If, therefore, we suppose the shells of _Foraminifera_
to be in course of deposition over the floor of the Pacific, at
certain depths they would remain unchanged, and would accumulate
to form a calcareous ooze; but at greater depths they would be
acted upon by the water, their lime would be dissolved out, their
form would disappear, and we should simply have left the small
amount of alumina which they previously contained. In process
of time this alumina would accumulate to form a bed of clay; and
as this clay had been directly derived from the decomposition
of the shells of animals, it would be fairly entitled to be
considered an organic deposit. Though not finally established,
the hypothesis of Sir Wyville Thomson on this subject is of the
greatest interest to the palaeontologist, as possibly serving to
explain the occurrence, especially in the older formations, of
great deposits of argillaceous matter which are entirely destitute
of traces of life.
It only remains, in this connection, to shortly consider the
rock-deposits in which _carbon_ is found to be present in greater
or less quantity. In the great majority of cases where rocks
are found to contain carbon or carbonaceous matter, it can be
stated with certainty that this substance is of organic origin,
though it is not necessarily derived from vegetables.
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