Petroleum or Rock. Oil 291 



Oozes in Tertiary Formations 



At the time of deposition of the Tertiary formations in Cali- 

 fornia in which petroleum now occurs the seas swarmed with 

 countless numbers of minute organisms, which on dying 

 dropped to the bottom and accumulated in the silts or in some 

 places formed oozes consisting almost entirely of their own 

 remains. Of these organisms the diatoms were the most nu- 

 merous, microscopic vegetable organisms which secrete siliceous 

 tests having a great variety of shapes. The foraminifers, some- 

 what larger than the diatoms, have tests of various rounded 

 and elongated shapes made of calcium carbonate (lime) . A few 

 radiolaria also occurred. The oozes which these organisms 

 formed are probably comparable to the globigerina, diato- 

 maceous, and radiolarian oozes now forming in the ocean. The 

 organic matter within the calcareous and siliceous tests slowly 

 decomposes and undergoes chemical change. The exact chem- 

 ical changes are not known, but it is thought they have been 

 influenced by geologic conditions, as of pressure and heat. 



The Monterey Group of Formations 

 an Important Source of Oil 



Oil occurs in many places in California. But it never occurs 

 in some rocks never in old granite, not in most of the older 

 geologic formations. Most of the oil in California has been 

 obtained from the Monterey group of formations, directly or 

 indirectly. California has been much rent and fractured. A 

 fault or break in the rocks may have caused the oil to drain 

 from its original source and accumulate in some other forma- 

 tion. The rocks that have been grouped under the name Mon- 

 terey, however, have come to be recognized as the principal 

 source of oil in California. The studies of Arnold, Anderson 

 and Johnson (see appendix) have shown that without much 

 doubt foraminiferous and diatomaceous shale is the source of 

 California oil. In fields on the west side of the Coast Ranges 

 the diatomaceous shale is carried in Monterey (Middle Miocene) 



