408 University of California Publications in Geology [Vol. 8 



In general, the deposits of a large part of the San Pablo in this 

 section may be classed as shallow-water deposits, in part of marine, 

 and in part of estuarine origin. Several unconformities within the 

 group probably indicate emergence and land conditions of short dura- 

 tion at several times during the epoch. 



Relation to Monterey Group. — The Monterey Group cannot every- 

 where be easily separated from the San Pablo, as the beds immediately 

 below and above the contact are quite similar. An irregular contact 

 was located by W. Kew, R. C. Stoner and J. H. Ruckman of the Sum- 

 mer Session held class of 1911 on the south side of the Black Hills 

 near Wall Point. This contact has been taken as the line of division 

 between the two groups. Along this contact bore-holes of the pholadid 

 type were found. A good fauna was not obtained from the upper 

 beds of the Monterey in this vicinity, but between two hundred and 

 three hundred feet below the contact are the coarse, gray sandstones 

 of the Scutella breweriana beds, which contain a typical fauna of that 

 horizon. 



Relation to Pinole Tuff and Orindan. — The San Pablo Group on 

 the south side of Mount Diablo is overlain unconformably in places by 

 the Pinole Tuff and elsewhere by the Orindan. In the vicinity of the 

 town of Walnut Creek the Pinole Tuff is about two hundred feet thick ; 

 it gradually thins out to the south, finally disappearing, and in the 

 vicinity of Tassajero Creek, is replaced by the Orindan which rests 

 directly upon the San Pablo. 



Detailed Section of the San Pablo Group on the South Side of Mount 

 Diablo as Measured in the Vicinity of Tassajero Creek 



Orindan Formation. Fresh-water beds. 



(TJneomformity) 

 San Pablo Group, 



Feet 



Shaly sandstone 150 

 Leaf -bearing shale 10 

 Hard, bluish sandstone and conglomerate; (zone of "river con- 

 glomerate") 50 

 (Irregular contact) 



Leaf -bearing shale 20 



Medium fine sandstone with shaly sandstone toward top 720 



Conglomerate containing Astrodapsis tumidus 15 

 ( Irregular contact) 



Leaf -bearing shale 20 

 Medium fine sandstone and bands of sandy shale, with Astrodapsis 



whitneyi, A. tumidus, Pinna alarnedensis, Trophon laiosoni, etc. 270 



