rate at which the falls are thus cutting their way back. Those
computations have varied greatly, but I believe I am speaking within the
bounds of prudence, if I assume that the falls of Niagara have not
retreated at a greater pace than about a foot a year. Six miles,
speaking roughly, are 30,000 feet; 30,000 feet, at a foot a year, gives
30,000 years; and thus we are fairly justified in concluding that no
less a period than this has passed since the shell-fish, whose remains
are left in the beds to which I have referred, were living creatures.

But there is still stronger evidence of the long duration of certain
types. I have already stated that, as we work our way through the great
series of the Tertiary formations, we find many species of animals
identical with those which live at the present day, diminishing in
numbers, it is true, but still existing, in a certain proportion, in the
oldest of the Tertiary rocks. Furthermore, when we examine the rocks of
the Cretaceous epoch, we find the remains of some animals which the
closest scrutiny cannot show to be, in any important respect, different
from those which live at the present time. That is the case with one of
the cretaceous lamp-shells (_Terebratula_) which has continued to exist
unchanged, or with insignificant variations, down to the present day.
Such is the case with the _Globigerinæ_, the skeletons of which,
aggregated together, form a large proportion of our English chalk. Those
_Globigerinæ_ can be traced down to the _Globigerinæ_ which live at the
surface of the present great oceans, and the remains of which, falling
to the bottom of the sea give rise to a chalky mud. Hence it must be
admitted that certain existing species of animals show no distinct sign
of modification, or transformation, in the course of a lapse of time as
great as that which carries us back to the Cretaceous period; and which,
whatever its absolute measure, is certainly vastly greater than thirty
thousand years.