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 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 Creta- 

 ceous period; while several characteristic families, such as the 

 Ammonitidcz, Belemnitidcz, and Hippuritid&i 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 tin's 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 Hippuritcs, the Inocerami, 

 and the diversified types of Reptiles which form such con- 

 spicuous features in the Cretaceous fauna, render the palaeon- 

 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 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 ani- 

 mals x 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. Seme day we 

 20 



