TERTIARV FAUNAS OF GHE PACIFIC COAST 523 
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 
any 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 anageofdiatoms. ‘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. 
