Chap. IX. 
GEOLOGICAL EECOED, 
295 
and likewise to reflect on the great changes of level, 
on the inordinately great change of climate, on the 
prodigious lapse of time, all included within this same 
glacial period. Yet it may be doubted whether in any 
quarter of the world, sedimentary deposits, including 
fossil remains, have gone on accumulating within the 
same area during the whole of this period. It is not, 
for instance, probable that sediment was deposited dur¬ 
ing the whole of the glacial period near the mouth of 
the Mississippi, within that limit of depth at which ma¬ 
rine animals can flourish; for we know what vast geo¬ 
graphical 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 Mis¬ 
sissippi 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 gene¬ 
rally have to be a very thick one; and the species un¬ 
dergoing 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 accumu¬ 
lated during a period of subsidence ; and to keep the 
depth approximately the same, which is necessary in 
