CENOZOIC ERA— AGE OF MAMMALS. 501 



CHAPTER V. 

 CENOZOIC ERA— AGE OF MAMMALS. 



This deserves the rank of a distinct era, and the corresponding 

 rocks that of a distinct system, because there is here a great break in 

 the rock-system, and a still greater break in the life-system. Between 

 the rocks of the Cretaceous and Tertiary there is, in Europe, almost 

 universal unconformity In America, on the contrary, especially on the 

 Western Plains and in California, there seems to be in some places a 

 continuous series of conformable rocks connecting the two eras (Hay- 

 den). The record seems to be continuous. Yet here, no less than in 

 Europe, there is at a certain horizon a rapid and most extraordinary 

 change in the life-system. This it seems impossible to explain on the 

 theory of evolution unless we admit periods of rapid evolution. The 

 reason why there is no general unconformity in America is, evidently, 

 that the movement here was continental, and not mere mountain-mak- 

 ing and strata-crushing. Such continental movements, however, would 

 produce very great changes in climate, and therefore in organic forms. 

 The end of the Jurassic was a period of mountain-making, and there- 

 fore of unconformity — the end of the Cretaceous, a time of continent- 

 making, and but little unconformity, but very great change of climate. 

 Therefore, although the interval lost in America seems greater at the 

 end of the Jurassic, the change of fauna and flora was far greater at 

 the end of the Cretaceous. 



General Characteristics of the Cenozoic Era. — As indicated by the 

 name, modern history commences here ; modern types were introduced 

 or became predominant; the present aspect of field and forest com- 

 mences, and the present adjustment of the relations of the great classes 

 and orders was established. Then, as now, the rulers of the seas were 

 great sharks and whales ; the rulers of the land, mammalian quadru- 

 peds ; and the rulers of the air, birds and bats. Many of the genera 

 and some of the species of both animals and plants were identical with 

 those still living. The dominant class becomes now Mammals : Rep- 

 tiles, therefore, in accordance with a necessary law, decrease in size and 

 number, and thus find safety in a subordinate position. In some of 

 these characteristics of the Cenozoic era was anticipated in the Upper 

 Cretaceous, in accordance with the law that the first beginnings of each 

 age is in the preceding age. 



Divisions. — The Cenozoic era, or age of Mammals, embraces two 

 periods, viz. — 1. The Tertiary * and 2. The QtjMenigjry- In the Ter- 

 tiary all the mammals are now wholly extinct, but the invertebrate 

 species are some of them still living, and an increasing percentage of 

 living species appears as time progresses. In the Quaternary most, 



