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.
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