310 HISTORICAL GEOLOGY 



geography of the west, because there were no great mountains on 

 the Pacific Coast to catch most of the moisture from the westerly 

 winds. 



Oligocene climate appears to have been somewhat cooler 

 (perhaps warm-temperate) in western North America, and tropical 

 in southeastern North America. In Europe the warm climate of 

 the Eocene seems to have continued, for Palms lived in northern 

 German}^. 



During Miocene time the climate of the Pacific Coast was 

 almost like that of today. On the Atlantic Coast a comparatively 

 cool current, apparently from the north, drove out the warm water 

 forms of the earlier Tertiary. In the western interior region of the 

 United States, subtropical plants gave way to temperate climate 

 plants. In Europe the warm climate of the earlier Tertiary 

 continued into the earlier Miocene and became distinctly cooler 

 (temperate) in the later Miocene. 



The Pliocene was in general cooler than the Miocene, in fact, 

 gradually increasing from temperate to sub-Arctic conditions in 

 the waters along the California coast, and even to Arctic conditions 

 in the British Isles region. Thus, passing upward in the British 

 Pliocene series, the number of Arctic fossil forms increases from 

 a few percent to 50 or 60 percent. An exception to the above 

 general conditions appears to have been along the Atlantic Coast 

 of the United States, where the marine water was rather warmer 

 than it had been during the Miocene. As judged by the plants, 

 the lands apparently had not become so correspondingly cold 

 during the Pliocene. 



Economic Products 



The largest production of petroleum in the United States comes 

 from southern California, where it is obtained mostly from Tertiary 

 shales and sandstones. It seems certain that this petroleum origi- 

 nated from the decomposition of countless numbers of Diatoms 

 in certain of the shales. 



Lignite (or brown coal) underlies thousands of square miles of 

 both the Gulf States and the western interior regions, as well as 

 smaller areas on the Pacific Coast. There are also important 

 lignite deposits in Europe, particularly in Germany. 



Many important gold deposits of California occur in Tertiary 



