396 University of California Publications in Geology [Vol. 8 



the sea connected with the ocean to the south, as San Pablo outcrops 

 are found fairly continuously from Mount Diablo southwestward to 

 the coast. 



The character of the fauna obtained from the San Pablo beds to 

 the east of San Francisco Bay shows that during a large part of San 

 Pablo time, true marine conditions existed. Intercalated with these 

 typically marine beds at several horizons are beds which show evidence 

 of having been deposited under estuarine or brackish-water conditions. 

 In several of these sections, unconformities are found which the writer 

 believes to be due probably to slight local oscillations of the earth's 

 crust during San Pablo time. These movements affected the shore 

 deposits but possibly not the beds deposited further off shore. To 

 determine whether the estuarine or brackish-water deposits may be 

 taken as evidence of general estuarine conditions in the San Pablo 

 at these horizons, or whether they are merely shore phenomena, it will 

 be necessary to study additional sections in detail, and more must be 

 known about the distribution of the San Pablo to the north. 



DESCRIPTION OF SECTIONS 



San Pablo Bay 



STRATIGRAPHY AND LITHOLOGY 



The study of the type section on San Pablo Bay shows, if we are 

 to accept the classification of the United States Geological Survey, 

 that the San Pablo should be classed as a group rather than as a 

 formation. Here the thickness of the beds, as measured by the writer, 

 is close to twenty-two hundred and fifty feet. In this section the San 

 Pablo is divisible into at least thirteen lithologic members. Good 

 exposures are found, so that every member may be advantageously 

 studied at some locality within the area. The basal beds are well 

 exposed along the bay shore in the vicinity of the Union Oil Works 

 to the west of Selby Station. Outcrops of these beds may be followed 

 for a distance of four or five miles to the southeast. The middle 

 portion of the San Pablo is not well represented along the bay shore, 

 but. about two to three miles inland, on the south side of the syncline, 

 beds of this part of the section may be found. The best exposures 

 of the upper part of the group are on the north side of the syncline, 

 about one mile to the southeast of the Union Oil Works. Also very 

 good exposures of the upper beds are found to the east of and a little 

 to the southwest of the town of Rodeo. At this last locality are the 



