of the attached valve must have remained exposed long enough
to allow of the growth of the whole coralline, since corallines do not
live imbedded in the mud.

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.
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