1915] 



Clark: Fauna of the San Pablo Group 



397 



Peeten beds described by Newberry in the earliest description of the 

 geology of the region. 



The section on the bay shows great variation in lithology ; and this 

 is quite generally true of the San Pablo throughout Middle California. 

 The beds are for the most part of a marine origin, as is proven by the 

 great abundance of sea-urchins at various horizons. Intercalated with 

 these marine beds are deposits which are to be classed as of estuarine 

 or brackish-water origin. Coarse conglomeratic sandstone predomi- 

 nates, with minor layers of shale and finer sandstone. The sandstones 

 in the upper part of the group are quite tuffaceous. as is also the case 

 in the region of Mount Diablo. 



LitJwlogy of Section. — At the base of the group are seven hundred 

 and fifty feet of coarse sandstone. These beds are made up of rather 

 hard, fossiliferous, coarse, gray sandstone, alternating with massive, 

 medium-coarse sandstones which are not fossiliferous. The harder beds 

 are usually well exposed. Some of the hard layers are from ten to 

 twenty feet in thickness while others are not more than a foot thick. 

 The harder layers are usually very fossiliferous, though the preserva- 

 tion of the fossils is generally poor. Toward the base of this lower 

 division the harder, fossiliferous strata are quite numerous and close 

 together, but higher up the massive sandstones predominate. 



The sandstones just described are overlain by about one hundred 

 and forty feet of medium fine, yellowish brown, concretionary sand- 

 stone, which is best exposed about two and one-half miles east of the 

 town of Rodeo (Univ. Calif. Loc. 1532). Here the brown sandstones 

 are overlain by a coarse, cross-bedded, tuffaceous, conglomeratic sand- 

 stone. Between the brown and the tuffaceous sandstones just de- 

 scribed there is a sharp line of contact which, at several places, is 

 irregular, the greatest difference of level noted being over three feet. 

 Irregularities, such as might have been made by some kind of boring 

 animal, were found along this contact, though no good evidence of true 

 pholas borings was obtained. "Whether this unconformity corresponds 

 to the one in the San Pablo Group on the north side of the mountain, 

 described by the writer. 18 or whether it comes at the same stratigraphic 

 position as a limestone on Rocky Ridge near the southeast corner of 

 the Concord quadrangle has not been determined. As will be seen 

 from the description of the different sections, the character of the 

 deposits of the San Pablo group in this part of the state is .such that 

 minor local unconformities are to be expected at various horizons. 



is Clark, Bruce L., Univ. Calif. Publ. Bull. Dept. Geo!., vol. 7, no. 4, p. 55. 



