1. _Definition._--Geology is the science of the origin and development
of the structure of the earth. It treats of the nature and mode of
formation of the various materials of which the earth's crust is
composed; it seeks to discover what mutations of land and water, and
what changes of climate, have supervened during the past; it endeavours
to trace the history of the multitudinous tribes of plants and animals
which have successively tenanted our globe. In a word, Geology is the
Physical Geography of past ages.
2. _Rocks._--Every one knows that the crust of the earth is composed of
very various substances, some of which are hard and crystalline in
texture, like granite; others less indurated and non-crystalline, such
as sandstone, chalk, shale, &c.; while yet others are more or less soft
and incoherent masses, as gravel, sand, clay, peat, &c. Now, all these
heterogeneous materials, whether they be hard or soft, compact or loose,
granular or crystalline, are termed _rocks_. Blowing sand-dunes,
alluvial silt and sand, and even peat, are, geologically speaking,
rocks, just as much as basalt or any indurated building-stone. The
variety of rocks is very great, but we do not study these long before we
become aware that many kinds which present numerous contrasts in detail,
yet possess certain characters in common. And this not only groups these
diverse species together, but serves also to distinguish them from other
species of rock, which in like manner are characterised by the presence
of some prevalent generic feature or features.
_Classification of Rocks._--All the rocks that we know of are thus
capable of being arranged under _five_ classes, as follows:
I. MECHANICALLY FORMED.
II. CHEMICALLY FORMED.
III. ORGANICALLY DERIVED.
IV. METAMORPHIC.
V. IGNEOUS.
3. The MECHANICALLY FORMED class comprises a considerable variety of
rocks, all of which, however, come under only two subdivisions--namely,
_Sedimentary_, and _Eolian_ or _Aerial_, the former being by far the
more important. Of the _Sedimentary_ group, there are three rocks which
may be taken as typical and representative--namely, _conglomerate_ or
_puddingstone_, _sandstone_, and _shale_. A short examination of the
nature of these will sufficiently explain why they come to be grouped
together under one head. _Conglomerate_ consists of a mass of
various-sized rounded stones cemented together; each stone has been well
rubbed, and rolled, and rounded. It is quite obvious that the now solid
rock must at one time have existed in a loose and unconsolidated state,
like gravel and shingle. Nor can we resist the conclusion that the
stones were at one time rolled about by the action of water--that being
the only mode in which gravel-stones are shaped. Again, when we have an
opportunity of examining any considerable vertical thickness of
conglomerate, we shall frequently observe that the stones are arranged
more or less definitely along certain lines. These, there can be no
question, are _lines of deposition_--the rounded stones have evidently
not been formed and accumulated all at once, but piled up gradually,
layer upon layer. And since there is no force in nature, that we know
of, save water in motion, that could so round and smooth stones, and
spread them out in successive layers or beds, we may now amplify our
definition of conglomerate, and describe it as a _compacted mass of
stones which have been more or less rounded, and arranged in more or
less distinct layers or beds, by the action of water_.
4. _Sandstone_ may at the outset be described as a _granular
non-crystalline rock_. This rock shews every degree of coarseness, from
a mass in which the constituent grains are nearly as large as
turnip-seed, down to a stone so fine in the grain that we need a lens to
discover what the particles are of which it is composed. When these
latter are examined, they are found to exhibit marks of attrition, just
like the stones of a conglomerate. Sharp edges have been worn off, and
the grains rounded and rubbed; and whereas lines of deposition are often
obscure, and of infrequent occurrence in conglomerate--in sandstone, on
the contrary, they are usually well marked and often abundant. We can
hardly doubt, therefore, that sandstone has also had an _aqueous_
origin, or in other words, that it has been formed and accumulated by
the force of water in motion. In short, sandstone is merely compacted
sand.
5. If it be easy to read the origin of conglomerate and sand in the
external character of their ingredients, and the mode in which these
have been arranged, we shall find it not less easy to discover the
origin of _shale_. Shale is, like sandstone, a granular non-crystalline
rock. The particles of which it is built up are usually too small to be
distinguished without the aid of a lens, but when put under a sufficient
magnifying power, they exhibit evident marks of attrition. In structure
it differs widely from sandstone. In the latter rock the layers of
deposition, though frequently numerous, are yet separated from each
other by some considerable distance, it may be by a few inches or by
many yards. But in shale the layers are so thin that we may split the
rock into _laminae_ or plates. Now we know that many sedimentary
materials of recent origin, such as the silt of lakes, rivers, and
estuaries, although when newly dug into they appear to be more or less
homogeneous, and shew but few lines of deposition, yet when exposed to
the action of the atmosphere and dried, they very often split up into
layers exhibiting division planes as minute as any observable in shale.
There is no reason to doubt, therefore, that shale is merely compacted
silt and mud--the sediment deposited by water. It becomes evident,
therefore, that conglomerate, sandstone, and shale are terms of one
series. They are all equally sedimentary deposits, and thus, if we
slightly modify our definition of conglomerate, we shall have a
definition which will include the three rocks we have been considering.
For they may all be described as _granular non-crystalline rocks, the
constituent ingredients of which have been more or less rounded, and
arranged in more or less distinct layers, by the action of water_.
6. The _Eolian_ or _Aerial_ group of rocks embraces all natural
accumulations of organic or inorganic materials, which have been formed
upon the land. The group is typically represented by _debris_, such as
gathers on hill-slopes and at the base of cliffs, by the _sand-hills_ of
deserts and maritime districts, and by _soil_. All these accumulations
owe their origin to atmospheric agencies, as will be more particularly
described in the sequel. As the _Sedimentary_ and _Eolian_ rocks are the
results of the _mechanical_ action of water and the atmosphere, they are
fitly arranged under one great class--the MECHANICALLY FORMED ROCKS.
7. CHEMICALLY FORMED ROCKS constitute another well-marked class, of
which we may take _rock-salt_ as a typical example. This rock has
evidently been deposited in water, but not in the manner of a
sedimentary bed. It is not built up of water-worn particles which have
been rolled about and accumulated layer upon layer, but has been slowly
precipitated during the gradual evaporation of water in which it was
previously held in solution. Its formation is therefore a chemical
process. Various other rocks come under the same category, as we shall
afterwards point out.
8. The ORGANICALLY DERIVED class comprises a number of the most
important and useful rock-masses. _Chalk_ may be selected as a typical
example. Even a slight examination shews that this rock differs widely
from any of those mentioned above. Conglomerate, sandstone, shale, &c.
are built up of pebbles, particles, grains, &c. of various inorganic
materials. But chalk, when looked at under the microscope, betrays an
organic origin. It consists, chiefly, of the hard calcareous parts of
animal organisms, and is more or less abundantly stocked with the
remains of corals, shells, crustaceans, &c. in every degree of
preservation; indeed, so abundant are these relics, that they go to form
a great proportion of the rock. _Coal_ is another familiar example of an
organically derived rock, since it consists entirely of vegetable
remains.
9. The METAMORPHIC class, as the name implies, embraces all those rocks
which have undergone some decided change since the time of their
formation. This change generally consists in a re-arrangement of their
constituent elements, and has frequently resulted in giving a
crystalline texture to the rocks affected. Hence certain sedimentary
deposits like sandstone and shale have been changed from granular into
crystalline rocks, and the like has happened to beds of limestone and
chalk. _Mica-schist, gneiss_, and _saccharoid marble_ are typical of
this class.
10. The IGNEOUS rocks are those which owe their origin to the action of
the internal forces of the earth's crust. Most of them have been in a
state of fusion, and betray their origin by their crystalline and
sometimes glassy texture, and also, as we shall see in another section,
by the mode of their occurrence. _Lava_, _basalt_, and _obsidian_ are
characteristic types of this group of igneous rocks. Another group
embraces a large variety of igneous rocks which are non-crystalline, and
vary in texture from fine-grained, almost compact, bedded masses, like
certain varieties of _tuff_, up to coarse, irregular accumulations of
angular stones imbedded in a fine-grained or gritty matrix, like
_volcanic breccia_ and _volcanic agglomerate_.