CENOZOIC ERA: AGE OF MAMMALS 581 



erates, was laid down. The variation in the lithological character 

 of the deposits within short distances is believed to have been caused 

 by the rather local elevation of land due to faulting and subsequent 

 stream rejuvenation. The California earthquake rift (Fig. 275, p. 277) 

 is first known to have been a plane of movement at this time. This 

 early (Vaqueros) sedimentation was followed by the deposition of 

 sandstone, volcanic ash, and limestone, and a great thickness of diato- 

 maceous material (Monterey). " It was an age of diatoms. These 

 small marine plants lived in extreme abundance in the sea and fell in 

 showers with their siliceous tests to add to the accumulating ooze 

 of the ocean bottom, just as they are forming ooze at the present day 

 in some oceanic waters. It is well known that diatoms multiply with 

 extreme rapidity. It has been calculated that, starting with a single 

 individual, the offspring may number one million within a month. 

 One can conceive that under very favorable life conditions such 

 as must have existed, the diatom frustules may have accumulated 

 rapidly at the sea bottom and aided the fine siliceous and argillaceous 

 sediments in the quick building-up of the thick deposits of the Middle 

 Miocene time, some of which are a mile through. These diatoma- 

 ceous shales are the source of some of the richest petroleum deposits 

 of California." (Arnold.) 



During much of the early portion of the Miocene and continuing 

 somewhat later, faulting, folding, and volcanic outbursts of consider- 

 able magnitude occurred. Great volcanoes were active from Wash- 

 ington and Oregon along the Pacific ranges of California, almost as 

 far south as Los Angeles. During the middle of the period mountain 

 building and great local deformations took place, the effects of which 

 were felt from Puget Sound to southern California. Extensive fault- 

 ing along the earthquake rift and other fault zones occurred, while 

 in other regions, low, broad folds were formed. The combined result 

 was the uplift of the Coast Ranges of California and Oregon to an alti- 

 tude of several thousand feet. The Cascades of Washington were 

 also increased in height. This stage of diastrophism was followed by 

 subsidence (Upper Miocene), as a result of which the northern part of 

 the Great Valley (Sacramento) was submerged and in it were depos- 

 ited sands and clays and beds of diatoms. 



By the close of the Miocene the Klamath and Sierra Nevada moun- 

 tains were peneplained, the material derived from them having been 

 deposited in the Great Valley and coastal belt of northern California, 

 forming the thick Tertiary strata now found there. These strata are 



