62 



University of California Publications. 



[Geology 



The two areas of serpentine on the north side of La Brea 

 Creek are interesting mainly for their peculiar relationship to 

 the overlying sedimentaries. The easterly of these two areas 

 appears on the map to lie between the two basal divisions of 

 the San Pablo formation. There is more or less overlapping 

 of the sedimentaries and it would appear that in lower San 

 Pablo time, while the basal conglomerates of that formation were 

 being deposited, the serpentine protruded above the sediments. 

 During this time depression kept pace with sedimentation and 

 the serpentine was covered by the second division of the San 

 Pablo. These two areas of serpentine, as has been mentioned, 

 are in all probability outcrops of a single mass which was 

 intruded into the Franciscan rocks. 



In general, the serpentine bodies appear to have the form 

 of laccolithic sills and prior to the great amount of erosion to 

 which the Franciscan rocks were subjected may have formed 

 one connected series of sills. 



MIOCENE SEEIE8. 



Lower Miocene. — The lowest Tertiary formation exposed in 

 the area mapped is probably of lower Miocene age. The forma- 

 tion lies conformably beneath a great thickness of Monterey shale 

 and is separated from it, in the discussion, on lithologic grounds. 

 There are several thick beds of siliceous shale in the terrane 

 which have tentatively been called Lower Miocene, but the 

 presence of large amounts of sandstone, clay shale and conglom- 

 erate distinguish it from the overlying Monterey. This series of 

 beds is very similar to the Temblor beds of the Monte Diablo 

 range described by F. M. Anderson . The marked overlapping 

 of the Monterey shales on the Temblor beds as noted by 

 Anderson is also a prominent feature with the Monterey shales 

 on the Lower Miocene of the Pajaro Valley region. 



Numerous casts of fossil remains in a poor state of preserva- 

 tion were found in these beds exposed along La Brea Creek ; 

 but most of these could not be identified and the few that were 

 recognizable were forms of wide range in the Tertiary. 



sProc. Cal. Acad. Sci., 3rd ser., 2, 1905, p. 168, et seq.; and 4th ser., 3, 

 1908, pp. 18-20. 



