220 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



diatoms can be seen with a hand lens, but as a rule it is not 

 of that extremely light porous variety which is found around 

 Santa Maria and Lompoc. Excepting in occasional shattered 

 zones, notably in the sandy shale or sandstone lenses relatively 

 near the top, there are no productive oil measures in the def- 

 initely recognized Santa Margarita, but according to a cur- 

 rent theory of the origin of the California oils, it is from this 

 and the underlying series of shales that the oil of the West 

 Side fields originated. 



In the South-End McKittrick area there are no indications 

 of the presence of productive oil zones in the Santa Marga- 

 rita, but oil stained silicious shales are abundantly developed. 

 Farther to the south, however, in the vicinity of Fellows, the 

 Santa Margarita contains sandy shales and sandstone lenses, 

 some of which are coarse and full of granite boulders. These 

 sand and boulder lenses in a few localities contain small 

 amounts of dry oil stains. 



We have, therefore, in this particular area, clearly defined 

 representatives of the Tulare, Etchegoin and Santa Margarita 

 formations, and by careful mapping we have been able to show 

 the outcrops of these different formations in the West Side 

 fields. The great importance of their separate mapping is 

 seen in connection with their relation to the oil deposits which 

 are now the sources of such great material wealth in this 

 region. 



II. Age and Stratigraphy of the Oil Horizons of the 

 West Side Fields 



A general conception of age and stratigraphic relationships 

 of the oil sands of the West Side fields can be gotten from a 

 glance at the accompanying idealized section. This section 

 was compiled from surface outcrops and from well data. In 

 order to make the information more complete the Coalinga 

 section was also included. The section is intended to show 

 the relative average thickness of the formations in the various 

 fields, their stratigraphic relationships and the position of the 

 known oil sands. 



It is to be noted that with the exception of the Coalinga 

 field the greatest production comes from the Etchegoin 

 (Lower Pliocene) sands. 



