1915] 



Clark: Fauna of the San Pablo Group 



415 



The section is of interest because of its contrast to the Mount 

 Diablo section which lies only a few miles to the east, these beds being' 

 made up almost entirely of sandstones, with little or no shale. Con- 

 glomerates and leaf-bearing shales such as are common in the Mount 

 Diablo section are lacking. The probable reason for the uniformity 

 of these beds is that they were deposited farther away from the shore 

 line. During much of San Pablo time the shore line was near the 

 present site of Mount Diablo and these slight oscillations would have 

 markedly affected the character of the depositions from time to time. 

 A few miles to the west, in deeper water," the same changes in level 

 would not have appreciably affected the uniform processes of sedi- 

 mentation. The estuarine deposits of the San Pablo in the vicinity of 

 Mount Diablo were undoubtedly formed in close connection with the 

 shore line. 



The beds in the lower four hundred feet of this section consist oi 

 fairly hard, coarse, fossiliferous sandstone, alternating with medium 

 coarse massive sandstones. They are lithologieally similar to the basal 

 beds of the series on San Pablo Bay. In ascending order above the 

 coarse sandstones are about six hundred feet of medium fine to medium 

 coarse, fairly homogeneous sandstones ; seven hundred feet consist of 

 fairly coarse, gray sandstones, some layers of which are fossiliferous ; 

 about 300 feet of fine, dark brown to gray sandstones ; and about 75 

 feet of coarse, tuffaceous sandstone. The San Pablo is overlain by the 

 Pinole Tuff and the Orindan series. 



FAUNA 



Scutella gabbii was found at the base of the section. A little above 

 the base, Astrodapsis tumidus, subsp. cierboensis was obtained, asso- 

 ciated with Tivela diabloensis. About one thousand feet above the base, 

 Astrodapsis tumidus and Astrodapsis whitneyi were collected from a 

 zone which at localities 1121 and 1127 yielded a fairly large fauna. 

 An upper San Pablo fauna was obtained at locality 2329. Macoma 

 andersoni, one of the common species at this locality, is thought to be 

 characteristic of the zone above the Astrodapsis whitneyi beds on the 

 south side of Mount Diablo, and this zone is supposed to be ecpiivalent 

 to the very fossiliferous horizon just above the shale in the bay section. 



