point to the presence of the chalk-flints in the
Tertiary strata. This last, of course, affords unquestionable proof
that the Chalk must have been subjected to enormous denudation
prior to the formation of the Tertiary beds, all the chalk itself
having been removed, and nothing left but the flints, while these
are all rolled and rounded. In the continent of North America,
on the other hand, the lowest Tertiary strata have been shown
to graduate downwards conformably with the highest Cretaceous
beds, it being a matter of difficulty to draw a precise line
of demarcation between the two formations.

In the second place, there is a marked break in the _life_ of
the Mesozoic and Kainozoic periods. With the exception of a few
_Foraminifera_, and one _Brachiopod_ (the latter doubtful), no
Cretaceous species is known to have survived the Cretaceous period;
while several characteristic _families_, such as the _Ammonitidoe,
Belemnitidoe_, and _Hippuritidoe_, died out entirely with the
close of the Cretaceous rocks. In the Tertiary rocks, on the
other hand, not only are all the animals and plants more or less
like existing types, but we meet with a constantly-increasing
number of _living species_ as we pass from the bottom of the
Kainozoic series to the top. Upon this last fact is founded the
modern classification of the Kainozoic rocks, propounded by Sil
Charles Lyell.

The absence in strata of Tertiary age of the chambered Cephalopods,
the Belemnites, the _Hippurites_, the _Inocerami_, and the
diversified types of Reptiles which form such conspicuous features
in the Cretaceous fauna, render the palaeontological break between
the Chalk and the Eocene one far too serious to be overlooked. At
the same time, it is to be remembered that the evidence afforded
by the explorations carried out of late years as to the animal
life of the deep sea, renders it certain that the extinction
of marine forms of life at the close of the Cretaceous period
was far less extensive than had been previously assumed. It is
tolerably certain, in fact, that we may look upon some of the
inhabitants of the depths of our existing oceans as the direct,
if modified, descendants of animals which were in existence when
the Chalk was deposited.

It follows from the general want of conformity between the Cretaceous
and Tertiary rocks, and still more from the great difference in
life, that the Cretaceous and Tertiary periods are separated, in
the Old World at