COAST FIELDS AND VALLEY FIELDS 679 



ing from a relatively small area in the southern Coast Eanges of Cali- 

 fornia, bnt that the potential supply lies there, and that it is from this 

 area that the oil which will be prodnced on the Pacific coast in future 

 years will come. 



The Coast Fields a^^d the Valley Fields 



In speaking of the oil fields of California it is quite usual to divide 

 them according to geographic position into two groups — the coast fields 

 and the valley fields. The coast fields are those that lie close along the 

 Pacific Ocean or in relatively short valleys that drain directly into the 

 ocean ; the valley fields embrace those productive fields that lie about the 

 southern end of the San Joaquin Valley, which is the southern half of 

 the great trough that occupies the central part of California. 



AVhen viewed broadly, the general features of the various productive 

 fields, both coast and valley fields, appear remarkably similar, for not only 

 is the oil uniformly associated with a certain type of formation (although 

 not necessarily with a formation of definite age) , but the general struc- 

 ture is, with a few notable exceptions, of a like character. It is this 

 knowledge that makes those of us who have worked in the California 

 fields feel that we can with a reasonable degree of assurance say that the 

 main productive areas within the State are at present outlined, and that 

 the future production of the State will come chiefly from within the 

 bounds of the fields that are now productive and from the areas imme- 

 diately adjacent to those fields. Various pools isolated from those now 

 yielding oil will no doubt be discovered from time to time in the south- 

 ern Coast Eanges, but the total production from these pools will not be 

 comparable with that which will be obtained in the fields now known. 

 Such a statement as this may appear somewhat premature when one re- 

 members that the geology of much of the State is only imperfectly known, 

 yet the general features that it appears characterize every successful oil 

 field in this region are fairly definite and may be recognized, even though 

 the details of the geology are not known. 



Origin" of the Oil 



The first, and probably the most striking, feature common to the fields 

 is the presence in or immediately adjacent to them of thick masses of 

 shale of organic origin. These shales are commonly known as diatoma- 

 ceous shales, because of tbe abundance of the remains of diatoms that they 

 contain. The oldest of these shales lies in the uppermost part of the 



