Canaries, the detonations would have been heard at
Gibraltar, at Lisbon, at Portsmouth, Southampton, Cork, and probably at
Dublin and Liverpool; or, again, supposing the eruption had taken place
on the coast of Iceland, the report would have been heard all over the
western and northern coasts of the British Isles, as well as at
Amsterdam and the Hague. The enormous distance to which the sound
travelled in the case of Krakatoa was greatly due to the fact that the
explosions took place at the surface of the sea, and the sound was
carried along that surface uninterruptedly to the localities recorded; a
range of mountains intervening would have cut off the sound-wave at a
comparatively short distance from its source.
(3) _Oceanic Waves._--As may be supposed, the eruption gave rise to
great agitation of the ocean waters with various degrees of vertical
oscillation; but according to the conclusions of Captain Wharton,
founded on numerous data, the greatest wave seems to have originated at
Krakatoa about 10 a.m. on the 27th of August, rising on the coasts of
the Straits of Sunda to a height of fifty feet above the ordinary
sea-level. This wave appears to have been observed over at least half
the globe. It travelled westwards to the coast of Hindostan and Southern
Arabia, ultimately reaching the coasts of France and England. Eastwards
it struck the coast of Australia, New Zealand, the Sandwich Islands,
Alaska, and the western coast of North America; so that it was only the
continent of North and South America which formed a barrier (and that
not absolute) to the circulation of this oceanic wave all over the
globe. The destruction to life and property caused by this wave along
the coasts of Sunda was very great. Combined with the earthquake shocks
(which, however, were not very severe), the tremendous storm of wind,
the fall of ashes and cinders, and the changes in the sea-bed, it
produced in the Straits of Sunda for some time after the eruption a
disastrous transformation. Lighthouses had been swept away; all the old
familiar landmarks on the shore were obscured by a vast deposit of
volcanic dust; the sea itself was encumbered with enormous quantities of
floating pumice, in many places of such thickness that no vessel could
force its way through them; and for months after the eruption one of the
principal channels was greatly obstructed by two islands which had
arisen in its midst. The Sebesi channel was completely blocked by banks
composed of volcanic materials, and two portions of these banks rose
above the sea as islands, which received the name of "Steers Island" and
"Calmeyer Island"; but these, by the action of the waves, have since
been completely swept away, and the materials strewn over the bed of the
sea.[9]
(_g._) _Atmospheric Effects._--But the face of nature, even in her most
terrific and repulsive aspect, is seldom altogether unrelieved by some
traces of beauty. In contrast to the fearful and disastrous phenomena
just described, is to be placed the splendour of the heavens, witnessed
all over the central regions of the globe throughout a period of several
months after the eruption of 1883, which has been ably treated by the
Hon. Rollo Russell and Mr. C. D. Archibald, in the Royal Society's
Report.
When the particles of lava and ashes mingled with vapour were projected
into the air with a velocity greater than that of a ball discharged from
the largest Armstrong gun, these materials were carried by the prevalent
trade-winds in a westerly direction, and some of them fell on the deck
of ships sailing in the Indian Ocean as far as long. 80° E., as in the
case of the _British Empire_--on which the particles fell on the 29th of
August, at a distance of 1,600 miles from Krakatoa. But far beyond this
limit, the finer particles of dust (or rather minute crystals of felspar
and other minerals), mingled with vapour of water, were carried by the
higher currents of the air as far as the Seychelles and Africa,--not
only the East coast, but also the West, as Cape Coast Castle on the Gold
Coast; to Paramaribo, Trinidad, Panama, the Sandwich Isles, Ceylon and
British India, at all of which places during the month of September the
sun assumed tints of blue or green, as did also the moon just before and
after the appearance of the stars;[10] and from the latter end of
September and for several months, the sky was remarkable for its
magnificent coloration; passing from crimson through purple to yellow,
and melting away in azure tints which were visible in Europe and the
British Isles; while a large corona was observed round both the sun and
moon. These beautiful sky effects were objects of general observation
throughout the latter part of the year 1883 and commencement of the
following year.
The explanation of these phenomena may be briefly stated. The fine
particles, consisting for the most part of translucent crystals, or
fragments of crystals, formed a canopy high up in the atmosphere, being
gradually spread over both sides of the equator till it formed a broad
belt, through which the rays of the sun and moon were refracted. Towards
dawn and sunset they were refracted and reflected from the facets of the
crystal, and thus underwent decomposition into the prismatic colours; as
do the rays of the sun when refracted and reflected from the particles
of moisture in a rain-cloud. The subject is one which cannot be fully
dealt with here, and is rather outside the scope of this work.
(_h._) _Origin of the Eruption._--The ultimate cause of volcanic
eruptions is treated in a subsequent chapter, nor is that of Krakatoa
essentially different from others. It was remarkable, however, both for
the magnitude of the forces evoked and the stupendous scale of the
resulting phenomena. It is evident that water played an important part
in these phenomena, though not as the prime mover;--any more than water
in the boiler of a locomotive is the prime mover in the generation of
the steam. Without the fuel in the furnace the steam would not be
produced; and the amount of steam generated will be proportional to the
quantity and heat of the fuel in the furnace and the quantity of water
in the boiler. In the case of Krakatoa, both these elements were
enormous and inexhaustible. The volcanic chimney (or system of
chimneys), being situated on an island, was readily accessible to the
waters of the ocean when fissures gave them access to the interior
molten matter. That such fissures were opened we may well believe. The
earthquakes which occurred at the beginning of May, and later on, on the
27th of that month, may indicate movements of the crust by which water
gained access. It appears that in May the only crater in a state of
activity was that of Perboewatan; in June another crater came into
action, connected with Danan in the centre of the island, and in August
a third burst forth. Thus there was progressive activity up to the
commencement of the grand eruption of the 26th of that month.[11] During
this last paroxysmal stage, the centre of the island gave way and sunk
down, when the waters of the ocean gained free access, and meeting with
the columns of molten matter rising from below originated the prodigious
masses of steam which rose into the air.
(_i._) _Cause of the Detonations._--The detonations which accompanied
the last great eruption are repeatedly referred to in all the accounts.
These may have been due, not only to the sudden explosions of steam
directly produced by the ocean water coming in contact with the molten
lava, but by dissociation of the vapour of water at the critical point
of temperature into its elements of oxygen and hydrogen; the reunion of
these elements at the required temperature would also result in
explosions.
The phenomena attending this great volcanic eruption, so carefully
tabulated and critically examined, will henceforth be referred to as
constituting an epoch in the history of volcanic action over the globe,
and be of immense value for reference and comparison.
[1] The eruption of Krakatoa has been the subject of an elaborate Report
published by the Royal Society, and is also described in a work by
Chevalier R. D. M. Verbeek, Ingenieur en Chef des Mines, and published
by order of the Governor-General of the Netherland Indies (1886). See
also an Article by Sir R. S. Ball in the _Contemporary Review_ for
November, 1888.
[2] Verbeek, _loc. cit._, p. 4.
[3] The account of this eruption is a free translation from Verbeek.
[4] Verbeek, _loc. cit._, p. 160.
[5] Judd, _Rep. R. S._
[6] A fuller account by Prof. Judd will be found in the _Report of the
Royal Society_, p. 14. Several vessels at anchor were driven ashore on
the straits owing to the strong wind which arose.
[7] Judd, _Report_, p. 21.
[8] _Report_, Part ii.
[9] In this eruption, 36,380 human beings perished, of whom 37 were
Europeans; 163 villages (_kampoengs_) were entirely, and 132 partially,
destroyed.--Verbeek, _loc. cit._, p. 79.
[10] Verbeek, _loc. cit._, p. 144-5. The dust put a girdle round the
earth in thirteen days.
[11] Verbeek, _loc. cit._, p. 30.
CHAPTER II.
EARTHQUAKES.
_Connection of Earthquakes with Volcanic Action._--The connection
between earthquake shocks and volcanic eruptions is now so generally
recognised that it is unnecessary to insist upon it here. All volcanic
districts over the globe are specially liable to vibrations of the
crust; but at the same time it is to be recollected that these movements
visit countries occasionally from which volcanoes, either recent or
extinct, are absent; in which cases we may consider earthquake shocks to
be abortive attempts to originate volcanic action.
(_a._) _Origin._--From the numerous observations which have been made
regarding the nature of these phenomena by Hopkins, Lyell, and others,
it seems clearly established that earthquakes have their origin in some
sudden impact of gas, steam, or molten matter impelled by gas or steam
under high pressure, beneath the solid crust.[1] How such impact
originates we need not stop to inquire, as the cause is closely
connected with that which produces volcanic eruptions. The effect,
however, of such impact is to originate a wave of translation through
the crust, travelling outwards from the point, or focus, on the surface
immediately over the point of impact.[2] These waves of translation can
in some cases be laid down on a map, and are called "isoseismal curves,"
each curve representing approximately an equal degree of seismal
intensity; as shown on the chart of a part of North America affected by
the great Charleston earthquake. (Fig. 37.) Mr. Hopkins has shown that
the earthquake-wave, when it passes through rocks differing in density
and elasticity, changes in some degree not only its velocity, but its
direction; being both refracted and reflected in a manner analogous to
that of light when it passes from one medium to another of different
density.[3] When a shock traverses the crust through a thickness of
several miles it will meet with various kinds of rock as well as with
fissures and plications of the strata, owing to which its course will be
more or less modified.
(_b._) _Formation of Fissures._--During earthquake movements, fissures
may be formed in the crust, and filled with gaseous or melted matter
which may not in all cases reach the surface; and, on the principle that
volcanoes are safety-valves for regions beyond their immediate
influence, we may infer that the earthquake shock, which generally
precedes the outburst of a volcano long dormant, finds relief by the
eruption which follows; so that whatever may be the extent of the
disastrous results of such an eruption, they would be still more
disastrous if there had been no such safety-valve as that afforded by a
volcanic vent. Thus, probably, owing to the extinction of volcanic
activity in Syria, the earthquakes in that region have been peculiarly
destructive. For example, on January 1, 1837, the town of Safed west of
the Jordan valley was completely destroyed by an earthquake in which
most of the inhabitants perished. The great earthquakes of Aleppo in the
present century, and of Syria in the middle of the eighteenth, were of
exceptional severity. In that of Syria, which took place in 1759, and
which was protracted during a period of three months, an area of 10,000
square leagues was affected. Accon, Saphat, Baalbeck, Damascus, Sidon,
Tripoli, and other places were almost entirely levelled to the ground;
many thousands of human beings lost their lives.[4] Other examples might
be cited.
(_c._) _Earthquake Waves._--We have now to return to the phenomena
connected with the transmission of earthquake-waves. The velocity of
transmission through the earth is very great, and several attempts have
been made to measure this velocity with accuracy. The most valuable of
such attempts are those connected with the Charleston and Riviera
shocks. Fortunately, owing to the perfection of modern appliances, and
the number of observers all over the globe, these results are entitled
to great confidence. The phenomena connected with the Charleston
earthquake, which took place on the 31st of August, 1886, are described
in great detail by Captain Clarence E. Dutton, of the U.S. Ordnance
Corps.[5] The conclusions arrived at are;--that as regards the depth of
the focal point, this is estimated at twelve miles, with a probable
error of less than two miles; while, as regards the rate of travel of
the earthquake-wave, the estimate is (in one case) about 3.236 miles per
second; and in another about 3.226 miles per second.
On the other hand, in the case of the earthquake of the Riviera, which
took place on the 23rd of February, 1887, at 5.30 a.m. (local time), the
vibrations of which appear to have extended across the Atlantic, and to
have sensibly affected the seismograph in the Government Signal Office
at Washington, the rate of travel was calculated at about 500 miles per
hour, less than one-half that determined in the case of Charleston; but
Captain Dutton claims, and probably with justice, that the results
obtained in the latter case are far more reliable than any hitherto
arrived at.
(_d._) _Oceanic Waves._--When the originating impact takes place under
the bed of the ocean--either by a sudden up-thrust of the crust to the
extent, let us suppose, of two or three feet, or by an explosion from a
submarine volcano--a double wave is formed, one travelling through the
crust, the other through the ocean; and as the rate of velocity of the
former is greatly in excess of that of the latter, the results on their
reaching the land are often disastrous in the extreme. It is the
ocean-wave, however, which is the more important, and calls for special
consideration. If the impact takes place in very deep water, the whole
mass of the water is raised in the form of a low dome, sloping equally
away in all directions; and it commences to travel outwards as a wave
with an advancing crest until it approaches the coast and enters shallow
water. The wave then increases in height, and the water in front is
drawn in and relatively lowered; so that on reaching a coast with a
shelving shore the form of the surface consists of a trough in front
followed by an advancing crest. These effects may be observed on a small
scale in the case of a steamship advancing up a river, or into a harbour
with a narrow channel, but are inappreciable in deep water, or along a
precipitous open coast.
(_e._) _The Earthquake of Lisbon, 1755._--The disastrous results of a
submarine earthquake upon the coast have never been more terribly
illustrated than in the case of the earthquake of Lisbon which took
place on November 1, 1755. The inhabitants had no warning of the coming
danger, when a sound like that of thunder was heard underground, and
immediately afterwards a violent shock threw down the greater part of
their city; this was the land-wave. In the course of about six minutes,
sixty thousand persons perished. The sea first retired and left the
harbour dry, so forming the trough in front of the crest; immediately
after the water rolled in with a lofty crest, some 50 feet above the
ordinary level, flooding the harbour and portions of the city bordering
the shore. The mountains of Arrabida, Estrella, Julio, Marvan, and
Cintra, were impetuously shaken, as it were, from their very
foundations; and according to the computation of Humboldt, a portion of
the earth's surface four times the extent of Europe felt the effects of
this great seismic shock, which extended to the Alps, the shores of the
Baltic, the lakes of Scotland, the great lakes of North America, and
the West Indian Islands. The velocity of the sea-wave was estimated at
about 20 miles per minute.
(_f._) _Earthquake of Lima and Callao, 28th October, 1746._--Of somewhat
similar character was the terrible catastrophe with which the cities of
Lima and Callao were visited in the middle of the last century,[6] in
which the former city, then one of great magnificence, was overthrown;
and Callao was inundated by a sea-wave, in which out of 23 ships of all
sizes in the harbour the greater number foundered; several, including a
man-of-war, were lifted bodily and stranded, and all the inhabitants
with the exception of about two hundred were drowned. A volcano in
Lucanas burst forth the same night, and such quantities of water
descended from the cone that the whole country was overflowed; and in
the mountain near Pataz, called Conversiones de Caxamarquilla, three
other volcanoes burst forth, and torrents of water swept down their
sides. In the case of these cities, the land-wave, or shock, preceded
the sea-wave, which of course only reached the port of Callao.
[Illustration: Fig. 37.--The lines represent isoseismal curves, or
curves of equal intensity, the force decreasing outwards from the focus
at Charleston, No. 10.]
(_g._) _Earthquake of Charleston, 31st August, 1886._--I shall close
this account of some remarkable earthquakes with a few facts regarding
that of Charleston, on the Atlantic seaboard of Carolina.[7] At 9.51
a.m. of this day, the inhabitants engaged in their ordinary occupations
were startled by the sound of a distant roar, which speedily deepened in
volume so as to resemble the noise of cannon rattling along the road,
"spreading into an awful noise, that seemed to pervade at once the
troubled earth below and the still air above." At the same time the
floors began to heave underfoot, the walls visibly swayed to and fro,
and the crash of falling masonry was heard on all sides, while
universal terror took possession of the populace, who rushed into the
streets, the black portion of the community being the most demonstrative
of their terror. Such was the commencement of the earthquake, by which
nearly all the houses of Charleston were damaged or destroyed, many of
the public buildings seriously injured or partially demolished. The
effects were felt all over the States as far as the great lakes of
Canada and the borders of the Rocky Mountains. Two epicentral _foci_
appear to have been established; one lying about 15 miles to the N.W. of
Charleston, called the _Woodstock focus_; the other about 14 miles due
west of Charleston, called the _Rantowles focus_; around each of these
_foci_ the isoseismic curves concentrated,[8] but in the map (Fig. 37)
are combined into the area of one curve. The position of these _foci_
clearly shows that the origin of the Charleston earthquake was not
submarine, though occurring within a short distance of the Atlantic
border; the curves of equal intensity (isoseismals) are drawn all over
the area influenced by the shock.
As a general result of these detailed observations, Captain Dutton
states that there is a remarkable coincidence in the phenomena with
those indicated by the theory of wave-motion as the proper one for an
elastic, nearly homogeneous, solid medium, composed of such materials as
we know to constitute the rocks of the outer portions of the earth; but
on the other hand he states that nothing has been disclosed which seems
to bring us any nearer to the precise nature of the forces which
generated the disturbance.[9]
[1] The views of Mr. R. Mallet, briefly stated, are somewhat as
follows:--Owing to the secular cooling of the earth, and the consequent
lateral crushing of the surface, this crushing from time to time
overcomes the resistance; in which case shocks are experienced along the
lines of fracture and faulting by which the crust is intersected. These
shocks give rise to earthquake waves, and as the crushing of the walls
of the fissure developes heat, we have here the _vera causa_ both of
volcanic eruptions and earthquake shocks--the former intensified into
explosions by access of water through the fissures.--"On the Dynamics of
Earthquakes," _Trans. Roy. Irish Acad._, vol. xxi.
[2] Illustration of the mode of propagation of earthquake shocks will be
found in Lyell's _Principles of Geology_, vol. ii. p. 136, or in the
author's _Physiography_, p. 76, after Hopkins.
[3] "Rep. on Theories of Elevation and Earthquakes," _Brit. Ass. Rep._
1847, p. 33. In the map prepared by Prof. J. Milne and Mr. W. K. Burton
to show the range of the great earthquake of Japan (1891), similar
isoseismal lines are laid down.
[4] Lyell, _loc. cit._, p. 163. Two Catalogues of Earthquakes have been
drawn up by Prof. O'Reilly, and are published in the _Trans. Roy. Irish
Academy_, vol. xxviii. (1884 and 1886).
[5] _Ninth Annual Report, U.S. Geological Survey_ (1888).
[6] _A True and Particular Account of the Dreadful Earthquake_, 2nd
edit. The original published at Lima by command of the Viceroy. London,
1748. Translated from the Spanish.
[7] I take the account from that of Capt. Dutton above cited, p. 220.
[8] Dutton, _Report_, Plate xxvi., p. 308.
[9] _Ibid._, p. 211. On the connection between the moon's position and
earthquake shocks, see Mr. Richardson's paper on Scottish earthquakes,
_Trans. Edin. Geol. Soc._, vol. vi. p. 194 (1892).
PART VII.
VOLCANIC AND SEISMIC PROBLEMS.
CHAPTER I.
THE ULTIMATE CAUSE OF VOLCANIC ACTION.
Volcanic phenomena are the outward manifestations of forces deep-seated
beneath the crust of the globe; and in seeking for the causes of such
phenomena we must be guided by observation of their nature and mode of
action. The universality of these phenomena all over the surface of our
globe, in past or present times, indicates the existence of a general
cause beneath the crust. It is true that there are to be found large
tracts from which volcanic rocks (except those of great geological
antiquity) are absent, such as Central Russia, the Nubian Desert, and
the Central States of North America; but such absence by no means
implies the non-existence of the forces which give rise to volcanic
action beneath those regions, but only that the forces have not been
sufficiently powerful to overcome the resistance offered by the crust
over those particular tracts. On the other hand, the similarity of
volcanic lavas over wide regions is strong evidence that they are drawn
from one continuous magma, consisting of molten matter beneath the
solid exterior crust.
(_a._) _Lines of Volcanic Action._--It has been shown in a previous page
that volcanic action of recent or Tertiary times has taken place mainly
along certain lines which may be traced on the surface of a map or
globe. One of these lines girdles the whole globe, while others lie in
certain directions more or less coincident with lines of flexure,
plication or faulting. The Isle of Sumatra offers a remarkable example
of the coincidence of such lines with those of volcanic vents. Not only
the great volcanic cones, but also the smaller ones, are disposed in
chains which run parallel to the longitudinal axis of the island
(N.W.-S.E.). The sedimentary rocks are bent and faulted in lines
parallel to the main axis, and also to the chains of volcanic mountains,
and the observation holds good with regard to different geological
periods.[1] Another remarkable case is that of the Jordan Valley.
Nowhere can the existence of a great fracture and vertical displacement
of the strata be more clearly determined than along this remarkable line
of depression; and it is one which is also coincident with a zone of
earthquake and volcanic disturbances.
(_b._) _Such Lines generally lie along the Borders of the Ocean._--But
even where, from some special cause, actual observation on the relations
of the strata are precluded, the general configuration of the ground and
the relations of the boundaries between land and sea to those of
volcanic chains, evidently point in many cases to their mutual
interdependence. The remarkable straightness of the coast of Western
America, and of the parallel chain of the Andes, affords presumptive
evidence that this line is coincident with a fracture or system of
faults, along which the continent has been bodily raised out of the
waters of the ocean. Of this elevation within very recent times we have
abundant evidence in the existence of raised coral-reefs and oceanic
shell-beds at intervals all along the coast; rising in Peru to a level
of no less than 3,000 feet above the ocean, as shown by Alexander
Agassiz.[2] Such elevations probably occurred at a time when the
volcanoes of the Andes were much more active than at present. Considered
as a whole, these great volcanic mountains may be regarded as in a
dormant, or partially moribund, condition; and if the volcanic forces
have to some extent lost their strength, so it would appear have those
of elevation.
(_c._) _Areas of Volcanic Action in the British Isles._--In the case of
the British Islands it may be observed that the later Tertiary volcanic
districts lie along very ancient depressions, which may indicate zones
of weakness in the crust. Thus the Antrim plateau, as originally
constituted, lay in the lap of a range of hills formed of crystalline,
or Lower Silurian, rocks; while the volcanic isles of the Inner Hebrides
were enclosed between the solid range of the Archæan rocks of the Outer
Hebrides on the one side, and the Silurian and Archæan ranges of the
mainland on the other. And if we go back to the Carboniferous period, we
find that the volcanic district of the centre of Scotland was bounded by
ranges of solid strata both to the north and south, where the resistance
to interior pressure from molten matter would have been greater than in
the Carboniferous hollow-ground, where such molten matter has been
abundantly extruded. In all these cases, the outflow of molten matter
was in a direction somewhat parallel to the plications of the strata.
(_d._) _Special Conditions under which the Volcanic Action
operates._--Assuming, then, that the molten matter, forming an interior
magma or shell, is constantly exerting pressure against the inner
surface of the solid crust, and can only escape where the crust is too
weak (owing to faults, plications, or fissures) to resist the pressure,
we have to inquire what are the special conditions under which outbursts
of volcanic matter take place, and what are the general results as
regards the nature of the _ejecta_ dependent on those conditions.
(_e._) _Effect of the Presence or Absence of Water._--The two chief
conditions determining the nature of volcanic products, considered in
the mass, are the presence or absence of water. Such presence or absence
does not of course affect the essential chemical composition of the
_ejecta_, but it materially influences the form in which the matter is
erupted. The agency of water in volcanic eruptions is a very interesting
and important subject in connection with the history of volcanic action,
and has been ably treated by Professor Prestwich.[3] At one time it was
considered that water was essential to volcanic activity; and the fact
that the great majority of volcanic cones are situated in the vicinity
of the oceanic waters, or of inland seas, was pointed to in confirmation
of this theory. But the existence in Western America and other volcanic
countries of fissures of eruption along which molten lava has been
extruded without explosions of steam, shows that water is not an
essential factor in the production of volcanic phenomena; and, as
Professor Prestwich has clearly demonstrated, it is to be regarded as an
element in volcanic explosions, rather than as a prime cause of volcanic
action. The main difficulty he shows to be thermo-dynamical; and
calculating the rate of increase in the elastic force of steam on
descending to greater and greater depths and reaching strata of higher
and higher temperatures, as compared with the force of capillarity, he
comes to the conclusion that water cannot penetrate to depths of more
than seven or eight miles, and therefore cannot reach the seat of the
eruptive forces. Professor Prestwich also points out that if the
extrusion of lava were due to the elastic force of vapour of water there
should be a distinct relation between the discharge of the lava and of
the vapour; whereas the result of an examination of a number of
well-recorded eruptions shows that the two operations are not related,
and are, in fact, perfectly independent. Sometimes there has been a
large discharge of lava, and little or no escape of steam; at other
times there have been paroxysmal explosive eruptions with little
discharge of lava. Even in the case of Vesuvius, which is close to the
sea, there have been instances when the lava has welled out almost with
the tranquillity of a water-spring.
(_f._) _Access of Surface Water to Molten Lava during Eruptions._--The
existence of water during certain stages in eruptions is too frequent a
phenomena to be lost sight of; but its presence may be accounted for in
other ways, besides proximity to the sea or ocean. Certain volcanic
mountains, such as Etna and Vesuvius, are built upon water-bearing
strata, receiving their supplies from the rainfall of the surrounding
country, or perhaps partly from the sea. In addition to this the ashes
and scoriæ of the mountain sides are highly porous, and rain or snow can
penetrate and settle downwards around the pipe or throat through which
molten lava wells up from beneath. In such cases it is easy to
understand how, at the commencement of a period of activity, molten lava
ascending through one or more fissures, and meeting with water-charged
strata or scoriæ, will convert the water into steam at high pressure,
resulting in explosions more or less violent and prolonged, in
proportion to the quantity of water and the depth to which it has
penetrated. In this manner we may suppose that ashes, scoriæ, and blocks
of rock torn from the sides of the crater-throat, and hurled into the
air, are piled around the vent, and accumulate into hills or mountains
of conical form. After the explosion has exhausted itself, the molten
lava quietly wells up and fills the crater, as in the cases of those of
Auvergne and Syria, and other places. We may, therefore, adopt the
general principle that in volcanic eruptions _where water in large
quantities is present, we shall have crater-cones built up of ashes,
scoriæ, and pumice; but where absent, the lava will be extravasated in
sheets to greater or less distances without the formation of such cones;
or if cones are fanned, they will be composed of solidified lava only,
easily distinguishable from crater-cones of the first class_.
(_g._) _Nature of the Interior Reservoir from which Lavas are
derived._--We have now to consider the nature of the interior reservoir
from which lavas are derived, and the physical conditions necessary for
their eruption at the surface.
Without going back to the question of the original condition of our
globe, we may safely hold the view that at a very early period of
geological history it consisted of a solidified crust at a high
temperature, enfolding a globe of molten matter at a still higher
temperature. As time went on, and the heat radiated into space from the
surface of the globe, while at the same time slowly ascending from the
interior by conduction, the crust necessarily contracted, and pressing
more and more on the interior molten magma, this latter was forced from
time to time to break through the contracting crust along zones of
weakness or fissures.
(_h._) _The Earth's Crust in a State of both Exterior Thrust and of
Interior Tension._--As has been shown by Hopkins,[4] and more recently
by Mr. Davison,[5] an exterior crust in such a condition must eventually
result in being under a state of horizontal thrust towards the exterior
and of tension towards the interior surface. For the exterior portion,
having cooled down, and consequently contracted to its normal state,
will remain rigid up to a certain point of resistance; but the interior
portion still continuing to contract, owing to the conduction of the
heat towards the exterior, would tend to enter upon a condition of
tension, as becoming too small for the interior molten magma; and such a
state of tension would tend to produce rupture of the interior part. In
this manner fissures would be formed into which the molten matter would
enter; and if the fissures happened to extend to the surface, owing to
weakness of the crust or flexuring of the strata, or other cause, the
molten matter would be extruded either in the form of dykes or volcanic
vents. In this way we may account for the numerous dykes of trap by
which some volcanic districts are intersected, such as those of the
north of Ireland and centre of Scotland.
From the above considerations, it follows that the earth's crust must be
in a condition both of pressure (or lateral thrust) towards the exterior
portion, and of tension towards the interior, the former condition
resulting in faulting and flexuring of the rocks, the latter in the
formation of open fissures, through which lava can ascend under high
pressure. These operations are of course the attempt of the natural
forces to arrive at a condition of equilibrium, which is never attained
because the processes are never completed; in other words, radiation and
convection of heat are constantly proceeding, giving rise to new forces
of thrust and tension.
It now remains for us to consider what may be the condition of the
interior molten magma; and in doing so we must be guided to a large
extent by considerations regarding the nature of the extruded matter at
the surface.
(_i._) _Relative Densities of Lavas._--Now, observation shows that, as
bearing on the subject under consideration, lavas occur mainly under two
classes as regards their density. The most dense (or basic) are those in
which silica is deficient, but iron is abundant; the least dense (or
acid) are those which are rich in silica, but in which iron occurs in
small quantity. This division corresponds with that proposed by Bunsen
and Durocher[6] for volcanic rocks, upon the results of analyses of a
large number of specimens from various districts. Rocks may be thus
arranged in groups:
(1) _The Basic_ (Heavier)--poor in silica, rich in iron; containing
silica 45-58 per cent. Examples: Basalt, Dolerite,
Hornblende rock, Diorite, Diabase, Gabbro,
Melaphyre, and Leucite lava.
(2) _The Acid_ (Lighter)--rich in silica, poor in iron; containing
silica 62-78 per cent. Examples: Trachyte, Rhyolite,
Obsidian, Domite, Felsite, Quartz-porphyry, Granite.
The Andesite group forms a connecting link between the highly acid and
the basic groups, and there are many varieties of the above which it is
not necessary to enumerate. Durocher supposes that the molten magmas of
these various rocks are arranged in concentric shells within the solid
crust in order of their respective densities, those of the lighter
density, namely the acid magmas, being outside those of greater density,
namely the basic; and this is a view which seems not improbable from a
consideration not only of the principle itself, but of the succession of
the varieties of lava in many districts. Thus we find that acid lavas
have been generally extruded first, and basic afterwards--as in the
cases of Western America, of Antrim, the Rhine and Central France. And
if the interior of our globe had been in a condition of equilibrium from
the time of the consolidation of the crust to the present, reason would
induce us to conclude that the lavas would ultimately have arranged
themselves in accordance with the conditions of density beneath that
crust. But the state of equilibrium has been constantly disturbed. Every
fresh outburst of volcanic force, and every fresh extrusion of lava,
tends to disturb it, and to alter the relations of the interior viscous
or molten magmas. Owing to this it happens, as we may suppose, that the
order of eruption according to density is sometimes broken, and we find
such rocks as granophyre (a variety of andesite) breaking through the
plateau-basalts of Mull and Skye, as explained in a former chapter.
Notwithstanding such variations, however, the view of Durocher may be
considered as the most reasonable we can arrive at on a subject which is
confessedly highly conjectural.
(_j._) _Conclusion as regards the Ultimate Cause of Volcanic
Action_.--Notwithstanding, however, the complexity of the subject, and
the uncertainties which must attend an inquiry where some of the data
are outside the range of our observation, sufficient evidence can be
adduced to enable us to arrive at a tolerably clear view of the ultimate
cause of volcanic action. So tempting a subject was sure to evoke
numerous essays, some of great ingenuity, such as that of Mr. Mallet;
others of great complexity, such as that of Dr. Daubeny. But more recent
consideration and wider observation have tended to lead us to the
conclusion that the ultimate cause is the most simple, the most
powerful, and the most general which can be suggested; namely, _the
contraction of the crust due to secular cooling of the more deeply
seated parts by conduction and radiation of heat into space_. Owing to
this cause, the enclosed molten matter is more or less abundantly
extruded from time to time along the lines and vents of eruption, so as
to accommodate itself to the ever-contracting crust. Nor can we doubt
that this process has been going on from the very earliest period of the
earth's history, and formerly at a greater rate than at present. When
the crust was more highly heated, the radiation and conduction must have
been proportionately more rapid. Owing to this cause also the
contraction of the crust was accelerated. To such irresistible force we
owe the wonderful flexuring, folding, and horizontal overthrusting which
the rocks have undergone in some portions of the globe--such as in the
Alps, the Highlands of Scotland and of Ireland, and the Alleghannies of
America. It is easy to show that the acceleration of the earth's
rotation must be a consequence of such contraction; but, after all, this
is but one of those compensatory forces of which we see several examples
in the world around us. It can also be confidently inferred that at an
early period of the earth's history, when the moon was nearer to our
planet than at present, the tides were far more powerful, and their
effect in retarding the earth's rotation was consequently greater.
During this period the acceleration due to contraction was also greater;
and the two forces probably very nearly balanced each other. Both these
forces (those of acceleration and retardation) have been growing weaker
down to the present day, though there appears to have been a slight
advantage on the side of the retarding force.[7]
[1] R. D. M. Verbeek, _Krakatau_, p. 105 (1886); also, J. Milne, _The
Great Earthquake of Japan_, 1891.
[2] _Bull. Mus. Comp. Zool._, vol. iii.
[3] _Proc. Roy. Soc._, No. 237 (1885); also, _Rep. Brit. Assoc._ (1881).
[4] Hopkins, _supra cit._, p. 218.
[5] C. Davison and G. H. Darwin, _Phil. Trans._, vol. 178, p; 241.
[6] Durocher, _Ann. des Mines_, vol. ii. (1857).
[7] See on this subject the author's _Textbook of Physiography_ (Deacon
and Co., 1888), pp. 56 and 122.
CHAPTER II.
LUNAR VOLCANOES.
The surface of the moon presented to our view affords such remarkable
indications of volcanic phenomena of a special kind, that we are
justified in devoting a chapter to their consideration. It is very
tantalising that our beautiful satellite only permits us to look at and
admire one half of her sphere; but it is not a very far-fetched
inference if we feel satisfied that the other half bears a general
resemblance to that which is presented to the earth. It is scarcely
necessary to inform the reader why it is that we never see but one face;
still, for the sake of those who have not thought out the subject I may
state that it is because the moon rotates on her axis exactly in the
time that she performs a revolution round the earth. If this should not
be sufficiently clear, let the reader perform a very simple experiment
for himself, which will probably bring conviction to his mind that the
explanation here given is correct. Let him place an orange in the centre
of a round table, and then let him move round the table from a
starting-point sideways, ever keeping his face directed towards the
orange; and when he has reached his starting-point, he will find that he
has rotated once round while he has performed one revolution round the
table. In this case the performer represents the moon and the orange
the earth.
Now this connection between the earth and her satellite is sufficiently
close to be used as an argument (if not as actual demonstration) that
the earth and the moon were originally portions of the same mass, and
that during some very early stage in the development of the solar system
these bodies parted company, to assume for ever after the relations of
planet and satellite. At the epoch referred to, we may also suppose that
these two masses of matter were in a highly incandescent, if not even
gaseous, state; and we conclude, therefore, that having been once
portions of the same mass, they are composed of similar materials. This
conclusion is of great importance in enabling us to reason from analogy
regarding the origin of the physical features on the moon's surface, and
for the purpose of comparison with those which we find on the surface of
our globe; because it is evident that, if the composition of the moon
were essentially different from that of our earth, we should have no
basis whatever for a comparison of their physical features.
When the moon started on her career of revolution round the earth, we
may well suppose that her orbit was much smaller than at present. She
was influenced by counteracting forces, those of gravitation drawing her
towards the centre of gravity of the earth,[1] and the centrifugal
force, which in the first instance was the stronger, so that her orbit
for a lengthened period gradually increased until the two forces, those
of attraction and repulsion, came into a condition of equilibrium, and
she now performs her revolution round the earth at a mean distance of
240,000 miles, in an orbit which is only very slightly elliptical.[2]
How the period of the moon's rotation is regulated by the earth's
attraction on her molten lava-sheets, first at the surface, and now
probably below the outer crust, has been graphically shown by Sir Robert
Ball,[3] but it cannot be doubted that once the moon was appreciably
nearer to our globe than at present. The attraction of her mass produced
tides in the ocean of correspondingly greater magnitude, and capable of
effecting results, both in eroding the surface and in transporting
masses of rock, far beyond the bounds of our every-day experience.
Of all the heavenly bodies, the sun excepted, the moon is the most
impressive and beautiful. As we catch her form, rising as a fair
crescent in the western sky after sunset, gradually increasing in size
and brilliancy night after night till from her circular disk she throws
a full flood of light on our world and then passes through her
decreasing phases, we recognise her as "the Governor of the night," or
in the words of our own poet, when in her crescent phase, "the Diadem of
night." Seen through a good binocular glass, her form gains in
rotundity; but under an ordinary telescope with a four-inch objective,
she appears like a globe of molten gold. Yet all this light is
derivative, and is only a small portion of that she receives from the
sun. That her surface is a mass of rigid matter destitute of any
inherent brilliancy, appears plain enough when we view a portion of her
disk through a very large telescope. It was the good fortune of the
author to have an opportunity for such a view through one of the largest
telescopes in the world. The 27-inch refractor manufactured by Sir
Howard Grubb of Dublin, for the Vienna observatory, a few years ago, was
turned on a portion of the moon's disk before being finally sent off to
its destination; and seen by the aid of such enormous magnifying power,
nothing could be more disappointing as regards the appearance of our
satellite. The sheen and lustre of the surface was now observed no
longer; the mountains and valleys, the circular ridges and hollows were,
indeed, wonderfully defined and magnified, but the matter of which they
seemed to be constituted resembled nothing so much as the pale plaster
of a model. One could thus fully realise the fact that the moon's light
is only derivative. Still we must recollect that the most powerful
telescope can only bring the surface of the moon to a distance from us
of about 250 miles; and it need not be said that objects seen at such a
distance on our earth present very deceptive appearances; so that we
gain little information regarding the composition of the moon's crust,
or exterior surface, simply from observation by the aid of large
telescopes.
Reasoning from analogy with our globe, we may infer that the exterior
shell of the moon consists of crystalline volcanic matter of the highly
silicated, or acid, varieties resting upon another of a denser
description, rich in iron, and resembling basalt. This hypothesis is
hazarded on the supposition that the composition of the matter of the
moon's mass resembles in the main that of our globe. During the process
of cooling from a molten condition, the heavier lavas would tend to fall
inwards, and allow the lighter to come to the surface, and form the
outer shell in both cases. Thus, the outer crust would resemble the
trachytic lavas of our globe, and their pale colour would enable the
sun's rays to be reflected to a greater extent than if the material were
of the blackness of basalt.[4] So much for the material. We have now to
consider the structure of the moon's surface, and here we find ourselves
treading on less speculative and safer ground. All astronomers since the
time of Schroter seem to be of accord in the opinion that the remarkable
features of the moon's surface are in some measure of volcanic origin,
and we shall presently proceed to consider the character of these forms
more in detail.
But first, and as leading up to the discussion of these physical
features, we must notice one essential difference between the
constitution of the moon and of the earth; namely, the absence of water
and of an atmosphere in the case of the moon. The sudden and complete
occultation of the stars when the moon's disk passes between them and
the place of the observer on the earth's surface, is sufficient evidence
of the absence of air; and, as no cloud has ever been noticed to veil
even for a moment any part of our satellite's face, we are pretty safe
in concluding that there is no water; or at least, if there be any, that
it is inappreciable in quantity.[5] Hence we infer that there is no
animal or vegetable life on the moon's surface; neither are there
oceans, lakes or rivers, snowfields or glaciers, river-valleys or
cañons, islands, stratified rocks, nor volcanoes of the kind most
prevalent on our own globe.
[Illustration: Fig. 38.--Photograph of the moon's surface (in part)
showing the illuminated "spots," and ridges, and the deep hollows. The
position of "Tycho" is shown near the upper edge, and some of the
volcanic craters are very clearly seen near the margin.]
Now on looking at a photographic picture of the moon's surface (Fig.
38), we observe that there are enormous dark spaces, irregular in
outline, but more or less approaching the circular form, surrounded by
steep and precipitous declivities, but with sides sloping outwards.
These were supposed at one time to be seas; and they retain the name,
though it is universally admitted that they contain no water. Some of
these hollows are four English miles in depth. The largest of these,
situated near the north pole of the moon, is called _Mare Imbrium_; next
to it is _Mare Serenitatis_; next, _Mare Tranquilitatis_, with several
others.[6] Mare Imbrium is of great depth, and from its floor rise
several conical mountains with circular craters, the largest of which,
_Archimedes_, is fifty miles in diameter; its vast smooth interior being
divided into seven distinct zones running east and west. There is no
central mountain or other obvious internal sign of former volcanic
activity, but its irregular wall rises into abrupt towers, and is marked
outside by decided terraces.[7]
The Mare Imbrium is bounded along the east by a range of mountains
called the _Apennines_, and towards the north by another range called
the _Alps_; while a third range, that of the _Caucasus_, strikes
northward from the junction of the two former ranges. Several circular
or oval craters are situated on, and near to, the crest of these ridges.
[Illustration: Fig. 39.--A magnified portion of the moon's surface,
showing the forms of the great craters with their outer ramparts. The
white spot with shadow is a cone rising from the centre of one of the
larger craters to a great height and thus becoming illuminated by the
sun's light.]
But the greater part of the moon's hemisphere is dotted over by almost
innumerable circular crater-like hollows; sometimes conspicuously
surmounting lofty conical mountains, at other times only sinking below
the general outer surface of the lunar sphere. On approaching the
margin, these circular hollows appear oval in shape owing to their
position on the sphere; and the general aspect of those that are visible
leads to the conclusion that there are large numbers of smaller craters
too small to be seen by the most powerful telescopes. These cones and
craters are the most characteristic objects on the whole of the visible
surface, and when highly magnified present very rugged outlines,
suggestive of slag, or lava, which has consolidated on cooling, as in
the case of most solidified lava-streams on our earth.[8] One of the
most remarkable of these crateriform mountains is that named
_Copernicus_, situated in a line with the southern prolongation of the
Apennines. Of this mountain Sir R. Ball says: "It is particularly well
known through Sir John Herschel's drawing, so beautifully reproduced in
the many editions of the _Outlines of Astronomy_. The region to the west
is dotted over with innumerable minute craterlets. It has a central,
many-peaked mountain about 2,400 feet in height. There is good reason to
believe that the terracing shown in its interior is mainly due to the
repeated alternate rise, partial congealation and retreat of a vast sea
of lava. At full moon it is surrounded by radiating streaks."[9] The
view regarding the structure of Copernicus here expressed is of
importance, as it is probably applicable to all the craters of our
satellite.
"When the moon is five or six days old," says Sir Robert Ball, "a
beautiful group of three craters will be readily found on the boundary
line between night and day. These are _Catharina_, _Cyrillus_, and
_Theophilus_. Catharina is the most southerly of the group, and is more
than 16,000 feet deep and connected to Cyrillus by a wide valley; but
between Cyrillus and Theophilus there is no such connection. Indeed
Cyrillus looks as if its huge surrounding ramparts, as high as Mont
Blanc, had been completely finished when the volcanic forces commenced
the formation of Theophilus, the rampart of which encroaches
considerably on its older neighbour. Theophilus stands as a well-defined
round crater, about 64 miles in diameter, with an internal depth of
14,000 to 18,000 feet, and a beautiful central group of mountains,
one-third of that height, on its floor. This proves that the last
eruptive efforts in this part of the moon fully equalled in intensity
those that had preceded them. Although Theophilus is on the whole the
deepest crater we can see in the moon, it has received little or no
deformation by secondary eruptions."
But perhaps the most remarkable object on the whole hemisphere of the
moon is "the majestic Tycho," which rises from the surface near the
south pole, and at a distance of about 1/6th of the diameter of the
sphere from its margin. Its depth is stated by Ball to be 17,000 feet,
and its diameter 50 miles. But its special distinction amongst the other
volcanic craters lies in the streaks of light which radiate from it in
all directions for hundreds and even thousands of miles, stretching with
superb indifference across vast plains, into the deepest craters, and
over the highest opposing ridges. When the sun rises on Tycho these
streaks are invisible, but as soon as it has reached a height of 25° to
30° above the horizon, the rays emerge from their obscurity, and
gradually increase in brightness until full moon, when they become the
most conspicuous objects on her surface. As yet no satisfactory
explanation has been given of the origin of these illuminated rays,[10]
but I may be permitted to add that their form and mode of occurrence are
eminently suggestive of gaseous exhalations from the volcano illumined
by the sun's rays; and owing to the absence of an atmosphere, spreading
themselves out in all directions and becoming more and more attenuated
until they cease to be visible.
The above account will probably suffice to give the reader a general
idea of the features and inferential structure of the moon's surface.
That she was once a molten mass is inferred from her globular form; but,
according to G. F. Chambers, the most delicate measurements indicate no
compression at the poles.[11] That her surface has cooled and become
rigid is also a necessary inference; though Sir J. Herschel considered
that the surface still retains a temperature _possibly_ exceeding that
of boiling water.[12] However this may be, it is pretty certain that
whatever changes may occur upon her surface are not due to present
volcanic action, all evidence of such action being admittedly absent.
If, when the earth and moon parted company, their respective
temperatures were equal, the moon being so much the smaller of the two
would have cooled more rapidly, and the surface may have been covered by
a rigid crust when as yet that of the earth may have been molten from
heat. Hence the rigidity of the moon's surface may date back to an
immensely distant period, but she may still retain a high temperature
within this crust. Having arrived at this stage of our narrative, we are
in a position to consider by what means, and under what conditions, the
cones and craters which diversify the lunar surface have been developed.
In doing so it may be desirable, in the first place, to determine what
form of crater on our earth's surface those of the moon do not
represent; and we are guided in our inquiry by the consideration of the
absence of water on the lunar surface. Now there are large numbers of
crateriform mountains on our globe in the formation of which water has
played an important, indeed essential, part. As we have already seen,
water, though not the ultimate cause of volcanic eruptions, has been the
chief agent, when in the form of steam at high pressure, in producing
the explosions which accompany these eruptions, and in tearing up and
hurling into the air the masses of rock, scoriæ, and ashes, which are
piled around the vents of eruption in the form of craters during periods
of activity. To this class of craters those of Etna, Vesuvius, and
Auvergne belong. These mountains and conical hills (the domes excepted)
are all built up of accumulations of fragmental material, with
occasional sheets and dykes of lava intervening; and where eruptions
have taken place in recent times, observation has shown that they are
accompanied by outbursts of vast quantities of aqueous vapour, which has
been the chief agent (along with various gases) in piling up the
circular walls of the crater.
It has also been shown that in many instances these crater-walls have
been breached on one side, and that streams of molten lava which once
occupied the cup to a greater or less height, have poured down the
mountain side. Hence the form or outline of many of these fragmental
craters is crescent-shaped. Such breached craters are to be found in all
parts of the world, and are not confined to any one district, or even
continent, so that they may be considered as characteristic of the class
of volcanic crater-cones to which I am now referring. In the case of the
moon, however, we fail to observe any decided instances of breached
craters, with lava-streams, such as those I have described.[13] In
nearly all cases the ramparts appear to extend continuously round the
enclosed depression, solid and unbroken; or at least with no large gap
occupying a very considerable section of the circumference. (See Fig.
38.) Hence we are led to suspect that there is some essential
distinction between the craters on the surface of the moon and the
greater number of those on the surface of our earth.
It is scarcely necessary to add that the volcanic mountains of the moon
offer no resemblance whatever to the dome-shaped volcanic mountains of
our globe. If it were otherwise, the lunar mountains would appear as
simple luminous points rising from a dark floor, over which they would
cast a conical shadow. But the form of the lunar volcanic mountains is
essentially different; as already observed, they consist in general of a
circular rampart enclosing a depressed floor, sometimes terraced as in
the case of Copernicus, from which rise one or more conical mountains,
which are in effect the later vents of eruption.
In our search, therefore, for analogous forms on our own earth, we must
leave out the craters and domes of the type furnished by the European
volcanoes and their representatives abroad, and have recourse to others
of a different type. Is there then, we may ask, any type of volcanic
mountain on our globe comparable with those on the moon? In all
probability there is.
If the reader will turn to the description of the volcanoes of the
Hawaiian group in the Pacific, especially that of Mauna Loa, as given by
Professor Dana and others, and compare it with that of Copernicus, he
will find that in both cases we have a circular rampart of solid lava
enclosing a vast plain of the same material from which rise one or more
lava-cones. The interiors in both cases are terraced. So that, allowing
for differences in magnitude, it would seem that there is no essential
distinction between lunar craters and terrestrial craters of the type of
Mauna Loa. Dana calls these Hawaiian volcanoes "basaltic," basalt being
the prevalent material of which they are formed. Those of the moon may
be composed of similar material, or otherwise; but in either case we may
suppose they are built up of lava, erupted from vents connected with the
molten reservoirs of the interior. Thus we conclude that they belong to
an entirely different type, and have been built up in a different
manner, from those represented by Etna, Vesuvius, and most of the
extinct volcanoes of Auvergne, the Eifel, and of other districts
considered in these pages.
Let us now endeavour to picture to ourselves the stages through which
the moon may be supposed to have passed from the time her surface began
to consolidate owing to the radiation of her heat into space; for there
is every probability that some of the craters now visible on her disk
were formed at a very early period of her physical history.
When the surface began to consolidate, it must also have contracted; and
the interior molten matter, pressed out by the contracting crust, must
have been over and over again extruded through fissures produced over
the solidified surface, until the solid crust extended over the whole
lunar surface, and became of considerable thickness.
It is from this epoch that, in all probability, we should date the
commencement of what may be termed "the volcanic history" of the moon.
We must bear in mind that although the moon's surface had become solid,
its temperature may have remained high for a very long period. But the
continuous radiation of the surface-heat into space would produce
continuous contraction, while the convection of the interior heat would
tend to increase the thickness of the outer solid shell; and this, ever
pressing with increasing force on the interior molten mass, would result
in frequent ruptures of the shell, and the extrusion of molten lava
rising from below. Hence we may suppose the fissure-eruptions of lava
were of frequent occurrence for a lengthened period during the early
stage of consolidation of the lunar crust; but afterwards these may be
supposed to have given place to eruptions through pipes or vents,
resulting in the formation of the circular craters which form such
striking and characteristic objects in the physical aspect of our
satellite.[14]
It is not to be supposed that the various physical features on the lunar
surface have all originated in the same way. The great ranges of
mountains previously described may have originated by a process of
piling up of immense masses of molten lava extruded from the interior
through vents or fissures; while the great hollows (or "seas") are
probably due to the falling inwards of large spaces owing to the escape
of the interior lava.
But it is with the circular craters that we are most concerned. Judging
from analogy with the lava-craters present on our globe, we must suppose
them to be due to the extrusion, and piling up, of lava through central
pipes, followed in some cases by the subsidence of the floor of the
crater. It seems not improbable that it was in this way the greater
number of the circular craters lying around Tycho, and dotting so large
a space round the margin of the moon, were constructed. (See Fig. 38.)
In general they appear to consist of an elevated rim, enclosing a
depressed plain, out of which a central cone arises. The rim may be
supposed to have been piled up by successive discharges of lava from a
central orifice; and after the subsidence of the paroxysm the lava still
in a molten condition may have sunk down, forming a seething lake within
the vast circular rampart, as in the case of the Hawaiian volcanoes. The
terraces observable within the craters in some instances have probably
been left by subsequent eruptions which have not attained to the level
of preceding ones; and where a central crater-cone is seen to rise
within the caldron, we may suppose this to have been built up by a later
series of eruptions of lava through the original pipe after the
consolidation of the interior sea of lava. The mamelons of the Isle of
Bourbon,[15] and some of the lava-cones of Hawaii, appear to offer
examples on our earth's surface of these peculiar forms.
Such are the views of the origin of the physical features of our
satellite which their form and inferred constitution appear to suggest.
They are not offered with any intention of dogmatising on a subject
which is admittedly obscure, and regarding which we have by no means all
the necessary data for coming to a clear conclusion. All that can be
affirmed is, that there is a great deal to be said in support of them,
and that they are to some extent in harmony with phenomena within range
of observation on the surface of our earth.
The far greater effects of lunar vulcanicity, as compared with those of
our globe, may be accounted for to some extent by the consideration that
the force of gravity on the surface of the moon is only one-sixth of
that on the surface of the earth. Hence the eruptive forces of the
interior of our satellite have had less resistance to overcome than in
the case of our planet; and the erupted materials have been shot forth
to greater distances, and piled up in greater magnitude, than with us.
We have also to recollect that the abrading action of water has been
absent from the moon; so that, while accumulations of matter had been
proceeding throughout a prolonged period over its surface, there was no
counteracting agency of denudation at work to modify or lessen the
effects of the ruptive forces.
[1] Correctly speaking, each attracts the other towards its centre of
gravity with a force proportionate to its mass, and inversely as the
square of the distance; but the earth being by much the larger body, its
attraction is far greater than that of the moon.
[2] The variation in the distance is only under rare circumstances
40,000 miles, but ordinarily about 13,000 miles.
[3] _Story of the Heavens_, 2nd edition, p. 525, _et seq._
[4] A series of researches made by Zöllner, of Leipzig, led him to
assign to the light-reflecting capacity of the full-moon a result
intermediate between that obtained by Bouguer, which gave a brightness
equal to 1/300000 part of that of the sun, and of Wollaston, which gave
1/801070 part. We may accept 1/618000 of Zöllner as sufficiently close;
so that it would require 600,000 full moons to give the same amount of
light as that of the sun.
[5] Schroter, however, came to the conclusion that the moon has an
atmosphere.
[6] A chart of the moon's surface, with the names of the principal
physical features, will be found in Ball's _Story of the Heavens_, 2nd
edit., p. 60. It must be remembered that the moon as seen through a
telescope appears in reversed position.
[7] _Ibid._, p. 66.
[8] As represented by Nasmyth's models in plaster.
[9] Ball, _loc. cit._, p. 67.
[10] Ball, _loc. cit._, p. 69.
[11] _Astronomy_, p. 78.
[12] _Outlines of Astronomy_, p. 285.
[13] At rare intervals a few crescent-shaped ridges are discernible on
the lunar sphere, but it is very doubtful if they are to be regarded as
breached craters.
[14] The number of "spots" on the moon was considered to be 244 until
Schroter increased it to 6,000, and accurately described many of them.
Schroter seems to have been the earliest observer who identified the
circular hollows on the moon's surface as volcanic craters.
[15] Drawings of these very curious forms are given by Judd,
_Volcanoes_, p. 127.
CHAPTER III.
ARE WE LIVING IN AN EPOCH OF SPECIAL VOLCANIC ACTIVITY?
The question which we are about to discuss in the concluding chapter of
this volume is one to which we ought to be able to offer a definite
answer. This can only be arrived at by a comparison of the violence and
extent of volcanic and seismic phenomena within the period of history
with those of pre-historic periods.
At first sight we might be disposed to give to the question an
affirmative reply when we remember the eruptions of the last few years,
and add to these the volcanic outbursts and earthquake shocks which
history records. The cases of the earthquake and eruption in Japan of
November, 1891, where in one province alone two thousand people lost
their lives and many thousand houses were levelled[1]; that of Krakatoa,
in 1883; of Vesuvius, in 1872; and many others of recent date which
might be named, added to those which history records;--the recollection
of such cases might lead us to conclude that our epoch is one in which
the subterranean volcanic forces had broken out with extraordinary
energy over the earth's surface. Still, when we come to examine into
the cases of recorded eruptions--especially those of great violence--we
find that they are limited to very special districts; and even if we
extend our retrospect into the later centuries of our era, we shall find
that the exceptionally great eruptions have been confined to certain
permanently volcanic regions, such as the chain of the Andes, that of
the Aleutian, Kurile, Japanese, and Philippine and Sunda Islands, lying
for the most part along the remarkable volcanic girdle of the world to
which I have referred in a previous page. Add to these the cases of
Iceland and the volcanic islands of the Pacific, and we have almost the
whole of the very active volcanoes of the world.
Then for the purposes of our inquiry we have to ascertain how these
active vents of eruption compare, as regards the magnitude of their
operations, with those of the pre-historic and later Tertiary times. But
before entering into this question it maybe observed, in the first
place, that a large number of the vents of eruption, even along the
chain of the earth's volcanic girdle, are dormant or extinct. This
observation applies to many of the great cones and domes of the Andes,
including Chimborazo and other colossal mountains in Ecuador, Columbia,
Chili, Peru, and Mexico. The region between the eastern Rocky Mountains
and the western coast of North America was, as we have seen, one over
which volcanic eruptions took place on a vast scale in later Tertiary
times; but one in which only the after-effects of volcanic action are at
present in operation. We have also seen that the chain of volcanoes of
Japan and of the Kurile Islands are only active to a slight extent as
compared with former times, and the same observation applies to those
of New Zealand. Out of 130 volcanoes in the Japanese islands, only 48
are now believed to be active.
Again, if we turn to other districts we have been considering, we find
that in the Indian Peninsula, in Arabia, in Syria and the Holy Land, in
Persia, in Abyssinia and Asia Minor--regions where volcanic operations
were exhibited on a grand scale throughout the Tertiary period, and in
some cases almost down into recent times--we are met by similar
evidences either of decaying volcanic energy, or of an energy which, as
far as surface phenomena are concerned, is a thing of the past. Lastly,
turning our attention to the European area, notwithstanding the still
active condition of Etna, Vesuvius, and a few adjoining islands, we see
in all directions throughout Southern Italy evidences of volcanic
operations of a past time,--such as extinct crater-cones, lakes
occupying the craters of former volcanoes, and extensive deposits of
tuff or streams of lava--all concurring in giving evidence of a period
now past, when vulcanicity was widespread over regions where its
presence is now never felt except when some earthquake shock, like that
of the Riviera, brings home to our minds the fact that the motive force
is still beneath our feet, though under restrained conditions as
compared with a former period.
Similar conclusions are applicable with even greater force to other
parts of the European area. The region of the Lower Rhine and Moselle,
of Hungary and the Carpathians, of Central France, of the North of
Ireland and the Inner Hebrides, all afford evidence of volcanic
operations at a former period on an extensive scale; and the contrast
between the present physically silent and peaceful condition of these
regions, as regards any outward manifestations of sub-terrestrial
forces, compared with those which were formerly prevalent, cannot fail
to impress our minds irresistibly with the idea that volcanic energy has
well-nigh exhausted itself over these tracts of the earth's surface.
From this general survey of the present condition of the earth's
surface, as regards the volcanic operations going on over it, and a
comparison with those of a preceding period, we are driven to the
conclusion that, however violent and often disastrous are the volcanic
and seismic phenomena of the present day, they are restricted to
comparatively narrow limits; and that even within these limits the
volcanic forces are less powerful than they were in pre-historic times.
The middle part of the Tertiary period appears, in fact, to have been
one of extraordinary volcanic activity, whether we regard the wide area
over which this activity manifested itself, or the results as shown by
the great amount of the erupted materials. Many of the still active
volcanic chains, or groups, probably had their first beginnings at the
period referred to; but in the majority of cases the eruptive forces
have become dormant or extinct. With the exception of the lavas of the
Indian-Peninsular area, which appear, at least partially, to belong to
the close of the Cretaceous epoch, the specially volcanic period may be
considered to extend from the beginning of the Miocene down to the close
of the Pliocene stage. During the Eocene stage, volcanic energy appears
to have been to a great degree dormant; but plutonic energy was
gathering strength for the great effort of the Miocene epoch, when the
volcanic forces broke out with extraordinary violence over Europe, the
British Isles, and other regions, and continued to develop throughout
the succeeding Pliocene epoch, until the whole globe was surrounded by a
girdle of fire.
* * * * *
The reply, therefore, to the question with which we set out is very
plain; and is to the effect that the present epoch is one of
comparatively low volcanic activity. The further question suggests
itself, whether the volcanic phenomena of the middle Tertiary period
bear any comparison with those of past geological times. This, though a
question of great interest, is one which is far too large to be
discussed here; and it is doubtful if we have materials available upon
which to base a conclusion. But it may be stated with some confidence,
in general terms, that the history of the earth appears to show that,
throughout all geological time, our world has been the theatre of
intermittent geological activity, periods of rest succeeding those of
action; and if we are to draw a conclusion regarding the present and
future, it would be that, owing to the lower rate of secular cooling of
the crust, volcanic action ought to become less powerful as the world
grows older.
[1] Admirably illustrated in Prof. J. Milne's recently published work,
_The Great Earthquake of Japan, 1891_.
APPENDIX.
A BRIEF ACCOUNT OF THE PRINCIPAL VARIETIES OF VOLCANIC ROCKS.
The text-books on this subject are so numerous and accessible, that a
very brief account of the volcanic rocks is all that need be given here
for the purposes of reference by readers not familiar with petrological
details.
Let it be observed, in the first place, that there is no hard and fast
line between the varieties of igneous and volcanic rocks. In this as in
other parts of creation--_natura nil facit per saltum_; there are
gradations from one variety to the other. At the same time a systematic
arrangement is not only desirable, but necessary; and the most important
basis of arrangement is that founded on the proportion of _silica_ (or
quartz) in the various rocks, as first demonstrated by Durocher and
Bunsen, who showed that silica plays the same part in the inorganic
kingdom that carbon does in the organic. Upon this hypothesis, which is
a very useful one to work with, these authors separated all igneous and
volcanic rocks into two classes, viz., the Basic and the Acid; the
former containing from 45-58 per cent., the latter 62-78 per cent. of
that mineral. But there are a few intermediate varieties which serve to
bridge over the space between the Basic and Acid Groups. The following
is a generalised arrangement of the most important rocks under the above
heads:--
_Tabular View of Chief Igneous and Volcanic Rocks._
BASIC GROUP.
1. Basalt and Dolerite.
2. Gabbro.
3. Diorite.
4. Diabase and Melaphyre.
5. Porphyrite.
INTERMEDIATE GROUP.
6. Syenite.
7. Mica-trap, or Lampophyre.
8. Andesite.
ACID GROUP.
9. Trachyte, Domite, and Phonolite.
10. Rhyolite and Obsidian.
11. Granophyre.
12. Granite.
In the above grouping, and in the following definitions, I have not been
able to follow any special authority. But the most serviceable
text-books are those of Mr. Frank Rutley, _Study of Rocks_, and Dr.
Hatch, _Petrology_; also H. Rosenbusch, _Mikroskopische Physiographie
der Mineralien_, and F. Zirkel's _Untersuchungen über mikroskopische
Structur der Basaltgesteine_. We shall consider these in the order above
indicated:--
1. BASALT.--The most extensively distributed of all volcanic rocks. It
is a dense, dark rock of high specific gravity (2.4-2.8), consisting of
plagioclase felspar (Labradorite or anorthite), augite, and
titano-ferrite (titaniferous magnetite). Olivine is often present; and
when abundant the rock is called "olivine-basalt." In the older rocks,
basalt has often undergone decomposition into melaphyre; and amongst the
metamorphic rocks it has been changed into diorite or hornblende rock;
the augite having been converted into hornblende.
When leucite or nepheline replaces plagioclase, the rock becomes a
leucite-basalt,[1] or nepheline-basalt. Some basalts have a glass paste,
or "ground-mass," in which the minerals are enclosed.
The lava of Vesuvius may be regarded as a variety of basalt in which
leucite replaces plagioclase, although this latter mineral is also
present. Zirkel calls it "Sanidin-leucitgestein," as both the
macroscopic and microscopic structure reveal the presence of leucite,
sanidine, plagioclase, nephiline, augite, mica, olivine, apatite, and
magnetite.[2]
_Dolerite_ does not differ essentially from basalt in composition or
structure, but is a largely crystalline-granular variety, occurring more
abundantly than basalt amongst the more ancient rocks, and the different
minerals are distinctly visible to the naked eye.
A remarkable variety of this rock occurs at Slieve Gullion in Ireland,
in which mica is so abundant as to constitute the rock a "micaceous
dolerite."
2. GABBRO.--A rather wide group of volcanic rocks with variable
composition. Essentially it is a crystalline-granular compound of
plagioclase, generally Labradorite and diallage. Sometimes the pyroxenic
mineral becomes hypersthene, giving rise to _hypersthene-gabbro_; or
when hornblende is present, to _hornblende-gabbro_; when olivine, to
_olivine-gabbro_. Magnetite is always present.
These rocks occur in the Carlingford district in Ireland, in the Lizard
district of Cornwall, the Inner Hebrides (Mull, Skye, etc.) of Scotland,
and in Saxony.
3. DIORITE.--A crystalline-granular compound of plagioclase and
hornblende with magnetite. When quartz is present it becomes (according
to the usual British acceptation) a _syenite_; but this view is
gradually giving place to the German definition of syenite, which is a
compound of orthoclase and hornblende; and it may be better to
denominate the variety as _quartz-diorite_. The diorites are abundant as
sheets and dykes amongst the older palæozoic and metamorphic rocks, and
are sometimes exceedingly rich in magnetite. Mica, epidote, and chlorite
are also present as accessories.
The rock occurs in North Wales, Charnwood Forest, Wicklow, Galway, and
Donegal, and the Highlands of Scotland. There can be little doubt that
amongst the metamorphic rocks of Galway, Mayo, and Donegal the great
beds of (often columnar) diorite were originally augitic lavas, which
have since undergone transformation.
4. DIABASE.--It is very doubtful if "Diabase" ought to be regarded as a
distinct species of igneous rock, as it seems to be simply an altered
variety of basalt or dolerite, in which chlorite, a secondary
alteration-product, has been developed by the decomposition of the
pyroxene or olivine of the original rock. It is a convenient name for
use in the field when doubt occurs as to the real nature of an igneous
rock. Melaphyre is a name given to the very dark varieties of altered
augitic lavas, rich in magnetite and chlorite.
5. PORPHYRITE (or quartzless porphyry).--A basic variety of
felstone-porphyry, consisting of a felspathic base with distinct
crystals of felspar, with which there may be others of hornblende, mica,
or augite. The colour is generally red or purple, and it weathers into
red clay, in contrast to the highly acid or silicated felsites which
weather into whitish sand.
6. SYENITE.--As stated above, this name has been variously applied. Its
derivation is from Syene (Assouan) in Egypt, and the granitic rocks of
that district were called "syenites," under the supposition (now known
to be erroneous) that they differ from ordinary granites in that they
were supposed to be composed of quartz, felspar, and hornblende, instead
of quartz, felspar, and mica. From this it arose that syenite was
regarded as a variety of granite in which the mica is replaced by
hornblende, and this has generally been the British view of the
question. But the German definition is applied to an entirely different
rock, belonging to the felstone family; and according to this
classification syenite consists of a crystalline-granular compound of
orthoclase and hornblende, in which quartz may or may not be present.
From this it will be seen that, according to Zirkel, syenite is
essentially distinct from diorite in the species of its felspar.[3] It
seems desirable to adopt the German view; and as regards diorites
containing quartz as an accessory, to apply to them the name of
_quartz-diorite_, as stated above, the name syenite as used by British
geologists having arisen from a misconception.
7. MICA-TRAP (LAMPOPHYRE).--A rock, allied to the felstone family, in
which mica is an abundant and essential constituent, thus consisting of
plagioclase and mica, with a little magnetite. Quartz may be an
accessory. This rock occurs amongst the Lower Silurian strata of
Ireland, Cumberland, and the South of Scotland; it is not volcanic in
the ordinary acceptation of that term. The term _lampophyre_ was
introduced by Gümbel in describing the mica-traps of Fichtelgebirge.
8. ANDESITE.--This is a dark-coloured, compact or vesicular,
semi-vitreous group of volcanic rocks, composed essentially of a glassy
plagioclase felspar, and a ferro-magnesian constituent enclosed in a
glassy base. According to the nature of the ferro-magnesian constituent,
the group may be divided into _hornblende-andesite_, _biotite-andesite_,
and _augite-andesite_. Quartz is sometimes present, and when this
mineral becomes an essential it gives rise to a variety called
_quartz-andesite_ or _dacite_.
These rocks are the principal constituents of the lavas of the Andes,
and the name was first applied to them by Leopold von Buch; but their
representatives also occur in the British Isles, Germany, and elsewhere.
Dacite is the lava of Krakatoa and some of the volcanoes of Japan.
9, 10. TRACHYTE and DOMITE, etc.--These names include very numerous
varieties of highly silicated volcanic rock, and in their general form
consist of a white felsitic paste with distinct crystals of sanidine,
together with plagioclase, augite, biotite, hornblende, and accessories.
When crystalline grains or blebs of quartz occur, we have a
quartz-trachyte; when tridymite is abundant, as in the trachyte of Co.
Antrim, we have "tridymite-trachyte."
The trachytes occupy a position between the pitchstone lavas on the one
hand, and the andesites and granophyres on the other.
(_b._) _Domite_ is the name applied to the trachytic rocks of the
Auvergne district and the Puy de Dôme particularly. They do not contain
free quartz, though they are highly acid rocks, containing sometimes as
much as 68 per cent. of silica.
(_c._) _Phonolite (Clinkstone)_ is a trachytic rock, composed
essentially of sanidine, nepheline, and augite or hornblende. It is
usually of a greenish colour, hard and compact, so as to ring under the
hammer; hence the name. The Wolf Rock is composed of phonolite, and it
occurs largely in Auvergne.
(_d._) _Rhyolites_ are closely connected with the _quartz-trachytes_,
but present a marked fluidal, spherulitic, or perlitic structure. They
consist of a trachytic ground-mass in which grains or crystals of
quartz and sanidine, with other accessory minerals, are imbedded. They
occur amongst the volcanic rocks of the British Isles, Hungary, and the
Lipari Islands, from which the name _Liparite_ has been derived.
(_e._) _Obsidian (Pitchstone)._--This is a vitreous, highly acid rock,
which has become a volcanic glass in consequence of rapid cooling,
distinct minerals not having had time to form. It has a conchoidal
fracture, various shades of colour from grey to black; and under the
microscope is seen to contain crystallites or microliths, often
beautifully arranged in stellate or feathery groups. Spherulitic
structure is not infrequent; and occasionally a few crystals of
sanidine, augite, or hornblende are to be seen imbedded in the glassy
ground-mass. The rock occurs in dykes and veins in the Western Isles of
Scotland, in Antrim, and on the borders of the Mourne Mountains, near
Newry, in Ireland.
11. GRANOPHYRE.--This term, according to Geikie, embraces the greater
portion of the acid volcanic rocks of the Inner Hebrides. They are
closely allied to the quartz-porphyries, and vary in texture from a fine
felsitic or crystalline-granular quartz-porphyry, in the ground-mass of
which porphyritic turbid felspar and quartz may generally be detected,
to a granitoid rock of medium grain, in which the component dull felspar
and clear quartz can be readily distinguished by the naked eye.
Throughout all the varieties of texture there is a strong tendency to
the development of minute irregularly-shaped cavities, inside of which
quartz or felspar has crystallised out--a feature characteristic of the
granites of Arran and of the Mourne Mountains.
12. GRANITE.--A true granite consists of a crystalline-granular rock
consisting of quartz, felspar (orthoclase), and mica; the quartz is the
paste or ground-mass in which the felspar and mica crystals are
enclosed. This is the essential distinction between a granite and a
quartz-porphyry or a granophyre. Owing to the presence of highly-heated
steam under pressure in the body of the mass when in a molten condition,
the quartz has been the last of the minerals to crystallise out, and
hence does not itself occur with the crystalline form.
True granite is not a volcanic rock, and its representatives amongst
volcanic ejecta are to be found in the granophyres, quartz-porphyries,
felsites, trachytes, and rhyolites so abundant in most volcanic
countries, and to one or other of these the so-called granites of the
Mourne Mountains, of Arran Island, and of Skye are to be referred.
Granite is a rock which has been intruded in a molten condition
amongst the deep-seated parts of the crust, and has consolidated
under great pressure in presence of aqueous vapour and with extreme
slowness, resulting in the formation of a rock which is largely
crystalline-granular. Its presence at the surface is due to denudation
of the masses by which it was originally overspread.
[Illustration: Plate I.]
EXPLANATION OF PLATE I.
MAGNIFIED SECTIONS OF VESUVIAN MINERALS.
Fig. 1. Section of leucite crystal from the lava of 1868, with
fluid cavities. Mag., 350 diams.
" 2, 3, 4, and 5. Sections of nepheline crystals from the lava of
1767, 1834, and 1854.
" 6. Section of sodalite crystal from the lava of 1794, with
belonites and crystals of magnetite enclosed.
" 7, 8, 9. Crystals of leucite with microliths and cavities
darkened by magnetite dust; also, containing crystals of
magnetite.
" 10. Group of leucite crystals of irregular form from the lava of
1855, congregated around a nucleus of crystals of plagioclase
and magnetite.
[Illustration: Plate II.]
EXPLANATION OF PLATE II.
MAGNIFIED SECTIONS OF VESUVIAN MINERALS.
Fig. 1. Section of augite crystal from the lava of 1794, with
numerous gas cells and delicately banded walls. The interior
contains two long prisms, probably of apatite.
" 2. Crystal of augite with banded walls, and indented by leucite
crystals, from the lava of 1794. Mag., 40 diams.
" 3, 4, 5. Sections of augite crystals from the lavas of 1794 and
1820.
" 6. Group of augite crystals from the lava of 1835.
" 7. Ditto from the lava of 1822, with encluded mica-flake (_a_)
and portion of the glass paste, or ground-mass, of the rock
(_b_), containing microliths and grains of magnetite.
Fig. 8. Two crystals of olivine from the lava of 1855; they are
intersected on one side by the plane of the thin section, and
are remarkable for showing lines of gas cells, and bands of
growth sometimes cellular. Mag., 40 diams.
" 9. Section of rock-crystal (quartz), with double terminal
pyramids, from the lava of 1850.
" 10. Twin crystal of sanidine from the lava of 1858. Mag., 40
diams.
" 11, 12, 13. Sections of plagioclase crystals (probably
labradorite) from the lava of 1855. Mag., 100 diams.
" 14. Section of olivine crystal from the lava of 1631--imperfectly
formed. Mag., 30 diams.
" 15. Section of mica-flake from the lava of 1822. Mag., 30 diams.
[Illustration: Plate III.]
EXPLANATION OF PLATE III.
MAGNIFIED SECTIONS OF VOLCANIC ROCKS.
1. Diorite dyke, traversing Assynt limestone, North Highlands.
2. Basalt from upper beds, near Giant's Causeway, County Antrim.
3. Hornblende-hypersthene-augite Andesite, from Pichupichu, Andes.
4. Augite-Andesite from Pichupichu, Andes.
5. Olivine dolerite, with hornblende and biotite, Madagascar.
6. Leucite basalt, with mellilite, Capo di Bove, Italy.
[Illustration: Plate IV.]
EXPLANATION OF PLATE IV.
MAGNIFIED SECTIONS OF VOLCANIC ROCKS.
1. Vesuvian lava, glass paste with numerous crystals of leucite;
others of augite and nepheline porphyritically developed; also
small grains of magnetite.
2. Vesuvian lava, glass paste with numerous crystals of leucite;
others of olivine, hornblende, and sanidine, porphyritically
developed; small grains of magnetite.
3. Trachyte from Hungary; felsitic paste with crystals of
hornblende and sanidine, and a little magnetite.
4. Gabbro, from Carlingford Hill, Ireland, consisting of anorthite,
augite, a little olivine, and magnetite.
5. Dolerite, from old volcanic neck, Scalot Hill, near Lame,
consisting of labradorite, augite, olivine, and magnetite.
6. Dolerite, Ballintoy, County Antrim, showing ophetic structure,
consisting of augite, labradorite, and magnetite.
[1] Mr. S. Allport has discovered this in the rock called the "Wolf
Rock" off the coast of Cornwall. The most important work on basalt is
that by F. Zirkel, _Unters. über mikros. Zusammensetzung und Structur
der Basaltgesteine_. Bonn (1870).
[2] Zirkel, _Die mikroskopische Beschaffenheit der Mineralien und
Gesteine_, p. 153. Leipsig (1873).
[3] Zirkel, _Petrog._, i. 578; B. von Cotta, p. 178 (Eng. Trans.).
INDEX.
INDEX.
Abyssinian table-lands, 190 _et seq._
Albano, Lake, 89
America, volcanic regions of North, 136 _et seq._;
of Western, 144
Andes, 18, 27, 227, 254
Andesite, 263
Antrim, 154 _et seq._
Arabia, dormant volcanoes of, 126-135
Arabian desert, 134
Archibald, C. D., 213
Arizona, volcanoes of, 137
Argyll, Duke of, 173
Ascension, 36
Ashangi, volcanic series of, 192
Atmospheric effects of Krakatoa eruption, 213-214
Auckland district, volcanoes of, 147
Auvergne, volcanic regions of, 14, 16, 92 _et seq._
Azores, 32
Ball, Sir R. S., 242, 244
Basalt, 260
Blanford, W. T., 188, 189
Bonneville, Lake, 141-142
British Isles,
Tertiary volcanic districts of, 154 _et seq._, 227;
pre-Tertiary volcanic districts of, 196 _et seq._
Buch, L. von, 6, 11, 24
California, volcanoes of, 140
Callirrhoë, springs of, 133
Cañon, the Grand, 138
Cantal, volcanoes of the, 99-101
Cape Colony, Basalts of, 194
Charleston earthquake, 218, 222, 224
Chambers, G. F., 246
Charnwood Forest, 198
Chimborazo, 18
Clermont, vale of, 96-97
Clinkstone, 263
Cordilleras of Quito, 25
Cotopaxi, 16-18, 24, 26
Crater-cones, Lava, 19
Crateriform cones, 13
Craterless domes, 15
Dana, Prof. J. D., 19, 39, 249
Darwin, 28, 30
Darwin, Prof. G. H., 9, 231
Daubeny, 7, 61, 69
Davison, C., 9, 231
Davy, Sir H., 11
Deccan trap-series, 187 _et seq._
Demavend, Mount, 24
Diabase, 262
Diorite, 261
Dolerite, 261
Domite, 263
Dore, volcanoes of Mont, 100-101
Doughty, C. M., 127
Durocher, 232
Dutton, Capt. C. E., 9, 220, 222
Dykes in Ireland, 169-170
Earthquakes, 217 _et seq._
Errigal, 10
Etna, 14, 61 _et seq._, 229
Fingal's Cave, 185
Forbes, D., 27
France, extinct volcanoes of, 92 _et seq._
Gabbro, 261
Gardner, J. S., 156
Geikie, Sir A., 8, 29, 143, 156, 160, 169, 172, 176, 177, 196
Giant's Causeway, 165-166
Granite, 264
Granophyre, 264; of Mull, 174
Green, Prof. A. H., 194
Hatch, Dr., 260
Haughton, Prof., 68
Haurân, volcanoes of the, 22, 129
Haute Loire, volcanic districts of, 101-105
Hawaii, volcanoes of, 39, 249, 251
Hecla, 32
Herschel, Sir J., 244
Hibbert, Dr. S., 6, 114, 124
Hochstetter, F. von, 147
Hopkins, 171, 217
Hull, Dr. E. G., 110
Humboldt, A. von, 20, 25
Hutton, James, 5
Iceland, volcanoes of, 30-32
Ireland, volcanic Tertiary rocks of, 154 _et seq._
Jaulân, 129
Johnston-Lavis, 52
Jordan valley, 126 _et seq._, 226
Jorullo, 24
Judd, Prof., 8, 68, 69, 71, 172, 178, 208
Krakatoa, eruption of, 206 _et seq._
Kurile Islands, volcanoes of, 28
Laacher See, 121-123
Lampophyre, 262
Lancerote, 34
Lasaulx, Prof. von, 68
Lavas, relative density of, 232-234
Lima in 1746, earthquake of, 222
Lipari Islands, volcanoes of, 69 _et seq._
Lisbon, earthquake of, 221
Lister, J. J., 38
Lunar volcanoes, 236 _et seq._
Lyell, Sir C., 30, 62, 78, 217
Mackowen, Col., 74
Magdala, volcanic series of, 192-193
Mallet, R., 9, 217
Mauna Loa, 19, 39, 249
Mica-trap, 262
Milne, Prof., 28, 218, 253
Moab, volcanic regions of, 132
Moon, volcanoes of, 236 _et seq._
Monte Nuovo, 85
Mull, 172 _et seq._
Neapolitan group of volcanoes, 28
New Zealand, volcanoes of, 146
Obsidian, 264
Ocean waves of seismic origin, 208, 220
O'Reilly, Prof., 9, 219
Orizaba, 21
Ovid, 3
Pacific, volcanic islands of, 37
Palestine, dormant volcanoes of, 126-135
Palmieri, Prof., 55
Pantelleria, 74
Phlegræan fields, 85
Phonolite, 263
Pitchstone, 264
Pliny, 2, 4
Porphyrite, 262
Powell, Major, 138
Pre-Tertiary volcanic rocks, 187 _et seq._;
of British Isles, 196 _et seq._
Puy de Dôme, 105-110
Pythagoreans on volcanoes, 2-3
Quito, Cordilleras of, 25
Rangitoto, 19, 149
Reyer, Dr. E., 17
Rhine valley, volcanoes of, 113 _et seq._
Rhyolite, 263
Riviera in 1887, earthquake of, 219
Rocca Monfina, 80
Roderberg, 119, 120
Rome, 88-89
Rosenbusch, H., 260
Roto Mahana, 151
Ruapahu, 151
Russell, Hon. Rollo, 213
Rutley, F., 260
St. Helena, 37
San Francisco, Mount, 138
Santorin, 76-83
Schehallion, 10
Schumacher, 127
Scotland, volcanic districts of, 172 _et seq._
Scrope, Poulett, 5, 73, 93, 98
Scuir of Eigg, 180-184
Seismic phenomena, special, 201 _et seq._, 217 _et seq._
Shasta, Mount, 140
Siebengebirge, 116-120
Skye, 177-179
Sleamish, 168
Smyth, Piazzi, 33
Snake River, volcanoes of, 142
Staffa, 185-186
Strabo on volcanoes, 3
Stromboli, 71-73
Sumatra, volcanic action in, 226
Syenite, 262
Symes, R. G., 167
Syria, earthquakes in, 219
Taupo Lake, 150
Taylor, Mount, 138
Tell el Ahmâr, 131
Tell el Akkasheh, 131
Tell el Farras, 131
Tell Abû en Nedâ, 130
Tell Abû Nedîr, 129
Templepatrick, quarry at, 160
Teneriffe, 33
Tertiary period, volcanic activity of, 255
Thucydides, 2
Tonga Islands, volcanoes of, 38
Tongariro, 151
Trachyte, 263
Trass of Brühl Valley, 123-125
Tristan da Cunha, 37
Tristram, Canon, 127, 131
Utah, volcanoes of, 137
Verbeek, R. D. M., 202
Vesuvius, 4, 14, 41-60, 67, 229
Volcanoes,
historic notices of, 1-5;
form, structure, and composition of, 10-19;
lines and groups of active, 20-29;
of mid-ocean, 30-40;
extinct or dormant, 84 _et seq._;
special volcanic and seismic phenomena, 201 _et seq._;
the ultimate cause of volcanic action, 225 _et seq._;
whether we are living in an epoch of special volcanic activity, 253-256;
brief account of volcanic rocks, 259-265
Vulcanists, 5
Vulcano, 69, 71
Wallace, A. R., 81
Waltershausen, W. S. von, 7, 61
Wellington, Mount, 149
Wharton, Capt., 212
Whymper, E., 18
Yarmûk, valley of the, 129, 131
Yellowstone Park, 145
Zirkel, F., 260
Zöllner, 240
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Transcriber's note:
Changed 'Kilarrea' to 'Kilauea' on page 19: Mauna Loa and Kilarrea.
Changed 'Kilanea' to 'Kilauea' on page 39: Kilanea, 4158 feet.
Made punctuation (semi-colons) consistent in caption to figure 16.
Changed 'Brionde' to 'Brioude' on page 94: till at Brionde it becomes.
Changed 'occuping' to 'occupying' on page 96: occuping a hollow.
Changed 'Rodesberg' to 'Roderberg' on page 118: old extinct volcano of
Rodesberg.
Changed 'Wolkenberg' to 'Wolkenburg' on page 118: and that of the
Wolkenberg.
Left the reference to Jeremiah, l. 25. in footnote to Part III Chapter
I, although Jeremiah, li. 25. seems more appropriate.
Changed 'fumarols' to 'fumaroles' on page 137: fumarols give evidence.
Removed extra comma on page 153: of the present, epoch.
Changed 'columnal' to 'columnar' on page 176: the columnal structure.
Changed 'groves' to 'grooves' on page 183: the groves and scorings.
Changed 'Angust' to 'August' on page 212: the 27th of Angust.
Changed 'mikroskopischen' to 'mikroskopische' on page 260: über
mikroskopischen Structur.
Changed 'become' to 'becomes' on page 260: the rock become a
leucite-basalt.
Left inconsistent spellings of 'Baalbec' and 'Baalbeck'; 'Harrat' and
'Harrât'; 'mètres' and 'metres'; 'pitchstone' and 'pitch-stone';
'prehistoric' and 'pre-historic'; 'Rhône' and 'Rhone'; 'sub-aerial',
'subaërial' and 'subaerial'; 'tableland' and 'table-land'.
Greek words were replaced with their transliterations: 'meson pyr' and
'Peri kosmou'.
The oe-ligature was expanded to the two separate characters: 'Euboea'
and 'Boeotia'.
Left the list numbering as is at the beginning of Chapter II of Part IV,
even though the list begins at item c, as if it continues the list which
began in the previous chapter.
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