Vol. 5] 



Weaver. — San Pablo Formation. 



253 



have all been correlated with the Pinole Tuff. Provisionally the 

 tuff here is also considered as the equivalent of the Pinole Tuff. 

 The average thickness of the San Pablo along this belt is about 

 one thousand feet. 



South of Mt. Diablo. — On the south side of Mount Diablo 

 there is a belt of San Pablo strata comparable in extent and 

 volume with that on the north side. It extends along the Black 

 Hills in a general northwest to southeast direction. When traced 

 to the northwest it extends up through Shell Ridge and passes 

 underneath the alluvium of San Ramon Valley. In the vicinity 

 of Shell Ridge it lies upon the Monterey, but to the south- 

 eastward the Monterey thins out and disappears and the San 

 Pablo rests upon the Tejon. The Orindan beds lie above it. 

 The Tejon, San Pablo, and Orindan have all been overturned 

 and in places the Tejon has been partly thrust over upon the 

 San Pablo. The average dip is about 80 degrees north, but near 

 the contact with the Tejon it decreases to 50 degrees with the 

 Tejon uppermost. These sections were measured across the strike 

 of the San Pablo belt at Tassajero Canon, Railroad Ranch, and 

 Green Valley. The average thickness is about twelve hundred 

 feet. The characteristic features of this belt are about the same 

 as those on the north side except that the white chalky shale at 

 the base on the north side is represented by a thick-bedded, buff- 

 colored, shaly sandstone. This is followed by conglomeratic 

 fossiliferous sandstone with a small amount of shale. Near the 

 middle of the formation there is a belt of conglomerate consisting 

 of pebbles made of quartz, chert, and volcanic rock, firmly 

 cemented together in a bluish-gray matrix. This belt has a 

 thickness of over one hundred and twenty feet, and is followed 

 by about one hundred feet of coarse-grained, gray, fossiliferous 

 sandstone containing lenses of a fine-grained conglomerate and 

 showing a distinct cross bedding. When followed westward this 

 band of conglomerate becomes less distinct. It passes into nu- 

 merous alternations of coarse sandstone and conglomerate and 

 finally into a conglomeratic sandstone. In the exposures along 

 the road from Railroad Ranch it appears only as conglomeratic 

 sandstone. Above this there are sandstones interbedded with 

 shales and near the top there is a series of thin-bedded sand- 



