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_.