ations formed in the bottoms of seas and at the mouths of
rivers have entombed remains of marine animals, more especially their
harder parts, as shells, corals, and bones, and also fragments or
entire specimens of land animals and plants. Hence, in any rock of
aqueous formation, we may find fossil remains of the living creatures
that existed in the waters in which that rock was accumulated or on
the neighboring land. If in the process of building up the continents,
the same locality constituted in succession a part of the bottom of
the ocean, of an inland sea, of an estuary, and a lake, we should find
in the fossil remains entombed in the deposits of that place evidences
of these various conditions; and thus a somewhat curious history of
local changes might be obtained. Geology affords more extensive
disclosures of this nature. It shows that as we descend into the older
formations we gradually lose sight of the existing animals and plants,
and find the remains of others not now existing; and these, in turn,
themselves disappear, and were preceded by others; so that the whole
living population of the earth appears to have been several times
renewed prior to the beginning of the present order of things. This
seems farther to have occurred in a slow and gradual manner, not by
successive great cataclysms or clearances of the surface of the earth,
followed by wholesale renewal. This doctrine of geological uniformity
is, however, to be understood as limited by the equally certain fact
that there has been progress and advance, both in the inorganic
arrangements of the earth's surface and in its organized inhabitants,
and that there have, in geological as in historical times, been local
cataclysms and convulsions, as those of earthquakes and volcanoes,
often on a very extensive scale. Farther, there are good reasons to
believe that there have been alternations of cold or glacial periods
and of warm periods, of periods of subsidence and re-elevation, and of
periods of greater and less activity of certain of the leading agents
of geological change. But as to the extent of these differences and
their bearing on the geological history, there is still much
uncertainty and difference of opinion.[141]

In the sediment _now_ accumulating in the bottom of the waters are
being buried remains of the existing animals and plants. A geological
formation is being produced, and it contains the skeletons and other
solid parts of a vast variety