Vol. 6] Jones: The Geology of the Sargent Oil Field. 



(11 



subject has been thoroughly presented by Lawson 4 , Ransome r ', 

 and others. 



There is more or less uniformity in the strike and dip of the 

 strata over the region in question. A general northwest- 

 southeast strike with a northeast dip of from 30 to 45 degrees 

 prevails. Within small limits, however, there is much variation 

 to the strike and dip, and the whole terrane exhibits evidences 

 of considerable twisting and shearing. 



This area of Franciscan rocks was evidently deeply buried 

 beneath the great thickness of Miocene shales but formed a 

 land area during at least the lower part of San Pablo time, and 

 was exposed to erosion then, as the basal divisions of the San 

 Pablo formation carry large amounts of Franciscan rock frag- 

 ments. 



SEEPENTINE. 



Little need be said in regard to the altered peridotites of 

 this region. The occurrences are all similar to those noted 

 elsewhere in the Coast Ranges where areas of Franciscan rocks 

 have been studied. The original peridotite in its altered con- 

 dition was not found. Some of the occurrences, however, are of 

 sufficient interest structurally to be worthy of note. 



There are four areas of serpentine on the geological map. 

 Two of these, on the north side of La Brea Creek, are evidently 

 connected and form one intrusive body. The most interesting 

 by far of these masses is the irregular one in the extreme 

 northern part of the area mapped. The structure of this body 

 was plainly evident in the field and its contacts with the intruded 

 sedimentaries are all clear and decisive. The lower part of the 

 mass is intruded between the bedding planes of Franciscan 

 sandstone, while the upper part lies across the broken edges of 

 the country rock and in cross-section shows a thin layer of 

 serpentine, the overlying sandstone having been removed by 

 erosion. 



The serpentine body just south of here is also intrusive 

 between the bedding planes and may have been at one time 

 structurally connected with the mass above it. 



4 15th Ann, Ept. U. S. G. S. 



5 Univ. Calif. Publ. Bull. Dept. Geol., 1, 1894, pp. 193-233. 



