240 RALPH ARNOLD 



fornia now occupied by the Coast Ranges and fairly deep water con- 

 ditions became prevalent. A large area embraced between the 

 Salinas and San Joaquin valleys and extending northward from the 

 Antelope and Cholame valleys well toward the Livermore Valley was 

 an exception to this general subsidence, and although much of it had 

 been under water in Vaqueros time it was probably dry land or at 

 least an area not subject to sedimentation during the Monterey. The 

 wearing-away of extended land-areas ceased as they became sub- 

 merged, and the material for the formation of coarse detrital deposits 

 was no longer plentiful. Although the total thickness of the Monterey 

 approximates a mile it is not probable that the depth of the sea at 

 ^ny time was as much as this, being more likely closer to half a mile. 



During the period of transition between the Vaqueros and the 

 Monterey, limestone was formed chiefly, but somewhat inclosed 

 basins where deposits of alkaline mud were laid down apparently 

 existed in places. Such a basin is indicated by the alkaline gypsifer- 

 ous clays on the south side of the Casmalia Hills, in northwestern 

 Santa Barbara County, probably representing upper Vaqueros. 



During the early part of the middle Miocene (Monterey) time 

 conditions were variable, calcareous and siliceous deposits alternating, 

 probably as a result of alternating temporary predominance in the 

 sea of organisms with calcareous or siliceous shells. As the period 

 progressed the siliceous organisms became more predominant and 

 remained so, making up a large fraction of the total bulk of the 

 Monterey formation. 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 1,000,000 within a month. One can con- 

 ceive 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 middle Miocene time, 

 some of which are a mile through. These diatomaceous shales are 

 the source of some of the richest petroleum deposits of California. 



