290 



HISTORICAL GEOLOGY 



Pliocene marine strata are far less extensively developed on the 

 Pacific Coast than the Miocene, the principal areas being in the 

 Coast Ranges of California, and two or three small areas on 

 the coast of Oregon. Pliocene fresh water beds are widely devel- 

 oped in the southern half of the Great Valley of California. The 



Fig. 179 



Soft white diatomaceous Miocene shale in southern California. 

 U. S. Geological Survey, Bui. 322.) 



(After Arnold, 



maximum thickness of the marine Pliocene is at least 4000 to 5000 

 feet to the south of San Francisco. 



Thickness of the Tertiary. — In the above descriptions some 

 details have been given regarding the thickness of Tertiary for- 

 mations. To summarize for the whole Tertiary, the maximum 

 thickness of the whole system (not including igneous rocks) on the 

 Atlantic Coast is less than 1000 feet; on the western Gulf Coast 

 between 3000 and 4000 feet; in the western interior many thou- 

 sands of feet, though usually not more than a few thousand feet 

 occur in any one locality, because in no case are all the formations 



