On careful examination, it is found that the materials of which each of
these layers of more or less hard rock are composed are, for the most
part, of the same nature as those which are at present being formed
under known conditions on the surface of the earth. For example, the
chalk, which constitutes a great part of the Cretaceous formation in
some parts of the world, is practically identical in its physical and
chemical characters with a substance which is now being formed at the
bottom of the Atlantic Ocean, and covers an enormous area; other beds of
rock are comparable with the sands which art; being formed upon
seashores, packed together, and so on. Thus, omitting rocks of igneous
origin, it is demonstrable that all these beds of stone, of which a
total of not less than seventy thousand feet is known, have been formed
by natural agencies, either out of the waste and washing of the dry
land, or else by the accumulation of the exuviae of plants and animals.
Many of these strata are full of such exuviae--the so-called
"fossils."
Remains of thousands of species of animals and plants, as perfectly
recognizable as those of existing forms of life which you meet with in
museums, or as the shells which you pick up upon the seabeach, have been
embedded in the ancient sands, or muds, or limestones, just as they are
being embedded now, in sandy, or clayey, or calcareous subaqueous
deposits.
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