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. But
there are circumstances which tend to show, that nothing like an inch of
chalk has accumulated during the life of a _Crania_; and, on any probable
estimate of the length of that life, the chalk period must have had a
much longer duration than that thus roughly assigned to it.

Thus, not only is it certain that the chalk is the mud of an ancient sea-
bottom; but it is no less certain, that the chalk sea existed during an
extremely long period, though we may not be prepared to give a precise
estimate of the length of that period in years. The relative duration is
clear, though the absolute duration may not be definable. The attempt to
affix any precise date to the period at which the chalk sea began, or
ended, its existence, is baffled by difficulties of the same kind. But
the relative age of the cretaceous epoch may be determined with as great
ease and certainty as the long duration of that epoch.