2g4 HISTORICAL PALAEONTOLOGY. 



* 



CHAPTER XVIII. 

 THE EOCENE PERIOD. 



Before commencing = the study of the subdivisions of the 

 Kainozoic series, there are some general considerations to be 

 noted. In the first place, there is in the Old World a com- 

 plete and entire physical break between the rocks of the 

 Mesozoic and Kainozoic periods. In no instance in Europe 

 are 'Tertiary strata to be found resting conformably upon any 

 Secondary rock. The Chalk has invariably suffered much 

 erosion and denudation before the lowest Tertiary strata were 

 deposited upon it. This is shown by the fact that the actually 

 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 

 Awmonitida, Belenmitida, and Hippuritida, 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 Sir 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 con- 



