The progress of knowledge may, one day, enable us to deduce from such
facts as these the maximum rate at which the chalk can have accumulated,
and thus to arrive at the minimum duration of the chalk period. Suppose
that the valve of the Crania upon which a coralline has fixed itself in
the way just described, is so attached to the sea-urchin that no part of
it is more than an inch above the face upon which the sea-urchin rests.
Then, as the coralline could not have fixed itself, if the Crania had
been covered up with chalk mud, and could not have lived had itself
been so covered it follows, that an inch of chalk mud could not have
accumulated within the time between the death and decay of the soft
parts of the sea-urchin and the growth of the coralline to the full size
which it has attained. If the decay of the soft parts of the sea-urchin;
the attachment, growth to maturity, and decay of the Crania; and the
subsequent attachment and growth of the coralline, took a year (which is
a low estimate enough), the accumulation of the inch of chalk must have
taken more than a year: and the deposit of a thousand feet of chalk
must, consequently, have taken more than twelve thousand years.
The foundation of all this calculation is, of course, a knowledge of the
length of time the Crania and the coralline needed to attain their full
size; and, on this head, precise knowledge is at present wanting.
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