hich are molten rocks formed in the
manner of modern lavas. So general has been this sorting, altering,
and disturbance of the substance of the earth's crust, that, though we
know its structure over large portions of our continents to the depth
of several miles, the geologist can point to no instance of a truly
primitive rock which can be affirmed to have remained unchanged and
_in situ_ since the beginning.

"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 had remained from the first in the state in which we
now see them--that they were created in their present forms and in
their present position. The geologist now 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 condition and configuration
gradually 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 lying buried in the crust of the
earth."[140]

2. Having ascertained that the rocks of the earth have thus been
produced by secondary causes, we next affirm, on the evidence of
geology, that a distinct order of succession of these deposits can be
ascertained; and though there are innumerable local variations in the
nature of the rocks formed at the same period, yet there is, on the
great scale, a regular sequence of formations over the whole earth.
This succession is of the greatest importance in the case of aqueous
rocks, or those formed in water; and it is evident that in the case of
beds of sand, clay, etc., deposited in this way, the upper must be the
more recent of any two layers. This simple principle, complicated in
various ways by the fractures and disturbances to which the beds have
been subjected, forms the basis of the succession of "formations" in
geology as deduced from stratigraphical evidence.

3. This regular series of formations would be of little value as a
history of the earth were it not that nearly all the aqueous rocks
contain remains of the contemporary animals and plants. Ever since
the earth began to be tenanted by organized beings, the various
accumul