It may be worth while briefly to consider a few of these collateral
proofs that the chalk was deposited at the bottom of the sea.
The great mass of the chalk is composed, as we have seen, of the
skeletons of Globigerinae, and other simple organisms, imbedded in
granular matter. Here and there, however, this hardened mud of the
ancient sea reveals the remains of higher animals which have lived and
died, and left their hard parts in the mud, just as the oysters die and
leave their shells behind them, in the mud of the present seas.
There are, at the present day, certain groups of animals which are never
found in fresh waters, being unable to live anywhere but in the sea.
Such are the corals; those corallines which are called Polycoa; those
creatures which fabricate the lamp-shells, and are called Brachiopoda;
the pearly Nautilus, and all animals allied to it; and all the forms of
sea-urchins and star-fishes.
Not only are all these creatures confined to salt water at the present
day; but, so far as our records of the past go, the conditions of their
existence have been the same: hence, their occurrence in any deposit is
as strong evidence as can be obtained, that that deposit was formed in
the sea. Now the remains of animals of all the kinds which have been
enumerated, occur in the chalk, in greater or less abundance; while not
one of those forms of shell-fish which are characteristic of fresh water
has yet been observed in it.
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