FORMATIONS AND PHYSICAL HISTORY 583 



been changed locally to lime phosphate. 1 The alteration appears 

 to have been effected through organic matter, especially the animal 

 cxiTi-ments accumulated about bird, seal, and perhaps other rook- 

 eries. The organic matter furnished the phosphoric acid, which, 

 carried down in solution, changed the carbonate of lime to the phos- 

 phate. The phosphate is used extensively as a fertilizer. In 

 Trxus part of the Miocene is non-marine. Much of the oil of Texas 

 and Louisiana comes from dolomized limestone which is probably 

 Miocene. 2 



The Pacific coast. 3 At the beginning of the period, the sea 

 encroached upon the Pacific coast, covering considerable areas 

 which were land during the Oligocene. It flooded the southern 

 part of the central valley of California early in the period, and 

 later the northern part as well. At about the beginning of the 

 period, faulting seems to have affected considerable parts of Cali- 

 fornia, and some of the planes of movement at that time have 

 served as planes of movement since. This was the time of the 

 first definitely recognized movement along the great earthquake 

 rift of California. Though subsidence was the rule in central and 

 southern California, local fault-blocks seem to have had notable 

 elevation. 



The Miocene history of the Pacific coast is divided into two 

 somewhat distinct epochs, separated by diastrophism and vulcan- 

 ism. During the first epoch, besides clastic formations and vol- 

 canic ash, there is a formation (Monterey] containing much diato- 

 maceous material which is an important source of oil. 4 The amount 

 of siliceous material ascribed to diatoms is prodigious, and seems 

 credible only when the extraordinary rate of reproduction of diatoms 

 is recalled. It has been estimated that a million individuals might 

 come from one, in the course of a month. If this is the fact, it is 

 perhaps not strange that large amounts of siliceous material accumu- 

 lated where conditions favored. 



After the early Miocene there were extensive igneous eruptions 

 in eastern Washington, Oregon, and the Coast ranges of California. 

 South of San Francisco, this w r as the time of the last important 



1 Penrose, Bull. 46, U. S. Geol. Surv. 



2 Hayes, Bull. 213, U. S. Geol. Surv., p. 346. 

 1 Arnold, Ralph, Jour. Geol., Vol. XVII. 



4 Eldridge, Bull. 213, U. S. Geol. Surv.; Arnold and Anderson, Bull. 322, U. S. 

 Geol. Surv M 



