106 LAND MAMMALS IN THE WESTERN HEMISPHERE 



side of the Ural Mountains from the Arctic Ocean and joined 

 the enlarged Mediterranean. During the existence of this 

 Ural Sea any land connection of North America with Europe 

 must necessarily have been by means of a North Atlantic 

 bridge, or by one across the Arctic Sea, since communication 

 with Asia by way of Alaska would not have reached eastern 

 Europe. 



Any such general statement of geographical conditions 

 during the Eocene as the foregoing sketch, cannot but be to 

 some extent misleading, because it brings together, as con- 

 temporary, arrangements which were, in some cases at least, 

 separated by considerable intervals of time and which were 

 subject to continual change. Along nearly all coasts the posi- 

 tion of the sea was quite different in the latter part of the epoch 

 from what it had been in the earlier portion. On the north 

 side of the Gulf of Mexico, for example, the sea retreated from 

 time to time, and the successive divisions of the Eocene rocks 

 are so arranged that the later ones are farther to the south. 

 Limitations of space, however, forbid the attempt to follow 

 out these minor changes. 



In the western interior are found extensive non-marine 

 or continental deposits of Eocene date, which must be con- 

 sidered more in detail, because of the highly important bearing 

 which they have upon mammalian history. With the excep- 

 tion of a few small areas in Colorado, these deposits are all 

 situated in the plateau region west of the Rocky Mountains, 

 and were made of the debris of older rocks washed down by 

 rain and rivers and deposited in broad basins. Some of them 

 are the sediments of shallow or temporary lakes, and one series, 

 at least, is made up of volcanic ash and dust showered upon 

 the land, or into water of no great depth. The oldest of these 

 Eocene stages, known as the Wasatch (see Table, p. 17) covers 

 a very large region, though in a discontinuous manner ; the 

 principal area begins in New Mexico, where it lies over the 

 Torrejon, of the Paleocene, and extends far to the north through 



