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Title: The Elements of Geology; Adapted to the Use of Schools and Colleges
Author: Loomis, Justin R.
Language:
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THE
ELEMENTS OF GEOLOGY;
ADAPTED TO THE USE OF
SCHOOLS AND COLLEGES.
BY
JUSTIN R. LOOMIS,
PROFESSOR OF CHEMISTRY AND GEOLOGY IN WATERVILLE COLLEGE.
WITH NUMEROUS ILLUSTRATIONS.
BOSTON:
GOULD AND LINCOLN,
59 WASHINGTON STREET.
1852
Entered according to Act of Congress, in the year 1852,
By GOULD & LINCOLN,
In the Clerk's Office of the District Court of the District of
Massachusetts.
Stereotyped by
HOBART & ROBBINS,
BOSTON.
PRESS OF G. C. RAND, CORNHILL, BOSTON.
PREFACE
In preparing the following work, it was intended to present a systematic
and somewhat complete statement of the principles of Geology, within
such limits that they may be thoroughly studied in the time usually
allotted to this science.
A sufficient number of leading facts has been introduced to enable the
learner to feel that every important principle is a conclusion to which
he has himself arrived; and yet, for the purpose of compression, that
fullness of detail has been avoided with which more extended works
abound. In furtherance of the same object, authorities are seldom cited.
The consideration of geological changes is made a distinct chapter,
subsequent to the one on the arrangement of materials. It should,
however, be remembered that these processes of arranging and disturbing
are not thus separated in time. In nature the two processes are always
going on together.
It seemed important to exhibit the science with as much unity and
completeness as possible; and hence, discussions upon debatable points
in Theoretical Geology, so interesting to mature geologists, would have
been out of place here; and yet those more intricate subjects have not
been omitted. A large proportion of the work is devoted to the
explanation of geological phenomena, in order to convey an idea of the
modes of investigation adopted, and the kind of evidence relied on.
Where diversities of opinion exist, that view has been selected which
seemed most in harmony with the facts; and the connection has not often
been interrupted to combat, or even to state, the antagonist view.
Technical terms have, in a few instances, been introduced, and
principles referred to, which are subsequently explained. The index
will, however, enable the student to understand them, without a separate
glossary.
Some may prefer to commence with the second chapter, deferring the study
of the elementary substances, minerals and rocks, to the last. Such a
course may be pursued without special inconvenience.
Questions have been added, for the convenience of those teachers who may
prefer to conduct their recitations by this means. But, when the
circumstances of the case admit of it, a much more complete knowledge of
the subject will be acquired by pupils who are required to analyze the
sections, and proceed with the recitation themselves; while the teacher
has only to correct misapprehension, explain what may seem obscure, and
introduce additional illustrations.
LIST OF ILLUSTRATIONS.
1. Columnar Trap, New Holland. (_Dana._)
2. The four divisions of rocks, and their relative positions. _A_,
Volcanic Rocks. _B_, Granite. 1, 2, 3, 4, Granite of different
ages. _C_, Metamorphic Rocks. _D_, Fossiliferous Rocks.
(_Lyell._)
3. Granite veins in slate, Cape of Good Hope. (_Hall._)
4. Granite veins traversing granite. (_Hitchcock._)
5. Extinct volcanoes of Auvergne. (_Scrope._)
6. Lava of different ages, Auvergne. (_Lyell._)
7. Strata folded and compressed by upheaval of granite.
8. Favosites Gothlandica.
9. Catenipora escharoides. (Chain coral.)
10. Caryocrinus ornatus. (_Hall._)
{ Leptæna alternate. Orthis testudinaria. }
11. { }(_Hall._)
{ Delthyris Niagarensis. }
12. Section of a chambered shell, showing the chambers and the
siphuncle.
13. Orthoceras.
14. Curved Cephalopoda, _a_, Ammonite; _b_, Crioceras; _c_,
Scaphite; _d_, Ancyloceras; _e_, Hamite; _f_, Baculite;
_g_, Turrilite. (_Agassiz and Gould._)
15. Trilobite.
16. Cephalaspis Lyellii. (_Agassiz._)
17. Pterichthys oblongus. (_Agassiz._)
18. Fault in the coal formation, _a a_, layers of coal,
_b b_, surface and soil.
19. Stigmaria ficoides; Newcastle. (_Lindley and Hutton._)
20. Trunk of sigillaria. (_Trimmer._)
21. Bark of sigillaria. (Natural size.)
22. Sphenopteris crenata. (_Lindley._)
23. Pachypteris lanceolata. (_Brongn._)
24. Sigillaria levigata. (_Brongn._)
25. Lepidodendron Sternbergii, Bohemia. (_Sternberg._)
26. Calamite.
27. Heterocercal fish. Homocercal fish.
28. Impressions of Raindrops, Wethersfield, Conn. (_Hitchcock._)
29. _b_, Bird tracks in the Conn. River Sandstone, _a_, Consecutive
tracks; _c_, Track of Cheirotherium (probably a reptile),
Penn. and Germany.
30. Section in the Isle of Portland. (_Buckland._)
31. Apiocrinites rotundus, Bradford, Eng. (_Miller._)
32. Gryphea incurva.
33. _a_, Outline of Ichthyosaurus; _b_, Plesiosaurus.
34. Pterodactyle.
35. _a_, Diploctenium cordatum; _b_, Marsupites; _c_, Salenia; _d_,
Galerites; _e_, Micraster cor-anguinum. (_Agassiz & Gould._)
36. _b_, Belemnite. _a_, Restored outline of the animal to which it
belonged.
37. Cerithium intermedium.
38. Murex alveolatus.
39. Conus concinnus.
40. Nummulite.
41. Outline of paleotherium.
42. Outline of anoplotherium.
43. Skeleton of the mastodon.
44. Univalve with entire mouth.
45. Univalve with notched mouth.
46. Unimuscular bivalve.
47. Bimuscular bivalve.
48. Parallel planes of cleavage intersecting curved strata.
(_Sedgwick._)
49. _a b_, A vein of segregation; _c d_, A dike.
50. Faults and denuded strata.
51. Vertical conglomerate. (_Lyell._)
52. Inclined strata in Dorsetshire, England. (_Buckland._)
53. Dip of strata.
54. Axes and valleys in disturbed strata.
55. Curved strata of slate, Berwickshire, Eng. (_Lyell._)
56. Folded strata.
57. Slope of mountains.
58. Europe at the Silurian epoch. (_Guyot._)
59. Europe at the tertiary epoch.
60. Area of elevation and depression in the Pacific and Indian
Oceans. (_Darwin._)
61. _c c_, Coral wall. (_Trimmer._)
62. _c c_, Coral wall above the sea-level; _c' c'_, Second coral
wall.
63. Coral wall after partial subsidence.
64. Atoll. The coral wall only appearing. The original island
entirely submerged.
65. Remains of the temple of Jupiter Serapis, near Naples.
66. Detached hills of old red sandstone, Rosshire, Scotland.
(_Lyell._)
67. Section of denuded strata, Mass. (_Hitchcock._)
68. Grooved and striated surface of rocks.
69. Artesian wells.
70. Segregated masses in rocks.
71. Columnar form taken by basalt on solidification.
72. Layers of limestone now forming, San Vignone, Italy. (_Lyell._)
73. Erosion of rock by the action of the waves.
74. Marine currents.
75. Sediment deposited in horizontal layers.
76. Section of greensand, Bedfordshire, Eng. (_Lyell._)
77. Glacier, with lateral and medial moraines, _a a_, Terminal
moraines.
78. Iceberg.
79. Volcanic Eruption. (_Trimmer._)
80. Fractures produced by upheaval.
81. Fossiliferous rock altered by contact with granite.
82. Consecutive changes by which horizontal strata become vertical.
TABLE OF CONTENTS.
Page
CHAPTER I.
OF THE MATERIAL WHICH COMPOSE THE CRUST OF THE EARTH.
SECTION I.--ELEMENTARY SUBSTANCES, 11
SECTION II.--SIMPLE MINERALS, 13
SECTION III.--THE MINERAL MASSES WHICH FORM THE CRUST OF THE EARTH, 16
CHAPTER II.
OF THE ARRANGEMENT OF THE MATERIALS WHICH COMPOSE
THE CRUST OF THE EARTH.
SECTION I.--THE CLASSIFICATION OF ROCKS, 21
SECTION II.--THE PLUTONIC ROCKS, 23
SECTION III.--THE VOLCANIC ROCKS, 25
SECTION IV.--THE NON-FOSSILIFEROUS STRATIFIED (OR
METAMORPHIC) ROCKS, 30
SECTION V.--THE FOSSILIFEROUS ROCKS, 32
SECTION VI.--FOSSILS, 57
SECTION VII.--THE TIME NECESSARY FOR THE FORMATION OF THE
STRATIFIED ROCKS, 63
CHAPTER III.
OF THE CHANGES TO WHICH THE CRUST OF THE EARTH HAS
BEEN SUBJECTED.
SECTION I.--CHANGES WHICH HAVE TAKEN PLACE AT GREAT DEPTHS
BELOW THE SURFACE, 67
SECTION II.--CHANGES IN THE MASS OF THE STRATIFIED ROCKS, 68
SECTION III.--CHANGES OF ELEVATION AND SUBSIDENCE, 73
SECTION IV.--CHANGES ON THE SURFACE OF THE EARTH, 85
SECTION V.--CHANGES OF CLIMATE, 88
SECTION VI.--ADVANTAGES RESULTING FROM GEOLOGICAL CHANGES, 91
CHAPTER IV.
OF THE CAUSES OF GEOLOGICAL PHENOMENA.
SECTION I.--ATMOSPHERIC CAUSES, 95
SECTION II.--CHEMICAL ACTION, 97
SECTION III.--ORGANIC CAUSES, 101
SECTION IV.--AQUEOUS CAUSES, 103
SECTION V.--AQUEO-GLACIAL ACTION, 120
SECTION VI.--IGNEOUS CAUSES, 127
CHAPTER I.
OF THE MATERIALS WHICH COMPOSE THE CRUST OF THE EARTH.
SECTION I.--ELEMENTARY SUBSTANCES.
There are about sixty substances known to the chemist which are
considered as elementary; but most of them are rarely met with, and only
in minute quantities. A few of them are, however, so abundant, in the
composition of the crust of the earth, as to render some attention to
them necessary.
_Oxygen_ is more widely diffused than any other substance. It is an
ingredient of water and of the atmosphere, the former containing
eighty-eight per cent., and the latter twenty-one. Nearly all rocks
contain oxygen in combination with the metallic and metalloid bases, and
the average proportion of oxygen which they contain is about forty-five
per cent.; so that it will not differ much from the truth to consider
the oxygen in the earth's crust as equal in weight to all the other
substances which enter into its composition.
_Hydrogen_ occurs in nature principally in combination with oxygen,
forming water. It is also an ingredient in bitumen and bituminous coal.
_Nitrogen_ is confined almost entirely to the atmosphere, of which it
forms four-fifths. It enters into the composition of some varieties of
coal, and is sparingly diffused in most fossiliferous rocks.
One of the most important substances in nature is _carbon_. It
constitutes the principal part of all the varieties of coal, as well as
of graphite, peat and bituminous matter. A much larger amount of carbon
exists in the carbonic acid which is combined with the oxides of the
metalloids and metals. The most abundant of these compounds is
limestone, which contains about twelve per cent, of carbon.
In the neighborhood of volcanoes _sulphur_ is found pure and in a
crystalline form. It is a constant ingredient in volcanic rocks, and in
several of the most important ores, particularly those of lead, copper
and iron. The most abundant sulphate is gypsum, which contains
twenty-six per cent, of sulphur. In small quantities it is widely
diffused in rocks, and in the waters of the ocean.
_Chlorine_ is found principally as an ingredient of rock-salt, which
contains sixty per cent, of it, and of sea-water, which contains one and
a half per cent.
_Fluorine_ is found, though very sparingly, in nearly all the
unstratified rocks. It forms nearly half of the mineral known as
Derbyshire spar.
Of the metals, _Iron_ is the only one that is found abundantly. It
enters into the composition of nearly all mineral substances. It is
generally combined with oxygen, and occurs less frequently as a
carbonate or sulphuret. Of volcanic rocks it forms about twenty per
cent. Its ores are sometimes found in the form of dikes or seams, having
been injected from below; at other times, in the form of nodules or
stratified masses, like other rocks of mechanical origin.
_Manganese_ is likewise extensively diffused, but in very small
quantity. The other metals are often met with, but their localities are
of very limited extent.
Of the metallic bases of the earths and alkalies, _Silicium_ is the most
abundant. It generally occurs in the form of silex, which is an oxide of
the metal. There are but few rocks in which it is not found in
considerable amount.
_Aluminium_ generally occurs as an oxide, in which form it is alumina.
It is the base of the different varieties of clay and clay-slate. It is
also a constituent of felspar and mica.
_Potassium_ is an ingredient of felspar and mica, and hence is found in
all the primary and in most of the volcanic rocks, as well as in the
stratified rocks derived from them.
_Sodium_ is a constituent of a variety of felspar which is somewhat
abundant in volcanic rocks. Its principal source is the extensive beds
of rock-salt, and the same substance in a state of solution in the
waters of the ocean.
_Calcium_ constitutes about forty per cent, of limestone, and is an
ingredient in nearly all igneous rocks. This metal, in the state of an
oxide, is lime.
_Magnesium_ is somewhat abundant, but less so than calcium. It is one of
the bases of dolomite and magnesian limestone, and is an ingredient of
talc and all talcose rocks.
The substances now enumerated constitute nearly the entire mineral mass
of the crust of the earth. They may be arranged in the following
order:--
I. NON-METALLIC SUBSTANCES.
Oxygen. Hydrogen. Nitrogen.
Carbon. Sulphur. Chlorine.
Fluorine.
II. METALS.
Iron. Manganese.
III. METALLIC BASES OF THE EARTHS AND ALKALIES.
Silicium. Aluminium. Potassium.
Sodium. Calcium. Magnesium.
These substances, chemically combined, form _Simple Minerals_.
SECTION II.--SIMPLE MINERALS.
All substances found in the earth or upon its surface, which are not the
products of art or of organic life, are regarded by the mineralogist as
_simple minerals_. About four hundred mineral species are known, and the
varieties are much more numerous; but only a small number of them are so
abundant as to claim the attention of the geologist. An acquaintance
with the following species is, however, necessary.
_Quartz_ is probably the most abundant mineral in nature. It is composed
wholly of silex. Its specific gravity is 2.65. It is the hardest of the
common minerals, gives sparks with steel, scratches glass, and breaks
into irregular angular fragments under the hammer. When crystallized,
its most common form is that of a six-sided prism, terminated by
six-sided pyramids. When pure, it is transparent or translucent, and its
lustre is highly vitreous. The transparent variety is called _rock
crystal_. When purple, it is _amethyst_. When faint red, it is _rose
quartz_. When its color is dark brown, or gray, and it has a conchoidal
fracture, it is _flint_. When quartz occurs in white, tuberous masses,
of a resinous lustre and conchoidal fracture, it is _opal_. The precious
opal is distinguished by its lively play of colors. _Jasper_ is opaque,
and contains a small per cent, of oxide of iron, by which it is colored
dull red, yellowish red or brown. The light-colored, massive,
translucent variety is _chalcedony_. The flesh-colored specimens are
_carnelian_. When composed of layers of chalcedony of different colors,
it becomes _agate_. Several of the varieties of quartz, such as
amethyst, opal, carnelian and agate, are used to considerable extent in
jewelry.
_Felspar_ is composed of silex, alumina and potassa. It resembles
quartz, but it is not as hard, cleaves more readily, and is not
generally transparent. Its specific gravity is 2.47. Its lustre is
feebly vitreous, but pearly on its cleavage faces. Its color is
sometimes green, but generally dull white, and often inclined to red or
flesh-color.
_Mica_ is composed of the same ingredients as felspar, together with
oxide of iron. Its specific gravity is nearly three. It is often
colorless, but frequently green, smoky, or black. It may be known by its
capability of division into exceedingly thin, transparent, elastic
plates.
_Hornblende_ is composed of silex, alumina and magnesia. Its specific
gravity is a little above three. Its color is generally some shade of
green. When dark green or black, whether in a massive or crystalline
state, it is _common hornblende_. When light green, it is _actinolite_.
The white variety is _tremolite_. When it is composed of flexible
fibres, it is _asbestus_; and when the fibres have also a silky lustre,
it is _amianthus_.
_Augite_ or _Pyroxene_ has, till recently, been considered as a variety
of hornblende. Its specific gravity is slightly different; its
composition is the same, and in general appearance it is not easily
distinguished from hornblende. It has, however, been made a distinct
species, because its crystalline form is different.
_Hypersthene_ is composed of silex, magnesia and oxide of iron. Its
specific gravity is 3.38. It closely resembles hornblende. The lustre of
its cleavage faces is metallic pearly. Its color is grayish or greenish
black.
_Talc_ is composed of silex and magnesia. Its specific gravity is 2.7.
It resembles mica in its general appearance and in its lamellar
structure, but it is easily distinguished from it by its plates being
not elastic, and by its soapy feel. Its color is generally some shade of
green. _Soapstone_ is an impure variety of talc, of a light gray color,
earthy texture, and is unctuous to the touch. _Chlorite_, another impure
variety, is a dark green rock, massive, easily cut with a knife, and
unctuous to the touch.
_Serpentine_ is composed of silex and magnesia. Its specific gravity is
2.55. It is generally massive, unctuous to the touch, and of a green
color. It is often variegated with spots of green of different shades.
With a mixture of carbonate of lime it forms the _verd antique marble_.
_Carbonate of Lime_, or common limestone, is composed of carbonic acid
and lime. Its specific gravity is 2.65. It presents a great variety of
forms. In a crystalline state it is generally transparent, and when so,
possesses the property of double refraction. It may be distinguished
from every other common species by its rapid effervescence with acids.
It readily cleaves parallel to all the faces of the primary form, which
is a rhombohedron.
_Sulphate of Lime_, or Gypsum, is composed of sulphuric acid and lime.
Its specific gravity is 2.32. When crystalline, it has a pearly lustre,
is transparent, and goes under the name of _Selenite_. _Common Gypsum_
resembles the other earthy limestones, but it is softer, and may be
readily distinguished by its not effervescing with acids.
To the minerals now enumerated may be added the following, which are of
frequent occurrence, but not in great quantities; namely, carbonate of
magnesia, oxide of iron, iron pyrites, rock-salt, coal, bitumen, schorl
and garnet.
These simple minerals, either in separate masses or mingled more or less
intimately together, compose almost wholly the earth's crust.
SECTION III.--THE MINERAL MASSES WHICH FORM THE CRUST OF THE EARTH.
That portion of the structure of the earth which is accessible to man is
called the _crust of the earth_.
The mineral masses which compose it, whether in a solid state, like
granite and limestone, or in a yielding state, like beds of sand and
clay, are called _rocks_.
The _unstratified rocks_ are Granite, Hypersthene rock, Limestone and
Serpentine, and the Trappean and Volcanic rocks.
_Granite_ is a rock of a light gray color, and is composed of quartz,
felspar and mica, in variable proportions, confusedly crystallized
together. The felspar is generally the predominant mineral. It is
sometimes of a very coarse texture, the separate minerals occurring in
masses of a foot or more in diameter. At other times it is so
fine-grained that the constituent minerals can scarcely be recognized by
the naked eye; and between these extremes there is every variety. The
term granite is not, however, confined to an aggregate of these three
minerals. In some instances the felspar so predominates as almost to
exclude the other minerals, when it is called _felspathic granite_. When
the quartz appears in the form of irregular and broken lines, somewhat
resembling written characters, in a base of felspar, it is called
_graphic granite_. When talc takes the place of mica, it is _talcose
granite_. When hornblende takes the place of mica, it is _syenite_.
Granite or any rock becomes _porphyritic_ when it contains imbedded
crystals of felspar.
There is a rock of crystalline structure, like granite, but of a darker
color, which is called _hypersthene rock_. It is composed of Labrador
felspar and hypersthene. The mineral species _serpentine_ and
_limestone_ often occur unstratified in considerable quantities.
_Volcanic rocks_ consist of the materials ejected from the craters of
volcanoes. They are composed of essentially the same minerals as trap
rocks. When the material has been thrown out in a melted state, it is
called _lava_. Lava, at the time of its ejection, contains a large
amount of watery vapor at a high temperature. Under the immense pressure
to which it is subjected in the volcanic foci, it may exist in the form
of water; but when the lava is thrown out at the crater, the pressure
cannot much exceed that of the atmosphere. The particles of water at
once assume the gaseous form. As lava possesses considerable viscidity,
the steam does not escape, but renders the upper portion of the mass
vesicular. This vesicular lava is called _scoriæ_. By the movement of
the stream of lava, these vesicles become drawn out into fine capillary
tubes, converting the scoriæ into _pumice-stone_.
A large part of the materials ejected from volcanoes is in the form of
dust, cinders and angular fragments of rock. These soon become
solidified, forming _volcanic tuff_, or _volcanic breccia_. In submarine
eruptions these fragments are spread out by the water into strata, upon
which other materials, not volcanic, are afterwards deposited. These
interposed strata are called volcanic grits.
The _trappean rocks_ are composed of felspar, mingled intimately and in
small particles with augite or hornblende. They also contain iron and
potassa. They are often _porphyritic_. When they contain spherical
cavities, filled with some other mineral, such as chlorite, carbonate of
lime or agate, they are called _amygdaloidal trap_.
The principal varieties of trappean rock are basalt, green stone, and
trachyte. In _basalt_, augite, or, in some cases, hornblende, is the
predominant mineral. It is a heavy, close-grained rock, of a black or
dark brown color. _Greenstone_ differs from basalt in containing a much
larger proportion of felspar. Its structure is more granular, and
frequently it assumes so much of the crystalline form as to pass
insensibly into syenite or granite. It is a dark colored rock, with a
slight tinge of green. Both green stone and basalt are disposed to
assume the columnar form, the columns being arranged at right angles to
the faces of the fissure into which the trap is injected. When it is
spread out into broad horizontal masses, the columns are vertical. (Fig.
1.) _Trachyte_ is composed principally of felspar, is of a grayish
color, and rough to the touch.
[Illustration: Fig. 1.]
Of the _stratified_ rocks the following are the most important:
_Gneiss_ is a rock closely resembling granite. It is an aggregate of the
same minerals, but the proportion of mica is somewhat greater. The only
distinction between them is that the gneiss is stratified, but the
stratification is often so indistinct that it passes insensibly into
granite. Generally, however, the stratification is so distinct as to
present a marked difference.
_Mica slate_ is such a modification of gneiss that the mica becomes the
predominant mineral, with a small intermixture of quartz and felspar.
Consequently the stratification becomes very distinct, so as sometimes
to render the mass divisible into thin sheets. The stratification is
often wavy, and sometimes much contorted.
_Sandstone_ consists of grains or fragments of any other rock, but more
frequently of siliceous rocks. The fragments are consolidated, sometimes
without any visible cement, but often by a paste of argillaceous or
calcareous substance. The color varies with that of the rock from which
it was derived. Generally, however, it is either drab or is colored red
by oxide of iron. The fragments are sometimes so minute as scarcely to
give the rock the appearance of sandstone. When they are of considerable
size and rounded, the rock is called _conglomerate_. When they are
angular, it is called _breccia_. _Greensand_ is a friable mixture of
siliceous and calcareous particles, colored by a slight intermixture of
green earth or chlorite.
_Limestone_ is a very abundant rock, and occurs in many different forms.
In transparent crystals it is _Iceland spar_. When white and
crystalline, it is _primary limestone_, _saccharine limestone_, or
_statuary marble_. When sub-crystalline it is generally more or less
colored. It is often _clouded_ with bands or patches of white in a
ground of some dark color. When its texture is close, and the
crystallization scarcely apparent, it is _compact limestone_. The white,
earthy variety is chalk. A variety of limestone composed of small
spheres is called _oölite_. _Lias_ is the name given to an impure
argillaceous variety of a brown or blue color. Any rock which contains a
considerable proportion of carbonate of lime, and which rapidly
disintegrates on exposure to the atmosphere, is called _marl_. Limestone
sometimes contains carbonate of magnesia. It is then _magnesian
limestone_, or _dolomite_.
_Clay_ consists of a mixture of siliceous and aluminous earth. It is
tough, highly plastic, and generally of a lead blue color. It is always
stratified, and often divided into very thin laminæ, which are
separated by sprinklings of sand only sufficient to keep them distinct.
_Clay slate_, or _argillaceous schist_, is composed of the same
materials as clay, and differs from it only in having become solidified.
Its color is gray, dark brown or black. In some beds it is purple.
_Shale_ is the same material in a state of partial solidification. On
exposure to the weather, it soon disintegrates, and is finally
reconverted into clay. All the varieties of argillaceous rock are easily
distinguished by a peculiar odor which they emit when breathed upon.
Argillaceous slate sometimes takes into its composition portions of some
other mineral, such as talc, mica, or hornblende. When any of these
minerals becomes so abundant as to constitute a considerable part of the
mass, the rock becomes _talcose_, _micaceous_, or _hornblende slate_.
Sometimes this last variety loses all appearance of a fissile structure,
and is composed almost wholly of hornblende. It is then called
_hornblende rock_.
_Diluvium_ is the name applied to masses of sand, gravel, and large
rocks, called boulders, heaped confusedly together on the surface of the
earth. It is also called _drift_.
CHAPTER II.
OF THE ARRANGEMENT OF THE MATERIALS WHICH COMPOSE THE CRUST OF THE
EARTH.
SECTION I.--THE CLASSIFICATION OF ROCKS.
In the first place, we divide rocks into _stratified_ and
_unstratified_. This division is one which will in general be easily
recognized, even by the most inexperienced observer; and the distinction
is important, because it separates the rocks of igneous origin from
those which have been produced by deposition of sediment from water.
It will be shown hereafter that a part of the unstratified rocks have
been formed at or near the surface of the earth; that is, they have
taken their present form by passing from a state of fusion to a solid
state above or between the stratified rocks, as in the case of lava
(Fig. 2, A). The other unstratified rocks have cooled so as to take the
solid form below the stratified rocks, as at B. The first are called
_epigene_, or _volcanic rocks_; the last, _hypogene_, or _plutonic
rocks_.
The lowest portion of the second division, the stratified rocks, are
termed _non-fossiliferous_, from the fact that they contain no evidence
of the existence of organic beings at the time when they were deposited.
Their relation to the other rocks is shown at C. It is supposed that
these rocks have been subjected to great changes by heat from the
igneous rocks below them. On this account Mr. Lyell proposes to call
them _metamorphic rocks_. The other portions of the stratified rocks are
_fossiliferous_, containing the remains of organic beings which lived at
the period when the rocks were deposited. They are represented at D. The
division of the last-named rocks info groups will be given hereafter.
[Illustration: Fig. 2.]
We have then four principal classes of rocks: _Plutonic Rocks_,
_Volcanic Rocks_, _Non-fossiliferous Stratified Rocks_ and
_Fossiliferous Rocks_.
SECTION II.--THE PLUTONIC ROCKS.
Granite is by far the most important of this class of rocks. Of its
thickness no estimate can be made, as no mining operations have ever
penetrated through it, and none of the most extensive displacements of
rocks by natural causes has brought to the surface any other rock on
which it rests. It may, therefore, be considered the foundation rock,
the skeleton of the earth, upon which all the other formations are
supported. The whole amount of granite in the earth's crust may be
greater than that of all other rocks, but it comes up through the other
formations so as to be exposed over only a comparatively small portion
of the surface, and this is generally the central portion of mountain
ranges, or the highest parts of broken, hill country. Still, it is not
unfrequently found in the more level regions, in the form of slightly
elevated ridges, with the stratified rocks reclining against it.
The structure of granite seems frequently to be a confused mixture of
the minerals which compose it, without any approach to order in their
arrangement; but in many cases it is found to split freely in certain
directions, and to work with difficulty in any other. This may result
from an arrangement of the integrant crystals, so that their cleavage
planes approach more or less nearly to parallelism. When this is the
case with the mica or felspar, it must diminish the cohesion in a
direction perpendicular to these planes, and thus facilitate the
cleavage of the mass.
[Illustration: Fig. 3.]
Granite is found to penetrate the stratified rocks in the form of veins.
The following section (Fig. 3) will show the relation of granite veins
to the granitic mass below. The granite which is quarried for
architectural purposes is often in comparatively small quantities,
disappearing at the distance of a few hundred yards beneath the
stratified rock; or else it exists in the form of isolated dome-shaped
masses. It is probable that, if they could be followed sufficiently far,
they would be found to be portions of dikes coming from the general mass
of granite below. Even the granite nuclei of the great mountain ranges
may be considered as injected dikes of enormous magnitude.
[Illustration: Fig. 4.]
Granite is itself intersected with granite veins more frequently,
perhaps, than any other rocks; but the vein is a coarser granite than
the rock which it divides. It is not uncommon to find one set of dikes
intercepted and cut off by a second set, and the second by a third. The
substance of the dikes was, of course, in a liquid state when it was
injected, and the first must have become solid before the second was
thrown in; hence the dikes are of different ages. The dikes _a b c_,
represented in Fig. 4, must have been injected in the order in which
they are lettered.
It is probable that, by the process of cooling, the liquid mass from
which these dikes have proceeded has been gradually solidifying from the
surface downwards. If so, it would follow that the granite nearest the
surface (1, Fig. 2) is the oldest, and the newest is that which is at
the greatest distance below (4). It is possible that at great depths
granite may be still forming, that is, taking the solid form, though of
this there can be no direct proof. There is, however, proof that it has
been liquid at periods of time very distant from each other; for the
dikes sometimes reach to the top of the coal formation (for example),
and then spread themselves out horizontally, as at _a_, showing that the
rock above the coal had not then been deposited. Another dike will
extend through the new red sandstone, as at _b_, and spread itself out
horizontally as before. These horizontal layers of granite, by their
position in strata whose ages are known, indicate the periods when
granite has existed in a liquid state. Granite veins have been
discovered in the Pyrenees as recent as the close of the cretaceous
period, and in the Andes they have been found among the tertiary rocks.
There are several other rocks, of minor importance, often found in
connection with granite. Hypersthene rock, in a few cases, forms the
principal part of mountain masses. Greenstone is more frequently
associated with the trappean rocks, but it sometimes passes
imperceptibly into syenite and common granite. Limestone is found in
considerable abundance, and serpentine in small quantities, as primary
rocks, and have evidently been formed like granite, by solidifying from
a state of fusion.
SECTION III.--THE VOLCANIC ROCKS.
The volcanic rocks consist of materials ejected from volcanoes. They
are, however, ejected in very _different states_; sometimes as dust,
sand, angular fragments of rock, cinders, &c., and sometimes as lava
streams. In some instances, the lava has so little fluidity that it
accumulates in a dome-shaped mass over the orifice of eruption, and
perhaps in a few instances it has been thrust upward in a solid state.
There are _two principal varieties_ of lava, the trachytic, consisting
mostly of felspar, and the basaltic, consisting of hornblende. When both
kinds are products of the same eruption, the trachytic lava is thrown
out first, and the basaltic last. The reason of this is, that felspar
is lighter than hornblende, and probably rises to the surface of the
lava mass at the volcanic focus, and the basaltic lava is therefore
reserved till the trachytic has been thrown off.
These, like other rocks, have been produced at different epochs. There
is, however, great difficulty in determining their age; There are some
differences of structure and composition observed, in comparing the
older and newer lavas; but the only method that can be relied on to
determine their age is their relation to other rocks. When they occur
between strata whose age is determined by imbedded fossils, they must be
of intermediate age between the inferior and superior strata.
1. _Modern Volcanic Rocks._--Some of the volcanic rocks are of modern
origin, and are produced by volcanoes now active. The total amount of
these, and of all the other volcanic rocks, is probably less than that
of either of the other principal divisions of rocks; yet they form no
inconsiderable part of the earth's crust. The number of active volcanoes
is not far from three hundred, and the number of eruptions annually is
estimated at about twenty. In some cases, the lava consists of only a
single stream, of but a few hundred yards in extent. It extends,
however, not unfrequently twenty miles in length, and two or three
hundred yards in breadth. The eruption of Mount Loa, on the island of
Hawaii, in 1840, from the crater of Kilauea, covered an area of fifteen
square miles to the depth of twelve feet; and another eruption of the
same mountain, in 1843, covered an area of at least fifty square miles.
The eruption in Iceland, in 1783, continued in almost incessant activity
for a year, and sent off two streams in opposite directions, which
reached a distance of fifty miles in one case, and of forty in the
other, with a width varying from three to fifteen miles, and with an
average depth of more than a hundred feet. The size of some of the
volcanic mountains will also assist in forming an idea of the amount of
volcanic rocks. Monte Nuovo, near Naples, which is a mile and a half in
circumference and four hundred and forty feet high, was thrown up in a
single day. Ætna, which is eleven thousand feet high, and eighty-seven
miles in circumference at its base, has probably been produced wholly
by its own eruptions. A large part of the chain of the Andes consists of
volcanic rock, but the proportion we have not the means of estimating.
2. _Tertiary Lavas._--There is another class of volcanic products, which
are so situated with reference to the tertiary strata that they must be
referred to that period. The principal localities of these lavas, so far
as yet known, are Italy, Spain, Central France, Hungary, and Germany.
They are also found in South America. Those of Central France have been
studied with the most care. They occur in several groups, but they were
the seats of volcanic activity during the same epoch, and formed parts
of one extensive volcanic region. Each of these minor areas, embracing a
circle of twenty or thirty miles in diameter, is covered with hills two
or three thousand feet in height, which are composed entirely of
volcanic products, like the cone of Ætna. On many of them there are
perfectly-formed craters still remaining. Numerous streams of lava have
flowed from these craters, some of which can now be traced, throughout
their whole extent, with as much certainty as if they were eruptions of
the present century. Some of the lavas have accumulated around the
orifices of eruption, forming rounded, dome-shaped eminences. These
lavas generally consist of trachyte, and have therefore a low specific
gravity, and imperfect fluidity. The basaltic lavas have often spread
out over broad areas, and, when they have been confined in valleys, have
reached a distance of fifteen miles or more from their source. There
still remain indications of a current of lava which was thirty miles
long, six broad, and in a part of its course from four to six hundred
feet deep. The above sketch (Fig. 5) will give some idea of the highly
volcanic aspect which the district of Auvergne, in France, presents.
[Illustration: Fig. 5.]
The unimpaired state of some of the cones and craters, and of the lava
currents, would lead to the impression that these regions have been the
theatre of intense volcanic action within a very recent period. But
there is good reason to believe that this has not been the case. "The
high antiquity of the most modern of these volcanoes is indeed
sufficiently obvious. Had any of them been in a state of activity in the
age of Julius Cæsar, that general, who encamped upon the plains of
Auvergne and laid siege to its principal city, could hardly have failed
to notice them."
It is equally certain that the commencement of their activity was at a
late period in the history of the earth. Lava currents are frequently
found in France resting upon the early tertiary strata, but no lava
current is found below them. The later tertiary strata contain pebbles
of volcanic rocks, showing that lavas had been previously ejected, but
none are found in the older strata of this formation. We must,
therefore, conclude that these volcanic tracts assumed their volcanic
character at some intermediate point in the tertiary period.
When we find that their activity commenced at so late a period and
closed so long ago, we might be led to suppose that it was of very short
duration. But a great number of facts, in the present condition of the
country, require that we should assign to them a very prolonged
activity. A single instance will be sufficient to show the nature of the
evidence upon which this conclusion rests. The heavy line (Fig. 6)
represents the present form of one of the valleys. A bed of lava forms
the highest point of land represented, and a second bed is found in an
intermediate part of the slope. The position of the upper bed must have
been a valley, when the lava flowed there. We may represent this valley
by the line _a b c_. The slow operation of natural denuding causes at
length excavated the valley _d e h_, when another lava current flowed
through it, covering its bed of pebbles, as before. The same denuding
causes have at length produced the present valley, _f g h_. These
remnants of lava-currents, as they have formed a very imperishable rock,
have protected the subjacent strata from erosion, and furnish evidence
of the position of the valley at different periods. When we consider
with what extreme slowness denuding causes produce changes on the
surface, and what extensive changes they have here nevertheless effected
in the interval between the production of the different lava currents,
we are compelled to feel that that interval was a very prolonged one.
Yet this period, however long it may have been, was evidently less than
the period of activity of these volcanoes.
[Illustration: Fig. 6.]
3. _Volcanic rocks of an earlier date_ are also found, sometimes as
distinct lavas, though generally as volcanic grits. They occur
interstratified with the cretaceous rocks, and with every other
formation of the fossiliferous series, showing that, from the earliest
times, these rocks have been accumulating as they now are.
_The trappean rocks_ may, in a general classification, be considered as
volcanic. It will be shown, hereafter, that they are the lavas of
submarine volcanoes. They do not, however, occur in the form of lava
currents, but in great tabular masses, generally between stratified
rocks, or in the form of dikes. They are also entirely unconnected with
cones or craters.
The trappean rocks occur more or less abundantly in all countries. One
of the most noted localities of this rock is a region embracing the
north of Ireland, and several of the islands on the western coast of
Scotland. It contains the celebrated Giant's Causeway, which consists of
a mass of columnar trap; also Fingal's Cave, which is produced by a
portion of the trap being columnar, and thus disintegrating more rapidly
than the rest, by the action of the waves. An immense mass of greenstone
trap, which has generally been considered as a vast dike, though often a
mile in thickness, is found extending from New Haven to Northampton, on
the west side of the Connecticut river. It then crosses to the east
side, and continues in a northerly direction to the Massachusetts line.
Under different names, it constitutes a nearly continuous and
precipitous mountain range for about one hundred miles. Dr. Hitchcock
supposes this greenstone range to be, not an injected dike, but a
tabular mass of ancient lava, which was spread out on the bed of the
ocean during the period of the deposition of the Connecticut river
sandstone. It was subsequently covered with a deposit of strata of great
thickness, and then by subterranean forces thrown into its present
inclined position.
There is a mass of basaltic rock in the valley of the Columbia river, in
the Oregon Territory, which extends without interruption for a distance
of four hundred miles. Its breadth and thickness is not known, but in
some places the river has cut a channel in this rock to a depth of four
hundred feet. Its age has not been determined, and it will, perhaps, be
found to be a tertiary or modern production.
SECTION IV.--THE NON-FOSSILIFEROUS STRATIFIED (OR METAMORPHIC) ROCKS.
1. _Gneiss_ is the most abundant rock in this class, and is generally
found reposing on granite. Its stratification is sometimes very
distinct, but it is often so imperfect that it can scarcely be
recognized. This is more frequently the case in the vicinity of granite
on which it rests, and into which it insensibly passes. A large part of
the material used for building purposes, under the name of granite, is
obscurely marked gneiss. In all primary countries it is an abundant
rock, occupying extensive districts, and sometimes forming mountain
masses.
2. _Mica slate_ lies next above gneiss, and is a very abundant rock. As
it differs from gneiss only in the proportion of mica which it contains,
and as the quantity of mica in it is very different in different places,
it is often difficult to make the distinction between them. It also
passes by insensible degrees into the argillaceous rocks. Many of the
argillaceous rocks are found, upon close examination, to contain mica in
minute scales in such abundance as to make it doubtful whether they
ought not to be regarded as mica slates; that is, the metamorphic action
by which argillaceous slate is converted into mica slate had proceeded
so far, before it was arrested, that it becomes impossible to say
whether the argillaceous or micaceous characters predominate.
3. _Argillaceous slate._--The last rock of this series is a slaty rock,
more or less highly argillaceous. It does not differ in lithological
characters from the same rock in the higher strata. It is doubtful
whether the roofing-slates should be considered as belonging to the
metamorphic series or not. They have been subjected to a very high
degree of metamorphic action, and yet strata intimately associated with
them have, in occasional instances, contained fossils.
It is not easy to fix the exact upper limit of this series. The fossils
are few, obscure, and seldom met with in the lowest fossiliferous
series; and the transition is very gradual from the distinctly
metamorphic to the fossiliferous rocks. This renders it impossible
always to determine accurately the line of separation.
The gneiss, mica slate and argillaceous slate, have the order of
superposition in which they are here named. They differ only in the
amount of metamorphic action to which they have been subjected; and the
gneiss which is most highly metamorphic has, by being the lowest, been
most acted upon,--the mica slate less, and the argillaceous slate least.
In a particular locality, however, the lowest rock which was subjected
to these causes of change, instead of having been of such a character as
to produce gneiss, may have been a limestone, and in that case the
lowest metamorphic rock would be a saccharine marble. In another
locality the lowest rock may have been a sandstone, which would be
converted into quartz rock. Hence there may occur, in any part of the
metamorphic series, crystalline limestone, quartz rock, hornblende
slate, chlorite slate, and talcose slate; and any one of these rocks may
be as abundant in any particular region, as gneiss, mica slate or
argillaceous slate, is in another.
The metamorphic rocks occur in all countries where there has been any
considerable amount of volcanic action, and their total amount is very
great; but their stratification is so confused and contorted, their
superposition so irregular, and denudations have been so extensive, that
no estimate can be made of their thickness. They are, perhaps, equal to
all the other stratified rocks.
SECTION V.--THE FOSSILIFEROUS ROCKS.
The fossiliferous rocks are divided into seven systems, which are
readily distinguished by the order of superposition, lithological
characters and organic remains. These systems are the Silurian, the Old
Red Sandstone, the Carboniferous, the New Red Sandstone, the Oölitic,
the Cretaceous, and the Tertiary systems. There is also an eighth system
now in process of formation.
It is the opinion of some geologists that there is another system
situated between the metamorphic rocks and the silurian system. It has
been called by Dr. Emmons, who has studied it with much care, the
"Taconic System," the Taconic Mountains, in the western part of
Massachusetts, being composed of these rocks. It is the lower part of
what has been called, in England and Wales, the _Cambrian system_.
The strata of this system have a nearly vertical position, and consist
principally of black, greenish and purple slates, of great thickness.
Granular quartz rock, however, occurs in considerable quantity, and in
this country two thick and important beds of limestone are found. These
limestones are occasionally white and crystalline. Generally, however,
as a mass, they are a dark, nearly black rock, with a network of lines
of a lighter color. All the clouded marbles for architectural and
ornamental purposes are from these beds, and our roofing and writing
slates are all obtained from the argillaceous portion of this system.
The number of species of organic remains contained in this system is
very small, and these, so far as discovered, belong to the annelida,
with a few doubtful cases of mollusca. This system of rocks is found
coming to the surface in a large part of New England, and the eastern
part of New York, also in the western part of England and Wales.
Those geologists who deny the existence of this system consider these
rocks as parts of the silurian system which have been most disturbed by
subterranean forces, and most altered by proximity to igneous rocks. The
annexed sketch (Fig. 7) will exhibit the relations here referred to.
Certain portions of the silurian rocks are supposed to have been thrown
into folds by the upheaval of the primary rocks. The plications nearest
to the intrusive granite would be most altered. That part of the figure
below the line _a a_ represents the outcropping edges as they now
appear, the upper portion of the folds having been removed by some
abrading cause.
[Illustration: Fig. 7.]
As it is yet uncertain which of these views is correct, convenience will
justify us in retaining the name of Cambrian system till further
investigations shall settle the question.
1. _The Silurian System._--The following tabular arrangement exhibits
the divisions of the system as recognized in England, in New York, in
Pennsylvania and Virginia, and in Ohio.
Key to Divisions
----------------
C - Cambrian Rocks.
S - Silurian System.
D - Devonian.
Ch - Champlain Division.
On - Ontario Division.
He - Helderberg Division.
Er - Erie Division.
Divisions as recognized Divisions as recognized Pennsylvania Ohio.
by English Authors. by the New York and Virginia.
Geologists.
/-------------------\ /-----------------------\ /----------\ /--------\
{ Upper Cambrian { { Potsdam Sandstone. } No. 1.
{ { {
C { Rocks, of Sedgwick{ { Calciferous Sandrock. }
{ { { Birdseye Limestone. } No. 2. }
{ (probably). {Ch{ Trenton Limestone. } } Blue
{ { } Limestone
{ Llandeilo Flags. { { Utica Slate. } } and Marl.
{ { { } No. 3. }
{ { Hudson River Group. }
{ {
{ { { Gray Sandstone. } No. 4.
{ { { Oneida Conglomerate. }
{ {On{
{ Caradoc Sandstone.{ { Medina Sandstone. } No. 5.
{ { { Clinton Group. }
{ { Niagara Group.
{ { } }
{ { { Oneida Salt Group. } :
{ { { Water-lime Group. } :
{ { { Pentamerus Limestone. } No. 6. :
{ { { Delthyris Shaly } :
{ { { Limestone. } :
S { { { Encrinal Limestone. } : Cliff
{ Wenlock Rocks. {He{ Upper Pentamerus } :
{ { { Limestone. : Limestone.
{ { { Oriskany Sandstone. } :
{ { { } No. 7. :
{ { { Cauda-Galli Grit. } :
{ { { Schoharie Grit. } :
{ { { Onondaga Limestone. Wanting. :
{ { { Corniferous Limestone. Wanting. }
{ }
{ { { Marcellus Shales. } } Black
{ { { Hamilton Group. } } Slate.
{ Upper and Lower { { } No. 8.
{ { { Tully Limestone. }
{ Ludlow Rocks and {Er{ }
: { { Genesee Slate.
D : the Devonian { { }
: { { Portage Group. } No. 9. } Waver
: System. { { } }
{ { {Chemung Group. } } Sandstone.
This name, _Silurian_, was first used to designate the lowest
well-characterized fossiliferous rocks in England. But it is now used to
embrace the whole system as it occurs elsewhere. It is well exhibited in
New York, both in consequence of its great development there, and
because the whole system is only slightly acted upon by disturbing
forces, so that the outcropping edge of each division extends over a
large surface.
This system is of great thickness, amounting, in places where it is well
developed, to twenty thousand feet.
The Champlain division commences with a quartzose sandstone, passing
gradually into limestone, which is succeeded by a very thick
argillaceous deposit, the Utica slate and Hudson River group. The
Ontario division in the lower part is a mass of sandstone. Above this is
the Clinton group, consisting of shales and sandstones. The most
important part of this group, in an economical point of view, is a
fossiliferous, argillaceous iron ore, coextensive with the group in this
country, and is worked to supply a large number of furnaces. The last of
the division is the Niagara group, which commences with a mass of shale,
and becoming at length calcareous, it terminates in a firm compact
limestone. This limestone has withstood the action of denuding causes
better than the shales either above or below it. It therefore presents a
bold escarpment at its outcrop, and occasions waterfalls wherever
streams of water cross it. The falls of Niagara are formed by this rock.
The Niagara limestone, in its extension westward, becomes the
lead-bearing rock of Missouri, Iowa and Wisconsin. The Helderberg
division is a succession of highly fossiliferous limestones, with the
intervention of only occasional beds of grits and shales. One member of
the series is the Onondaga Salt group. The water obtained from this
group in New York annually furnishes immense quantities of salt. The
Erie division consists of a thick mass of shales and sandstones.
[Illustration: Fig. 8.]
[Illustration: Fig. 9.]
[Illustration: Fig. 10.]
The fossils of this system are very numerous, but consist mostly of the
lower forms of animal life. _Corals_ (Figs. 8 and 9) are abundant, and
constitute in some places a large proportion of the limestones. The
_Crinoidea_, or lily-shaped animals, consist of a jointed stem
permanently attached, and bearing at the free extremity of the stem an
expanded portion, which is the pelvis, or digestive cavity. The mouth is
surrounded with a series of leaf-like tentacula, which serve the purpose
of seizing and holding food. Fig. 10 represents the pelvis of one of the
silurian fossils. The general character of the animal is better
represented by Fig. 30. The most abundant fossils of this period are the
lowest orders of _bivalve mollusca_ (Fig. 11). The _Cephalopoda_ are
characterized by having the organs of locomotion attached to the head.
The shell of several species is peculiar in being divided into distinct
cells, or chambers (Fig. 12, _b d_), perforated by a tube (siphuncle
_a_). These fossil shells are sometimes straight, as the _Orthoceras_
(Fig. 13), or curved, as shown in the several forms of Fig. 14. The
_Trilobite_ was an articulated, crustaceous animal, having two lines
along the back dividing it into three lobes, from which circumstance its
name is derived. It is found in great numbers in the Silurian rocks
(Fig. 15). In a few instances remains of fishes have been found, but
they by no means characterize the system.
[Illustration: Fig. 11.]
[Illustration: Fig. 12.]
[Illustration: Fig. 13.]
[Illustration: Fig. 14.]
[Illustration: Fig. 15.]
The geographical range of this system is probably greater than that of
any system of rocks above it. It is found occupying a large part of the
territory west of the Alleghany Mountains, from Canada, through New
York, and the other states, to Alabama; and extending westward to and
beyond the Mississippi river. It occupies a large district in the west
of England, and is found in great force in the north and east of Europe.
2. _The Old Red Sandstone._--This formation consists almost entirely of
a sandstone of a red color. It admits of division into three parts,
though the characters vary in different places. The lowest is a
thin-bedded argillaceous sandstone, consisting of finely levigated
material, and easily splitting into thin sheets. From this circumstance
it has received the name of _tilestone_. The middle portion is composed
of nodules or concretions of limestone imbedded in a paste of red sand
and shale. This has been called by English geologists, _cornstone_, and
though very partially developed in some regions where the system is
found, it is yet a very persistent member. The highest member of this
formation is a mass of red sandstone, often passing into a coarse
_conglomerate_. In England the thickness of the Old Red Sandstone is not
less than ten thousand feet. In this country it is scarcely three
thousand feet.
[Illustration: Fig. 16.]
The fossils of this system are a few shells, a small number of vegetable
species, and in particular localities the remains of fishes in great
abundance. The system is characterized principally by fossils of this
last kind. The fishes of this system have a cartilaginous skeleton, but
are covered with plates of bone, which were faced externally with
enamel. The jaws, which consisted of solid bone, were not covered with
integument. The exterior bony covering seems to have been the true
skeleton, as is, in part, the case with the tortoise. In some of the
fishes of this period there is a wing-like expansion on each side of the
neck, which has given them the name of _Pterychthis_ (Fig. 17). In
others, as the _Cephalaspis_, the plate of bone on the back is so large
as to cover nearly the whole body, and make it resemble a trilobite
(Fig. 18).
This system has an extensive geographical range. In England, it occupies
a band of several miles in width, extending from the Welsh border
northward through Scotland to the Orkney Islands. In this country, it
forms the Catskill Mountains, in New York, and extends south and west so
as to underlie the coal-fields of Pennsylvania and Virginia.
3. _The Carboniferous System._--This system consists of three parts,
distinguished by lithological and fossil characters.
The _carboniferous limestone_ is a dark-colored, compact limestone,
forming the base of the system, and reposing on the old red sandstone.
Its thickness is from six hundred to one thousand feet, often with
scarcely any intermixture of other rock; but it sometimes loses its
character of a limestone, and becomes a sandstone, or conglomerate. It
generally contains the ores of lead in considerable quantity, and from
this circumstance has been called _metalliferous limestone_. In England
it is the principal repository of these ores. In the Western States it
is the upper portion of the lead-bearing strata.
[Illustration: Fig. 17.]
The fossils are marine, and very numerous. Corals and crinoidea are very
abundant. The crinoidea, in some localities, form so large a part of the
rock as to have given to it the name of encrinal limestone. The
orthoceras and trilobite are found, but become extinct with this
formation. Several species of bivalves, such as Delthyris and Leptæna,
are also common.
Next above the limestone lies the sandstone, sometimes called _millstone
grit_. It is generally drab-colored, but occasionally red. Its thickness
is often equal to that of the limestone. Sometimes it is fine-grained
and compact; but generally it is coarse-grained, and often passes into a
conglomerate. It contains but few fossils, and those of vegetable
origin.
The highest part of the system is the _coal measures_. They consist of
beds of sandstone, limestone, shale, clay, ironstone and coal, occurring
without much uniformity in their order of superposition. The coal
measures have a thickness of about three thousand feet. The sandstones
and limestones are not distinguishable from the sandstones and
limestones in the lower part of the system. The ironstone either occurs
in concretionary nodules, often formed around some organic nucleus, or
it is an argillaceous ore, having a slaty structure. In either case, it
consists of subordinate beds in the shale. The coal consists of several
beds distributed through the measures. The beds vary in thickness from a
few lines or inches to several feet. In a few cases beds have been found
measuring fifty or sixty feet in thickness. The workable beds are
ordinarily from three to six feet thick.
[Illustration: Fig. 18.]
The carboniferous formation is very much disturbed by dikes, faults
(Fig. 18; see also Fig. 50), and other dislocations. The amount of
change of position in the strata, by faults, is very various; frequently
but a few feet. In one case in England there is a fault of nearly a
thousand feet. There is a case of dislocation in Belgium where the
strata are bent into the form of the letter Z, so that a perpendicular
shaft would cut through the same bed of coal several times.
The characters and order of superposition which have now been given may
be regarded as the general type of the carboniferous formation. There
are, however, several important modifications. 1. Beds of coal sometimes
alternate with beds of millstone grit. Thus, in Scotland and in the
north of England, this intermediate member of the system disappears, or,
rather, is incorporated with the coal measures. The same is true, to
considerable extent, in this country. 2. Sometimes the carboniferous
limestone also disappears as a distinct member of the system, partly by
becoming arenaceous, and partly by the intercalation of beds of coal. In
this last case, the whole formation from the old to the new red
sandstone becomes a series of coal measures. In this country the
carboniferous limestone is found very generally to underlie the coal
strata. 3. The fractures and faults, which were formerly supposed to be
characteristic of the coal formation, are seldom found in the great
coal-fields of this country, except in those of the anthracite coal of
Pennsylvania; and even there they are much less common than in the
coal-fields of Europe.
There are three principal varieties of coal, distinguished by the
different proportions of bitumen which they contain. The common
bituminous coal kindles readily, emits much smoke, and throws out so
much liquid bitumen that the whole soon cakes into a solid mass. It
contains about forty per cent, of bitumen. The second kind, or cannel
coal, contains twenty per cent., and inflames easily, but does not
agglutinate. The stone-coal, or anthracite, contains scarcely any
bitumen, ignites with difficulty, emits but little smoke, and produces a
very intense heat. The bituminous varieties are always found in the
least disturbed portions of the coal districts; and the anthracite is
found in the more broken and convulsed portions, where we may suppose
that the subterranean heat has been sufficient to drive off the volatile
bituminous part, and reduce it to the anthracite form. Hence the eastern
Pennsylvania coal-fields, which lie near the principal axes of elevation
of the Appalachian Mountains, furnish only anthracite; while the same
coal-seams, in their extension to the western part of the state, are
bituminous.
Where coal is quarried in large quantity, a shaft is sunk through the
overlying strata to the coal-beds, and the coal is raised to 'the
surface by steam power. After the coal has been quarried to some
distance from the shaft, pillars of unquarried coal are left to support
the overlying strata. Fatal accidents have sometimes occurred by the
giving away of these supports. Over a large part of the coal-fields of
the United States it has not yet become necessary to sink shafts. The
quarrying is commenced at the outcrop of the coal-bed; and, till the
cover becomes of considerable thickness, it has been found economical to
"strip" off the overlying rock, rather than to work a subterranean
gallery.
Brine-springs are often found in the coal measures of sufficient
strength to be used in the manufacture of salt. This is now done to
considerable extent in Ohio. In the valley of the Kenhawa river,
Kentucky, the rocks of which belong to the carboniferous system, the
brine is nearly saturated with salt; and in some of the borings they
have even discovered beds of rock-salt of great thickness and purity.
There is no other part of the geological series so obviously connected
with national prosperity as the coal formation. While a country is new,
the forests furnish an abundant supply of fuel; but in the course of a
few years these are consumed. This country will soon be principally
dependent upon its coal-mines for fuel, even for domestic purposes; and,
in carrying on the great branches of national industry, such as the
smelting and working of iron, and in the formation of steam for the
purposes of manufacture and transportation, we are already mainly
dependent upon mineral coal. A nation which does not possess an abundant
supply of this mineral, _or which does not use it_, cannot long maintain
a high degree of national prosperity.
In these inexhaustible masses of coal, accumulated ages before the
existence of the human race, is a most obvious prospective arrangement
for securing our happiness and improvement. And this arrangement
embraces not only the accumulation of a combustible material in such
abundance, but also its juxtaposition with an equally inexhaustible
accumulation of iron ore, and the limestone which is necessary as a flux
in the reduction of the ore. So bulky and heavy materials as coal and
iron ore could neither of them have been transported to any
considerable distance for the manufacture of iron; and without the
manufacture of iron on a large scale, the present operations in
manufactures and transportation could never have been entered upon. A
large proportion of the iron furnaces in this country, and nearly all of
them in Great Britain, employ mineral coal for fuel, and obtain their
ore from the beds contained in the coal measures.
The fossils of the coal measures are almost entirely of vegetable
origin, and are very abundant. They are seldom found in the coal-beds,
but in the strata of shale immediately above or below the solid coal.
[Illustration: Fig. 19.]
The _Stigmaria_ (Fig. 19) is found most abundantly, and in a large
proportion of cases to the exclusion of every other form, in the lower
shales. It consisted of a large dome-shaped mass, often three or four
feet in diameter, with trailing branches, or roots, spreading off
horizontally to a distance of twenty feet. In a few instances tree ferns
have been found, petrified in a horizontal position, and being
apparently a mere continuation of the stigmaria. Hence the stigmaria has
been supposed to be the base of the tall tree ferns, the leaves of which
so abound in the upper shales. If this is not the case, there are no
forms of the existing flora of the earth analogous to the stigmaria. It
is always found in connection with the coal-beds of the carboniferous
formation, and never with the coal-beds which sometimes occur in the
later formations.
[Illustration: Fig. 20.]
[Illustration: Fig. 21.]
The tree ferns (Fig. 20) attained a height of fifty or sixty feet, and a
diameter of four feet. They have received the name of _Sigillaria_ in
consequence of the seal-like impressions (Fig. 21) with which the
surface is covered, and which are the scars left where the fronds have
fallen off. These fronds (fern leaves) are the most abundant fossil of
the series. They are distinguished by some peculiarity in form, as the
Sphenopteris (wedge-shaped fern leaf), Pachypteris (thick fern leaf),
&c. (Figs. 22 and 23.)
[Illustration: Fig. 22.]
[Illustration: Fig. 23.]
[Illustration: Fig. 24.]
[Illustration: Fig. 25.]
There was another kind of Sigillaria (Fig. 24), in which the surface was
fluted, and the markings are superficial, and occur on the ridges. It
reached as great a size as the tree ferns, but to what general class of
plants it belonged is still doubtful.
The _Lepidodendron_ (scale-covered tree) (Fig. 25) is the fossil which
most nearly resembled in general appearance our present forest trees.
Specimens are found four feet in diameter and seventy feet in height. In
botanical characters it resembled, in some respects, the trailing
club-mosses, while in others it was very similar to the Norfolk Island
pine.
[Illustration: Fig. 26.]
The _Calamite_ (Fig. 26) was a plant resembling, in its jointed and
striated surface, the equisetum (rush), but was sometimes twelve inches
in diameter.
The carboniferous formation exists more or less abundantly in all the
great divisions of the earth. It occurs in nearly all of the countries
of Europe. The largest deposits known are, however, in the United
States; especially in the States of Pennsylvania and Virginia, and in
Ohio.
4. _The New Red Sandstone._--The lower division of this formation,
called the Permian system, consists of a thick mass of sandstones,
generally of a red color, with occasional alternations of argillaceous
rock, succeeded by a series of magnesian limestones. The upper division,
or Triassic system, is composed of a red conglomerate, a limestone which
has received the name of Muschelkalk (shelly limestone), and a series of
variegated marls and sandstones.
The ores of copper are found, to considerable extent, in this formation.
The rich copper mines of Germany are in the magnesian limestone, or, as
it is there called, Zechstein (minestone). The Lake Superior copper
mines occur in a red sandstone formation, which will probably be found
to belong to this system.
The salt-beds, salt springs, and beds of gypsum, are so, generally found
in this rock in England, that it has been called by the English
geologists the "saliferous system." It is, however, found that in other
countries these minerals occur in equal abundance in formations of an
earlier and later date.
[Illustration: Fig. 27.]
The fossils of this system are not abundant. In the Permian portion,
impressions of fishes are found, always with the peculiarity that the
tail is _heterocercal_ (Fig. 27); that is, with the spine continued into
the upper lobe. The same peculiarity prevails in the carboniferous and
all the earlier formations. Fishes with the tail _homocercal_ begin to
appear in the Triassic portion of this system, and are found in all the
subsequent formations. The remains of saurians also occur in this
formation.
[Illustration: Fig. 28.]
The red sandstones seem to have been better adapted to retain the forms
which were impressed upon them than to preserve the organic remains
which were deposited in them. Hence, while they contain but few fossils,
the strata are often covered with ripple marks, with sun cracks,
occasioned by contraction while drying, or with depressions produced by
rain-drops, and the pits are sometimes so perfect as to show the
direction of the wind when the drops fell. (Fig. 28.) The tracks of
animals are also well preserved. Some of them were produced by reptiles
(Fig. 29, _c_), and some probably by marsupial animals, but most of them
by birds (_a_, _b_). President Hitchcock has distinguished the tracks of
more than thirty species in the sandstones of the Connecticut valley.
Birds, reptiles and marsupial animals, seem to have been first
introduced during this period.
[Illustration: Fig. 29.]
The new red sandstone is well developed in all its members on the
continent of Europe. In England, all the members are present, except the
Muschelkalk. The Triassic portion of it occurs in North America. It is
found in detached portions, probably as parts of a continuous formation,
in Nova Scotia, the eastern part of Maine, the Connecticut valley, and
from New Jersey southward through Pennsylvania, Maryland, &c., to South
Carolina.
5. _The Oölitic System._--The lower portion of this system is the Lias,
and consists of a series of fissile, argillaceous limestone, marl, and
clays. The _Oölite_ forms the intermediate member of the system, and
consists of alternations of clay, arenaceous rock and limestone. Some of
the limestones have an oölitic structure, and the whole system takes its
name from this circumstance, though this structure is not found in all
parts of it, and is often found in other formations. The central part of
the oölite, the coral rag, is principally a mass of corals and
comminuted shells. The _Wealden_, the highest member of the oölitic
system, is an estuary deposit, consisting of calcareous beds, followed
by sandstone, and terminated by the Wealden clay.
This system is throughout highly calcareous, and furnishes, wherever it
is developed, valuable materials for architectural and ornamental
purposes.
This system is distinguished for the great amount and variety of its
organic remains. The _vegetable productions_ were intermediate between
those of the coal period and those of the present time. The upper
oölite, in the south of England, contains the stumps of trees and other
plants, rooted in a black carbonaceous layer, evidently the soil from
which they grew. These stumps and prostrate trunks are the remains of
coniferous trees of large growth. (Fig. 30.)
Corals occur in great abundance; also encrinites (Fig. 31), mollusks
(Fig. 32), and cephalopoda.
[Illustration: Fig. 30.]
[Illustration: Fig. 31.]
[Illustration: Fig. 32.]
But this system is specially characterized by the remains of saurian
reptiles. The Ichthyosaurus (Fig. 33, _a_) was a marine animal, having
the general form of a fish, while its head, and especially its teeth,
resemble those of the crocodile. It was an air-breathing animal like
the cetacea, and was furnished with similar paddles. It was carnivorous,
and was undoubtedly the largest and most formidable animal existing in
the earlier part of the oölitic period. Its length could not have been
less than thirty or forty feet.
[Illustration: Fig. 33.]
The _Plesiosaurus_ (Fig. 33, _b_) was also a marine animal, and in ninny
respects similar to the Ichthyosaurus; but its general form was more
slender, its head was small, and its neck was of great length, the
cervical vertebræ exceeding in number those of the swan.
[Illustration: Fig. 34.]
The _Pterodactyle_ (Fig. 34) was a small saurian, of the size, probably,
of our largest eagle. The finger-bones, which in the other saurians form
the paddles, are in the Pterodactyle very much lengthened, so as to
support a membranous expansion, like that of the bat. These wings were
of sufficient size to enable it to sustain itself in the air, and to
make a rapid and easy flight.
The _Iguanodon_ is a Wealden fossil, remarkable for its great magnitude.
It is estimated that its length was seventy feet. It was a lizard,
adapted for motion on land, and was herbivorous.
This formation is well developed in England, and, with the exception of
the Wealden, on the continent of Europe. It has been supposed that no
part of the oölitic series was to be found in this country; but there is
a highly arenaceous rock occupying the valley of the James river, in the
vicinity of Richmond, Virginia, of considerable extent, and a thousand
feet in thickness, containing a bed of coal of forty feet in thickness,
which, from its fossils, must be referred to the oölitic series.
6. _The Cretaceous Formation._--The lower part of this formation
consists of _greensand_, interstratified with beds of clay. The
intermediate portion is a mixture of argillaceous greensand and _impure
chalk_. The upper part is composed of _chalk_, which is a friable,
nearly pure carbonate of lime. The strata of chalk are separated, at
intervals of from three to six feet, by layers of flint, either in the
form of nodules or of continuous strata.
These characters, by which the cretaceous system is known in England,
are but partially recognized elsewhere. Thus, in the Alps, the
"Neocomian System," consisting of crystalline limestones, is the
equivalent of the English greensand; while the greensand of this country
is the equivalent of the white chalk of England.
[Illustration: Fig. 35.]
[Illustration: Fig. 36.]
The fossils of the cretaceous formation are very different from those of
the oölite, and are such as to show that it was deposited in deep seas.
_Microscopic shells_ are often so abundant as to constitute a large
proportion of the mass. _Zoöphytes_ are very numerous, such as sponges,
corals, star-fishes (Fig. 35, _d e_), and a few crinoidea (_b_).
Mollusks were also abundant, and cephalopoda, consisting of
chamber-shells and belemnites (Fig. 36). The belemnite probably
resembled the existing cuttle-fish; but the remains consist, in most
cases, of a partially hollow calcareous substance (_b_), which was
contained within the animal, and formed its skeleton.
The chalk and greensand are largely developed in England; and the same
formation, with different lithological characters, is found in great
force flanking the principal mountain ranges of southern Europe, and
extending into Asia. In this country the system commences with the
greensand and friable limestones of New Jersey, and following the
Alleghany range to its southern termination, it bends around into a
north-western direction, and is continued into Missouri.
7. _The Tertiary System._--The tertiary strata embrace the formations
from the cretaceous to the human era. They consist of clay, sand,
sandstone, marl and limestone, and are distinguished from the lower
rocks by being less consolidated; though the limestones are in some
instances solidified, and resemble the strata of earlier origin. The
tertiary strata are generally of less thickness than the older
formations, and less continuous, being local deposits formed in lakes
and estuaries. In a few instances they have been thrown into inclined
positions, though in most cases they have been but slightly disturbed,
and raised but a few hundred feet above the present level of the sea.
The late tertiary strata seldom overlap the older, so as to indicate
their relative ages by superposition. They have therefore been separated
into groups according to the proportions of living and extinct species
of shells which they are found to contain. The oldest tertiary or
_Eocene formation_[A] contains only four per cent, of living species,
the _Miocene_ contains seventeen per cent., the Pleiocene forty per
cent., and the _Pleistocene_ ninety per cent.
[A] _Eös_, dawn, and _kainos_, recent. The formation
which commenced at _the dawn of the recent period_,
containing but a small number of living species.
Miocene (_meion_, less), less recent than the Pleiocene
(_pleion_, more). Pleistocene (_pleistos_, most), most
recent.
During the pleistocene period, peculiar conditions existed, by which a
great amount of loose material, known by the name of _drift_, was spread
over the northern portions of both hemispheres. In America it is found
from Nova Scotia nearly to the Rocky Mountains, and extending as far
south as Pennsylvania and the Ohio river. In Europe, it is found from
the Atlantic to the Ural Mountains, and reaching south into Germany and
Poland. It is also found in the colder portions of South America, and in
the vicinity of several mountains, as the Alps.
It consists of irregular accumulations of earthy substances of different
degrees of fineness, but characterized by containing masses of rock of
considerable size, often of many tons weight, called boulders. Rocks
having the same lithological characters exist in situ north of where the
boulders and other drift are now found, though at a distance often of
one or two hundred miles. There can be no doubt but that the drift has
been transported from these northern localities; and the polished,
striated and grooved condition of the rocky surface, wherever the drift
is distributed, has obviously been produced by the passage of the drift
materials over it.
Towards the close of this period, while the land was a few hundred feet
below its present level, there were deposited in the valleys of the
drift region beds of blue and gray clay, materials which are used in
making bricks and coarse pottery; also beds of sand, sometimes evenly
spread out, but often thrown into irregular mounds and ridges.
In regions which are not covered with drift,--as the south of Europe and
the United States,--the pleistocene deposits are succeeded, without
apparent change of conditions, by those which are now taking place.
The formations of the tertiary period are distinguished from those of
the cretaceous period by the absence of deep-sea fossils, and from the
oölite by the absence of its characteristic saurians. The mollusks are
also very different, such genera as the cerethium (Fig. 37), murex (Fig.
38), and conus (Fig. 39), which abound in the present seas, first
appearing in the tertiary period. The nummulite (Fig. 40), a peculiar
form of chambered shell, is so abundant as to constitute in some places
almost the entire rock.
The period is however characterized by the existence of a large number
of pachydermatous animals, of which the tapir, hog, horse and elephant,
are examples of living species.
[Illustration: Fig. 37.]
[Illustration: Fig. 38.]
[Illustration: Fig. 39.]
[Illustration: Fig. 40.]
The _Paleotherium_ (Fig. 41) resembled, in most respects, the tapir. It
was furnished with a short proboscis, and the foot was divided into
three toes. The length of the largest species was about that of the
horse; but its body was larger, and it was of less height.
[Illustration: Fig. 41.]
[Illustration: Fig. 42.]
The _Anoplotherium_ (Fig. 42) was a more slender animal, and resembled
in size and general form the gazelle.
The _Megatherium_, an animal of the late tertiary epoch, was larger than
the existing species of elephant, and in its general structure and
habits resembled the sloth.
The _Mastodon_ (Fig. 43) lived during the latest portion of the tertiary
epoch. Its remains are found most abundantly where the animal seems to
have perished by sinking into the soft marshy ground near the brackish
springs of New York and Kentucky. But they are found also in Europe and
Asia. It was larger than any existing land animal, and was nearly allied
in structure and habits to the elephant.
[Illustration: Fig. 43.]
The _Mammoth_ was a species of elephant, now extinct, of which remains
are found with those of the mastodon, but in the greatest abundance in
Europe and Asia. A large number of skeletons, many of them imperfect,
have been discovered in the low grounds in the south-east of England. It
was this animal which was found encased in ice and sand in Siberia, in
1804.
Contemporaneously with the existence of these huge animals, a near
approach was made to the present fauna of the earth, by the introduction
of ruminant animals resembling the ox and deer, and especially by the
existence of the class of animals which in anatomical characters stands
next to man, the apes and monkeys.
The tertiary system, though not generally so continuous over extended
areas as the older formations, yet constitutes the surface of a very
large part of Europe. (See Fig. 59.) In the United States the earlier
portion is found along the seaboard, from New Jersey to Louisiana, and
extending back towards the mountains to a distance varying from ten to
one hundred miles. The later deposits are found in detached portions
throughout the Eastern and Middle States. It covers a large surface in
South America, and is found in India.
8. _The Recent Formation._--It is intended to embrace in this term
strata which have been formed since the creation of man. It is, however,
impossible to separate them by any well-defined characters from those of
the tertiary period. The recent formation consists of land which is
forming by the filling up of lakes, and by the increase of deltas from
the accumulated sediment which rivers have furnished.
There is, however, no doubt but that formations on a large scale have
continued in progress over extensive areas of the bed of the sea; and
they have been no less rapid, we may presume, than they were in earlier
periods. But, though they are preserving the records of the present era,
they will probably remain in a great measure inaccessible for many ages.
These deposits, so far as they are accessible, are found to contain the
remains of plants and animals (including man) now living in the vicinity
where the deposits are forming.
SECTION VI.--FOSSILS.
Any organic substance imbedded in a geological formation, or any product
of organic life, as a coprolite or a coin, or any marking which an
organic substance has given to a rock, is regarded as a _fossil_. The
study of fossils, as a branch of practical geology, requires an
acquaintance with the principles and the minute details of botany and
zoology. Without this knowledge, however, many of the general
conclusions to which the study of fossils has led may be understood.
1. _Fossils are preserved in different ways._--When any organic
substance is imbedded in a forming rock, it may itself remain; or it may
be removed by the infiltration of water, or other causes, so gradually
as to leave its form, and even its most delicate markings, in the rock;
or some mineral substance may have been substituted, and fill the space
which the organic substance once occupied; that is, it may be an organic
substance preserved, it may be an impression of it, or it may be a cast
of it.
2. The process by which the substitution in this last case is effected
is called _mineralization_. The mineralizing ingredient is generally
derived from the contiguous rock. In siliceous rocks it is silex. In
calcareous rocks it is carbonate of lime. When iron is diffused through
a rock, it often becomes the mineralizer. The substituted mineral is
generally a very perfect representation of the original fossil. We
cannot therefore suppose that the original substance was entirely
removed before any of the mineral matter was deposited. The substitution
must have taken place particle by particle, as the organic matter was
removed. Fossils are, in fact, often found, in which the mineralization
has been arrested after it had commenced, so that the fossil is in part
an organic and in part a mineral substance. It has been proved, by
direct experiment, that these changes of removal and substitution are
simultaneous. Pieces of wood were placed in a solution of sulphate of
iron. After a few days, the wood was found to be partially mineralized,
and after the remaining ligneous matter had been removed by exposing it
to a red heat, "oxide of iron was found to have taken the form of the
wood so exactly, that even the dotted vessels, peculiar to the species
employed, were distinctly visible under the microscope."
3. As the fossiliferous strata are generally of marine origin, it is to
be presumed that only a small proportion of terrestrial animals are
preserved; and our knowledge of the organic remains which are preserved
is yet so imperfect, that discoveries are constantly making, as
examinations are extended. Still, enough is known to enable us to draw
some satisfactory conclusions as to _the order in which living beings
were created upon the earth_.
Though most of the earlier organic forms which have been preserved are
of animal origin, yet vegetable remains occasionally occur in connection
with them, and we must suppose vegetables to have been produced
abundantly. For all animal food consists of vegetable substances, or of
animal substances which have once existed in the vegetable form. No
animal is capable of effecting those combinations of inorganic matter
upon which its growth and sustenance depend. We may therefore conclude
that _the introduction of animals and vegetables was contemporaneous_.
The greatest development of vegetable life was, however, during the
carboniferous period. The design of this abundant growth was
prospective. It was not produced for the support of animal life, but for
fuel, and stored till man should be introduced, and so far advanced in
civilization as to make this supply of carbonaceous matter subservient
to his wants and happiness.
In the earlier periods, the lower forms of animal life were, beyond all
comparison, the most abundant; yet the four great divisions of the
animal kingdom, Radiated, Articulated, Molluscous, and Vertebrated
animals, were all represented. There is, however, no evidence that any
vertebrated animals, except fishes, were created till after the
carboniferous period. In the next formation, the new red sandstone, we
find the tracks of reptiles and birds, and probably of marsupial
animals. The first evidence of the existence of mammalia in great
numbers is in the tertiary period, when the pachydermata and edentata
were so much more abundant than they have ever been since, and when the
bimana first appear.
But there is no evidence from geology that man existed till after the
close of the tertiary period. The grounds upon which contrary statements
have sometimes been made are untenable. In Ohio a very perfect
impression of a human foot was found on a slab of limestone of the
silurian age. But it was subsequently ascertained to have been common
for the aborigines, in the vicinity of their encampments, to cut in the
rocks, with surprising accuracy, the forms of the tracks of man and
other animals.
There is a human skeleton in the British Museum imbedded in solid
limestone, and another in Paris, both taken from Guadaloupe. It was at
one time supposed, from the degree of solidification of the limestone,
that it must have been formed at an early geological period; but it is
found that the beach-sand of that island now solidifies rapidly, from
the carbonate of lime which the waters there hold in solution. It is
rendered probable that the skeletons found there have not been buried
more than a century and a half.
4. As many parts of the bed of the present seas, which are probably
receiving detrital matter constantly, are unfavorable for the
development of animal life, while other parts are highly favorable, it
might be presumed that animal life would be equally scanty in particular
localities while the earlier rocks were forming, and in other localities
very abundant. Hence some strata, for hundreds of feet in thickness, are
composed almost entirely of fossils, while other strata are nearly or
quite destitute of them. The same member of a formation may in one place
be full of fossils, and in another without them. _The distribution of
fossils_ is therefore subject to no general law; at least, none of which
we can avail ourselves, in the search for them.
5. The value of fossils in geology consists in the use which is made of
them in determining _the origin and age of strata_.
As the animal species which inhabit bodies of fresh water are always
different from those found in the sea, their remains constitute the best
means of determining whether a formation is of _fresh water_ or _marine
origin_. In order to decide this point, it may, in some cases, be
necessary to be acquainted with the habits of particular species. In
most cases, however, it will be sufficient to remember that in
fresh-water formations, first, there are no sponges, corals, or
chambered shells; second, the univalves all have entire mouths (Fig.
44). Third, the bivalves are all bimuscular (Fig. 47). If, therefore, a
formation is found to contain sponge, coral, a chambered shell, a
univalve with a deeply notched mouth (Fig. 45), or a unimuscular bivalve
(Fig. 46), it must be considered a marine formation.
[Illustration: Fig. 44.]
[Illustration: Fig. 45.]
[Illustration: Fig. 46.]
[Illustration: Fig. 47.]
We have seen that the same formation, as exhibited in different places,
differs in its thickness, composition and degree of solidification. If
we could trace the strata through all the intermediate space, we might
be certain of their being the same formation, notwithstanding the change
in lithological characters. But this can seldom be done, even for a few
miles in extent. Sections of the strata are obtained only occasionally,
where rivers have cut through them, or where, over limited areas, the
soil has been removed from the outcropping edges. It is also frequently
the case that the strata are so much disturbed that their position will
furnish no aid in determining their age. When folded axes occur (as here
represented), the older strata are often the uppermost. There is an
instance in the Alps in which strata of vast thickness have been
inverted during the process of upheaval, and now rest on a bed of rock
formed from the debris which they had supplied.
[Illustration]
And yet it is important to determine what formations are of the same
age, notwithstanding their displacements, difference in lithological
characters, and separation by great distances and by mountains or
oceans. This determination can be made only by a comparison of the
imbedded fossils. It is found that every formation, and every important
member of a formation, contains an assemblage of fossils peculiar to
itself. When very widely separated, the species of fossils may not be
identical, but so very similar that they are regarded as equivalent
species. _The identification of formations_ consists in the
identification of fossils. It is for this purpose mainly that fossils
are regarded as of so great importance.
6. If each formation is characterized by the presence of new species, it
follows that _the work of creation was a progressive one_, continued
through long periods of time. The latest creation of which we have any
geological evidence is that of man. And if the leading design of the
existence of this earth was as a theatre for the development of moral
character, it is to be presumed that the work of creation ceased when a
species possessing moral capacities had been introduced.
It follows also, from what has been said, that there has been a constant
disappearance, a death, of species. It would seem that each species has
a life assigned to it, which is to be completed and surrendered. Though
its continuance is many times longer than the life of any individual of
the species, yet _it is the course of nature that species should
disappear_.
There may be something in the constitution of each species by which its
continuance is limited, making an old age and death necessary, as it is
in individuals. But there are other causes by which the duration of
species may often be terminated. The subsidence of New Holland would
cause the destruction of a large number of species. The preservation of
the human species was at one time effected only by a special and
miraculous interference. Slowly operating causes are now at work, by
which many species, such as the elephant, wolf and tiger, will at length
become extinct. Their existence in a natural state cannot long be
continued in a civilized country. The forest, their natural abode,
disappears, and some are intentionally destroyed, because they render
life and property unsafe. Under the operation of these causes, the Irish
elk (cervus giganteus) has become extinct, probably within the human
era. The Dodo, a gallinaceous bird, found living when maritime
communication between Europe and the East Indies was first established,
is now extinct. The Apteryx, a bird belonging to New Zealand, has
probably become extinct since the commencement of the present century.
SECTION VII.--THE TIME NECESSARY FOR THE FORMATION OF THE STRATIFIED
ROCKS.
There are no means of which the geologist can avail himself to determine
the antiquity of the earth, or the amount of time since the sedimentary
deposits commenced. But a nigh degree of antiquity may yet be shown.
The materials for all the stratified rocks have been obtained by the
destruction of previously solidified igneous rocks. This destruction may
have been accomplished in part by the operation of volcanic forces, but
much of it is the result of slow disintegration, and of the eroding
power of running water; and we can scarcely conceive of a period
sufficiently protracted for such results.
This conclusion of the high antiquity of the earth is confirmed by
observing that the stratified rocks consist of layers often not thicker
than sheets of paper, and probably not averaging the tenth of an inch;
and yet each layer is separate from the rest, in consequence of some
change in the conditions under which it was deposited. Each layer was
probably produced by the deposition of all the sediment furnished at one
time, and hence only as many layers would be formed in a year as the
number of freshets in the rivers which furnished the materials. If we
consider the fossiliferous and metamorphic rocks to be each forty
thousand feet in thickness,--which is not too large an estimate,--we
must reckon the years by hundreds of thousands to make the time
sufficiently extended for the result.
All the formations of any considerable extent now above the surface of
the sea existed before the creation of man, for none of them contain any
evidence of the existence of human beings; and if they had existed while
these strata were forming, sufficient evidence would have been left of
the fact, either in the form of fossilized human bones, or of works of
human art. Hence, whatever be the estimate which we form of the
antiquity of the earth, from the slowness of denudation, or from the
thickness of the strata, we must now add to that estimate the period
elapsed since the creation of the human species.
We have seen that at different periods of the earth's history different
species of animals inhabited it. We are unable to fix with accuracy the
ordinary duration of species. But the species which are now extinct
probably had an existence as long-continued as will be enjoyed by
species now living. Many recent species are known to have existed at
least nearly six thousand years, without, in most cases, any indications
of their soon becoming extinct. Whatever period be assigned as the
ordinary duration of species, that period has been several times
repeated; for the earth has been several times re-peopled, and every
time by species which had not before existed.
Moreover, the _amount_ of organic matter in the strata must have
required long periods of time for its accumulation. The vegetable
deposits, now converted into coal, are generally several feet thick, and
often over a hundred feet, and are known to extend over several thousand
square miles, both in this country and in Europe. Many of the
sedimentary rocks consist almost entirely of animal remains. The
mountain limestone, for instance, is eight hundred feet or more in
thickness, and in some places consists of the exuviæ of encrinites and
testacea.
In other cases the length of time required is shown, not from the amount
of organic remains, but from the evidence that they were deposited very
slowly. The polishing stone called tripoli is found in beds of ten or
twelve feet in thickness, and is composed entirely of the siliceous
shells of animalcules, so minute that, according to the estimate of
Ehrenberg, the number in a cubic inch is forty-one billions. Several
other rocks, such as semi-opal and flint, are sometimes found to have a
similar constitution. The time necessary for the accumulation of beds
several feet thick by the shells of animalcules so minute must have been
very great.
Each of these facts carries us back to a period immeasurably anterior to
the creation of man, as the epoch when the sedimentary deposits
commenced. There are no facts in geology which point to a different
conclusion. It is of the utmost importance to the geological student to
familiarize himself with this principle. It will assist him in
comprehending the greatness of geological changes, and in applying other
principles in explanation of geological phenomena.
This principle, so obvious to any one who allows himself to reason from
the facts which geology presents, has sometimes been regarded as at
variance with the Mosaic account of the creation. And if this account
really assigns an antiquity to the earth of not more than six thousand
years, the difficulty exists.
The statements made by Moses are found, upon examination, to be of the
most general character. They assert, in the first place, simply that "In
the beginning God created the heaven and the earth." The time which
elapsed after this first act, and previously to the acts of creation
subsequently recorded, is not limited by the sacred narrative. It may
have been during this indefinite lapse of time that God gave existence
and enjoyment to a large number of animal species on the surface of the
earth, and at the same time effected most of those physical changes in
the crust of it which have rendered it a fit abode for intellectual and
moral beings.
But if the word _day_, in the first chapter of Genesis, be considered to
mean a prolonged period (and philologists regard such an interpretation
as admissible), then that chapter is a record of the most important
events in the history of the earth up to and including the introduction
of man. And the account, thus understood, coincides with the results of
geological examinations.
Instead, then, of discrepancy between the works and the word of God, we
have this remarkable fact, that a history of the earth, written long
before the science of geology was known, is not contradicted, but
confirmed, by the progress of science thus far.
CHAPTER III.
OF THE CHANGES TO WHICH THE CRUST OF THE EARTH HAS BEEN SUBJECTED.
SECTION I.--CHANGES WHICH HAVE TAKEN PLACE AT GREAT DEPTHS BELOW THE
SURFACE.
The lowest change of winch we can gain any information is the _formation
of granite_. It will be shown hereafter that it has been in a melted
state, and that it has taken its present form on cooling. But whether
any considerable portions of the granitic masses, or of the melted
masses now below the surface, have resulted from the fusion of
stratified rocks, we have not the means of determining. It is, however,
not improbable, that in the changes of level to which the crust of the
earth has been subjected, the stratified rocks may have gone down so far
as to become melted. At the same time, the melted rock which is thrown
to the surface by volcanoes is subjected to the various destroying
agencies by which it becomes sedimentary matter, to be deposited as
mechanical strata. Thus, as the igneous rocks from below are brought up
to furnish materials for mechanical strata, there must be an equal
amount of depression of the mechanical strata towards the seats of
igneous action. And if this change takes place more rapidly than the
thickness of crust increases, then portions of the sedimentary rocks
must be undergoing fusion.
Next above the granite an immense thickness of rock occurs, which
exhibits, from its stratification and from the water-worn fragments
which it contains, distinct evidence of its mechanical origin. And yet
it is very different from the later mechanical formations. It is more
highly crystalline; it has, to a great extent, assumed a cleavage
distinct from the planes of stratification, and chemical affinity has
been so far active as to produce new combinations, and give to them
their peculiar crystalline form, as in the case of garnets, iron
pyrites, &c. These strata also differ from those above them in
containing no organic remains. It is not certain that organic life
existed on the earth at the time when these rocks were deposited. Either
it did not, or the evidence of it in the strata of that period has been
obliterated. The changes have at least been sufficient to justify their
being characterized as _metamorphic rocks_.
SECTION II.--CHANGES IN THE MASS OF THE STRATIFIED ROCKS.
1. The stratified rocks were deposited as mud or sand, and were at first
in a yielding state. Most of these deposits have become _solidified
rock_, such as limestone, clay slate and sandstone. The chalk of England
is, however, but imperfectly consolidated, the great sandstone formation
of New Holland is a friable mass easily disintegrated, and occasionally
beds of clay in a plastic state are found as far down as the coal. Among
the later rocks the solidification is less general, though there is some
degree of hardening in all except the most superficial layers. The
_fissile structure_ results from the solidification of the particles
composing each layer separately.
2. Since the solidification of the strata, or perhaps in connection with
it, there has been something of movement among the particles, resulting
in mineral veins, conchoidal structure, &c. One of the most general
changes of this kind is that by which a mass becomes separable into thin
sheets, independent of the stratification, and not parallel with it.
This structure is represented by Fig. 48, in which the heavier lines are
those of stratification, and the lighter of _cleavage_.
3. The strata have been everywhere more or less broken, and the
_fractures_, nearly vertical, extend to groat depths. When a fracture
reaches the surface, it often becomes a channel for water. It is thus
widened by the erosion, the deepest parts become filled with debris, and
it becomes a _gorge_, _ravine_ or _valley_.
[Illustration: Fig. 48.]
If the fracture does not come to the surface, it becomes a _cavern_. In
limestone, caverns which are formed in this way are very frequent, and
extend for many miles. There is generally a stream of water running
through them, but not of sufficient volume to have produced the erosion
which has been effected.
When the sides of the fracture are but little separated, some mineral
often separates itself from the adjacent rock, and filling up the space,
reunites the broken parts. It is then called a _vein of segregation_
(Fig. 49, _a b_). But the fracture is more frequently filled with some
volcanic rock injected from below. It is then a _dike_ (_c_ _d_), and
may have a width of many rods, though it often diminishes in width till
it is a mere thread. A dike of which the injected material is a metallic
ore is a _mineral vein_.
[Illustration: Fig. 49.]
4. The uplifting force by which the fracture is produced has frequently
raised the rock on one side higher than it has on the other. This is
called a _fault_. (Fig. 50.) The unequal movements by which the fault is
produced seem in some instances to have been repeated several times, and
the grinding of the broken edges upon each other has polished and
striated the sides of the fracture.
[Illustration: Fig. 50.]
[Illustration: Fig. 51.]
[Illustration: Fig. 52.]
5. Sedimentary rocks are often found with the planes of their strata
more or less _inclined_. It is evident that they were not thus formed.
The depositions of sediment from water will always be horizontal, or, at
most, only slightly inclined. But there is often evidence in the rock
itself that its strata were once horizontal. It is frequently observed
that vertical strata contain pebbles with their longer axes in the plane
of the strata. (Fig. 51.) When these pebbles were deposited, the longer
axes would take, on an obvious mechanical principle, a horizontal
position. Their present vertical position must have resulted from a
change in the position of the strata in which they are enclosed. The
same thing is shown by the position of a petrified forest in the south
of England, known as the Portland dirt-bed. Some parts of it are
inclined at an angle of forty-five degrees. The position of the
vegetable remains (Fig. 52) shows that when they were growing the
surface was horizontal.
The line _b d_ (Fig. 53), on inclined strata which makes with the
horizon the greatest angle, is called the _direction of the dip_. The
angle thus formed (_a b d_) is the _angle of inclination_. When inclined
strata come to the surface, the exposed edge, _b c_, is the _outcrop_,
and the line of outcrop on a horizontal surface is called the _strike_
of the strata.
[Illustration: Fig. 53.]
When the inclined position is produced by an uplift of the strata, along
a given line, so that they dip in opposite directions, this line is
called an _anticlinal axis_, as at Fig. 54. If, however, the strata are
fractured along this line, as at _b_, the fracture becomes a _valley of
elevation_.
[Illustration: Fig. 54.]
If depression take place along a given line, as at _c_, the strata will
dip towards this line, and it will be a _synclinal axis_. The depression
will be a _valley of subsidence_. A synclinal axis would also be
produced by an elevation of the strata, as at _d_ and _e_, on each side
of it, and the valley thus produced is one of elevation.
When successive sets of strata, as _f_ and _d_, Fig. 53, are not
parallel, they are said to be _unconformable_.
[Illustration: Fig. 55.]
[Illustration: Fig. 56.]
6. When the strata are subjected to displacement, they do not always
take a merely inclined position, but are often _contorted_ (Fig. 55), or
folded together (Fig. 56). These _folded axes_ frequently succeed each
other for many miles. (See Figs. 7 and 82.) In the case represented by
Fig. 56, if the highest portion has been removed, so that the line _a b_
represents the actual surface, we shall have apparently a succession of
deposits, of which those at _b_ would be the newest, and the oldest
would be found at _a_, when in fact the strata at the extremities are
parts of the same layer.
It is probable that disturbances like those now mentioned have been
taking place continually, in different places, from the earliest times.
There have been no periods of universal disturbance, and none of
universal repose. On the contrary, the periods of disturbance in one
part of the world have been periods of repose in another. For example,
the coal measures of Europe were much broken and disturbed before the
deposition of the new red sandstone, and the close of the coal period
was at one time supposed to have been a period of general convulsion. It
is now ascertained that the principal coal-fields in this country were
not much disturbed at that period, and have not been since.
SECTION III.--CHANGES OF ELEVATION AND SUBSIDENCE.
The continents, if we except the more rugged and broken portions, rise
from the sea with an almost imperceptible ascent; and even the mountains
have a much gentler slope than we are apt to suppose, so that a section
of the earth parallel to the equator would be almost a perfect circle.
The slope of a mountain, from its base to its highest point, rarely
forms with the horizon an angle of as much as twelve degrees. In the
following figure (57), A represents the peak of Chimborazo, B of
Teneriffe, C of Ætna, and D of Mount Loa, the principal volcano of the
Sandwich Islands. The highest mountains would be represented on a
twelve-inch globe by an altitude of less than the one-hundredth of an
inch above the level of the sea. But the rising and sinking of these
masses, though so small compared with the dimensions of the earth, are
yet geological changes on the largest scale.
1. _The Elevation of Mountains._--Mountains have formerly been covered
with the waters of the ocean. This is evident, in the case of some
mountains, from the existence of stratified rocks reaching to the
summits. The stratification could have been produced only by deposition
from water. It is, moreover, evident from the existence of marine
fossils, distributed through these strata, so abundantly, that they
cannot be accounted for on any other hypothesis than that the animals
lived and died where the remains of them are now found. These strata
must therefore have formed the bed of the sea while the fossils were
accumulating.
[Illustration: Fig. 57.]
There is no _direct_ evidence that the granitic mountain peaks were ever
submerged. But there is reason for believing that the sedimentary strata
which now occupy the lower slopes were, at the time of their deposition,
continuous,--the igneous rock having subsequently broken through
them,--so that the waters of the ocean once rested on the whole area
which the mountain now occupies!
If the ocean could ever have been above its present level sufficiently
to have covered all the sedimentary rocks, we might assume that the
height of mountains has not been changed. But the level of the ocean
cannot be subject to much variation. The total amount of water on the
globe is always the same. If the continents and mountains were all
submerged at once, and the waters were expanded by the highest
temperature consistent with the liquid form, there would not be a change
of level of more than two hundred and fifty feet. We may assume, then,
that the ocean level has always been essentially the same that it now
is. We must therefore conclude that the sedimentary rocks, and the
mountains of which they form a part, have been elevated to their present
position from the bed of the sea.
Different mountain ranges have been elevated at different periods. The
silurian and carboniferous formations were deposited before the
Alleghany Mountains, which they contributed to form, were elevated;
while the new red sandstone and the cretaceous and tertiary formations
were deposited subsequently to the upheaval. They are accordingly found
at the base of the range, nearly horizontal, and have risen above the
level of the ocean only as the continent generally has risen. The
Pyrenees were elevated after the deposition of the cretaceous rocks, and
have carried them up so that they appear at a high angle, while the
tertiary rocks at the base are horizontal, as in the United States. The
Andes have carried up the tertiary rocks with them, and their elevation
must therefore belong to a recent period. It appears that they are even
yet rising.
It has recently been shown that the Alps have been subjected to upheaval
at several distinct periods. At the close of the silurian period they
formed a cluster of islands. At the commencement of the tertiary period
they became a mountain range, and at the close of that period they were
thrown up some two thousand feet higher, to their present position.
Nearly the same things will probably be found true of other mountain
ranges, when their structure has been minutely studied.
The elevation of contiguous parallel ridges will necessarily leave
intervening _valleys of elevation_. As mountain ranges generally consist
of several such ridges, valleys of this description are numerous, and
they are often of great extent.
It is obvious that there are mountains in the sea of as great height
above the lowest valleys as the mountains of continents are above the
level of the sea. If a new continent should hereafter be formed by the
elevation of a large area of the bed of the sea, the existing mountains,
now appearing in the form of islands, would partake of the general
movement, and the new continent would have the same general diversities
of surface as existing continents. The mountains would have existed long
before the continent. It is therefore to be supposed that the mountains
of the present continents were elevated before the continents, and that
they stood for long periods as islands, exposed to the action of waves,
tides, and marine currents.
2. _The Elevation of Continents._--Continents have been elevated by so
slow a movement that it has not generally been perceived, even when they
have been peopled by nations advanced in civilization. And yet
satisfactory evidence is always left of former sea-levels.
Almost every seaboard furnishes examples of beaches, evidently once
washed by the sea, but now elevated more or less above high water.
At Lubec, near the northern extremity of the coast of Maine,
barnacles[B] are found attached to the rocks eighteen feet above high
water. The pilots at that place, and for a hundred miles north and south
of it, speak of the ship-channels as diminishing in depth, though it is
certain that they are not filling up. Such facts are to be explained
only by supposing that the coast is rising.
[B] The barnacle is a marine animal, permanently fixed
to the rocks, and live but a short time without being
surrounded by sea-water.
Lakes are numerous throughout the northern portions of North America,
which are receiving annually large quantities of sediment, and must
ultimately become alluvial plains. Those of moderate depth, as Lake
Erie, cannot require periods very protracted to fill them. Their
continuance in such abundance indicates that the elevation of the
continent to its present height is comparatively recent. This conclusion
is confirmed by evidence of another kind. Throughout this region of
lakes, beds of clay containing the remains of existing species of marine
animals, are found at all elevations from the sea-coast, to the height
of about four hundred feet, but not higher. These clay beds are very
recent, and were deposited when the surface was four hundred or five
hundred feet lower than it now is; and this amount of elevation has left
the existing lakes scattered over the surface.[C]
[C] "It is remarkable that on the shores of the great
lakes there are certain plants the proper station of
which is the immediate neighborhood of the ocean, as if
they had constituted part of the early flora of those
regions when the lakes were filled with salt water, and
have survived the change that has taken place in the
physical conditions of their soil."--_Torrey's Flora of
the State of New York._
[Illustration: Fig. 58.]
The following (Fig. 58) exhibits Europe as it was during the Silurian
epoch, and Fig. 59 as it was at the commencement of the tertiary epoch.
The land, as it then existed, is represented by the white surface, the
present waters by the dark shading, and the land which has been
reclaimed from the ocean by elevation since those periods by the lighter
shading.
[Illustration: Fig. 59.]
The whole southern part of South America, embracing an area equal to
that of Europe, has been elevated within a very recent period; and some
parts of it, if not all of it, are still rising. The shells found on the
plains from Brazil to Terra del Fuego, and on the Pacific coast, at a
height of from one hundred to thirteen hundred feet, are identical with
those now inhabiting the adjacent seas. And "besides the organic
remains, there are, in very many parts, marks of erosion, caves, ancient
beaches, sand-dunes, and successive terraces of gravel," all which must
have resulted from the action of the waves at a period not remote. At
Lima, articles of human skill peculiar to the original inhabitants of
Peru were found imbedded in a mass of sea-shells eighty-three feet above
the present sea level. The elevation on the Pacific coast has been in
part by sudden uplifts of a few feet at a time; but it is found, from
time to time, that there has been a change of level, amounting to a
foot or more in a year, when there have been none of these sudden
movements.
A considerable portion of Europe, reaching from North Cape in Norway to
near the southern part of Sweden, more than a thousand miles, and from
the Atlantic to St. Petersburg, more than six hundred miles, has been
rising at the rate of about three feet in a century, for at least two
centuries, and probably much longer. This change is proved by the
occurrence, at considerable elevations above the sea, of shells now
found in the Baltic; by rocks once sunken, now raised above the surface
of the sea, and by ancient seaports having become inland towns. To
determine the truth by actual measurement, the Royal Academy of
Stockholm, about thirty-five years since, caused marks to be cut in the
rocks along the coast, to indicate the ordinary level of the water. This
is easily ascertained, as the Baltic is nearly a tideless sea. The
present level of the sea, compared with that indicated by the marks
before mentioned, leaves no doubt that the country is rising.
3. _The Subsidence of Land._--Elevations can be shown to have taken
place by fossils, and by other evidences of former sea levels which are
left on the surface; but depressions leave but few indications of change
of level. It is yet doubtful whether the depression is equal to the
elevation; that is, whether the amount of land remains nearly constant,
or whether there has been an augmentation of the dry land within the
tertiary and recent periods. We are certain that the augmentation, if
any, has not been equal to the elevation, for subsidences to a great
amount are known to have taken place.
There are occasional instances of submerged forests seen at low tide, at
some distance from the shore. There are several near the coast of
England and Scotland, and near the coast of Massachusetts. They are but
a few feet below low water, and do not indicate a subsidence of more
than about twenty feet.
Numerous instances are on record of the sinking down of wharfs and
buildings near the sea during earthquakes. Almost every violent
earthquake is accompanied by a change of level. The changes of this kind
which have been noticed are in seaport towns, because greater facilities
are there afforded for detecting them, and because loss of property
awakens attention to them; but there is every reason to suppose that
these changes of level extend to great distances both into the country
and into the sea.
[Illustration: Fig. 60.]
An immense area in the Indian and Pacific Oceans, probably ten millions
of square miles, is undergoing change of level. The lines A B and D G
(Fig. 60) represent nearly the axes of depression; while an intermediate
and two exterior parallel lines would represent axes of elevation. The
evidence of these changes is found principally in the peculiarities of
the wall of coral rock encircling the islands.
The following figures represent, in sections, modifications of form of
the same island. The coral wall built up around the island by the
polyps, from the depth of fifty, or at most of a hundred feet, is shown
at _c c_ (Fig. 61). If the island is elevated, this wall becomes a
_fringing reef_ (Fig. 62), _b'_ becoming the level of the sea, and the
animal begins a new wall at the same depth as before. But if the island
is gradually sinking, the wall is kept built up to the surface, and
becomes a _barrier reef_ (Fig. 63). A channel is thus left between the
island and the reef, which, though gradually filling up with broken
coral or other sediment, is generally deep enough for a ship-channel. If
the island continue to subside till it disappears, and the coral wall is
still kept at the surface, it then becomes an _atoll_, a circular coral
island (Fig. 64), often of many leagues in diameter, beaten by the surf
on the outer edge, but enclosing a quiet lake, which communicates only
by occasional channels with the ocean.
[Illustration: Fig. 61.]
[Illustration: Fig. 62.]
[Illustration: Fig. 63.]
[Illustration: Fig. 64.]
The islands contiguous to the lines A B and C D (Fig. 60) are uniformly
atolls, or are surrounded by barrier reefs, and are therefore subsiding;
while the islands at a distance from these lines are surrounded by
fringing reefs, which indicate that they are rising.
A well-authenticated instance of gradual subsidence is that of
Greenland. The entire western coast, from its southern extremity to
Disco Island, a distance of six hundred miles, has for the last two
centuries been slowly subsiding. The dwelling-houses and places of
worship built by the early European settlers are now in part or entirely
submerged. The natives are said to be aware of the subsidence, and never
build their huts near the sea.
4. We have thus seen that both elevation and depression may take place.
There is reason to believe that these changes of level have, in some
cases, been several times repeated. In one of the eastern ranges of the
Andes, opposite to Chili, there is a mass of marine strata of five
thousand feet in thickness. About the middle of the series there occurs
a silicified forest. In one place a clump of coniferous trees was found
of more than fifty in number, and a foot or more in diameter. The base
of the strata must have been twenty-five hundred feet below the surface
of the sea, in order to admit of the deposition of the first half of it.
It was then elevated, so that a forest grew upon its surface. It was
then depressed at least twenty-five hundred feet, more, to admit of the
deposition of the subsequent strata, and the whole is now uplifted to
form a mountain range of eight thousand feet in height.
[Illustration: Fig. 65.]
The temple of Jupiter Serapis, near Naples, in Italy, was built near the
sea, about eighteen hundred years ago. It was gradually submerged, and
finally lost by the deposition of sediment nearly to the top of the
columns. It was afterwards elevated, so as to be entirely above the
level of the sea. The remains of the temple (Fig. 65) were afterwards
discovered by the columns projecting a little above the ground. The
sediment was removed to the depth of forty-six feet, when the workmen
came to the base of the columns, and to a pavement seventy feet in
diameter. In 1807 an artist was employed to take drawings of the ruin.
The pavement was then above the level of the sea. Sixteen years
afterwards the same artist found the pavement covered with water, and
the depth has continued to increase since that time. It is considered
that for the last forty years the depression has been three-fourths of
an inch a year.
Instances enough have now been given to show how extensively the system
admits of change. They are sufficient to justify us in searching for
indications of great revolutions in past times, even where no such
indications have as yet been discovered. They will serve as a key to
many otherwise inexplicable phenomena, In order to the interpretation of
such phenomena readily, we must cease to look upon these as exceptional
cases, and regard them not only as facts, but as facts of frequent
occurrence.
From the examples which have now been given, as well as from
speculations upon the cause of these changes, it seems highly probable
that all the surface of the solid portion of the earth, whether land or
the bed of the sea, is undergoing changes of level. It may be so gradual
that in the life of an individual it would be imperceptible, even where
the best means of detecting it exist. These means are generally the
works of man, and they are themselves so liable to change, that it would
be scarcely possible to detect variations of level, which amount to but
a few inches in a century.
If we admit that the relations of land and water have always been
variable, it is impossible to arrive at any certain conclusion as to the
amount, position or form, of the dry land at any former period. We may
determine, with some degree of certainty, what portions of the present
continents were submerged at particular epochs. Thus, we may infer that
most of this country was submerged during the silurian period, from the
great extent of the Silurian rocks; and, from the limited extent of the
chalk formation in this country, we know that during the cretaceous
period most of the continent was above the surface of the sea. But we
have absolutely no data for determining what portions of the bed of the
sea were at any time dry land.
It is supposable that the land has been principally confined to the
equatorial regions at one period, and to the polar at another. At still
a different period the land may have existed as islands scattered
through a general ocean. These relations may, therefore, be assumed to
have existed, if there are geological phenomena which best accord with
such relations.
SECTION IV.--CHANGES ON THE SURFACE OF THE EARTH.
1. The principal changes of this class consist in the wearing down and
removing immense quantities of the surface rock. The form in which the
_igneous rocks_, of which the entire crust of the earth was originally
composed, now appear, furnishes no assistance in judging of the amount
of denudation which they have suffered. We can judge only from the
amount of rock for which they have furnished the materials, and these
are the whole sedimentary series which exist both as dry land and as the
bed of the sea.
2. The _sedimentary rocks_ have also been subject to great denudation;
and we often have, in what is left, some indications of how much has
been removed. One of these indications consists in the now level surface
of those portions of country in which large _faults_ exist. By the
excavations for coal, in England, faults have been discovered of five or
six hundred feet. At the time that they were formed, the surface must
have presented precipitous escarpments (as represented by the dotted
lines in Fig. 50) of a height equal to the dislocation; but the whole is
now reduced to a general level (_z z_), denuding causes having removed
the elevated portions.
The extent of _valleys_ will often give some idea of the amount of
denudation to which a region has been subjected. In the north-west of
Scotland there is a succession of hills of about three thousand feet in
elevation, consisting, for the upper two thousand feet, of horizontal
strata of old red sandstone. (Fig. 66.) We cannot conceive that these
mountain masses were deposited in their present isolated form. The whole
intervening spaces must have been filled with strata continuous with
those by which the elevations are formed.[D]
[D] "I entertain little doubt that when this loftier
portion of Scotland, including the entire Highlands,
first presented its broad back over the waves, the
upper surface consisted exclusively, from one
extremity to the other, of a continuous tract of old
red sandstone; though, ere the land finally emerged,
the ocean currents of ages had swept it away, all
except in the lower and last raised borders, and in
detached localities where it still remains, as in the
pyramidal hills of Western Rosshire, to show the
amazing depth to which it had once overlaid the
inferior rocks."--_Miller, Old Red Sandstone_,
_p. 22_.
[Illustration: Fig. 66.]
[Illustration: Fig. 67.]
A somewhat similar instance occurs in the Connecticut river sandstone,
in the central part of Massachusetts. The following figure (Fig. 67)
represents two mountains of 1 the sandstone, between which the
Connecticut river flows. The dotted lines indicate a depth of one
thousand feet of the rock which has been swept away. It is also thought
that a bed of equal depth has been removed from this section southward,
through the State of Connecticut, to the sea-coast.
3. _Valleys_, and even many of the larger valleys, are produced by the
wearing down of the surface. The lower portion of the Connecticut valley
is one of denudation, though in its upper part it is a valley of
elevation, resulting from the upheaval of the Green and White Mountains.
The water-courses from the mountains are transverse to the direction of
the ranges, and generally consist of valleys of denudation. These
valleys were no doubt originally fractures, produced while the mountains
were rising. The fractures have been subsequently widened by denudation
into valleys.
[Illustration: Fig. 68.]
4. The rocky surface, beyond the fortieth parallels of latitude, and in
the vicinity of glacier-producing mountains, is generally covered with
_grooves_ and _striæ_ (Fig. 68), varying from several inches in depth to
the finest perceptible lines. Rocks that are of a soft consistence, or
which have been long exposed to atmospheric agents, seldom exhibit these
marks, though there are probably few places, outside of the parallels
before mentioned, where the rocky surface, if it has been protected from
atmospheric decay, does not contain such grooving.
5. Another change at the surface consists in the formation of a _soil_;
that is, of a superficial layer, of no great thickness, of earthy
matter, a large proportion of which is always in a minutely divided
state. In some instances it is common sediment, unsolidified; in
others, it consists of the surface rock in a state of disintegration;
but a large part of the soil within the region where the grooved
surfaces are found consists of materials transported from a distance.
Soils are distinguished according to their predominant minerals, as
siliceous, aluminous and calcareous. If siliceous matter is in excess,
it will be a light, warm soil, and allow the water to pass through it
too freely. If the clay predominates, the soil is cold, stiff, and too
retentive of moisture. A proper admixture of these three ingredients
constitutes the best soils. There are some other mineral ingredients
essential to the productiveness of soils, but they are always in small
proportion. In addition to the inorganic part which is common to the
upper soil, and the subsoil, there is required, in order to render the
upper layer productive, a large admixture of decaying animal and
vegetable matter.
SECTION V.--CHANGES OF CLIMATE.
Our means of determining the climate of any former period consists in a
comparison of the fossils of such period with the existing forms of life
in warm and cold climates.
The earliest abundant _vegetation_ consisted principally of ferns,
rushes and mosses, and a larger growth was attained than is attained by
any of the allied forms of the present time. We may infer that the
circumstances under which these lower forms of vegetable life are now
produced in the largest proportion, compared with other forms, and under
which they grow to the largest size, are the circumstances approaching
most nearly those under which the early vegetation was produced. These
circumstances are found to be a position elevated but little above the
level of the sea, a humid atmosphere, and the highest terrestrial
temperature. Such facts favor the conclusion that during the coal period
an ultra-tropical climate prevailed, and that the land existed in the
form of low islands, thickly set in a general ocean.
The peculiar characters of some of the _animal fossils_, from the
earliest fossiliferous to the tertiary series, indicate that a warmer
climate prevailed during their formation than now exists. The remains of
marine animals, such as the cephalopoda, are found in great numbers and
in high latitudes, in a fossil state; but similar species, as the
nautilus, now abound only between the tropics. The same is true of the
crinoidea. Coralline limestone is also found in great abundance and in
high northern latitudes; but the stone-producing coral now exists only
in very warm seas. The remains of saurian reptiles are numerous in the
oölite and Wealden; but all the larger recent species of the lizard
tribe, such as the crocodile, are confined to the warmer regions of the
earth.
A former warm climate in Siberia is indicated by the occurrence there of
the remains of elephants. These animals were so abundant that their
tusks are now collected as an article of commerce. The abundance and
high state of preservation of these remains seem to preclude the
explanation that they were conveyed there, from the present tropical
regions, by any great geological convulsion. The species must therefore
have inhabited the country, though the elephant is now found only
between the tropics. The Siberian elephant was a different species from
any now existing, and, unlike the recent species, had a covering of
coarse hair. There is, however, no reason to conclude that it could
endure a continued low temperature; and its sustenance would have been
impossible, from the very stinted vegetation which that region now
affords. We must therefore suppose that Siberia enjoyed, at the period
when it supported these animals in such abundance, a tropical climate.
Most of the facts which go to prove a change of climate have been
observed in the northern hemisphere; but the explorations in South
America and New Holland furnish ground for believing that the geological
phenomena of the two hemispheres are essentially alike, and that the
indications of climate are the same for the same periods.
Such is, in general, the evidence in reference to climate; and it leads
to the conclusion that a highly tropical climate prevailed in the
temperate, and for some distance, at least, into the polar zones, in the
early geological periods; while there is no reason for supposing that
the tropical regions experienced a temperature too high for physical
life to endure it. The climate of the earth was characterized then by a
higher temperature than now, and by greater uniformity. This was the
climate, with perhaps a gradual reduction of temperature, till the later
portions of the tertiary period.
Before the close of the tertiary period, a change occurred, and probably
a rapid one, to a more rigorous climate than now exists. The destruction
of the elephant in Siberia was evidently sudden, and was followed by
extreme cold; for the animals are in some cases entirely preserved in
ice, and in so perfect a state that, when the ice which surrounds them
becomes melted, the flesh is devoured by carnivorous animals. There are
occasionally found, in the drift of the boulder period, shells similar
to those of the Arctic regions, and in a condition to show that they
have not been transported. The clay beds of the northern portion of the
United States and of Canada were deposited during the last depression of
that portion of the continent, and they contain the remains of marine
animals identical in several instances with species now living, but
confined to more northern regions. It must therefore be admitted that
the interval between the middle tertiary and the modern era was one of
great cold. It is generally referred to as the _Glacial period_.
Very considerable local changes of climate have also occurred within the
historical period. Thus the mean temperature of the Alps has been so
reduced that the ancient passes have in modern times become choked up
with snow, and other passes have been sought,--a result, perhaps, of
additional upheaval. It would seem that Siberia is now receiving a
milder climate. The ice in which elephants have for centuries been
imbedded has been slowly melting for at least thirty years.
SECTION VI.--ADVANTAGES RESULTING FROM GEOLOGICAL CHANGES.
1. The division of the general surface into land and water, as well as
the diversified form of the land, the existence of mountains and low
lands, and the consequent modifications of climate, the waterfalls, and
the river-systems, constituting the drainage of continents, are all
results of the process of upheaval.
2. A large part of the mineral substances employed for architectural and
economical purposes are oceanic deposits, such as the marbles, slates,
sandstones and mineral salt, and would have been inaccessible if they
had not been elevated from the position in which they were formed. And
the elevation of them above the bed of the sea would have exposed only
the superficial layer, if they had not been either irregularly uplifted,
as at _e c_ (Fig. 69), or unequally worn down, as at _b_.
[Illustration: Fig. 69.]
The granitic rocks, as they were formed below the aqueous rocks, must
have remained unknown and useless, if they had not been brought to the
surface, as at _c_, by the most convulsive efforts of nature of which we
have any knowledge. Thus, natural mechanical forces have effected for
man what the mechanical forces under his control would be entirely
insufficient to accomplish.
3. It is by changes of this kind that we become acquainted with the
geological structure of the crust of the earth. Mining operations have
never extended to a greater depth than three thousand feet, while the
inclined position of the strata exposes for examination, along their
outcropping edges, _e a c_, the whole series, even to the primary rocks.
The upheaval of the granitic rocks, and the removal by denudation of the
overlying deposits, shows us the crystalline character which the earthy
materials take, when subjected to pressure and cooled from fusion with
extreme slowness. Thus we have, exposed to observation, the process of
nature in the formation and modification of rocks for several miles in
depth. Of the central portions, however, including by far the largest
part of the mass of the earth, we have no knowledge whatever.
4. _Springs_, and the other means of obtaining water for domestic
purposes, depend in part upon the inclined position of strata, and the
broken and uneven condition of the surface, and in part upon the
alternation of permeable and impermeable strata. If all the strata were
porous, like the sandstones, the water which falls upon the surface
would gradually settle through them to the level of the sea; or, if they
were all impermeable, like the clays, the water would pass over the
surface, and be collected in lakes or the ocean. As it is, the porous
structure of the soil and of some rocks acts as a reservoir, from which
the water is gradually discharged, and the intervention of impermeable
strata prevents its taking a perpendicular direction downwards. Thus, if
the stratum _e b_ (Fig. 69) consists of porous rock, and the one below
is impermeable, the water which is absorbed at _e_ will appear at _b_ as
a spring. Or, if the line _a d_ is a fracture, the water received at c
may reappear as a spring at _a_. If the strata were perforated by boring
at _e_ till the porous stratum _a_ is reached, the water will rise to
the surface, constituting an _Artesian well_. An ordinary well consists
of an excavation continued till a stratum is reached which is
permanently saturated with water.
5. Most of the _metallic ores_ which occur in the stratified rocks, with
the exception of iron, are found in fractures or as dikes. Without
these disturbances of the strata, the ores would have remained either
sparingly diffused throughout the adjacent strata, or as a part of the
melted mass at the volcanic centres. The ores and metals which are found
in the primary rocks are accessible only by the bringing up of these
rocks to the surface.
The fracturing, displacement, and elevation of the strata, attended, as
is often the case, with the destruction of property and of the life both
of man and the inferior animals, might, at first view, be thought an
unnecessary, if not a wanton infringement upon arrangements already
established. But the results which we have noticed, though by no means a
full enumeration of the advantages resulting from geological changes,
are sufficient to show that even the more violent disturbances to which
the crust of the earth has been subjected constitute an important part
of that series of adjustments which has rendered it a suitable abode for
human beings. These changes are therefore neither useless nor
accidental, but are essential parts of a wise and beneficent system.
CHAPTER IV.
OF THE CAUSES OF GEOLOGICAL PHENOMENA.
An exhibition of the _composition and structure of the earth_, together
with an account, as far as there is reliable evidence, of the
modifications which they have undergone, has been the object of the
preceding chapters. They are mainly a collection and classification of
observed facts. No reference has been made to causes or modes of
operation, except in a few cases where it was necessary in order that a
statement or description, might be intelligible.
If the facts have been given with sufficient clearness and detail to
convey a correct general idea of the crust of the earth, we are prepared
to inquire what are the agencies employed, and how they have operated in
producing it. It is the province of the geologist to question every
known power in nature, and to ascertain what geological effects each one
is now producing; and, observing what effects are produced by given
causes, he is to judge of the causes which have produced like effects in
past geological periods.
Some of these causes are in their nature limited, and effects can be
referred to them only within those limits. Thus, the congelation of
water expands it by a certain proportion of its volume, and beyond that
it can have no effect. But the expansive power of steam varies with the
temperature; and hence the effects referred to it may be equally varied.
Thus, we are not to expect exact uniformity of results in all past
times, but _the results will vary only as the circumstances vary_ upon
which the operation of these causes depends.
Geological causes, in most instances, operate with extreme slowness; and
therefore it will require a series of observations, continued for a long
time, to ascertain what are the capabilities of these causes. But a
single instance of their effects proves their capabilities thus far.
Hence, one instance of the deposition of a stratum of salt in a salt
lake; of the filling of a fracture with fluid lava; of a volcanic
eruption, like that of Iceland in 1783; of the subsidence of a volcanic
mountain, as that of Papandayang in Java; or of the rising of a large
area of land, as in Sweden, as fully proves that natural causes exist
capable of producing these effects, as if the effects were produced
daily. As these effects increase in number, and careful observations are
made and authentic accounts preserved, the means of correctly explaining
geological phenomena will increase. The causes thus far known are
Atmospheric Causes, Chemical Action, Organic Agency, and Aqueous,
Aqueo-glacial and Igneous Action.
SECTION I.--ATMOSPHERIC CAUSES.
The oxygen of the atmosphere is capable of uniting with some of the
constituents of rocks, by which their cohesion is weakened or destroyed.
This is the cause of the rapid disintegration of some varieties of
granite. The protoxide of iron which they contain is converted, by
contact with the atmosphere, into the peroxide. Its volume is thus
increased, and portions of the rock are separated from the mass. When
granite or limestone contains sulphuret of iron, the oxygen of the
atmosphere, in connection with moisture, combines with the sulphur,
forming sulphuric acid, by which limestone and the felspar of granite
are rapidly decomposed. Hence, a rock which contains an oxide or
sulphuret of iron should not be used for architectural purposes.
Carbonic acid is another constituent of the atmosphere which operates as
a decomposing agent. The water that falls from the atmosphere is charged
with it, and thus becomes capable of dissolving calcareous rocks.
Carbonic acid is thus indirectly the means of the rapid destruction of
rocks of this class. It is also believed that carbonic acid enters into
direct combination with some of the constituents of rocks, and
particularly felspar; for it is found that in those countries where
carbonic acid issues in great quantities from the earth, the rocks,
especially those which contain felspar, disintegrate rapidly. Masses of
many tons' weight, which appear to be solid granite, after being broken
are found to be in such a state of decay that fragments may be reduced
to sand between the fingers.
The moisture of the atmosphere has some effect as a decomposing agent.
Rocks which are exposed to frequent alternations of moisture and dryness
soon crumble into fragments. Rain, falling upon the surface of rock,
produces, mechanically, a destroying effect, which is not to be
overlooked.
Variations of temperature, especially those alternations above and below
the freezing point, have greater influence than any other cause in the
destruction of rocks. When the water with which a rock is saturated
congeals, the resulting expansion tends to enlarge the interstices, and
thus to separate the particles of the rock. When the ice melts, the
particles fail to resume the closeness of arrangement with which they
were before packed. By frequent repetition of this action, the
superficial portion loses its cohesion, and disintegrates. It is also
found that in the region of perpetual snow the surface of the mountain
masses is covered with rock in a disintegrated or fragmentary state, in
greater abundance than below the snow line; but no explanation of this
fact has yet been found.
In mountainous regions, electrical discharges and violent storms have
some destroying effect. Winds have considerable power in changing the
place of earthy matter in a disintegrated state. In deserts, the sands
are carried in great quantities to great distances.
The causes now enumerated, when considered separately, and as acting for
only limited periods of time, seem hardly worthy of notice; but when
considered as operating conjointly, and for indefinite periods of time,
they must have produced important changes on the surface of the earth.
From these causes, the surface and ornaments of castles and other
ancient edifices, and of boulders, and all insulated rocks, are found to
be decayed, and often to a considerable depth. It is from these causes
that a soil is produced on every surface of rock which is not so exposed
to the action of currents that the debris is removed as fast as it is
formed. Hence it is, also, that a slope of detritus is formed at the
base of every declivity, so that the ledge appears only at the highest
points.
It is from a combination of these atmospheric causes that a large part
of the sediment is furnished which brooks and rivers carry away. And
when cohesion is not entirely overcome, it is so far weakened that other
causes are much more effectual than they would otherwise be, in
effecting the disintegration of rocks.
SECTION II.--CHEMICAL ACTION.
All those changes in which the action is molecular,--that is, between
the molecules as such, and not between the masses,--including the
effects of the imponderable substances, we regard as resulting from
chemical agency.
Under the control of these molecular forces the crystalline rocks have
taken their form; and if the crust of the earth could have remained in a
fixed condition, in which these forces would have been in equilibrium,
no further chemical action could have taken place. But, instead of being
in a fixed condition, the present system is one of perpetual change.
Various disturbances of this equilibrium of forces,--such, for instance,
as the diurnal and annual changes of temperature at the surface, and the
still greater secular changes of temperature at great depths,--will
bring the chemical forces into operation. The mechanical disintegration
of the crystalline rocks, and the deposition of them in strata
independently of the chemical affinity of their particles, will give
occasion for chemical changes,--that is, for a rearrangement of the
particles in accordance with their affinities,--whenever any movement of
the particles among themselves can take place. These movements take
place, to a very great extent, under the influence of electrical
currents, and of change of temperature, even while the masses retain
their solid form.
Chemical affinity has exhibited itself on the largest scale in the
formation of the various mineral species of which the crust of the earth
is composed; but we may also refer to the same cause the formation of
divisional planes in rocks, the concretionary arrangement, and mineral
veins.
1. _Divisional Planes._--It has before been stated, that the older
rocks, in many cases, cleave freely in planes not parallel with the
stratification. (See Fig. 48.) In some instances, in beds of lava, a
similar cleavage exists, sufficiently perfect to allow of its use as a
roofing material. In these cases, there must have been a rearrangement
of the particles, so that their axes of greatest attraction would lie in
parallel planes; the same arrangement which exists in mica and other
crystalline substances, which have one and but one free cleavage.
A similar arrangement has sometimes taken place under such circumstances
as to submit the process to more careful scrutiny. In the gold mines of
Chili, the powder from which the gold has been washed is "thrown into a
common heap. A great deal of chemical action then commences; salts of
various kinds effloresce on the surface, and the mass becomes hard, and
divides into fragments which possess _an even and well-defined slaty
structure_." When a portion of clay, worked into a paste with a very
weak acid, is submitted to a weak voltaic action for several months, and
then dried, it is found to have acquired a distinct though imperfect
cleavage structure.
It appears, then, that both electrical currents and ordinary chemical
action are capable of arranging the particles of an earthy mass into
separable layers. We may then regard this change in the older rocks as
an imperfect crystallization, and probably induced by electro-chemical
agency.
It is also found that all rocks are divided into huge blocks by seams
not parallel with the cleavage, and too regular to be considered as
fractures. These seams bear an analogy to the secondary faces of
crystals, which are never parallel to the cleavage.
2. _Concretionary Formations._--There exist in many rocks concretions
which differ from the mass of the rocks. In most of the tertiary clays
there are small concretionary nodules, which contain more calcareous
matter than the mass of clay around them. In the coal formation, the
nodular iron ore consists of concretionary masses. In the chalk
formation, nodules of flint abound, and generally in layers. In many of
these cases, particularly in the clays and coal, the nodules have an
organic nucleus, and, although concretionary, they retain the marks of
stratification of the adjacent rocks. Hence they could not have been
deposited in the form of nodules. There must therefore have been in the
rock, though in the solid state, such motion among the molecules that
particles of a particular mineral have separated from the mass and
rearranged themselves in concretionary layers, yet so gradually as not
to disturb the lines of original stratification.
[Illustration: Fig. 70.]
There are other instances, similar to the last in all respects, except
that the segregated portion does not take the concretionary form. When
gypsum is distributed in small proportion through a formation, there
seems very little reason to doubt but that it is, by a molecular action,
segregated from the strata in lenticular masses, as at a (Fig. 70). Many
of the limestone strata contain irregular aggregations of quartz. It is
presumed that the siliceous and calcareous matter was deposited together
as sediment, and that the aggregation has resulted from a movement among
the particles similar to that by which the concretionary structure is
produced.
The columnar structure of basalt seems to have resulted from a peculiar
molecular action, at first resembling a concretionary arrangement, while
the mass was cooling from a state of fusion. In experimenting to
ascertain the cause of this structure, Mr. Watt fused in a furnace seven
hundred pounds of basalt. When cooled, he found that "numerous spheroids
had been formed, and that when two of them came in contact, they did not
penetrate each other, but were mutually compressed and separated by a
well-defined plane, invested with a rusty coating. When several met,
they formed prisms." (Fig. 71.)
[Illustration: Fig. 71.]
3. _Mineral Veins._--The phenomena of veins are such that they cannot
all be referred to the same cause. In some, the vein-stuff has been
protruded as a dike, differing from ordinary dikes only in the
accidental circumstance that it contains a metal or a metallic ore.
Mineral veins are not, however, generally filled by injection from
below. It is found that those veins only are productive which have an
east and west direction. But injected dikes run in all directions. The
ore often varies in richness at different depths in the vein, or passes
into ore of some other metal. The ore also varies in kind and quality,
according to the character of the rock through which the vein passes.
These phenomena are best explained by supposing that the sediment of
which the strata were formed contained the mineral substances of these
veins in small proportion. After they were solidified, and fractures had
been formed, the mineral substance was transferred by molecular action
to the fissures, and deposited.
It was shown by the early experiments of Davy, that voltaic currents are
capable of taking up mineral substances from their solutions, and
removing them from one cup to another. It has been ascertained that in
most mineral veins a proper apparatus will detect the existence of
electric currents. It may be regarded as certain, that the unequal
heating of different parts of the surface at the same time, by the sun,
causes a vast current of feeble intensity to circulate around the earth
once in twenty-four hours. The unequal distribution of heat below the
surface may also produce currents subject to other laws. We should
expect that these currents would take up the mineral substances diffused
through rocks, and deposit them by themselves. It seems probable,
therefore, that the molecular action, from which the segregation of
metallic veins has resulted, was that of voltaic currents.
SECTION III.--ORGANIC CAUSES.
The effects of all organic causes in producing geological changes are
inconsiderable, compared with those of inorganic causes. With the
exception of the coral formation, the most important of these effects
are those produced by human agency. We find examples of this agency in
the distribution of animals and plants beyond the regions where they are
indigenous; in the increased numbers of certain species, and in the
diminution, if not extinction, of others; in the modifications of
climate, dependent on the destruction of the forests and the cultivation
of the soil; in controlling the course of rivers; in arresting by
embankments the encroachments of the sea; in breaking up and changing
the place of great quantities of rock by mining and engineering
operations; and in the increased quantity of sediment furnished to
streams by cultivating the surface, and thus preventing the protecting
influence which the matted roots of trees and the smaller vegetables
would otherwise have. Such effects, though attributable mainly to man,
are produced in some degree by all other animals.
Besides these general effects, it is the existence of organic forms that
has conferred on all the sedimentary rocks their fossiliferous
character. _The records_ of the climate of each geological period, of
the physical geography, of the vegetable productions, and of the animal
forms by which the earth was peopled, consist in the remains of the
living beings of these several periods, imbedded in the contemporaneous
rock formations. But in the sediment deposited since the human era
there must have been furnished both the remains of human beings and
works of art, such as implements of labor and war, pottery, coins,
fragments of ships, &c.
Moreover, the _quantity of material_ which has been furnished by organic
causes is by no means small. The coal-beds are the product of vegetable
growth exclusively. We not unfrequently find strata of great extent
consisting almost entirely of the shells of molluscous animals, of the
stems of encrinites, or of the shields of microscopic animalcules.
But the most abundant rock which can be regarded as the product of
animal organization is the _coral formation_. It consists of immense
walls of coral limestone, separating either an atoll or the land of an
island or continent from the open sea. The base of this wall has a width
varying from a hundred feet to a mile or more, and the outer edge of it
is at such a distance from the shore as to give a depth not much
exceeding a hundred feet. Over this area of the bed of the sea, which
forms the base of the wall, the coral polyp commenced its work.
Attaching itself in immense numbers over this area, it deposits
calcareous matter from its under surface, and thus, by degrees, elevates
itself towards the surface of the water, till it reaches a level a
little above low-water mark. The height of the wall would not, with
these conditions, exceed one hundred feet; but some hundreds of the
islands surrounded by coral walls are gradually subsiding. The
depositions of the polyps keep pace with the subsidence, so that this
wall has reached an elevation from its base of a thousand feet, and in
one instance of two thousand feet. (See Figs. 61, 62, 63, 64.)
Most of the islands of the torrid zone are thus surrounded with coral
reefs, except a few where the cold polar currents reduce the temperature
too low to admit of their growth. In one instance, along the north-east
coast of New Holland, there is a coral reef, some twenty-five miles from
the land, which has a continuous extension, excepting occasional inlets
of no great depth, of a thousand miles. The reef along the island of New
Caledonia is four hundred miles long. A large number of other reefs have
a nearly equal extension. There is thus an area of several thousands of
square miles covered to a great depth with this coralline limestone.
Some limestone formations of great extent among the older rocks were the
work of similar animals. These lower forms of organization have,
therefore, always been important geological agents, both in collecting
the carbonate of lime from its solution in the waters of the ocean, and
in depositing it as solid rock.
SECTION IV.--AQUEOUS CAUSES.
Water is, next to heat, the most important geological agent. All the
stratified rocks are aqueous deposits, and their total amount is in some
respects a measure of the influence which this agent has exerted. The
materials have been obtained from the destruction of preëxisting rocks,
transported by water, and deposited in layers.
When the first strata were formed, the sediment must have been obtained
entirely from igneous rocks, because only those rocks existed; but now
it is obtained from every kind of rock which is exposed to abrading or
decomposing agencies. Hence, many of the later formations contain
fragments, and sometimes within the fragments well-characterized
fossils, of earlier formations.
The sediment which is ultimately to become stratified rock is deposited
on the beds of the ocean, and other great reservoirs of water. The
formation of most of the aqueous rocks, therefore, as well as of the
igneous rocks, is deep below the surface; and neither of these
operations, on the large scale, is directly exposed to our observation.
We may, however, learn by observation, how the sediment is furnished to
the waters and transported by them, and we can form some correct ideas
of the manner in which it will be laid down on the bed of the ocean, and
solidified.
I. _The Furnishing of Sediment._
1. Almost all the minerals which occur in the geological formations are,
to some slight extent, _soluble in water_. Hence, rain water, by
passing through a stratum of earth or rock and reäppearing as a spring,
loses the insipidity which it had as pure water, and becomes palatable.
It is then found to hold in solution some small proportion of earthy
substances, upon which this change of taste depends. Although the
proportion of dissolved matter is very small, yet the surface of earth
upon which this distilled water is shed is one-fourth of the surface of
the globe, and solution below all that surface is constantly taking
place. No inconsiderable amount must thus have been furnished, from the
existing rocks of each period, towards the formation of the strata of a
later period.
There are some substances which are soluble in water, in large
quantities. _Rock-salt_ is an example. It is not found in any very large
proportion in rocks generally, but a very large aggregate amount has
been taken up by the waters which have filtered through the strata. The
ocean gathers into itself, by degrees, all the soluble substances which
are thus taken up. It receives supplies of water charged with these
substances from springs, rivers and lakes. It returns as much water as
it receives; but it is always in the form of vapor, and is therefore
pure water. Hence the saline properties of the ocean, and of those
inland seas which have no outlets. There is thus gathered the materials
for the rock-salt deposits.
But many substances which are not considered soluble in water become so
by some modification of the water. Water of a high temperature is
capable of dissolving silex. In Iceland and other volcanic regions, the
hot springs are charged with silex, which is deposited as the water
cools. Thus, siliceous formations accumulate around springs of this
kind. The various agates may have been deposited from such solutions.
In the decomposition of mica, felspar and volcanic rocks, a considerable
amount of potassa is set free. Potassa or soda renders the water in
which it is dissolved capable of dissolving silex in large quantity. In
these ways water removes, with some degree of rapidity, one of the most
insoluble minerals which rocks contain.
In volcanic countries, and in coal districts, carbonic acid is abundant,
both in spring-water and in the gaseous form. Water charged with this
gas becomes capable of dissolving limestone. Where the water is exposed
to the air, the gas gradually escapes, and the calcareous matter is
deposited. Many accumulations of this kind are now taking place. Some
have already extended several miles in length, and they are often of
great thickness, in one instance, in Italy, two hundred feet (Fig. 72).
It is also probable that many calcareous springs issue below the surface
of lakes and seas, and thus, both fresh-water and marine deposits would
now be forming. These formations are distinctly stratified, and are
white and crystalline, and become solid at the time of deposition.
[Illustration: Fig. 72.]
These dissolved materials are less observed than others, because they do
not render the water turbid; but there is reason to believe that several
of the aqueous formations, particularly the limestones, have been built
up chiefly from them.
2. The _abrading action of rivers_ furnishes considerable detrital
matter. The general form of the river courses is determined by other
causes than the agency of the river itself, yet a river which has a
rapid current is continually deepening its channel. We have proof of
this by observing, when the water is low, that irregularity of surface
which running water always produces, by wearing away the softer parts of
the rock, and leaving the harder in relief. Hence, a river will have its
rapids either where the hardest strata occur, and which therefore wear
down least rapidly, or where the rock has been hardened by the intrusion
or near proximity of dikes.
The abrading power of rivers occasionally becomes greatly increased by
waterfalls. The force which the water acquires in its descent is such as
to excavate a deep cavity at the foot of the fall, reaching back under
the ledge from which the water descends. The ledge is therefore
constantly being undermined. The cataract of Niagara is peculiar, in
having the rock at its base of a soft and friable texture, so that it is
rapidly worn away, while the upper rock is a compact siliceous
limestone. If the order of superposition had been the reverse, the falls
would have been converted into a series of rapids. It is now preserved
as a single fall, and as such it has probably cut the gorge, about two
hundred feet deep and seven miles in length, through which its waters
now reach Lake Ontario. A few years since, a large mass, perhaps half an
acre in area, fell from the centre of the horse-shoe fall. Another mass
of equal size has recently fallen from the western extremity of the
ledge. Thus the fall is gradually receding.
But the foreign substances, such as drift-wood, ice, sand and gravel,
with which the waters of a river are occasionally charged, contribute
more than everything else to its abrading power. At such times its
volume is generally greatest, and its current the most rapid. Its bed is
then sometimes perceptibly deepened and widened in a few hours.
Much the greater part, however, of the earthy matter which rivers convey
in such quantity to the ocean, is furnished by other means than the
eroding action of the river itself. It is the loose material, the soil
and alluvium, to which the solid rocks have been reduced by the
imperceptible but incessantly operating atmospheric agencies, from which
most of the sediment of rivers is obtained. After a rain, every
tributary rivulet is turbid with suspended earthy matter, and it is from
these sources that the larger streams receive the most of their
sediment.
Some observations have been made for the purpose of ascertaining the
quantity of sediment which rivers annually carry into the sea. The
Kennebec furnishes materials which, if spread evenly on an area of one
mile square, and consolidated into rock of the specific gravity of
granite, would have a thickness of six inches. The Merrimac furnishes
about two-thirds as much, the Ganges about two hundred and fifty times
as much, and the Mississippi two thousand times as much.
Thus, the tendency is, to reduce the highest parts of the land, and to
fill up the depressions of the sea; and though we have not data enough
to form any reliable estimate of the total annual discharge of sediment
into the ocean by rivers, yet they are sufficient to show that the
effects of this kind are on a large scale, and to relieve us from any
impression that existing agencies are inadequate to the production of
the stratified rocks.
3. _The action of waves_ is another means by which detrital matter is
furnished. Wherever the shore consists of loose materials, and is
favorably situated to be acted upon by the waves, there is annually a
sensible encroachment of the sea. Such encroachments are rapidly making
in many places; and thus a large amount of sediment is delivered to the
waters of the ocean.
The waves also encroach upon the coast when it consists of rocks, even
of the most indestructible kinds. They continually beat upon it,
undermine the cliffs, and precipitate them into the sea. The tides
increase the power of the waves, by varying the place of their action,
so as to present the same surface of rock alternately to the action of
water and of the air, frost and sun. During storms, the waves have
sufficient force to break off fragments of rock from the escarpment,
sometimes in masses weighing twenty tons or more, and remove them many
rods inland.
A bold, rocky coast always exhibits evidence of a great amount of
erosion. The steep escarpments and the high rugged shafts of rock (Fig.
73) against which the waves now beat are the remnants of masses of rock
which once extended further into the sea, but have been worn away by the
waves. It is by such agency that the deep inlets and harbors of the
coast of New England and Nova Scotia have been excavated.
[Illustration: Fig. 73.]
This more violent action of the waves is only occasional; but when of
less power, they are incessantly rolling the loosened fragments of rock
upon each other, and thus wearing them down to particles small enough to
be carried away by the water.
4. The action of waves is confined to the coast, and never extends to
great depths. But _marine currents_ act principally on the bed of the
sea. The temperature of the mass of the ocean is much higher in the
equatorial than in the polar regions. At the surface, the difference
amounts to sixty degrees. The waters of the torrid zone are thus
expanded, and flow over the colder waters of the north and south; while
these colder waters of the polar seas flow back, in an under current,
towards the equator.
For the same reason,--a difference of temperature,--there will be, in
the higher regions of the atmosphere, a current of warm and moist air
flowing from the equator north and south, while the cold and dry air
conies in from the polar regions towards the equator. In this way the
equatorial waters are carried, in a state of vapor, towards the poles,
where they are condensed, and go to increase the currents of water
moving towards the equator.
Such are the general causes of the oceanic movements in a north and
south direction; but these currents at once become deflected westward,
by the diurnal revolution of the earth, as the trade winds do. Hence
there results a Pacific equatorial current, which has a motion of about
thirty miles a day, and an Atlantic equatorial current, moving from
sixty to seventy miles a day. The principal marine currents are shown in
Fig. 74.
The currents moving towards the poles are superficial, and therefore do
not produce any marked geological effects. But the polar currents, and
those which are produced from them, are of great depth, and there is no
reason to suppose that they do not move, from their commencement, along
the bed of the ocean. There is also reason to suppose that they exist at
great depths, where the opposing superficial currents entirely conceal
them.
Wherever these currents come to the surface, their motion is undoubtedly
greater than it is at the bottom, where it is retarded by the friction
which the moving waters encounter, and by the irregularities of the bed
of the ocean. It should, however, be remembered, that they move with the
weight of the whole superior body of water; and therefore, though the
motion be very slow, it will still possess great power.
Any irregularities in the bed of the ocean beneath such a current must
be subject to very rapid abrasion. We shall sea hereafter, that
earthquake vibrations often shiver the rocks at the solid surface; and
if any of these ridges at the bottom of the ocean were thus acted upon,
the loosened portions would be swept away by the current and deposited
at lower levels, or where the current subsides. If, in any instance
during an earthquake convulsion, a fault should be produced across one
of these marine currents, like the great fault of over five hundred feet
in England, the abutment thus thrown up would soon be worn down; and if
it consisted of unconsolidated matter, it would be swept away almost
bodily.
[Illustration: Fig. 74.]
The effect of such currents will be greatest where they are deflected by
a continent or island. Thus, a marine current sets from near New Holland
in a direct line to the north of the island of Madagascar, where it is
arrested by the African coast, and deflected into the narrow Mozambique
channel, and there acquires a velocity of four or five miles an hour. It
is impossible that any kind of rock should receive the constant force of
such a body of water without being rapidly worn away; and, if there
should be any difference of texture in this rocky barrier, the softer
portions would yield the most rapidly, and thus valleys might be formed.
It is not improbable that the deep indentation on the western coast of
Africa may have been due, in a great measure, to the coast current from
the Cape of Good Hope; and that the Caribbean Sea and the Gulf of Mexico
may have been excavated by the force of the Atlantic equatorial current
being thrown into this angle.
We may regard these currents as oceanic rivers; and it is obvious that
the volume of the terrestrial rivers would bear no comparison with that
of these currents, and their effects would be equally small in the
comparison. The Gulf Stream, and the Mozambique and other similar
currents, must be wearing down the valleys through which they flow, to
such an extent as to furnish an immense amount of detrital matter for
the formation of new rocks.
It is principally to the agency of these deep marine currents that we
are to refer those extensive denudations, so abundant on the present
continents, such as the wearing out of the intermediate masses of rock
between the hills already referred to (Fig. 66), the denudation of the
Connecticut river sandstone, and, perhaps, the excavations which have
formed Lake Erie and Lake Ontario.
II. _The Transportation of Sediment._
The detrital matter obtained in these several ways is swept away by
running water. The specific gravity of rocks does not, in general,
exceed two and a half. Hence, to keep them suspended in water, will
require a force of only three-fifths of what would be necessary to
suspend them in the atmosphere. In the case of river currents, the
velocity and irregularity of motion are generally sufficient to keep all
the finer sediment equally distributed.
There will, however, be a division of the sediment according to the
strength of the current. Hence, the bed of a mountain stream, if there
is any loose material, always consists of pebbles. As it approaches the
alluvial region, the bed is sandy; and when the current becomes very
sluggish, it consists of a fine mud.
Rivers never deposit all their sediment, some of them none of it, along
their course. Large rivers continue partially distinct from the ocean
water to a considerable distance beyond their mouths. The waters of the
Amazon have been recognized at a distance of three hundred miles. This
depends in part upon the volume and velocity of the river; more,
however, upon the fact that river water is lighter than sea water. This
extension of a river will, in most cases, be sufficient to deliver a
part of its sediment into a marine current. When such a current sweeps
very near the mouth of a river, as it does to that of the Niger, the
Amazon, or the Mississippi, it is probable that most of its sediment is
carried away by it.
The transporting power of a marine current is greater than that of a
river, in consequence of the greater specific gravity of its water; but
it has scarcely any of that irregular motion of rapid rivers, upon which
their transporting power in a great degree depends. The force of the
current alone, when it reaches the bottom, is, however, sufficient to
remove every form of loose earthy matter. Thus it may be presumed that
the Gulf Stream sweeps all the sediment from its bed until it reaches
the latitude of Cape Hatteras, where the cold waters from the north
begin to underlie it, and it takes the character of a surface stream.
But the transporting power of marine currents depends mostly upon the
depth of water. It is found, by experiment, that ordinary river sediment
will sink in water about one foot in an hour. A current, therefore, of
a thousand feet in depth, which moves a mile in an hour, would carry its
sediment a thousand miles. It is obvious, then, that there is no part of
the bed of the sea which may not be receiving sediment.
III. _The Deposition of Sediment._
From what has been said of the weight of sediment, it follows that it
will be deposited whenever the water in which it is suspended is at
rest. Hence, when a river increases in breadth so as to form a lake, the
waters at the outlet are seldom turbid. The earthy matters with which
the principal and tributary streams were charged all settle to the
bottom, and go to lessen the capacity of the reservoir. Thus lakes are
continually diminishing in depth and area. In many instances, they are
already filled with sediment, and are thus converted into alluvial
plains, through which the river flows in a narrow channel.
It is frequently the case that a river, as it approaches the sea, has so
slow a motion that its sediment is deposited on the bed of the stream.
Thus the bed will be raised, and the banks will also be raised, by the
deposition of sediment upon them at periods of overflow. The river will
then be raised above the adjacent country. The river Po, for the last
part of its course, is from ten to twenty feet above the adjacent lands.
The same is true of the Mississippi, and many other rivers. The streets
of New Orleans are several feet below the surface of the river. In an
uninhabited country, such a river would soon seek a new and lower
channel; but in a populous country, it becomes a matter of interest and
safety to confine the river in its old channel, by artificial
embankments.
But the principal part of the sediment of rivers is conveyed to the sea.
It here mingles with the debris which the waves have furnished, and a
part of it is deposited to form deltas. The remaining part is taken up
by marine currents, mingled with the debris which they have furnished,
and is spread out on the bed of the ocean.
Of the extent of these deposits we can form no estimate. Those of rivers
and lakes are comparatively unimportant, as they are in the older
formations. Some of the delta deposits are already of great extent. That
of the Ganges contains an area of twenty-six thousand square miles, that
of the Niger twenty-five thousand, and that of the Nile twelve thousand.
The delta of the Rhone has increased its area by three hundred square
miles in the last thousand years. The Po has encroached upon the
Adriatic two thousand square miles in the last two thousand years, and
the Mississippi has enlarged its delta by one hundred square miles in
the last hundred years. In the deep valleys of the ocean accumulations
may be taking place on as large a scale as they ever have been in former
times.
IV. _Character of the Formations thus produced._
Sedimentary matter thus deposited would take the form of _strata_. Thus,
a delta deposit may receive at one time from a river a layer of coarse
gravel and pebbles, and in the course of a few hours the current may be
so reduced that it will convey to the same place only fine sand and
silt. Or, if a depositing current receive its sediment only at
intervals, the heaviest particles would be thrown down first, and the
more finely levigated particles would continue to fall, till the water
became transparent. Another supply would furnish another similar
stratum, and so on. The same arrangement might result from the sediment
being furnished by different rivers. Thus, if sediment were furnished to
the Gulf Stream by the Merrimac river, and the streams emptying into the
Bay of Fundy, the freshets would occur earlier in the season in the
Merrimac, and it would furnish a supply of sediment from a region of
primary rocks. A later supply would come from the red sandstone region
of Nova Scotia, and the stratification would be indicated by the
different kinds of rock produced. Thus stratification will result from
difference in the color, composition, or size of the particles of which
rocks consist. A great variety of causes, both general and local, may
therefore give to a deposit this character. Hence, as stratified rocks
are produced by the sediment now laid down from water, we may conclude
that the older stratified rocks are the sediment deposited in like
manner, in former times.
The occurrence of layers of different composition, as one way in which
the stratification is indicated, is produced by local and frequently
recurring causes. There are, however, other alternations of much greater
extent; those, for example, nearly twenty in number, distinguished by
striking differences in lithological character, into which the New York
system of rocks is divided. These alternations have resulted from more
general causes. The physical geography of a wide region must have been
so different, at the different periods during which these several
formations were deposited, as to change, at each period, the kind of
sediment furnished to the forming currents, and modify the types of
animal life.
We have seen that the same causes that determined the stratified
arrangement will determine the alternations of strata of coarse and fine
materials.
It is obvious that the stratification of the marine deposits will be
nearly horizontal. If the surface were very irregular upon which the
deposition commenced, the irregularity would constantly diminish; for
the movement of the water over this surface, however slow, would tend to
remove the accumulations from the highest points, and leave them at the
lowest (Fig. 75). Delta and lake deposits will, however, dip somewhat,
though never at a high angle, towards the deep water. In certain
situations, where a river and a tidal wave, coming in conflict, cause,
in succession, eddies and currents in opposite directions, we should
expect to find the stratification very irregular (Fig. 76); sometimes
false stratifications (_a b_), sometimes the strata cut off abruptly,
and at other times contorted or dipping in opposite directions within
short distances.
[Illustration: Fig. 75.]
Wherever sediment is deposited, it will entomb whatever of the remains
of animal or vegetable life may be mingled with it. They will be at once
protected against the influence of all the ordinary decomposing
agencies, and will continue for ages to retain their peculiar markings,
and even their colors. They will thus constitute, in all future time, a
record of the present condition of the organic world. The lacustrine
deposits can contain only fresh-water species of animals, marine
deposits only marine animals, while deltas may contain the remains of
marine life mingled with those which have been washed down by rivers.
The remains of birds, insects, and terrestrial animals, may occasionally
occur, in every kind of deposit. Sediment deposited in deep water will
never contain fossils in abundance, the deep parts of the ocean being
almost wholly destitute of animal or vegetable life. It is only in water
of a few fathoms that the greater number of species and of individuals
occur. In all these particulars the deposits now forming sustain a close
resemblance to the older formations.
[Illustration: Fig. 76.]
There are certain formations, as that of the coal, which required
conditions for their formation different from those of ordinary
sedimentary deposits. Coal consists of mineralized vegetable matter. Its
vegetable origin is proved by the uniform occurrence of vegetable
fossils almost exclusively in the coal measures. When reduced to thin
slices and examined under a high magnifying power, a structure very
similar to the ligneous tissue of existing coniferæ is sometimes found
to exist. There are probably vegetable deposits now taking place not
altogether unlike those which produced the coal measures.
We know that many rivers--the Mississippi, for example--now carry into
the sea great quantities of ligneous matter. Before the country was
inhabited by man, the quantity was undoubtedly much greater than it now
is. It floats for a time; but the ligneous tissue itself is heavier than
water, and as soon as the air is excluded from the pores, and they are
filled with water, it will sink. The woody and earthy matters are swept
into the sea together; but, as they sink under different circumstances,
they will be deposited separately. _Thus wood may continue to accumulate
in particular places in the sea_ for long periods, with but little
intermixture of earthy substances.
It is, however, to be expected that, in the progress of geological
changes, the places which at one time receive deposits of wood will at
another receive detrital matter, and thus _the wood will become deeply
buried_ beneath sedimentary strata.
Wood thus situated will become converted into coal. Trees which had been
covered to considerable depth with earth have been found near the
Mississippi river changed to lignite, a substance resembling charcoal.
In this case, the wood had been exposed to no greater heat than is
common to the crust of the earth at the depth where it was found; and
yet it had undergone this change since the country has been known to
Europeans, as it retained the marks of the axe when it was discovered.
It has also been found by experiment that vegetable matter, by long
submersion in water, passes into the state of lignite. This is the first
step in the conversion of wood into mineral coal.
When lignite is exposed to moderate heat and great pressure, it loses
the characters of lignite, and becomes mineral coal. This is shown by
facts observed in Germany, Ireland and Iceland, where beds of lignite
have been overspread by basalt. The upper portions of the lignite are
changed to mineral coal. The lower portions, which the heat did not
reach, retain the characters of lignite.
Beds of vegetable matter, with a great thickness of rock deposited above
them, would therefore be subject to all the conditions necessary to
convert them into coal, namely, pressure from the superincumbent mass,
and the heat which the strata uniformly assume at great depths.
It is not improbable, therefore, that coal-beds are now forming, and
that they have been formed at every geological period since an abundant
terrestrial vegetation commenced. Accordingly, there occurs in Virginia
an extensive coal-field in the oölite formation. Coal-fields also occur
in England, of less extent, in the same formation. In France, and other
parts of Europe, there are extensive beds of lignite in the tertiary
formation.
We have therefore no difficulty in accounting, in a general way, for the
formations of the carboniferous period. The vegetables were probably
less woody than those of the present time of equal size, and were
therefore more easily prostrated and committed to the waters. They grew
rapidly in moist ground, and perhaps in shoal-water, and required an
atmosphere charged with moisture and of a high temperature. Thus much is
inferred from the conditions most favorable for the growth of recent
species analogous to the coal-plants. These recent species are tropical
plants, and grow in moist insular situations, conditions which would
have existed at the carboniferous period, if the present coal-fields
were then an archipelago dotted with low islands.
Such being regarded as the origin of the coal-beds, the _alternations_
of the earthy and carbonaceous strata may be referred, provisionally, to
those great changes in physical geography upon which the other
alternations of strata on a large scale depend. But the regularity with
which the coal-seams and sandstone succeed each other presents some
difficulties which, in the present state of knowledge, we cannot
satisfactorily account for.
_Beds of salt_ occur, interstratified with other rocks, in nearly all
countries. Still, it is not a sedimentary deposit, and its formation
must depend upon peculiar circumstances. In New York, saline, together
with earthy matter, constitutes the Onondaga limestone, one of the
formations of the New York system. In Kentucky, the strata of rock-salt
are in the coal formation; in England, they are in the new red
sandstone; in Spain, they are in the greensand, and in Poland they are
in tertiary strata. The conditions of its formation have therefore
existed in connection with the deposition of every fossiliferous rock.
It has been shown that the ocean is the principal reservoir of the
saline matters which are taken up whenever water percolates through
rocks. It must happen not unfrequently, in the course of submarine
elevations, that a basin of sea-water will be cut off from its
communication with the sea; and from this basin the evaporation might be
more rapid than the supply of water. The great salt-lake of Utah is
undoubtedly a basin of this kind. The Mediterranean Sea is another such
basin, not yet wholly separated from the ocean. The evaporation exceeds
the supply of water from the rivers, and a powerful stream is therefore
continually thrown in from the ocean, through the Strait of Gibraltar.
The waters of the Mediterranean are already more highly charged with
salt than ordinary sea-water. This sea may ultimately become a saturated
solution, and begin to deposit salt. But whether it does, or not, it
indicates the way in which salt-beds may be formed.
V. _Solidification of Aqueous Deposits._
Sediment is generally deposited as a soft mud, but in nearly all the
older formations it has become solidified. When rocks are deposited from
a chemical solution, they take at once the solid form. Such is the case
with rock-salt and with limestone, when the material has been held in
solution. Solidification takes place in nearly the same way when water
which holds carbonate of lime or oxide of iron in solution filters
through beds of sand or gravel. The substance held in solution is
deposited in the interstices till they become filled, and the whole is
changed to solid rock.
Some rocks are composed of such materials that they _set_, like
hydraulic cement, when they are deposited. Other rocks become solid
simply by drying. Thus a deposit now forming in Lake Superior becomes,
by drying, nearly as hard as granite. Such a deposit will therefore
become solid whenever it shall be elevated above the water.
The pressure to which all but the upper layers are subjected is probably
sufficient to reduce most rocks to the solid state. Dry and pulverized
clay is reduced by artificial pressure, for a moment, almost to stone.
The pressure upon the deep-seated rocks is constant, and greater than
any artificial pressure can be.
In addition to these causes, all the older rocks have been subjected to
a high temperature, some of them nearly to that of fusion. By this means
the solidification of every kind of rock would be promoted, and probably
some may have been reduced by it to the solid state, which would
otherwise have remained as an incoherent mass.
SECTION V.--AQUEO-GLACIAL ACTION.
1. _Glaciers._--A glacier is a mass of ice occupying the bed of a
mountain valley, having a slow progressive motion, and reaching somewhat
lower in the valley than the line of constant snow. (Fig. 77.) The
Glacier des Bois, which may be regarded as a specimen of the Alpine
glaciers, covers an area of about seventeen square miles. In its
lowest portion, when all its branches have become united into one
stream, it has an average width of half a mile, and is five miles long.
It is estimated that the glaciers of the Alps cover an area of fourteen
hundred square miles. These have been the most carefully studied, though
glaciers are found in the valleys of various other ranges of mountains.
In the higher valleys, the snow, which falls at all seasons of the year,
accumulates in immense quantities, and the steep mountain sides
contribute, by frequent avalanches, to this accumulation. The snow, when
thus increased, does not become a compact, adhesive mass; but, changing
into particles of solid ice, it resembles sand rather than snow. It is
this _névé_ which constitutes the upper part of every _glacier_, and
which, in a modified form, constitutes the lower part.
The valleys descend rapidly towards the base of the mountains; and this
snow-ice, having no cohesion between its particles, _moves slowly down
the slope of the valley_, like a very imperfect liquid. After descending
below the line of perpetual snow, the surface will melt during the day;
and the water, sinking into the porous mass, becomes frozen, and
converts the whole into more or less compact ice, yet never into a rigid
mass. Influenced by its own weight, and by the pressure of the snow-ice
behind, it still continues its motion, and conforms itself to the shape
and curves of the valley through which it passes. The average movement
per annum may be stated at about five hundred feet.
The temperature of the rocky bed of the valley will be a little, and but
a little, higher than thirty-two degrees. There will therefore be but
little melting at the bed of this river of ice. As it receives continual
accessions from the atmosphere, it will therefore increase in volume
till it descends to the level of perpetual snow. Below this line _the
waste exceeds the addition_; and as it approaches the lower and
cultivated portions of the valley, it rapidly diminishes, till it
finally loses the solid form, and becomes a rivulet. The terminus of the
glacier is determined principally by the general climate of the country.
Any considerable variation of climate will cause it to recede, or
descend lower down the valley. The terminus varies, however, somewhat
with the seasons, being lower in winter than in summer, though the
motion is much less in the cold season than in the warm; and it descends
many rods further some seasons than it does others.
The glacier consists principally of snow, more or less modified in
structure; but it also contains whatever else may have been thrown upon
its surface, or into the snows by which it is fed. Tributary glaciers
extend up through all the gorges into which the irregular surface of the
mountain-top is divided. On these rough peaks there are always fragments
of rock, varying in size from fine sand to masses weighing many tons;
some of them loosened when the mountain was upheaved, some by subsequent
earthquake vibrations, and others still by tempests, lightnings, and
changes of temperature. When the snow has accumulated to a certain
extent on the steep slopes, it falls in avalanches into the valleys,
carrying with it loosened masses of rock, and often breaking off large
fragments from the rocky escarpments against which it strikes. These
avalanches are almost constantly descending, and hence a glacier always
contains considerable _earthy matter distributed through it_.
[Illustration: Fig. 77.]
The friction of the glacier, at its edges and along its bed, separates
more or less of the rock over which it moves; and hence there is always
a layer of mud and pebbles under the glacier, and a line of loose
fragments, called a _lateral moraine_, at the sides. When two glaciers
unite, the two lateral moraines, thus brought together, come to the
surface, forming a medial moraine, and show the line of junction
sometimes for miles.
The friction of the glacier on the bed of rock, assisted by the layer of
pebbles, will wear down the prominent portions, and everywhere polish
the surface. Fragments of rocks may be frozen into the glacier at all
depths. Those which lie near the lower surface of the glacier would, by
slight melting of that surface, project downward so as to act as a
graver's tool on the rock over which it passes. Hence, when the
extremity of the glacier has receded beyond its ordinary limit, the
surface of rock exposed is found, upon examination, to be _polished_,
_striated_, and _occasionally grooved_ an inch or two deep.
Since the waste is almost wholly superficial, earthy matter, which was
at first concealed in the mass of the glacier, is continually coming to
view, as the surface melts and runs off. Thus, none of the freight of
the glacier is left along its course, but all is carried to its terminus
and discharged there. Hence, at the lower extremity of the glacier there
is always an embankment of earth, pebbles, and boulders. If the glacier
recedes a few yards at one season of the year, and leaves its earthy
fragments scattered over this surface, they will be pushed forward into
a ridge, as the glacier again advances. This ridge is called a _terminal
moraine_, and consists wholly of substances which have been separated
from the mountain mass, often at the highest beginnings of the glacier.
At the terminus of all the Alpine glaciers, there is a series of these
moraines (_a a a_, Fig. 77) marking the successive limits of the glacier
in former times.
There is a ridge of boulders on the north side of the Swiss valley, near
the base of the Jura Mountains, resembling a terminal moraine. These
boulders consist of several groups, distinguished by peculiarities of
structure and composition; and each group lies opposite to the
particular Alpine valley which now furnishes the same kind of
fragments. It has been thought that, at a former period of more severe
climate, the Swiss valley was filled in part with ice, and that the
present glaciers extended across it to the Jura Mountains.
It is found that the polished and striated surfaces of the rocks in the
Alpine valleys are precisely like the surface of the rock, which has not
been exposed to atmospheric influences, in the north of Europe and
America. It has been proposed to extend the glacier theory, and account
for these phenomena by supposing that the north polar regions were, at
the ice period, capped with a glacier-mass, extending as far south as
the drift phenomena appear.
It is not to be doubted that the phenomena of polished surfaces and
transported materials in the immediate vicinity of the Alps, and near
other high mountains, are correctly referred to glacial action. This
theory has therefore solved, in part, one of the most difficult problems
in geology; but there is great difficulty in extending it so as to
account for the drift phenomena in general. If the motion depends upon
gravitation only, the origin must have a much greater elevation than the
terminus, which would not be the case in the great glacier supposed to
extend southward from the Arctic regions. Elevation of temperature, it
has been thought, might account for the movement of the mass southward.
2. _Icebergs._--In very high latitudes, the ice, which makes out from
the land into the sea during the cold season, suffers but little waste
at any time. This sheet of ice continues to increase in breadth and
thickness, by congelation, from year to year. The spray and the snows of
each succeeding year will also add to the mass. It thus accumulates to
the height of several hundred yards. It will also reach down a good many
feet below the surface of the sea, and will extend back on the land, or
lie heaped up against a precipitous escarpment, and firmly frozen to it.
After a certain amount of extension over the sea, the accumulated weight
of the ice and snow would tend to depress it, and break it loose from
the shore. The waves would tend to the same result, and would act at
greater mechanical advantage, as its extension from the shore becomes
greater. _Hence, it would ultimately become separated from the shore_,
and float in the water.
At its commencement, the earth, pebbles and rocks, which may lie along
the shore, and as far down into the sea as the congelation extends, are
frozen into it. In many situations its mass would be increased by
avalanches while it remained attached to the land, and these would
supply also masses of earth and rocks, as they do to glaciers. When it
becomes loosened from the shore, it will break off, and carry with it
some of the earthy portions of the coast, or the less firmly fixed
masses of rock from the escarpment against which it formed. Thus every
iceberg becomes _freighted_, more or less, _with earth and rocks_. This
has almost uniformly been found to be the case, when they have been
landed upon by ships' crews and examined.
We have seen that the general tendency of the waters of the ocean, and
of the lower stratum of the atmosphere, is to a motion from the poles
towards the equator. However irregular, therefore, the course of an
iceberg may be, its general _movement_, influenced both by the
prevailing winds and by ocean currents, will be _towards the equator_.
[Illustration: Fig. 78.]
These floating ice-mountains (Fig. 78) are formed in _great numbers, and
of vast size_. The relative specific gravity of ice and water are such
that nine cubic feet of ice, below the surface of water, will support
one cubic foot above it. As icebergs are often one or two hundred feet
high, their vertical depth must be a thousand feet at least; and their
area is equal to a square mile, and sometimes it is much greater. In
1840, the United States Exploring Expedition, in the extreme southern
ocean, coasted for eighty miles along a single iceberg. They are never
absent from the polar seas; and at certain seasons they are so abundant
along the usual course of vessels from New York to Liverpool, as greatly
to obstruct and endanger navigation.
An iceberg may continue for some time to increase in size, while
floating in the polar seas, but will at length reach a latitude where
the waste will exceed the additions, in consequence of the temperature
both of the air and of the water. It will, therefore, drop gradually the
earthy matters which it contains, upon the bed of the ocean.
It is not improbable that icebergs may often reach down so far as to
strike the highest points of the bed of the sea. The ice would be
lifted, and glide over the elevation, without suffering any perceptible
deviation from its general course. It would thus affect the surface of
rocks exactly like a glacier. If, however, the iceberg becomes
permanently stranded, and melts in one place, its earthy matters will be
thrown down upon the elevation which first arrested it.
If the bed of the sea, between the fortieth and sixtieth degrees of
latitude, could be exposed for examination, the rocky surface would be
found to be polished and striated by the icebergs which have passed over
it, and the whole surface would be strewed with boulders and drifted
materials brought from Arctic and Antarctic lands. Sometimes it would be
accumulated in heaps, and sometimes spread nearly over the surface.
We have seen that very recently, probably about the close of the
tertiary period, the portion of Europe and America over which the
northern drift is found, has been depressed several hundred feet. It may
be presumed that at that time icebergs floated over it, polished the
surface of the rocks, and distributed the boulders and other drift which
is now found upon it.
SECTION II.--IGNEOUS CAUSES.
I. _Of the Temperature of the Mass of the Earth._--Heat has been the
most efficient agent in determining and modifying the structure of the
earth; and, in order that the explanations of the phenomena referable to
this cause may be intelligible, some idea must be formed of the actual
present condition of the mass of the earth with respect to heat.
At any point of the surface there are variations of temperature,
depending on external causes. But these variations are found to extend
only a little way below the surface,--never more than a hundred feet. At
greater depths, it is found that the temperature invariably increases
with the depth. Deep mines have always a temperature above the mean
annual temperature at the surface. The water obtained by deep boring is
always tepid when it comes to the surface. The thermal springs, so
abundant in this country and in Europe, are so situated as to justify
the impression that their waters come from great depths. To make these
general observations of any value, we must determine the law by which
the temperature increases. The result of all the observations yet made,
in mines and upon wells and springs, is that, below the first hundred
feet, the temperature increases by one degree of Fahrenheit's scale for
every forty-five feet.
Regarding this law of increment as applicable to all depths, at ten
miles below the surface we should have a temperature above that produced
by the combustion of wood; and at twenty-five miles, a temperature of
three thousand degrees, by which nearly all mineral substances would be
reduced to a state of fusion.
The general conclusion of a temperature sufficient to melt the mineral
substances of which rocks are composed, at no considerable distance
below the surface, is confirmed by the fact that portions of the
interior of the earth--at least, at the volcanic centres--are in a
melted state. The intimate connection between some volcanoes situated a
hundred miles or more apart, so that they are alternately in a state of
activity and rest, indicates that these centres are connected,--that
subterranean melted lava extends from one to the other, so that when one
is active, the elastic force is relieved at the other. These deep-seated
lakes of lava must therefore underlie large areas.
We are justified, then, in concluding that the mass of the earth, with
the exception of a comparatively thin superficial layer, has a very high
temperature.
By way of accounting for this temperature, it is now generally assumed
that the earth was originally in a state of fusion; that it was a mass
of liquid lava (if, indeed, it had not a temperature sufficient to
reduce it to the aëriform state). Starting with this assumption, there
must necessarily be a gradual reduction of temperature by radiation, and
a time must arrive when the surface would be crusted over with
solidified lava; and this crust would increase in thickness as the
cooling advanced, the interior still retaining its heat and liquidity.
The present condition of the crust of the earth, its form, that of an
oblate spheroid, with the exact difference of the equatorial and polar
diameters which is found to exist, as well as the phenomena of volcanic
eruptions, will all admit of explanation on this hypothesis.
It has, however, been rejected by some; and, to account for the heat of
the interior of the earth, it is suggested that, if the bases of the
earths and alkalies, particularly potassium, sodium and calcium, exist
in their metallic state beneath the surface, the rapid oxidation of them
by the access of water would generate heat of sufficient intensity to
melt the oxidized materials, and thus account for the phenomena
attributable to heat.
Either of these hypotheses may be adopted; but it is not necessary to
account at all for the existence of this temperature. The fact is
susceptible of proof; and, though we may not be able to frame any
hypothesis to account for its existence, we may yet employ the fact in
the explanation of other phenomena.
II. _The Action of Internal Heat in producing Volcanoes._
The phenomena of volcanoes and earthquakes are evidently produced by
some force operating from below. The effect of heat alone would be to
reduce the rock to a liquid state. There is no reason to suppose that it
is ever sufficient to reduce them to the aëriform state. The elastic
force must therefore depend upon some other substance associated with
the lava, and this substance is water.
This will be shown by an examination of lavas. At the time of their
ejection, they are in a fluid or semi-fluid state; but it is not a
complete fusion. Even the most fluid lavas contain particles of minerals
in a solid state. The liquidity depends upon the fusion of the more
fusible portions, and upon the steam of water at a high temperature,
which fills the interstices between the solid particles. The porous
character of cooled lavas is produced by the steam which filled the
cavities previous to solidification. Steam always escapes from the
surface of a lava current while it is cooling, and it is always
discharged in immense volumes from the orifice of eruption, in
connection with the lava, and especially at the close of an eruption.
The geographical position of volcanoes, also, leads to the conclusion
that water is essential to their activity. There are five principal
lines of volcanic activity. One, commencing at the southern extremity of
South America, extends northward along the Andes and Cordilleras to
California or Oregon. The second has a north-east and south-west
direction, from the Aleutian Islands through the Kurule, Japanese, and
Philippine islands, till it meets the third line, lying in a nearly east
and west direction, embracing Sumatra, Java, and most of the Pacific
volcanic islands. A fourth band commences in the Grecian islands, and
extends westward so as to include the volcanoes of Italy and the
adjacent islands, and the Azores. The fifth band embraces the volcanic
islands of the West Indies, crosses Mexico in about the latitude of the
city of Mexico, and extends into the Pacific. There are also some
isolated centres of volcanic activity, such as Iceland. These volcanic
bands embrace about three hundred volcanoes. It will be seen that they
must nearly all be in close proximity to the ocean, or to large seas.
About two-thirds of them are on islands. Moreover, the volcanic vents
which are wholly submarine are probably very numerous.
This circumstance of the position of volcanoes establishes a presumption
that they cannot exist at a distance from some large body of water; and,
taking it in connection with the constant presence of aqueous vapor in
lava, we are justified in the conclusion that _the presence of water is
an essential condition of volcanic activity_.
Knowing that heat and water exist at the volcanic centres, it is not
difficult to form an idea of _their mode of operation_. The water,
diffused through the interstices of the lava, and subjected to a
temperature sufficient to melt the lava, would possess an _elastic
power_, which, though never computed, we may well suppose capable of
overcoming any resistance which the crust of the earth might present.
The repressing force will be the tenacity and weight of the
superincumbent strata. Whenever the elasticity is superior to this
repressing force, it will manifest itself in the fracture of the strata,
and often in the ejection or lava to the surface.
This fracturing of the strata, produced by an uplifting subterranean
force, is believed to be the cause of the noise and the vibratory motion
which are the chief phenomena of earthquakes. The elastic force may
raise lava to the surface, and thus the fracture would become a volcano.
But the force may expend itself by the discharge of vapor into the
fissure, or by merely filling it with lava. In either case, the only
evidence of the existence of the volcanic force would be the noise and
the wave-like motion experienced at the surface. The cause of the
volcano and earthquake is therefore the same, though the phenomena which
characterize them are different.
When the strata are is thus fractured, lava may for a time be discharged
along the whole line. By the cooling of lava in the fracture, it would
become partially reunited. Still, this would be the line of least
resistance. It would therefore be again burst through in certain places,
which would long continue to be orifices of discharge, and thus the
original fracture would determine _a line of volcanic activity_.
The repressing force may become greater at an orifice of eruption than
at some other point, either by the great accumulation of ejected
materials around the opening, or by the dormancy of the volcano long
enough for the complete solidification of the lava with which the
channel was filled. The least resistance may then be far from any
previous vent, when a new orifice of discharge will be opened, and _a
new volcano make its appearance_. It seems probable, also, that
volcanoes may become extinct by the reduction of temperature at the
volcanic centre, and that new volcanic centres may be formed; but the
cause of this change of temperature is not yet well understood. New
volcanoes have broken out in the sea, near Iceland, in several
instances; others in the volcanic line east of Asia. Graham Island,
situated between Sicily and Africa, was formed by an eruption which
broke out in the bed of the sea where the soundings were more than one
hundred fathoms. The island was at one time two hundred feet 'above the
sea, and three miles in circumference. It was, however, gradually
destroyed by the action of the waves, and now remains a dangerous reef,
covered by less than two fathoms water. The volcano of Jorullo, in
Mexico, was formed in this way. Previous to the formation of the
mountain, the region where it now is was a cultivated table-land. During
the year 1759 volcanic action commenced and continued, until, at the
expiration of twelve months, a cone had been formed having an elevation
of sixteen hundred feet above the adjacent plain.
An orifice of eruption is at first but little elevated above the general
surface; but, by the accumulation of ejected matter, a cone is at length
formed around the vent. The upper portion of a cone always consists of
these materials, but there may also be in progress a general elevation
of that part of the earth's crust, and the cone will partake of that
general elevation. The cones of the Andes owe their height, in a great
measure, to a general movement of elevation; those of Ætna and
Vesuvius, in a greater degree, to accumulation of ejected matter.
In either way, the height may become so great that the force necessary
to raise a column of lava to the top would be greater than the sides of
the cone, weakened as they always are by fractures in all directions,
can sustain. Hence, the highest craters of Ætna and South America have
long been closed, and the lava escapes through fissures at a lower
level, and _lateral cones_ are produced.
[Illustration: Fig. 79.]
The form which the materials have, when ejected from volcanoes, depends
mainly upon the degree of liquidity of the lavas at the volcanic foci.
If the liquidity is very perfect, the aqueous vapor will readily rise
through the lava. The steam thus separated will drive before it whatever
rocks, or previous lavas, may obstruct it. In their progress they would
be reduced to sand and powder, and ejected as _volcanic cinders_. (Fig.
79.) If the lava possess considerable viscidity, the aqueous vapor will
separate with more difficulty, and the lava and vapor will ascend the
channel together. Large bubbles of vapor will, however, collect with
more or less of frequency; and, as they rise through the lava, will
drive forward a portion of it, and cause the overflow to take place by
pulsations. As the bubbles reach the surface, their bursting causes the
loud reports, which are compared to the discharge of heavy artillery.
With each explosion some of the lava will be projected violently into
the air, and, cooling, will fall to the surface as scoriæ,--or, if the
lava be highly vitreous, it will be drawn out into fibres, and descend
as volcanic glass.
III. _Geological Phenomena referable to Volcanic Action._
Volcanic agency has probably never been less than it is now, and we
ought therefore to find its effects very general and important.
1. The most obvious of these effects are the _fractures_ with which the
crust of the earth is everywhere intersected. The uplifting force upon
which all volcanic phenomena depend would necessarily fracture the
crust, and the wave-like motion resulting from the fracture would cause
numerous secondary fractures, having a parallel direction. They are
often of such extent, during earthquakes, as to endanger life. During
the great earthquake at Lisbon, in 1755, a fracture opened of sufficient
width to swallow up the quay, and several thousands of persons who had
fled there for safety. The chasm remained permanently open to the depth
of six hundred feet. The earthquakes with which the valley of the
Mississippi was visited in 1811 so often fissured the surface, that the
inhabitants protected themselves by clinging to the trunks of trees,
which they felled transversely to the direction of the fissures.
[Illustration: Fig. 80.]
The first fracture which is produced by the upheaving force will open
upwards, and scarcely reach down to the seat of the force. But there
will be other parallel fractures, dependent upon the first, and opening
downward. Thus, the primary fracture at _a_ (Fig. 80) will be at once
followed by the fracture _b_, opening toward the lava, which will be
injected into it, and which, on cooling, will form a _dike_. Their
formation is mostly concealed from observation, but not always. During
the eruption of Ætna, in 1669, numerous fissures opened, one of which
was six feet wide and twelve miles in length; and the light emitted from
it indicated that it was filled with lava to near the surface. The
process was as perfectly seen as from the nature of the case it could
be.
2. The conversion of the lower sedimentary strata into _metamorphic
rocks_ has been effected by volcanic heat. The material of which dikes
consist has been injected in a highly-heated state; and, by observing
the effect which they have had upon the adjacent rocks, we may judge of
the effect which subterranean heat must have upon the lower mechanical
strata. Wherever the dikes are of considerable thickness, they have
converted the adjacent shales into primary slate, the sandstones into
quartz rock, and the dark and friable limestones into granular marble,
and destroyed the organic impressions. In the southern extremity of
Norway there is a district in which granite protrudes in a large mass
through fossiliferous strata. These strata are invariably _altered_ to a
distance of from fifty to four hundred yards from the granite. The
shales have become flinty, and resemble jasper; and near the granite
they contain hornblende. The siliceous matter of the shales has become
quartz rock, which sometimes contains hornblende and mica, and therefore
constitutes a kind of granite. The limestone, which at points remote
from the injected rock is an earthy, blue, coralline limestone, has
become a white, granular marble, near the granite, and the corals are
obliterated. The altered shales and limestones in many places contain
garnets, ores of iron, lead, &c. The annexed (Fig. 81) is a plan of this
granite and altered rock.
[Illustration: Fig. 81.]
One of the most instructive examples of metamorphic action in this
country is found in the White Mountains of New Hampshire. These
mountains have, till recently, been thought to consist principally of
granite; but it is now ascertained that this supposed granite is an
altered rock of the silurian period. It is represented as "intersected
by veins of felspathic granite; and the general mass is itself in many
parts converted into a near approximation to a binary granite, composed
of distinctly developed quartz and white felspar, with a few sparsely
scattered specks of mica. In its weathered surfaces it wears a close
resemblance to some fine-grained granites; but, upon inspecting a fresh
fracture with a magnifier, we instantly perceive many rounded grains of
quartzose sand, and the felspar is imperfectly formed, though the mica
has more nearly reached the condition which it has in granite. In some
of the coarse varieties of this white rock, small rounded pebbles of
quartz are to be seen, giving unequivocal evidence, even to the naked
eye, of its being an altered sandstone. We feel no hesitation in
deciding it to have been a silico-argillaceous white sandstone, now
almost granitized by extensive metamorphic action."
Similar illustrations, on a small scale, may be seen in every country
where the strata have been cut through by intrusive dikes. Sir James
Hall has shown the same by actual experiment. He exposed pulverized
chalk to heat sufficient to melt it, and under sufficient pressure to
prevent the escape of the carbonic acid. After cooling, the chalk was
found to have taken the form of crystallized limestone. But instances
enough have been given to show what changes should be looked for
wherever the sedimentary rocks have been exposed to a high temperature.
The lower strata must have been exposed, for long periods of time, to
such a temperature. We do not know at what depth below the surface of
the earth the rocks become liquid; but above the line of actual fusion
there must be a mass of rock not melted, yet scarcely retaining the
solid form. For a great thickness, perhaps for several miles, it would
be in a more or less yielding state. As there is not actual fusion, the
stratification is not destroyed, but such a degree of mobility among
the particles exists, that some degree of crystallization takes place,
and the elastic forces below easily bend, throw into folds, compress,
and in every way contort these strata. At the same time, any organic
matters which they may contain are decomposed, and the impressions of
them are obliterated. And such is the condition in which the metamorphic
strata are actually found.
3. _Denudation_ is, in a great measure, dependent on volcanic action. It
results from the billowy motion peculiar to the earthquake. This is not
simply a violent horizontal motion, but an equally violent vertical one.
It is a series of waves,--a succession of alternate elevations and
depressions of the solid crust. The height of these waves can only be
judged of by their effects; but it is difficult to account for some of
these effects, without supposing the waves to have been several yards in
height, and their velocity, in the few instances in which the time has
been accurately determined, was twenty miles a minute.
That such earthquake waves actually exist there can be no doubt. During
the earthquake in Calabria, in 1783, the flagstones in many of the towns
were lifted from their places and thrown down inverted, and trees bent
so that their tops touched the ground. During the great earthquake in
Chili, in 1835, the walls of houses, which were parallel to the line of
oscillation, were thrown down, while those that were at right angles to
it, though greatly fractured, were often left standing. Wherever careful
observations have been made, during and after severe earthquakes,
analogous facts have been noticed. Persons are generally affected with
sea-sickness. The sea is violently agitated. It often retires to an
unusual distance, and then returns upon the shore with most destructive
waves. Incredible, therefore, as it may seem, that the solid crust of
the earth should be thrown into such wave-like undulations, the fact is
well established.
With a velocity of twenty miles an hour, the successive waves may be
some miles apart, and yet be sufficient to account for all the
phenomena. It is evident, therefore, that the curvature of the wave
will be very slight, and yet enough to break into fragments all the
rocks thus curved. During the earthquake in Chili, before referred to,
"the ground was fissured, in many parts, in north and south lines. Some
of the fissures near the cliffs were a yard wide. Many enormous masses
had fallen on the beach. The effect of the vibrations on the hard
primary slates was still more curious. The superficial parts of some
narrow ridges were as completely shivered as if they had been blasted by
gunpowder." Similar phenomena seem everywhere to be exhibited by
earthquakes.
It may be presumed that almost all parts of the earth have, at different
periods, been subject to these earthquake waves. Accordingly, we find
that the crust of the earth is nowhere in an entire state, but is
divided by irregular lines into comparatively small fragments. By this
means, the deep fissures produced by fractures opening upwards would be
filled with fragments of rock shattered from the uplifted edges. In this
way the boulder masses were originally loosened from their parent beds,
and exposed to the action of ice, or any other transporting agencies. In
the same way the rocky bed of the ocean is, to a considerable depth,
reduced to a disintegrated mass. In this condition it will be rapidly
removed by marine currents, more or less broken, worn and comminuted, by
the movement, and deposited elsewhere. The materials have thus been
furnished for a very large proportion of the sedimentary rocks, and
especially of those which are composed of distinct fragments of other
rocks. By this means, also, wherever the rock formations come to the
surface, they are so broken that limestone, sandstone or granite,
suitable for architectural purposes, is seldom found, except at
considerable depths. This fragmentary condition of the surface rock is
such as exposes it to be acted upon readily by any powerfully abrading
causes, or to be more rapidly disintegrated by atmospheric and aqueous
causes.
4. We have already assumed that one principal division of rocks--the
unstratified--is of igneous origin. We have the proof of actual
observation, that lavas, and the accompanying _tufas_ and _grits_, are
volcanic products. The peculiarities of these products, in situation,
structure, and form, and in the imbedded minerals, are so great, that
whenever we find these peculiarities in the rocks of a country not now
volcanic, we still regard these rocks as of volcanic origin. We thus
have lavas, as well as stratified rocks, of different ages. There has
probably been no time in the earth's history when they have not been
forming.
The _trappean rocks_ are also of igneous origin. It is evident, from
their occurring in the form of dikes, that they have been in a melted
state. As they rest upon rocks of a sedimentary origin, they must have
been thrown up by volcanic forces. Yet they differ from ordinary lavas.
They are not vesicular in their structure, are more crystalline, and
there is in no case evidence that they have flowed from craters. If we
regard them as the lavas of submarine volcanoes, we shall have
conditions which will account for all their peculiarities. At a certain
depth the pressure of the water would be sufficient to prevent the
formation and escape of vapor, and therefore the lavas thus ejected
would not be vesicular. As the rapid cooling of lavas depends, in a
great degree, upon the escape of watery vapor, submarine lavas would
cool slowly, in consequence of the pressure. The liquidity depending in
part upon the retention of the heat, and in part upon the retention of
the aqueous vapor, they would consequently remain in a liquid state much
longer than the lavas of sub-aërial volcanoes. They would therefore take
a more highly crystalline form. All the loose materials thrown out
during the eruption would be removed by oceanic currents, and hence no
cone would be built up around the orifice of eruption. We may therefore
regard the trappean rocks as the lavas of submarine volcanoes. The
present volcanoes of this kind are necessarily producing the same kind
of rocks, though there will be no other proof that they exist, except
the existence of the volcano, till the bed of the sea becomes dry land.
The _granitic rocks_ are also the product of igneous causes. Granite is
the most abundant of these crystalline rocks; and the others, such as
crystalline limestone, are so intimately associated with granite that
they must have had the same origin. Granite is everywhere found to send
off dikes into the overlying rocks, and must therefore have been in a
state of fusion; that is, it must have existed as lava beneath the
surface. It is obvious that fluid lava always exists in great quantity
beneath areas of energetic volcanic activity.
Portions of this lava must in succession take the solid form. Wherever
the surface is elevated along a line of fracture, the lava which is
accumulated beneath rises above the level of the general reservoir of
lava, and will therefore part with its heat more rapidly. On cooling, it
becomes the granitic nucleus of the mountain. We ought also to suppose
that, by the extremely slow process of the transmission of heat to the
surface, the crust of the earth is everywhere increasing in thickness;
that is, the upper portion of the great lava mass is solidifying.
Sir James Hall has shown, by experiment, that earthy substances, reduced
to a state of fusion, become more highly crystalline as they are allowed
to cool more slowly, and are subjected to greater pressure. It is
difficult to conceive of these conditions existing in a higher degree
than they do in the cooling masses of lava below the stratified rocks.
These lavas must therefore take the highly crystalline form which the
granitic rocks are found to have.
All the igneous rocks have therefore existed as subterranean lavas. The
volcanic rocks have become vitreous, the granitic are crystalline, and
the trappean are intermediate in structure, coinciding with the
circumstances of pressure and rate of cooling under which they have
severally been formed.
5. _The Elevation of Mountains_ is another result of volcanic action.
The height of mountains depends, in part, upon general elevation. Yet
there is a different action, upon which the existence of the mountain,
as such, depends. Whenever igneous action becomes intense under any
portion of the earth's surface, and the elastic force greater than the
repressive, the solid crust will be broken and raised up, and along this
line of fracture the lava will rise above its general level elsewhere.
This lava, thus lifted out of the general mass, in time solidifies, and
forms the nucleus of a mountain. At successive periods the elevating
force is renewed, and adds somewhat to the mountain mass before
supplied. In this way the mountain is ultimately formed.
So far as observations have been made, the elevation of mountains seems
not to be gradual, but spasmodic; and yet the elevating force probably
accumulates constantly and uniformly. The repressing force consists of
the weight of the strata above, which may be regarded as constant, and
their strength, which is variable. When the elevating force becomes
greater than both the repressing forces, the crust is fractured. The
strength of the strata then becomes nothing, and the repressing force is
the weight alone. The elastic mass below at once expands, and the
requisite space is furnished by the uplifting of the strata along the
line of fracture. As the ridge of lava which fills this additional space
cools, it recloses, in part, the original fracture, and the repressing
force again consists of the two elements,--weight and strength. There
will therefore be no further elevation till the elevating force is again
superior to these two forces. Thus the elevating force, though it may
accumulate at a uniform rate, will manifest itself only at considerable
intervals.
As the accumulation of lava along the line of fracture is the cause of
the upheaval, every mountain must have a central granitic axis.
Sometimes this granitic mass is pushed up through the fissure, as in the
case of Mont Blanc. At other times, the stratified rock, which formed
the original surface, is carried up so as to form the surface rock
nearly to the top. In either case, the strata are lifted along the line
of fracture, and left in an _inclined position_. In this position the
older rocks are always found, wherever there has been any considerable
amount of igneous disturbance.
In some instances, the additional space required by the expansion of the
igneous mass below is furnished, not by the uplifting of the strata, but
by their compression into folds between two lines of upheaval. The
igneous rock is elevated but little above the stratified through which
it had burst; but the stratified rocks have taken the undulatory form,
and the widening of the igneous mass along the lines of fracture has
compressed the undulations, until the planes of _the strata have become
vertical_. Fig. 82 will give an idea of the successive changes by which
the vertical position of the strata has been produced.
[Illustration: Fig. 82.]
The force by which mountains are elevated being the elasticity of the
vapor diffused through the subjacent lava, it may happen, if the lava
have a high degree of fluidity, that this vapor will collect in large
masses, and rise as far as the lava is in a fluid state. The irregular
flow of lava from craters during an eruption is undoubtedly due to the
rapid ascent of such steam bubbles through the lava. Such an
accumulation of vapor under a mountain mass, if it cannot escape, would
support it as long as the temperature remained unchanged. But, upon a
reduction of temperature, the mass which had been upheaved by it would
be unsupported, and liable at any time to sink. Instances of
_subsidence_ on a comparatively small scale will admit of explanation in
this way. Papandayang, one of the loftiest volcanic mountains of Java,
sunk down four thousand feet in the year 1772. The area engulfed was
sixteen miles long and six broad. The crater of Kilauea, in one of the
Sandwich Islands, was evidently formed in this way. It is situated on
the side of a mountain, and consists of a chasm eight miles in
circumference and a thousand feet in depth. Liquid lava can always be
seen boiling in the small craters at the bottom; and at times it rises
so as to overflow them, and fill the chasm to within four hundred feet
of the top, when lateral subterranean passages are opened, by which it
is discharged. The same explanation--a depression of the central
portion--may be given of the formation of the large craters in the
Canary and Grecian islands. It is also probable that Lake Avernus and
others, in Italy, and some in Germany, have had a similar origin.
The subsidence of Papandayang is of importance as a historical fact; and
it is not at all unreasonable to suppose that larger chasms of great
depth were also sudden subsidences of a similar character. Lake Superior
has a depth considerably greater than the elevation of its surface above
the level of the sea. The bottom of the Dead Sea is two thousand six
hundred feet below the surface of the Mediterranean. And at one place in
the Atlantic Ocean a sounding was attempted with more than six miles of
line, without reaching bottom. These sunken areas, however, though of
great extent, occupy only an insignificant portion of the entire surface
of the earth.
6. _The Elevation of Continents._--The causes of change of level which
have been given will not explain those _slow vertical movements_ which
are now taking place in Greenland and the north of Europe, or those by
which the present continents have been elevated and the bed of the sea
depressed. Any cause which will account for these movements must be one
operating for long periods, under large areas, and with great
uniformity.
The cause which fulfils all these conditions most satisfactorily is a
_variation of temperature_ in the mass of rock underlying the portion of
the surface whose level is changing. It has before been shown that the
temperature increases as we descend below the surface; but there is also
reason to suppose that it undergoes great variations. The volcanic grits
interstratified with the silurian rocks of England show that at the
silurian period volcanic fires were active below that portion of the
surface. When the early fossiliferous rocks of this country were
deposited, the Alleghany Mountains had not been elevated; but before the
tertiary period they had taken nearly their present form. Some portion
of the intermediate period was therefore one of volcanic upheaval. The
trappean rocks are also evidence of intense volcanic action existing
here. France, during the tertiary period, was a highly volcanic country;
but all volcanic activity has now subsided. The Andes have been mostly
elevated since the tertiary period, and are still rising. It is evident,
then, that at different periods volcanic heat may vary from its highest
to its least degree of activity, below any portion of the earth's
surface.
This variation of temperature must be followed by variation of volume of
the earth's crust; that is, it _must produce expansion or contraction_.
Experiments have been made, under the direction of the United States
government, to determine the expansion of the several kinds of rock used
in our public works. It was found that granite expands nearly one two
hundred thousandth of its length for every degree of increased
temperature, limestone somewhat more than that, and sandstone about
twice as much. Taking the expansion of the granite as the basis of
calculation, and supposing the crust for a hundred miles in thickness to
be undergoing change of temperature, there would be a resulting
difference of level exceeding two and a half feet for each degree of
change in temperature, or more than two thousand five hundred feet for a
change of one thousand degrees.
This calculation is made upon the supposition that the law of expansion
is the same for all temperatures, and that no new conditions are
introduced at high temperatures by the presence of aqueous particles. We
know, however, that solids expand more rapidly at high temperatures than
at low, and the elasticity of aqueous vapor at high temperatures must
increase the rate of expansion of the rock through which it is diffused.
Although we are not able to introduce, numerically, the effect of these
two circumstances, yet it is obvious that they must be considerable.
The mean elevation of land above the level of the sea is about nine
hundred feet, the mountain masses above that level not being included;
and the estimated mean depth of the ocean, not including its chasms,
does not exceed two thousand six hundred feet. The _total elevation of
the continental masses_, for which it is necessary to account, does not
therefore exceed three thousand five hundred feet. This amount of
vertical movement may evidently be produced by the expansion and
contraction resulting from changes of temperature.
These changes of level must, however, be very gradual. Any diminution of
temperature must result from the transfer of heat to the surface; and
the conducting power of rocks is very imperfect. The lava in a crater is
often so cooled on the surface that it can be walked on, while but a few
feet below it is still liquid. Lava currents continue in gradual motion
long after the surface is nearly cold. This was the case with one of the
currents from Ætna for more than nine months after its eruption, and
with another for ten years. Humboldt visited Jorullo forty years after
it was thrown up, when the lava around the mountain was still in a
heated state, the temperature in the fissures being on the decrease from
year to year; but twenty years after its ejection the heat was still
sufficient to light a cigar at the depth of a few inches. If so long a
period is insufficient to solidify a comparatively small quantity of
melted rock when the circumstances for cooling are most favorable, we
may well suppose that centuries would be required to abstract sufficient
heat from the earth's crust to produce any material change in the areas
of continents.
If this account of the elevation and subsidence of continents is
correct, it would seem that they ought to be constantly undergoing
change of level. And their _apparent stability_ may be regarded as an
objection to it. If in any place there is absolutely no vertical
movement, then those conditions must exist in which, for the time being,
there is no change of temperature.
But it is doubtful whether there ever is absolute stability of any
portion of the surface for long periods of time. Of the minor vertical
movements of the interior of continents, there can, from the nature of
the case, be no evidence whatever. Changes of level, where they are
known to be taking place, are so slow, that they are hardly perceptible
in the period of a human life. Such changes had been going on for
centuries in Sweden before they were suspected. As accurate observations
have increased in number, and historical records become available, it is
becoming known that a very large amount of the seaboard is undergoing
change of level. It becomes probable, then, that these extremely slow
changes of level are constantly and everywhere taking place.
That portion of the crust of the earth constituting the present
continents, being further removed from the centre, would part with its
heat more rapidly, and receive heat from the central mass more slowly,
than that portion which at present constitutes the bed of the sea. The
continents are therefore in a situation to undergo contraction and
depression, and the bed of the sea is most favorably situated for
rising. If the distribution of water through the mass has any influence
in promoting its expansion, then the bed of the sea would receive this
supply most abundantly, and the continents the least so. We see, then,
in nature, those provisions for an alteration of level, which, from the
character of the several rock formations, we know to have taken place.
When any portion of the earth's surface is covered with the sea, the
conditions exist which will at length elevate it. When it becomes dry
land, the conditions exist which will in time depress it below the level
of the ocean. Hence, those impressions in regard to the land, as stable
beyond the possibility of change, we ought to abandon; and those
vertical movements, which, when proved, we are accustomed to regard as
extraordinary, we shall, at length, consider as only particular
instances of one of the most general laws of nature.
7. _Variations of Climate._--The only sources of heat by which climate
can be affected are the sun and the heated interior of the earth.
If the former melted condition of the entire mass of the earth be
assumed, the temperature of the surface must have been increased, by
conduction of heat from within, for long periods after the superficial
stratum had become solid. It is, however, susceptible of proof, that the
present climates are not sensibly affected by interior heat, though at a
little more than a mile below the surface the temperature is equal to
that of boiling water. At any time, therefore, after the waters had
become condensed, collected into oceans, and become sufficiently cool to
support the animal life of which the remains are now found, it is not
probable that the climate was, to any considerable extent, influenced by
the heat conducted from the interior.
Still, there have been great changes of climate since those early
organic forms existed; and, since we have no ground for supposing that
the temperature of the sun's rays has suffered any reduction, we have to
inquire whether the means of retaining the heat from the sun could at
any time have been different. _The relative position of land and water_
depends, as we have seen, upon igneous causes, and has been very
different at different times. We shall find that climate must have been
greatly modified by these changes; for the land radiates and absorbs
heat freely, and water possesses this power in a very low degree.
Let us suppose the zone comprised between the tropics to be occupied by
land, and the portions without these limits to be covered with water.
Under these conditions, the land, having a nearly vertical sun the
whole time, would accumulate heat to a degree scarcely compatible with
the existence of animal life. This is sufficiently proved by the
oppressive tropical climates of the present time, influenced as they are
by polar lands and contiguous seas.
Under the same conditions, the sea would be heated by contact with the
land, and the heat would be distributed by marine currents to the polar
regions. But the water thus distributed would not part with its heat,
because it has but little radiating power, and nowhere comes in contact
with polar land. It follows, then, that both land and water would be
subjected to a very high temperature.
But, if we suppose the land confined to the polar regions, and the sea
to the equatorial, the opposite results would follow. The equatorial sea
would absorb but a small proportion of the solar heat which would be
thrown upon it. The land would receive the sun's rays too obliquely to
receive much elevation of temperature, as the present polar climates
show. Hence, the temperature of the earth would differ but little from
that of the planetary spaces, which is fifty-eight degrees below zero, a
temperature too low to allow of any considerable development of organic
life.
These are the conclusions to which we are led by considering the
different powers of land and water to absorb and radiate heat, and we
shall find that the existing climates are in accordance with these
conclusions. America has a lower temperature than Europe in the same
latitudes. It has also a smaller proportion of land in the equatorial
regions, and a greater proportion in the north polar regions. The
eastern continent is colder in Asia than in Europe in the same
latitudes. It has also less equatorial and more polar land. The southern
is colder than the northern hemisphere at equal distances from the
equator. There is also less land near the equator on the south side, and
probably as much land around the south as the north pole.
Hence, we see that there may have been such a relation of land and water
as to account for all the variations of temperature which are known to
have existed. We cannot say that such actually has been the case. We can
tell, with some degree of accuracy, what portions of the present
continents were land at the several geological periods; but
three-fourths of the surface of the earth is covered with water, and of
the condition of this portion during those periods we have no means even
of conjecturing. We can only say, that, by the operation of known
causes, the relative position of land and water may have been such as to
produce the climates known to have existed at former periods of the
history of the earth.
INDEX.
Page
A.
Abundance of vegetable products of the coal period, 59
Accumulation of vegetable matter, 117
Actinolite, 15
Action of internal heat, 128
Action of waves, 107
in forming harbors, 108
Advantages of geological changes, 91
Ætna, 26, 73
Agate, 14
Age of rocks, doubtful--
from change of lithological character, 61
from distance, 61
from disturbance, 61
Alternation of coarse and fine material, 115
Aluminium, 12
Amethyst, 14
Amygdaloidal structure, 17
Ancient volcanic rocks, 29
Andes, granite veins in, 25
Angle of inclination, 71
Anoplotherium, 55
Anticlinal axis, 71
Aqueous causes, 103
Aqueo-glacial action, 120
Argillaceous schist, 20, 31
Arrangement of materials in the crust of the earth, 21
Artesian wells, 92
Asbestos, 15
Atmospheric causes, 95
Atolls, 81
Augite, 15
Auvergne, volcanic district of, 28
B.
Basalt, 18
Bed of the sea--
sunken areas in the, 142
why elevated, 145
Belemnites, 52
Breccia, 19
Brine springs--
in Silurian rocks, 35
in the carboniferous formation, 43
in the new red sandstone, 47
C.
Calamite, 47
Calcium, 13
Cambrian system, 32
Carbon, 11
Carbonate of lime, 15
Carbonate of magnesia, 19
Carbonic acid a cause of disintegration of rocks, 95
Carboniferous formation, 39
essential to national wealth, 43
extent of, 47
a prospective arrangement, 43
faults in, 41
not always disturbed by faults, 42
Carboniferous limestone, 39
sometimes becomes a coal-bearing rock, 42
fossils of the, 40
Carnelian, 14
Cause of internal heat, 128
Cause of stratification, 114
Caverns, 69
Cephalaspis, 39
Cephalopoda, 36
in oölite, 50
Chalcedony, 14
Chalk, 52
Changes of climate, 88
how produced, 146
Changes in the crust of the earth, 67
of temperature a disintegrating agent, 96
at the surface, 85
Chemical action, 97
in solids, 99
in crystallization, 97
Chlorine, 12
Chlorite, 15
Chlorite slate, 32
Classification of rocks, 21
Clay, 19
Clay slate, 20
Cleavage structure, 68, 98
Coal, 16
varieties of, 42
mode of quarrying, 42
origin of, 116
conversion of vegetable matter into, 117
now forming, 118
Coal measures, 131
fossils of the, 44
Coal plants, tropical character of, 88
Coal and iron associated, 43
Clouded marble, 33
Columnar structure, 18, 99
Compact limestone, 19
Concretionary formations, 99
Conglomerate, 19
of old red sandstone, 38
Connecticut valley--
one of denudation, 87
trap of, 30
Continents--
mean elevation of, 144
total elevation of, 144
elevated gradually, 144
why depressed, 145
Contorted strata, 72
Coral formation, 81, 102
extent of, 102
Coral reefs--
fringing, 81
barrier, 81
Coral rag, 49
Corals in silurian rocks, 35
Copper mines of Lake Superior, 47
Creation, a progressive work, 62
Cretaceous formation, 52
Cretaceous formation, fossils of, 52
geographical range of, 53
Crinoidea in Silurian rocks, 36
Crust of the earth, 16
expansion and contraction of, 143
D.
Delta deposits, 114
Denudation of igneous rocks, 85
Denudation of sedimentary rocks, 85
Denudation produced by earthquake waves, 136
Deposition of sediment, 113
Diluvium, 20
Dike, 69, 133
Divisional planes, 68, 98
Dolomite, 13, 19
Drift, 20
extent of, 53
connected with striated surface of the rocks, 54
connected with subsidence, 54, 126
E.
Earth in a state of change, 97
Earthquakes, 130
wave-like motion of, 136
Earthquake waves, rocks shivered by, 137
Effect of atmospheric agencies, 95
Electrical discharges, effect of, 96
Elementary substances, 11
Elevation and subsidence, 73
Elevation and subsidence several times repeated, 82
Elevation of mountains, 73
cause of, 139
spasmodic, 140
gradual, 75
Elevation of different mountains at different times, 75
Elevation of continents, 76
cause of, 142
Elevation of North America, 76
Elevation of the coast of Maine, 76
Elevation of Europe, 77
Elevation of South America, 78
Encrinites, 36, 50
F.
Fault, 41, 69
Felspar, 14
Filling up of lakes, 76
Fingal's cave, 30
Fissile structure, origin of, 68
Flint, 14
in chalk, 52, 99
Fluorine, 12
Folded axes, 61, 72
Formation of soils, 87
Fossils--
definition of, 57
how preserved, 57
mineralization of, 58
use of, 60
order in which animals appeared, shown by, 58
animal and vegetable, created together, 59
as a record of climate, 88, 101
Fossiliferous rocks, 32
classification of, 32
Fractures, 42, 68, 130
opening downward, 133
G.
Garnet, 16
Geological causes, how far uniform, 94
Geological causes, slow operation of, 95
Geological investigations aided by displacement of strata, 92
Geological periods, prolonged, 63
shown by amount of strata, 63
shown by duration of species, 64
shown by amount of organic matter, 64
shown by microscopic accumulations, 65
Geology and Revelation, 65
Giant's Causeway, 30
Glacial period, 90
Glacial theory, 124
Glaciers--
how formed, 120
cause of motion, 121
when they decrease, 122
earthy matter on them, 122
lateral moraines, 123
surfaces grooved by, 123
terminal moraines, 123
Gneiss, 18, 30
Gorge, 69
Graham Island, 131
Granite, 16
varieties of, 16
thickness of, 23
structure of, 23
formation of, 67
igneous origin of, 138
Granite veins, 24
in granite, 24
Granite of different ages, 25
Granitic axes of mountains, 24, 140
Greensand, 19, 52
Greenstone, 18, 30
Grooved surfaces of rock, 54, 87, 126
Gypsum, 15
in new red sandstone, 47
beds, how produced, 99
H.
Hall, Sir James, experiments, 135, 139
Heterocercal tails of fishes, 48
Homocercal tails, 48
Hornblende, 14
Hornblende slate, 20, 32
Hydrogen, 11
Hypersthene, 15
Hypersthene rock, 17, 25
I.
Icebergs--
how formed, 124
earthy materials in, 125
motion of, 125
size and number of, 125
effect in distributing drift, 126
grooving the surface, 126
Iceland spar, 19
Iceland, volcanic eruption in, 26
Ichthyosaurus, 50
Igneous causes, 127
Iguanodon, 51
Inclined position of strata produced by upheaval, 70, 140
Increase of temperature below the surface, 127
Iron, 12
J.
Jasper, 14
Jorullo, 131
K.
Kilauea, 26, 142
L.
Lakes, filling up of, 76, 113
Lava, 17, 25, 137
varieties of, 25
tertiary, 27
elastic vapors contained in, 130, 132
great quantity of modern, 26
Lead-bearing strata, 35, 40
Lepidodendron, 46
Lias, 19, 49
Limestone, 15, 19
as a primary rock, 25
metalliferous, 40
Local changes of climate, 90
M.
Magnesian limestone, 19, 47
Magnesium, 13
Mammoth, 56
Man--
recently created, 59
as an agent in producing geological changes, 101
impressions of the feet of, 59
skeleton of from Guadaloupe, 59
Manganese, 12
Marine currents, 108
Marine currents, cause of, 109
abrading power of, 111
Marl, 19
Mastodon, 55
Megatherium, 55
Metamorphic changes, 67
Metamorphic rocks, 30
amount of, 67
origin of, 134
order of superposition, 31
upper limit variable, 13
localities of, 32
Metallic ores, 92
Mica, 14
slate, 18, 31
Millstone grit, 40
fossils of, 41
becomes coal measures, 42
Mineral--
definition of, 13
Mineral veins, 69
formation of, 100
Modern formation, 57
why but little known, 57
fossils, 57
Moisture of the atmosphere a disintegrating agent, 96
Monte Nuovo, 26
Mount Loa, eruption of, 26
N.
Neocomian system, 52
New red sandstone, 47
fossils of, 48
ores of, 47
geographical range of, 49
Niagara Falls, how preserved, 106
Nitrogen, 11
Nummulite rock, 54
O.
Oceanic mountains, 75
Ocean level, nearly permanent, 74
Old red sandstone, 38
fossils of, 39
extent of, 39
Oölite, 19
Oölitic structure, 49
Oölite system, 49
calcareous, 49
fossils of, 50
localities of, 51
Opal, 14
Organic causes, 101
Orthoceras, 36, 40
Outcrop, 71
Oxide of iron, 16
Oxygen, 11
P.
Paleotherium, 55
Papandayang, 95, 142
Permian system, 47
Plesiosaurus, 51
Porphyritic structure, 17
Potassium, 12
Primary limestone, 19
Pterodactyle, 51
Pumice-stone, 17
Pyroxene, 15
Q.
Quartz, 14
rock, 32
R.
Raindrops, impressions of, 48
Raised beaches, 76
Ravine, 69
Recent elevation in Europe, 79
Recent formation, 57
Ripple marks, 48
Rivers--
beds of, raised, 113
continued into the sea, 112
abrading action of, 105
abrading action of promoted by foreign substances, 106
Rock crystal, 14
Rock salt, 16, 118
Rocks, denned, 16
Rose quartz, 14
S.
Saccharine limestone, 19, 32
Saliferous system, 47
Saline properties of the ocean, how obtained, 104
Salt beds--
where found, 118
how formed, 119
Sandstone, 19
Schorl, 16
Scoriæ, 17, 133
Sediment--
amount of in rivers, 107
deposition of, 113
sorted by rivers, 112
Selenite, 16
Serpentine, 15
a primary rock, 25
Shale, 20
Siberia, remains of elephants in, 89
Sigillaria, 45
Silicium, 12
Sinking of wharves, towns, &c., 79
Silurian system, 34
tabular arrangement of, 34
divisions of, 35
fossils of, 36
geographical range, 38
Slaty structure--
in the gold washings of Chili, 98
produced by electric currents, 98
Slope of mountains, 73
Soapstone, 15
Sodium, 13
Solidification of rocks, 68, 119
Soluble materials of rocks, 103
Solution of mineral substances--
promoted by heat, 104
promoted by an alkali, 104
promoted by carbonic acid, 105
Sources of the sedimentary materials, 103
Sources of the sediment of rivers, 97
Species--
disappearance of, 62
causes of the disappearance of, 62
Springs, 92
Stability of continents only apparent, 145
Statuary marble, 19
Stigmaria, 44
Strata--
horizontal, 70, 115
permeable and impermeable, 92
irregular, how produced, 115
Striated surfaces, 87
Submerged forests, 79
Subsidence of land, 79, 145
Subsidence of land in Greenland, 82
Subsidence and elevation in the Pacific, 80
Sulphate of lime, 15
Sulphur, 12
Sun-cracks, 48
Sunken areas, 142
Syenite, 17
Synclinal axes, 72
T.
Taconic system, 32
Talc, 15
Talcose slate, 20, 32
Temperature at great depths, 127
Temple of Jupiter Serapis, 83
Tertiary system, 53
age, how determined, 53
fossils, 54
divisions of, 53
geographical range, 56
Tilestones of the old red sandstone, 38
Trachyte, 18
Tracks in new red sandstone, 49
Transportation of sediment, 111
Trappean rocks, 17
localities of, 29
Tremolite, 15
Trias, 47
Trilobite, 37, 40
V.
Valley, 69
of elevation, 71, 75
of subsidence, 72
Valley of denudation, 87
Vein of segregation, 69
Verd-antique marble, 15
Volcanic rocks, 17, 25
of different ages, 26
in what states ejected, 25
Volcanic mountains, dimensions, 26
Volcanic activity--
regions of, 129
water essential to, 130
Volcanic cones--
formation of, 131
lateral, 132
Volcanic cinders, scoriæ, glass, 132
Volcanic action, effects of, 133
Volcanic origin--
of trappean rocks, 138
of granitic rocks, 138
Volcanoes--
number, 26
linear arrangement of, 131
near the sea, 129
new, 131
of the tertiary period, long active, 28
W.
Watt on fusion of basalt, 100
Waves, action of, 107
Wealden, 49
Wind a geological agent, 96
Z.
Zechstein, 47
QUESTIONS
TO
ELEMENTS OF GEOLOGY
QUESTIONS.
CHAPTER I.
SECTION I.
How many elementary substances are known?
In what combinations is oxygen found? What proportion of the earth's
crust consists of it?
In what combinations does hydrogen occur? Nitrogen? Carbon? Sulphur?
Chlorine? Fluorine? Iron? Manganese?
How does silicium occur? Aluminium? Potassium? Sodium? Calcium?
Magnesium?
How are these elementary substances classified? (Silicium, or silicon,
has but a doubtful claim to be regarded as metallic.)
SECTION II.
What is a simple mineral? How many are known?
What are the physical properties of quartz? How are the several
varieties distinguished?
What are the physical properties of felspar? Mica? Hornblende? How are
its varieties distinguished? Augite? Hypersthene? Talc? How are its
varieties distinguished? Serpentine? Carbonate of lime? Gypsum? Its
varieties?
What other minerals are mentioned?
SECTION III.
Define the crust of the earth. Rocks.
What are the unstratified rocks?
What is the structure of granite?
How are the varieties distinguished?
What is the porphyritic structure?
Describe hypersthene rock.
What are volcanic rocks? Lava? Scoriæ? Pumice-stone?
How is the vesicular structure produced?
What are volcanic breccias? Volcanic grits?
What is the composition of the trappean rocks?
What is the amygdaloidal structure?
What are the three varieties of trappean rocks, and how are they
distinguished?
[Illustration: Fig. 1.]
Name the stratified rocks. Describe gneiss. Mica slate. Sandstone.
Conglomerate. Greensand. Describe the varieties of limestone.
What is dolomite? Of what does clay consist? Clay slate? What
modifications does clay slate present? What is diluvium?
CHAPTER II.
SECTION I.
What is the primary division of rocks?
Upon what principle are the unstratified rocks divided?
Upon what principle are the stratified rocks divided?
Why are the non-fossiliferous called metamorphic rocks?
Name the four classes of rocks.
SECTION II.
What is the most abundant plutonic rock?
How is its thickness ascertained?
What is its amount?
Where is it found?
What is its ordinary structure?
What peculiarity of structure facilitates the cleavage of granite?
[Illustration: Fig. 2.]
The granitic masses are generally deep below the surface; in what other
position does granite appear?
[Illustration: Fig. 3.]
[Illustration: Fig. 4.]
In what classes of rocks are granite veins found?
Were they all produced at the same time?
How is this demonstrated?
What is the relative position of the older and newer granites?
What other plutonic rocks occur in considerable quantities?
SECTION III.
Of what do volcanic rocks consist?
In what states are they ejected?
What are the principal varieties of lava, and how are they
distinguished?
Why is the basaltic lava the last to be ejected?
How is the age of the volcanic rocks determined?
What are the three divisions of the volcanic rocks, as dependent upon
age?
What is the proportion of the volcanic to other rocks?
How many active volcanoes exist?
Describe the eruptions of Kilauea.
Describe the eruption in Iceland in 1783.
What are the dimensions of Mount Ætna, and how has it been produced?
How are the tertiary lavas known to be such?
Where have they been most studied?
What is the evidence that these rocks in France are volcanic?
Have these lavas been produced within the historic period?
[Illustration: Fig. 5.]
Were they produced at an early period in the earth's history?
Give the evidence that their activity was long-continued.
[Illustration: Fig. 6.]
What is the form of the earlier volcanic rocks?
What circumstances distinguish the trappean from other volcanic rocks?
What are some of the prominent localities of the trappean rocks?
How do they occur in the islands west of Scotland?
How in the valley of the Connecticut river?
SECTION IV.
What is the lowest metamorphic rock?
Describe it.
How does mica slate differ from gneiss?
Is it well distinguished from argillaceous slate?
What is the third rock in the metamorphic series?
Why is it difficult to determine the upper limit of this series?
Why do the principal rocks of this series occur in the order here given?
What other rocks may take the place of these principal rocks?
Where do the metamorphic rocks occur?
What is their thickness and amount?
SECTION V.
How many systems of fossiliferous rocks are there, and what are they?
What other system is provisionally introduced?
What is its position?
[Illustration: Fig. 7.]
Describe it.
What materials of value are obtained from this system?
[Illustration: Fig. 8.]
What fossils does it contain?
In what localities is it found?
What explanation, in reference to these rocks, is given by those who
deny that they constitute a distinct system?
[Illustration: Fig. 9.]
[Illustration: Fig. 10.]
In what respects does the State of New York present the best facilities
for studying the Silurian system?
Describe the Champlain division.
[Illustration: Fig. 11.]
The Ontario division.
The Helderberg division.
[Illustration: Fig. 12.]
[Illustration: Fig. 13.]
[Illustration: Fig. 14.]
[Illustration: Fig. 15.]
[Illustration: Fig. 16.]
Describe the Erie division.
What are the fossils of this system?
[Illustration: Fig. 17.]
Describe the Crinoidea.
The Cephalopoda, and the two forms.
[Illustration: Fig. 18.]
The Trilobite.
What higher forms of animal life existed during the silurian period?
The geographical range of the system?
Of what does the Old Red Sandstone consist?
Describe its three divisions.
What are its fossils?
[Illustration: Fig. 19.]
Describe the fishes of that period.
What was the peculiarity of the Pterichthys?
Of the Cephalaspis?
Where are the rocks of this system found?
How is the carboniferous system divided?
Describe the carboniferous limestone.
What ores are found in it?
Describe, its fossils.
Describe the millstone grit.
Of what do the coal measures consist?
How does the ironstone occur?
Describe the coal beds.
How is the continuity of the strata interrupted?
What variations from this general type occur in the formation?
[Illustration: Fig. 20.]
[Illustration: Fig. 21.]
Describe the several varieties of coal.
How is the coal quarried?
[Illustration: Fig. 22.]
What mineral springs occur in this formation?
To what uses is coal applied?
(The coal was deposited thousands of years ago, and has served no useful
purpose, that we know of, till very recently. Its formation was planned
and completed to meet a want which was not to be felt till the lapse of
many ages. It is a notable instance of the wisdom and forethought, as
well as of the benevolence, of God.) In what does this prospective
arrangement consist? What are the character and position of the fossils
of the coal measures?
[Illustration: Fig. 23.]
[Illustration: Fig. 24.]
[Illustration: Fig. 25.]
[Illustration: Fig. 26.]
What are the four most abundant forms?
Describe the Stigmaria. The Sigillaria. The Lepidodendron. The Calamite.
Where are the beds of coal found?
What is the fourth formation of rocks?
Into what two portions is it divided?
Of what does the Permian portion consist?
The Trias?
What minerals are found in this formation?
What springs?
[Illustration: Fig. 27.]
[Illustration: Fig. 28.]
[Illustration: Fig. 29.]
What fossils?
How are the fishes of the earlier and later portions distinguished?
What peculiarity of the red sandstones is mentioned?
By what kinds of animals were the tracks, which they contain, made?
Give localities of the new red sandstone.
What are the three divisions of the Oölitic system?
[Illustration: Fig. 30.]
[Illustration: Fig. 31.]
[Illustration: Fig. 32.]
Describe the Lias.
The Oölite.
The Wealden.
What are the general peculiarities of the system?
[Illustration: Fig. 33.]
What are the fossil animals of the system?
By which class of fossil animals is the system characterized?
[Illustration: Fig. 34.]
Describe the Ichthyosaurus. The Plesiosaurus. Pterodactyle. The
Iguanodon.
Where is the system developed?
What are the divisions of the Cretaceous system?
How are the layers of chalk separated?
What is the geological position of the Neocomian system, and the
greensand of this country?
What are the fossils of this system?
[Illustration: Fig. 35.]
[Illustration: Fig. 36.]
What the geographical range?
How are the tertiary deposits distinguished from the older formations?
Upon what principle is the tertiary system divided?
What are these divisions called, and what does each, name signify?
[Illustration: Fig. 37.]
[Illustration: Fig. 38.]
[Illustration: Fig. 39.]
[Illustration: Fig. 40.]
[Illustration: Fig. 41.]
[Illustration: Fig. 42.]
[Illustration: Fig. 43.]
In what portion of the tertiary period was the drift deposited
What is the geographical range of the drift?
Of what does it consist?
What is the latest tertiary deposit?
What are the fossils of the tertiary system?
Describe the Paleotherium. The Anoplotherium. The Megatherium. The
Mastodon. The Mammoth.
What other animals belonged to this period?
Where are the tertiary deposits found?
What formations are regarded as recent?
What formations of this class are accessible?
What others are in progress?
What are the fossils of this formation?
SECTION VI.
What is a fossil?
In what ways are they preserved?
When is a fossil said to be mineralized?
Describe the process of mineralization.
How is it proved that the removal of the organic matter and substitution
of mineral particles are simultaneous?
Were animals created before vegetables?
How is this shown?
At what period was the vegetable growth the greatest?
What forms of animal life were most abundant during the earlier periods?
What vertebrated animals belonged to these periods?
What advance is made in the new red sandstone period?
During what period do the mammalia first appear in abundance?
During what geological period was man created?
How are the footprints and skeletons of human beings hi solid rocks
accounted for?
Why are not fossils distributed uniformly through all the formations,
and through all the parts of each formation?
In what does the importance of fossils consist?
How are the fresh-water and marine formations distinguished?
What circumstances render it difficult to identify rocks of the same age
in different localities?
How are formations identified?
Was the work of creation one of short duration?
What was the last work of creation of which we have any geological
evidence?
Why may we presume that no more species will be created?
Do all the animal and vegetable species which have been created still
exist?
[Illustration: Fig. 44.]
[Illustration: Fig. 45.]
[Illustration: Fig. 46.]
[Illustration: Fig. 47.]
What causes are operating to destroy species?
SECTION VII.
How long has it been since the creation of the earth?
How does the amount of stratified rock indicate the great antiquity of
the earth?
How does the stratification show the same thing?
What is the proof that the principal strata were deposited before the
creation of man, and how does this fact bear upon the question of the
antiquity of the earth?
Give the argument drawn from the successive creations and disappearance
of animal and vegetable species.
The argument drawn from the amount of organic matter in the stratified
rocks.
The argument from slow accumulation.
What is the general conclusion from these facts?
Why is this conclusion an important one?
What objection to it has been raised?
How is this objection answered?
What additional explanation is given?
CHAPTER III.
SECTION I.
What is the deepest geological change of which we have any knowledge?
What are the reasons for supposing that the lowest stratified rocks are
undergoing fusion?
Why are the lowest stratified rocks regarded as of mechanical origin?
What changes have they undergone?
SECTION II.
In what state were the stratified rocks deposited? What change have they
undergone in this respect? How is the fissile structure produced? How is
the cleavage structure produced?
[Illustration: Fig. 48.]
What is the third class of changes?
What do fractures at the surface become by the erosion of water? How are
caverns formed?
Describe a vein of segregation. A dike. A mineral vein. What is a fault?
Were the inclined strata thus deposited?
How is it proved that they have taken the inclined position since they
were deposited?
What is the direction of the dip?
What lines form the angle of inclination?
What is the outcrop of inclined strata? The strike?
[Illustration: Fig. 49.]
[Illustration: Fig. 50.]
[Illustration: Fig. 51.]
[Illustration: Fig. 52.]
[Illustration: Fig. 53.]
Describe an anticlinal axis. A synclinal axis. A valley of elevation. A
valley of subsidence.
[Illustration: Fig. 54.]
[Illustration: Fig. 55.]
[Illustration: Fig. 56.]
When are strata unconformable?
What other disturbances have taken place in the strata?
When did these various disturbances take place?
How is it known that there has been no period of universal disturbance?
SECTION III.
How is it known that the mountains have been covered by the ocean?
[Illustration: Fig. 57.]
[Illustration: Fig. 58.]
Were the granitic ridges thus covered?
Has the level of the sea been, to any considerable extent, fluctuating?
How, then, have the rocks, of which the mountain masses consist, been
covered by sea?
Give the evidence that different mountains were elevated at different
times.
Has the process of upheaval been sudden or gradual?
How are the mountain valleys, which have the direction of the mountain
ranges, been produced?
[Illustration: Fig. 59.]
How is the existence of submarine mountains shown?
What is the movement by which continents are elevated?
State the evidence of the elevation of continents from the existence of
elevated sea-beaches.
The evidence of the elevation of the coast of Maine.
The evidence of elevation from the existence of lakes.
From the geographical range of the older strata.
The evidence of the recent elevation of South America.
Of the rising of the north of Europe.
State the proof of subsidence from the occurrence of submerged forests.
Why are these changes but little observed?
[Illustration: Fig. 60.]
What are the grounds for asserting that a change of level is taking
place over a large area in the Pacific and Indian Oceans?
[Illustration: Fig. 61.]
[Illustration: Fig. 62.]
[Illustration: Fig. 63.]
[Illustration: Fig. 64.]
What is the present state of the coast of Greenland in this respect?
Have the changes of level of the same place always been in the same
direction?
Give the evidence of elevation and depression in South America. In
Italy.
[Illustration: Fig. 65.]
What general conclusion may we draw in respect to the stability of the
earth's surface?
To what extent can we ascertain the geography of past epochs?
What former relations of land and water are suggested as not improbable?
SECTION IV.
How can we estimate the denudation which the igneous rocks have
suffered?
How do faults indicate the denudation of the stratified rocks?
How do valleys indicate denudations?
Describe the instance in Scotland.
What is the evidence of denudation in the Connecticut valley? How are
valleys produced?
[Illustration: Fig. 66.]
What is the condition of the surface rock in the colder portions of the
temperate zones?
[Illustration: Fig. 67.]
[Illustration: Fig. 68.]
With what is the surface rock generally covered?
How are soils formed?
How may soils be improved?
What is necessary to render soils fertile?
SECTION V.
What means have we of judging of the climate of former periods?
What was the climate of the coal period?
What animal fossils indicate a former warm climate?
What evidence that Siberia once enjoyed a milder climate?
Do similar indications appear in the southern hemisphere?
When has the climate of the earth been most uniform?
Has the climate been growing gradually colder to the present time?
What is the evidence of a somewhat recent period of intense cold?
What recent local changes of climate are mentioned as having occurred?
SECTION VI.
State the general advantages of geological changes.
By what changes have the coal-beds and other stratified rocks become
accessible?
What advantage from these elevating forces in reference to the granitic
rocks?
[Illustration: Fig. 69.]
How do these changes affect our means of knowing the structure of the
earth?
Explain the origin of springs, wells, and artesian wells?
By what changes have the metallic ores become accessible?
In what light, then, are we to regard disturbances of geological
structure?
CHAPTER IV.
What is the object of the preceding chapters?
How can we arrive at a knowledge of the causes which have produced
geological phenomena?
Have geological causes always operated with the same intensity?
How are the means of forming correct geological theories increasing?
SECTION I.
How does oxygen become an agent in the disintegration of rocks?
How does carbonic acid operate in the disintegration of rocks?
What is the effect of moisture and rain?
What is the effect of variations of temperature?
What other atmospheric causes are mentioned?
How do these causes become important?
What are some of their effects?
SECTION II.
What are the changes which are to be referred to chemical agency?
Mention some of the disturbances which give rise to chemical changes.
What are the principal effects of chemical action?
How is the cleavage structure accounted for?
[Illustration: Fig. 70.]
[Illustration: Fig. 71.]
Mention instances which show that a cleavage may be established in a
body in a solid state.
In it a crystalline arrangement of the particles of the mass?
What other divisional planes exist in rocks?
Mention instances of concretionary formations.
Why may not these concretions have been deposited as nodules?
How have these concretions been formed?
Mention instances of segregation without the concretionary structure.
How was the segregation in these instances effected?
How is the columnar structure produced?
What is the origin of the mineral veins which are first mentioned?
How is it shown that other veins are not injected?
How were these veins formed?
What is the force by which these molecular changes have been effected?
SECTION III.
In what ways are geological changes produced by human agency?
Of what are the organic remains, in rocks, the record?
What rocks contain organic materials in large quantity?
What is the most abundant organic product?
Explain the mode of growth of corals.
Give instances of extensive coral reefs.
What is the total amount of surface covered by the coral reefs?
SECTION IV.
What degree of importance is attached to water as a geological agent?
What are the sources of the sediment which water deposits?
Why is not the formation of the sedimentary rocks capable of being
observed?
What is the first mode in which solid matter is taken up by water?
Why are the waters of the ocean saline?
What effect has the temperature of water in the solution of silex?
What effect has an alkaline condition of water?
[Illustration: Fig. 72.]
What rock is soluble in water charged with carbonic acid?
Give an instance of limestone formation from such solutions.
How do rivers furnish sediment for the stratified rocks?
What determines the position of rapids in rivers?
What is the effect of waterfalls on the abrading action of rivers?
What is the peculiarity of rock at Niagara which has prevented the fall
from becoming a succession of rapids?
What other circumstance increases the abrading action of rivers?
What is the principal source of the sediment which is transported by
rivers?
What is the annual amount of sediment furnished by the Kennebec? The
Merrimac? The Mississippi? The Ganges?
What is the general tendency of these abrading forces?
What is the effect of waves upon the coast, when it consists of
unsolidified materials?
Describe their effect upon rocky coasts.
How is the encroachment upon such coasts shown?
[Illustration: Fig. 73.]
What is the effect of waves of less power?
How are marine currents produced?
How are they increased by the evaporation of the torrid zone?
What are the most important marine currents?
Which class of currents have the greater depth?
Upon what does the power of deep currents depend?
How would the effect of these currents be increased by earthquakes?
Where will the effects of these currents be greatest?
Mention instances of these effects.
[Illustration: Fig. 74.]
What must be the effect of such currents as the Gulf-stream and
Mozambique channel?
Mention, generally, the effects of these currents.
Why does detrital matter remain suspended in the water of rivers?
How is the coarse and fine sediment separated?
Why do river currents extend some distance into the sea?
What effect does this have in distributing the sediment which the rivers
furnish?
Upon what does the transporting power of marine currents depend?
When a river enters a lake, why is its sediment deposited?
Describe the effect.
When is sediment deposited in the beds of rivers?
Describe the effects of this deposition.
Where is most of the sediment deposited?
Give the area of some delta deposits.
How do the deep-sea deposits now forming compare in extent with the
earlier formations?
State the several circumstances by which a succession of deposits would
be arranged in strata.
How are those differences produced upon which the separation into
independent formations depends?
Why are marine deposits nearly horizontal?
[Illustration: Fig. 75.]
How are the irregular stratifications produced?
What peculiarity in the fossils will distinguish the lacustrine and
marine deposits?
What peculiarity in reference to fossils will characterize the deep-sea
deposits?
How is coal shown to be of vegetable origin?
Why will the drift wood of the sea accumulate in particular localities,
and why will it sink?
Why will it become buried beneath earthy matter?
How is it known that wood thus buried will, at length, become lignite?
How is lignite converted into mineral coal?
What is the proof of it?
Have beds of coal been formed at other periods, besides the
carboniferous?
Is it probable that coal-beds are now forming?
How did the flora of the carboniferous period differ from the existing
flora?
[Illustration: Fig. 76.]
Are the alternations of the earthy and coal strata satisfactorily
explained?
In what portions of the geological series are the deposits of salt
found?
Where is saline matter principally stored?
Explain the conjectural formation of salt in the Mediterranean Sea.
What form do rocks take when deposited from a chemical solution?
How is sand or gravel solidified by the infiltration of mineral waters?
What is the effect of drying upon the solidification of rocks?
What is the effect of pressure?
What of heat?
SECTION V.
What is a glacier?
What is the extent of the glaciers of the Alps?
What change does the mass of snow in the higher valleys of the glacier
mountains undergo?
What is the source of supply to the glacier?
What is the cause of the motion of the glacier?
What is the usual annual motion?
Why will the glacier melt but little at its under side?
Where will the waste at the surface just equal the addition?
What circumstances vary the position of the terminus of the glacier?
[Illustration: Fig. 77.]
What, besides snow and ice, enters into the composition of a glacier?
How are these materials supplied?
How is a lateral moraine formed?
What effect has the motion of the glacier on the rocky surface over
which it passes?
What is the material by which this effect is produced?
How is the terminal moraine produced?
How may the moraines on the Jura Mountains be explained?
How has it been proposed to explain the striated surfaces of rocks found
in the north of Europe and America?
What is the objection to this extension of the glacier theory?
How does the ice accumulate along the coast in high latitudes to form
icebergs?
Why does it ultimately separate from the shore?
How does it become freighted with earthy matter?
In what direction do the icebergs float, and why?
What are the dimensions of an iceberg, estimated from the part that is
visible?
[Illustration: Fig. 78.]
Where does the mass of ice increase, and where diminish?
What will be the effect of its melting?
How is it supposed that icebergs may have striated the rocky surface?
What is probably the condition of the bed of the seas over which
icebergs now float?
Has the north of Europe and America been so depressed, during a period
comparatively recent, as to admit of this explanation of the drift
phenomena?
SECTION VI.
What is the condition of the interior of the earth with respect to heat?
How do the observations made in deep mines and wells prove this?
How far is the temperature influenced from the surface?
What is the general law of increment of temperature?
At what depth would most mineral substances be melted?
How is this conclusion confirmed?
What was probably the original state of the mass of the earth?
What other explanation may be given of this interior heat?
What is the elastic force upon which volcanic phenomena depend?
Upon what does the fluidity of lava depend?
Upon what does its porous structure, when cooled, depend?
Why are volcanoes situated near the sea?
Describe the principal lines of volcanic activity.
What are the forces tending to repress the elasticity of the mass below?
What will be the effect when the elastic is greater than the repressing
force?
What produces the phenomena of the earthquake?
What is a volcano?
Why are volcanoes generally arranged a linear direction?
Under what circumstances will a new volcano be formed?
What instances are cited?
How is a volcanic cone formed?
Why are lateral cones produced?
How are volcanic cinders formed? Scoriæ? Volcanic glass?
[Illustration: Fig. 79.]
Give instances of fractures as results of volcanic action. How are dikes
formed?
[Illustration: Fig. 80.]
By what agency have the changes in the metamorphic rocks been effected?
Give the instance of metamorphic action from intrusive granite in
Norway.
What instance is given as occurring in New Hampshire?
Give the experiment by which it is shown that these changes will result
from a high temperature.
[Illustration: Fig. 81.]
What must be the condition of the lowest stratified rocks in regard to
temperature?
Why is not the stratification destroyed?
What changes are produced by this high temperature?
Explain the connection of denudation and earthquake action.
What is the evidence that the surface of the earth is thrown into
undulations during earthquakes?
What is the velocity of these undulations?
Give the instance which occurred in Chili.
To what parts of the earth are these undulations limited?
What condition of the surface may be regarded as resulting from this
cause?
What is the class of rocks most obviously referable to volcanic agency?
How do the trap rocks differ from ordinary lavas?
Why are they not vesicular?
Why more crystalline?
Why were cones never formed?
What is the proof that the granitic rocks have once been in a melted
state?
Why does not the mass of melted rock below the surface retain
permanently its liquid form?
Why does it, on cooling, become more crystalline than lava?
State the process by which mountains are formed.
By what law does the elevating force accumulate?
Why, then, is the process of elevation spasmodic, and not constant?
How is the inclined position of strata produced?
How are strata brought into a vertical position over large areas?
Why do subsidences occasionally follow these movements of elevation?
Mention instances.
[Illustration: Fig. 82.]
What explanation is suggested of deep and extensive chasms?
What conditions must exist together, in the force by which continents
are produced?
What cause fulfils these conditions?
What is the proof that the temperature under given localities is
variable?
What will be the result of these variations?
What is the law of expansion of rocks, as obtained by experiment?
What amount of change of level may be thus accounted for?
What circumstances would probably increase this amount?
What amount of vertical movement must be accounted for?
Why must these changes of level be very slow?
Under what conditions would there be no change of level?
Is it probable that these conditions exist to any great extent?
Why, then, are not the changes of level observed?
Why is the bed of the sea most likely to experience the change of
elevation?
Why are the continents most favorably situated to undergo depression?
What are the sources of heat upon which climate depends?
Does the interior temperature sensibly affect the present climates?
What cause may be assigned for the changes of climate which are known to
have taken place?
What are the relations of land by which the highest temperature would be
produced?
How would this distribution of land affect the temperature of the waters
of the ocean?
What would result if the opposite relations of land and water existed?
What confirmation of these conclusions is drawn from the existing
climates of different parts of the earth?
Is there any reason to suppose that the relations of land and water
which would have produced a warmer climate in former times did not
exist?
*** End of this LibraryBlog Digital Book "The Elements of Geology; Adapted to the Use of Schools and Colleges" ***
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