GEOGRAPHICAL DEVELOPMENT OF THE AMERICAS 113 



3. Oligocene Epoch 



North America. — The Oligocene, or third of the Tertiary- 

 epochs, was a time of great significance in the history of the 

 American mammals and of great geographical changes in the 

 West Indian and Central American regions, but in North 

 America proper the changes were riot so widespread. On the 

 Atlantic coast the marine Oligocene is but scantily displayed 

 except in the Florida peninsula, where it is found in a thick- 

 ness of some 2000 feet, but it is well developed along the north 

 shore of the Gulf of Mexico, where the coast-hne followed 

 that of the Eocene, only a Uttle farther to the south, marking 

 the retreat of the sea at the end of the Eocene. The Gulf 

 Stream entered the Atlantic over the site of northern Florida 

 and flowed northward nearer the coast than it does to-day, 

 in consequence of which warm-water conditions extended far 

 to the north and West Indian shells flourished on the New 

 Jersey coast. In the middle OUgocene part of northern Florida 

 was elevated into an island and the water over much of the 

 remainder of the peninsula became shallower, but this did 

 not greatly alter the course of the Gulf Stream. The Pacific 

 encroached upon the western shore of Oregon and British 

 Columbia and very extensively upon that of Alaska, where 

 strata no less than 10,000 feet thick are assigned to this 

 epoch. 



In the western interior OUgocene formations are among 

 the most important and widely spread of the continental 

 Tertiaries and are divisible into two principal stages and each 

 of these again into three substages. Of these, the older or 

 White River stage covers a vast region in northeastern Colo- 

 rado, western Nebraska, eastern Wyoming and southern 

 South Dakota, with separate areas in the Black HUls, North 

 Dakota and the Northwest Territory of Canada. The de- 

 posits are believed to be chiefly of fluviatile origin, and many 

 of the ancient stream-channels, some of great size, may still 



