606 BRUCE L. CLARK 
Paleogeography.—The sediments of the Briones and Cierbo 
groups were deposited in a limited arm of the sea confined to the 
San Francisco Bay region (Figs. 8 and g). Where all the groups 
of the San Pablo series are present, erosion contacts are found 
separating them. 
With the opening of the Santa Margarita there was a great 
inundation, somewhat comparable to that of the Temblor, though 
Fic. 8.—Briones (lower San Pablo, Miocene) 
the basins of the former were more local. Beds of Santa Margarita 
age are found from a little north of San Francisco to the region 
just north of Los Angeles (Fig. 10). The deposits are found on 
both the eastern and the western sides of the Coast ranges, and 
over large areas to the south of San Francisco these deposits 
are composed very largely of organic shales of considerable 
thickness. 
