682 R. W. PACK OIL FIELDS OF THE PACIFIC COAST 



within the oil, which would tend both to increase the specific gravity and 

 to make the resulting- oil much more viscous. 



Effect of Uncon^formities on Accumulation of Oil 



Structural features of lesser importance than the anticlines, but still 

 of very considerable importance in the consideration of the accumulation 

 of oil J are the unconformities that separate the various formations, and 

 particularly those that separate any one of the diatomaceous shales from 

 the formation that rests upon it. The hydrocarbons, whetlier they be in 

 the liquid or gaseous state in leaving the diatomaceous shales, tend natu- 

 rally to follow the bedding planes, or the sandy beds intercalated with 

 the shale, for it is along these lines that the resistance to movement is 

 least. If the shales are truncated along the plane of unconformity, these 

 hydrocarbons tend gradually to accumulate in tlie usually sandy bed that 

 foiTQS the lowest member of the formation renting unconformably upon 

 the shale, and it is the beds occupying this position that are the chief oil 

 sands in the productive fields. 



Geologic Features determining Presence of Oil 



These, then, are the more important general geologic features of the 

 California oil fields, the features that seem to determine whether or not 

 oil occurs at all in the region, and, if it exists there, in what parts of the 

 region it has probably accumulated in considerable amounts: (1) the 

 deposits of diatomaceous shale in which the oil had its source; (2) the 

 anticlines or equivalent structures furnishing the traps for holding the 

 oil, and (3) the unconformable relationship existing between the shale 

 and the overlying beds, affording easy passage for the oil through the 

 rocks to an entrance into these traps. 



The Province of the Geologist 



Although it seems certain that the position of the main oil-bearing 

 lands in California is known, and that the pools which will be discovered 

 in the future will probably not be comparable with those now known, still 

 the work of the geologist is by no means ended there ; rather has it only 

 just begun. Too frequently the work of the petroleum geologist is 

 thought to consist only of the estimation of untested lands, of the search 

 for new fields to conquer, and of new pools for the operator to drain. A 

 great deal has been said about the aid a geologist may be to the pros- 



