1915J 



Clark: Fauna of the San Pablo Group 



411 



stones about 100 feet in thickness. This is overlain by a fine, white, 

 tuffaceous shale, which is approximately seventy-five feet thick. Many 

 leaf impressions are contained in the shale, and, toward the top, Cyrena 

 calif ornica, Mytilus perrini and Macoma pabloensis were found, sug- 

 gesting that these deposits were either of brackish-water or estuarine 

 origin. 



Above the shale is about two hundred feet of medium-grained, blue 

 sandstones containing Astrodapsis whitneyi, associated with a very 

 meagre fauna. Above the Astrodapsis whitneyi beds is a horizon of 

 conglomerate, followed by massive gray sandstones and shaly sand- 

 stones. Some of the most important fossil localities in the sandstones 

 and conglomerates are 1942, 1947, 1948 and 1950. The most common 

 species are Astrodapsis tumidus, Glycimeris coalingensis, Tivela gabbi 

 and Mulinia densata. Some of the species in this zone that have not 

 been found associated with Astrodapsis whitneyi, are Chione diabloen- 

 sis, Macoma diabloensis, Tivela gabbi, Astralium raytnondi, Calliostoma 

 bicarinatum, Crepidula pabloensis and Nassa pabloensis. Some of the 

 species in the Astrodapsis whitneyi zone, that have not been found as- 

 sociated with the fauna mentioned above, are Astrodapsis whitneyi, 

 Venus martini, Chione pabloensis, Dosinia merriami var. oeeidentalis, 

 Tellina englishi, Cancellaria pabloensis and Trophon lawsoni. 



There are good reasons for believing that the two minor faunal 

 zones just described are also represented in the San Pablo Bay section. 



The assemblage of species in the most fossiliferous zone above the 

 shale is very similar to that in the beds just above the Astrodapsis 

 whitneyi zone of Shell Ridge. Tivela gabbi is present with a number 

 of other species which have not been found associated with Astrodapsis 

 whitneyi in the Mount Diablo section, but are found in the horizon 

 above. The evidence for the correlation of the beds below the shale 

 of the Bay Section with the Astrodapsis whitneyi zone of the Mount 

 Diablo section is not so good. Astrodapsis whitneyi has not been found 

 in that section, but the species Trophon lawsoni, Avhich has been found 

 only in the Astrodapsis whitneyi zone on the south side of Mount 

 Diablo, is present. 



In the Shell Ridge section the beds above the so-called river con- 

 glomerates have not yielded many fossils, but farther to the south, in 

 the region of Pine Canon, a few species that may prove to belong to a 

 minor faunal zone were collected near the top of the series. These are 

 Astrodapsis tumidus large variety, Mytilus merriami and Trophon 

 ponderosum var. pabloensis. The meagreness of the fauna from these 



