In other cases, we may find, scattered through the rock, and
still in their natural position, the valves of shells such as
we know at the present day as living buried in the sand or mud
of the sea-shore or of estuaries. In other cases, the bed may
obviously have been an ancient coral-reef, or an accumulation of
social shells, like Oysters. Lastly, if we find the deposit to
contain the remains of marine shells, but that these are dwarfed
of their fair proportions and distorted in figure, we may conclude
that it was laid down in a brackish sea, such as the Baltic, in
which the proper saltness was wanting, owing to its receiving
an excessive supply of fresh water.
In the preceding, we have been dealing simply with the remains
of aquatic animals, and we have seen that certain conclusions
can be accurately reached by an examination of these. As regards
the determination of the conditions of deposition from the remains
of aerial and terrestrial animals, or from plants, there is not
such an absolute certainty. The remains of land-animals would,
of course, occur in "sub-aerial" deposits--that is, in beds,
like blown sand, accumulated upon the land. Most of the remains
of land-animals, however, are found in deposits which have been
laid down in water, and they owe their present position to the
fact that their former owners were drowned in rivers or lakes,
or carried out to sea by streams.
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