The Students Elements of Geology | Page 9

plants which have, at different
periods of the past, inhabited the globe.
All are aware that the solid parts of the earth consist of distinct
substances, such as clay, chalk, sand, limestone, coal, slate, granite, and
the like; but previously to observation it is commonly imagined that all
these had remained from the first in the state in which we now see
them-- that they were created in their present form, and in their present
position. The geologist soon comes to a different conclusion,
discovering proofs that the external parts of the earth were not all
produced in the beginning of things in the state in which we now
behold them, nor in an instant of time. On the contrary, he can show
that they have acquired their actual configuration and condition
gradually, under a great variety of circumstances, and at successive
periods, during each of which distinct races of living beings have
flourished on the land and in the waters, the remains of these creatures
still lying buried in the crust of the earth.
By the "earth's crust," is meant that small portion of the exterior of our
planet which is accessible to human observation. It comprises not
merely all of which the structure is laid open in mountain precipices, or
in cliffs overhanging a river or the sea, or whatever the miner may
reveal in artificial excavations; but the whole of that outer covering of

the planet on which we are enabled to reason by observations made at
or near the surface. These reasonings may extend to a depth of several
miles, perhaps ten miles; and even then it may be said, that such a
thickness is no more than 1/400 part of the distance from the surface to
the centre. The remark is just: but although the dimensions of such a
crust are, in truth, insignificant when compared to the entire globe, yet
they are vast, and of magnificent extent in relation to man, and to the
organic beings which people our globe. Referring to this standard of
magnitude, the geologist may admire the ample limits of his domain,
and admit, at the same time, that not only the exterior of the planet, but
the entire earth, is but an atom in the midst of the countless worlds
surveyed by the astronomer.
The materials of this crust are not thrown together confusedly; but
distinct mineral masses, called rocks, are found to occupy definite
spaces, and to exhibit a certain order of arrangement. The term ROCK
is applied indifferently by geologists to all these substances, whether
they be soft or stony, for clay and sand are included in the term, and
some have even brought peat under this denomination. Our old writers
endeavoured to avoid offering such violence to our language, by
speaking of the component materials of the earth as consisting of rocks
and SOILS. But there is often so insensible a passage from a soft and
incoherent state to that of stone, that geologists of all countries have
found it indispensable to have one technical term to include both, and
in this sense we find ROCHE applied in French, ROCCA in Italian, and
FELSART in German. The beginner, however, must constantly bear in
mind that the term rock by no means implies that a mineral mass is in
an indurated or stony condition.
The most natural and convenient mode of classifying the various rocks
which compose the earth's crust, is to refer, in the first place, to their
origin, and in the second to their relative age. I shall therefore begin by
endeavouring briefly to explain to the student how all rocks may be
divided into four great classes by reference to their different origin, or,
in other words, by reference to the different circumstances and causes
by which they have been produced.

The first two divisions, which will at once be understood as natural, are
the aqueous and volcanic, or the products of watery and those of
igneous action at or near the surface.
AQUEOUS ROCKS.
The aqueous rocks, sometimes called the sedimentary, or fossiliferous,
cover a larger part of the earth's surface than any others. They consist
chiefly of mechanical deposits (pebbles, sand, and mud), but are partly
of chemical and some of them of organic origin, especially the
limestones. These rocks are STRATIFIED, or divided into distinct
layers, or strata. The term STRATUM means simply a bed, or any
thing spread out or STREWED over a given surface; and we infer that
these strata have been generally spread out by the action of water, from
what we daily see taking place near the mouths of rivers, or on the land
during temporary inundations. For, whenever a running stream charged
with mud or sand, has its velocity checked, as when it enters a lake or
sea,