tance,
probable that sediment was deposited during the whole of the glacial
period near the mouth of the Mississippi, within that limit of depth at
which marine animals can flourish; for we know what vast geographical
changes occurred in other parts of America during this space of time.
When such beds as were deposited in shallow water near the mouth of
the Mississippi during some part of the glacial period shall have been
upraised, organic remains will probably first appear and disappear at
different levels, owing to the migration of species and to geographical
changes. And in the distant future, a geologist examining these beds,
might be tempted to conclude that the average duration of life of the
embedded fossils had been less than that of the glacial period, instead
of having been really far greater, that is extending from before the
glacial epoch to the present day.

In order to get a perfect gradation between two forms in the upper
and lower parts of the same formation, the deposit must have gone on
accumulating for a very long period, in order to have given sufficient
time for the slow process of variation; hence the deposit will generally
have to be a very thick one; and the species undergoing modification
will have had to live on the same area throughout this whole time.
But we have seen that a thick fossiliferous formation can only be
accumulated during a period of subsidence; and to keep the depth
approximately the same, which is necessary in order to enable the same
species to live on the same space, the supply of sediment must nearly
have counterbalanced the amount of subsidence. But this same movement
of subsidence will often tend to sink the area whence the sediment
is derived, and thus diminish the supply whilst the downward movement
continues. In fact, this nearly exact balancing between the supply of
sediment and the amount of subsidence is probably a rare contingency;
for it has been observed by more than one palaeontologist, that very
thick deposits are usually barren of organic remains, except near their
upper or lower limits.

It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation composed
of beds of different mineralogical composition, we may reasonably
suspect that the process of deposition has been much interrupted, as
a change in the currents of the sea