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 {296} 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 and a supply of sediment of a different nature will
generally have been due to geographical changes requiring much time. Nor
will the closest inspection of a formation give any idea of the time which
its deposition has consumed. Many instances could be given of beds o