1915] 



Clark: Fauna of the San Pablo Group 



113 



determined, the coarse, basal sandstones of the one grade downward 

 into the medium fine sandstones of the other. Here again is a section 

 in which no structural break has been found separating the San Pablo 

 from the upper Monterey. 



Lithology. — The coarse, massive, non-fossiliferous sandstones of 

 the basal San Pablo outcrop as jutting ledges along the high, nearly 

 perpendicular canon walls. These sandstones are followed above by 

 a series of fine and coarse sandstones, with several interbedded layers 

 of fine conglomerate, which yielded fossils at two levels. 



A noteworthy feature of the San Pablo in this area is the presence 

 of small lenticular beds of limestone near the middle of the group ; 

 two outcrops were found, both approximately at the same stratigraphic 

 position. At one locality near the south end of Section 35, T 1 S, R 

 2 W, Mount Diablo base line, there is an outcrop of the limestone 

 which is less than a quarter of a mile in length, and from a maximum 

 thickness of eight feet it thins rapidly in both directions. The lime- 

 stone grades downward into a muddy shale and sandstone and is over- 

 lain by a considerable thickness of fine gravel. No difference in dip or 

 strike of the beds above and below the lens could be detected. Some 

 of the limestone is pure white and of a very fine, porcelaneous texture, 

 fracturing conchoidally like flint, and some of it is more impure and 

 of a darker color. No shells were found, but the impressions of leaves 

 and rushes are very abundant. 



The other limestone locality is near the southwest corner of Section 

 5, T 2 S, R 1 W, Mount Diablo base line, about one and a half miles 

 southeast of the first locality. 



The question arises — Under what conditions were the beds of this 

 character deposited? The presence of great quantities of dicotyledon- 

 ous leaves and rushes shows conclusively that the limestones are not 

 of typical marine origin. These limestones closely resemble those of 

 fresh-water lake deposits, but it is not certain that they could not lie 

 formed under estuarine conditions. Limestones lithologieally almost 

 identical with those found in the San Pablo occur in the overlying 

 Orindan in this same section. That these upper limestones are of 

 lacustral origin is shown by the fresh-water shells which they contain. 



The beds above the limestone lenses in the San Pablo are mostly 

 medium fine sandstones with occasional subordinate layers of shale 

 similar to the leaf -bearing shale seen in the section on the south side 

 of the Mount Diablo anticline. 



