808 GEOLOGY 



the first definitely recognized movement along the great earth- 

 quake 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 l\vo 

 somewhat distinct epochs, the earlier and the later, the separation 

 being marked by diastrophism and vulcanism. 



The lowest (Vaqueros) formation of the early Miocene in south- 

 ern California is largely clastic, but later beds (Monterey) contain 

 a large amount of diatomaceous material, and the diatomaceous 

 beds are an important source of oil. 1 The amount of silicious 

 material ascribed to diatoms is prodigious, and only seems credible 

 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 silicious material accumulated in 

 places where conditions were favorable for diatoms. Voljunir 

 ash is also a constituent of the Lower Miocene. As implied above, 

 the Miocene is generally unconformable on the Eocene or on older 

 formations. 



After the early Miocene, igneous eruptions were extensive in 

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

 South of San Fransisco, this was the time of the last important 

 eruptions in the Coast ranges, though vulcanism continued 1, 

 in Oregon and Washington, and perhaps to some extent in northern 

 California. The igneous eruptions accompanied the Mid-Miocene 

 diastrophism referred to above, which consisted in the readjust- 

 ment of fault-blocks and folds throughout the Pacific coast region. 

 This readjustment went so far that even high mountains v 

 developed locally, as shown by the coarseness of the sod in i. 

 which followed. 



After the diastrophism referred to, the extension of the 

 over present lands on the Pacific coast was <;i-eater than a! 

 time since the Eocene. It was not until this time that the northern 

 part of the Central valley of California (Sacramento valley) \ 



1 Eldridge, Bull. 313, U. S. Geol. Surv.; Arnold and Anderson, Bull 

 U. S. Geol. Surv. 



