not live
imbedded in 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 of that
epoch.

You will have heard of the interesting discoveries recently made, in
various parts of Western