ea during the whole of this period. It is
not, for instance, 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 best flourish: for we know
that great 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 migrations
of species and to geographical changes. And in the distant future, a
geologist, examining these beds, would 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
continuously accumulating during a long period, sufficient for the slow
process of modification; hence, the deposit must be a very thick one;
and the species undergoing change must have lived in the same district
throughout the whole time. But we have seen that a thick formation,
fossiliferous throughout its entire thickness, can accumulate only
during a period of subsidence; and to keep the depth approximately the
same, which is necessary that the same marine species may live on the
same space, the supply of sediment must nearly counterbalance the
amount of subsidence. But this same movement of subsidence will tend to
submerge 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 widely different mineralogical composition, we may
reasonably suspect that the process of deposition has been more or less