THE EOCENE PERIOD. 285 
eroded surface of the Chalk can often be seen ; or, failing this, 
that we can 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 Ze of 
the Mesozoic and Kainozoic periods. With the exception of 
a few Foraminifera, and one Lrachiopod (the latter doubtful), 
no Cretaceous sfecies 1s known to have survived the Creta- 
ceous period ; while several characteristic famzlies, such as the 
Ammonitide, Belemnitide, and Hippuritide, 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 ving 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 Sir Charles Lyell. 
The absence in strata of Tertiary age of the chambered 
Cephalopods, the Belemnites, the Wippurites, the Inocerami, 
and the diversified types of Reptiles which form such con- 
spicuous features in the Cretaceous fauna, render the palzeon- 
tological break between the Chalk and the Eocene one far too 
serious to be overlooked. At the same time, it is to be re- 
membered that the evidence afforded by the explorations car- 
ried out of late years as to the animal life of the deep sea, ren- 
ders it certain that the extinction of marine forms of lifé 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 ani- 
mals 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 any rate, by an enormous lapse 
of unrepresented time. How long this interval may have been, 
we have no means of judging exactly, but it very possibly was 
as long as the whole Kainozoic epoch itself. Some day we 
