110 University of California Publications in Geology [Vol. ll 



The San Lorenzo beds in the area to the north of Mount Diablo 

 are in general different from those to the west of the mountain. To 

 begin with, here the maximum thickness is over thirty-seven hundred 

 feet, while to the west of the mountain it is only a little over five 

 hundred feet. 



The series to the north of Mount Diablo is divided by the writer 

 into two formations, the lower of which is the Markley, the upper, 

 the Kirker formation. These formations both represent more or less 

 of a heterogeneous assemblage of beds, most of which were formed 

 under shallow-water conditions of sedimentation. The Markley for- 

 mation is possibly composed, in part, of continental beds ; this is 

 suggested by the arkosic character of the sandstones, the grains of 

 which tend to be angular, by the presence of considerable carbonaceous 

 material of plant remains, and by the lack of marine fossils. Evi- 

 dences of true marine conditions were obtained at least at one horizon. 

 A thin band of typical marine diatomaceous shale was found at about 

 five hundred feet from the top of the formation. 



The Kirker formation is separated from the Markley formation 

 by a disconf ormity ; this is shown by an erosional contact, a true basal 

 conglomerate, fluviatile deposits between the two formations, and 

 Pholad borings. 



The Kirker formation is composed largely of rhyolitic tuffs and 

 tuffaceous sandstones. It is overlain unconformably by the San 

 Pablo (Upper Miocene), the Monterey group, which is present to 

 the west of the mountain, being absent. 



The Kirker formation contains a number of species common to 

 the Kirker tuffs of the Sobrante anticline ; this, together with the 

 similarity of lithology, makes the correlation reasonably certain. The 

 Markley formation is provisionally correlated with the San Ramon 

 formation to the west of the mountain. The lithology and thickness 

 of the two, however, are so different that some objection may possibly 

 be made to this. 



Summary of Correlation 



The San Lorenzo series of the region of Mount Diablo belongs to 

 the same general period of deposition as the San Lorenzo formation 

 of the Santa Cruz Mountains and the Kreyenhagen shales found 

 along the west side of the San Joaquin Valley to the north and to 

 the south of the town of Coalinga. These, in turn, are correlated 



