68 
SANTA MARIA OIL DISTRICT, CALIFORNIA.. 
to the main portions of the quiet sea bottom which was formerly 
the surface of the land and which had been given a comparatively low 
relief by the long period of erosion that preceded the submergence. 
During the period of transition between the Vaqueros and the Mon¬ 
terey, 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 gypsiferous clays on 
the south side of the Casmalia Hills, probably representing upper 
Vaqueros. In some places, as, for instance, in the San Rafael Moun¬ 
tains, sandstone beds were formed early in Monterey time, probably in 
the neighborhood of locally unsubmerged areas. But later very little 
sand was deposited anywhere. Further submergence no doubt took 
place during the period, removing the sources of this sand and 
allowing to be deposited under fairly constant conditions a thickness 
of beds greater than a mile. It is not probable, however, that the 
depth of the sea was at any time as much as this, being more likely 
closer to half a mile. 
During the early part of 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 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 middle Miocene time. A principal obstacle 
to the rapid accumulation of the diatoms might be the limited supply 
of silica from which these alg*e derive the material of their tests. 
Other organisms with their shells and skeleton v ere ] so present 
to aid in building up the shale beds. They e Red olaria and 
Foraminifera; sponges with their spicule; > > v abundant; 
Crustacea; fishes, the remains of which are nuna * is m the shales; 
and mollusks with delicate shells, vdscii .v, 001 on, though poorly 
preserved. 
Volcanic eruptions, p *ssil !v .*<■ a;a; , broke out at different 
times during the latl a { ■>' i i Awer Miocene (Vaqueros) and 
the early part of th< n d lie Miocene (Monterey.) They may have 
accompanied movements tl a took place during the transition 
