THE NEOZOIC AGES. 



237 



same kind occurs in America. On the Atlantic coast 

 the marine beds of the Older Tertiary cover the Creta- 

 ceous, and little elevation seems to have occurred. 

 Farther west the elevation increases, and in the upper 

 part of the valley of the Mississippi it amounts to 

 1700 feet. Still farther west, in the region of the 

 Rocky Mountains, there is evidence of elevation to the 

 extent of as much as 7000 feet. Throughout all these 

 regions scarcely any disturbance of the old Cretaceous 

 sea-bottom seems to have occurred until after the 

 deposition of the older Tertiary, so that there was first 

 a slow and general elevation of the Cretaceous ocean 

 bottom, succeeded by gigantic folds and fractures, and 

 extensive extravasations of the bowels of the earth in 

 molten rocks, in the course of the succeeding Tertiary 

 age. These great physical changes inaugurated the 

 new and higher life of the Tertiary, just as the similar 

 changes in the Permian did that of the Mesozoic. 



The beginning of these movements consisted of a 

 great and gradual elevation of the northern parts of 

 both the Old and New Continents out of the sea, 

 whereby a much greater land surface was produced, 

 and such changes of depth and direction of currents 

 in the ocean as must have very much modified the 

 conditions of marine life. The efiect of all these 

 changes in the aggregate was to cause a more varied 

 and variable climate, and to convert vast areas pre- 

 viously tenanted by marine animals into the abodes of 

 animals and plants of the land, and of estuaries, lakes, 

 and shallow waters. Still, however, very large areas 



