TERTIARY AGE. 



521 



The same kind of change, from constant submergence to an era of 

 occasional emergences, occurred over the eastern Rocky Mountain 

 slopes, even into the Mississippi valley ; and also in California on the 

 ♦vest ; for the Lignitic beds of Mississippi and Tennessee, and those 

 of California, show that the marine era of the Cretaceous was there 

 followed by one of fresh-water or terrestrial depositions, in which leaf- 

 bearing and lignite-bearing beds were formed. Further, the rocks 

 which next follow the Lignitic beds — those of the Claiborne and 

 Vicksburg series — give evidence that, after the Lignitic era, the subsi- 

 dence was again renewed ; for the deposits are again marine in the 

 Mississippi valley and about the Mexican gulf; and, although fresh- 

 water over the Rocky Mountains, they have there a thickness of sev- 

 eral thousands of feet, as evidence of the subsidence in progress. 



Ffer. 929. 



North America in the Period of the Middle Tertiary. 



The epoch of cold, which terminated the life of the Cretaceous Con- 

 tinental seas, would not necessarily have been attended by breaks in 

 the series of rocks ; and no such breaks are found in the Rocky Moun- 

 tain region. The cold winds and oceanic currents appear to have done 

 thoroughly the work of extermination over Europe and eastern North 

 America, but less completely in the seas bordering the Pacific; and 



