1915] 



Clark: Fauna of the San Pablo Group 



407 



at the same horizon as the big shale member in the upper part of 

 the group as described in the San Pablo Bay section. 



In the vicinity of Sycamore Canon, on the south side of the Black 

 Hills, the leaf-bearing shale mentioned above is overlain by a heavy 

 fossiliferons conglomerate, the pebbles of which are composed mostly 

 of andesite. In Sycamore Canon a well-marked, irregular contact was 

 found between the shale and the conglomerate, with borers of the 

 pholadid type extending from the conglomerate into the shale. This 

 unconformity was noted at two different localities over a mile apart. 



The beds above the shale member just described are even more 

 variable than in the lower part of the section, and consist of alternating 

 layers of pearl-gray shale and blue sandstones, together with lenticular 

 beds of conglomerate. 



In the vicinity of Tassajero Creek, there is a zone of irregularly 

 bedded conglomerate and sandstone about two hundred feet strati- 

 graphically below the top of the group. The observed thickness of this 

 zone is over fifty feet at several localities. The materials grade rapidly 

 along the strike from sandstone to conglomerate and vice versa. The 

 zone was traced for a distance of over fifteen miles, southeastward from 

 Walnut Creek, but its maximum extension in that direction was not 

 determined. No fossils were found in this zone. The pebbles in the 

 conglomerate are noticeably round and some of them are two to three 

 inches or more in diameter. In the field, they were referred to as the 

 river conglomerates. 



On Kailroad Kanch, about three miles northeast of the town of 

 Danville and about one mile due south of Wall Point, there is a 

 heavy, well-rounded, non-fossiliferous conglomerate, approximately in 

 the same stratigraphic position as the conglomerate in the vicinity of 

 Tassajero Creek. Lying beneath this is a pearl-gray shale containing 

 an abundance of leaf impressions. 



At one locality just below the reservoir on the Mount Diablo road, 

 a well-marked irregular contact appears between the conglomerate 

 and the shale. 



Two miles southeast of the town of Walnut Creek and southwest 

 of Shell Ridge (Concord sheet) there is a non-fossiliferous conglom- 

 erate about one hundred and fifty feet below the top of the San Pablo, 

 which at one locality is nearly fifty feet thick, but which rapidly 

 thins out along the strike. This is very much like that seen in the 

 region of Tassajero Creek and south of Wall Point, and probably 

 represents the same zone. 



