398 University of California Publications in Geology [Vol. 8 



The portion of the San Pablo, above the contact just described, has 

 a thickness of about thirteen hundred and fifty feet. As the reader 

 will see from the section on page 399, it consists of a series of coarse 

 sandstones, fine sandstones and shales. At the base are one hundred 

 and fifteen feet of coarse gray, tuffaceous sandstone, already men- 

 tioned as lying above the fine concretionary sandstone. Above this 

 are two hundred and twenty-five feet of medium fine, gray sandstone, 

 followed by two hundred and fifty feet of coarse, cross-bedded, tuffa- 

 ceous sandstones. The upper part of this last layer of coarse sand- 

 stone is marked by abundance of the species Mulinia densata. At one 

 locality on the south side of the syncline, near the top of this member 

 of the group, a layer of pure white tuff about a foot in thickness was 

 found. 



The coarse, tuffaceous sandstone is followed above by about ninety 

 feet of pearl-gray shale which forms one of the most persistent mem- 

 bers of the group on San Pablo Bay. The contacts between this shale 

 and the underlying and overlying sandstones are cpute sharp. Both 

 above and below this member the change from coarse sandstone to 

 shale takes place within a few inches. At one locality, about two miles 

 to the east of Rodeo, a conglomerate, in which fairly large pebbles 

 occur, lies just above the shale. Some of the pebbles are two to three 

 inches in diameter ; most of them are made up of very fine sandstone 

 from which one or two undeterminable fossils were obtained. Many 

 of the pebbles contain bore-holes of the pholadid type. 



Above the shale member, the outcrops consist of alternating harder 

 and softer sandstones with minor amounts of shale. The harder sand- 

 stones contain abundant specimens of the echinoderm, Astrodapsis 

 tumidus. In this part of the section, coarse sandstones predominate. 

 The beds are very tuffaceous and in places show a distinctly blue 

 color. 



Relation to Monterey Group. — In the region of San Pablo Bay 

 there appears to be no appreciable difference in dip and strike between 

 the Monterey Group and the San Pablo. At the point where the 

 separation is made there is a fairly well marked change in lithology,. 

 the upper Monterey being a medium-fine, yellow-gray sandstone, while 

 the lower San Pablo is a coarse conglomeratic sandstone. Dr. C. E. 

 Weaver 19 described an unconformity in this region between the Monte- 

 rey and the San Pablo. He states that "areal mapping shows the 

 different divisions of the Monterey to extend diagonally beneath the 



is Weaver, C. E., Stratigraphy and palaeontology of the San Pablo Forma- 

 tion in Middle California, Univ. Calif. Publ. Bull. Dept. Geol., vol. 5, no. 16, 

 p. 251, 1909. 



