60 



THE AGE OF MAMMALS 



of the Age of Reptiles; they had only attained a height of four or five 

 thousand feet when the Age of Mammals commenced; they continued to 

 rise during the entire period. But consid(T the map of Europe and Asia 

 at the beginning of Eocene time and r(>alize that the great mountain 

 systems of the Pyrenees, the Alps, and the Himalayas were still unborn, 

 level surfaces in fact, partly washed by the sea. As shown in the dia- 

 gram, the birth of the Pyrenees was at the beginning of the Oligocene. At 

 this time Switzerland was still a comparatively level plain, and not until 



Fig. 14. — Duration of the .\ge of MainmaLs a.s measured by the evolution of the horse. 

 Skeleton of the Eocene four-toed horse, Eohippus, and of the Texas Lower Pleistocene horsa, 

 Equus scotti. In the American Museum of Natural History. 



the close of the Oligocene did the mighty system of the Swiss Alps begin 

 to rise. Central Asia was even yet a plain and upland, and only during 

 the Miocene did the Himalayas, the noblest existing mountain chain, begin 

 to rise to their present fellowship with the sky. In North America again, 

 since the close of the Eocene the region of the present Grand Canon of 

 the Colorado has been elevated 11,000 feet and the river has carved its 

 mighty canon through the rock to its present maximum depth of 6500 feet. 

 Those who have been impressed with a sense of the antiquity of these 

 wonders of the world and will imagine the vast changes in the history of 

 continental geography and continental life which were involved, will be 

 ready to concede that the Age of Mammals alone represents an almost 

 inconceivable period of time. 



