252 



University of California Publications. 



[Geology 



North of Mt. Diablo. — On the north side of Mount Diablo the 

 San Pablo occurs as a long narrow belt having a width of about 

 three-fourths of a mile and a length of about fifteen miles which 

 extends in a general northwest and southeast direction. It is 

 underlain by the Monterey formation and overlain by the Pinole 

 Tuff and Orindan formation. All of these formations dip north. 

 Two sections were measured in detail across the strike of the San 

 Pablo formation. One of these is located along the east side of 

 Kirker's Creek and the other along Markeley Canon. In gen- 

 eral the character of the strata at the same relative positions 

 in the two sections is the same. However, the thick beds of 

 white shale which outcrop so prominently at the base of the 

 section in Kirker's Creek, are not well represented at the base 

 in Markeley Canon. Here the San Pablo and Monterey at the 

 line of contact are apparently conformable. In the Kirker's 

 Creek section some thick-bedded coarse gray sandstones which 

 very closely resemble the characteristic San Pablo sandstones 

 occur between the Monterey shales and the lower white shale 

 member of the San Pablo. In the western portion of Contra 

 Costa County the Monterey formation is separated into nine well 

 defined divisions of sandstones and shale, but to the east in the 

 vicinity of Mount Diablo these divisions lose their identity. 

 However, it has seemed best to consider the sandstone at the 

 base of the white San Pablo shale to be the equivalent of the 

 upper sandstone member of the Monterey farther west and to 

 make the base of the San Pablo the line separating the sandstone 

 from the white ash beds. Thus we have, as the lower portion of 

 the San Pablo, strata composed of white chalky shale followed by 

 coarse thick-bedded sandstones and conglomerates. Overlying 

 these are several feet of shale containing fossil leaves. These 

 are followed by conglomerates and an extensive series of coarse- 

 grained, conglomeratic, gray sandstones in which are numerous 

 fine bands of conglomerate. These sandstones are very often 

 cross-bedded. Besides these there are beds of shale, shaly sand- 

 stones, and fossils. Above these are great thicknesses of bluish- 

 gray sandstones. These sandstones are overlain by tuffs inter- 

 mixed with conglomerates. At San Pablo Bay, Carneros Creek, 

 and Pleasant 's Valley, tuffs rest upon the San Pablo and they 



