94 University of California Publications in Geology [Vol. ll 



coarser beds represent continental deposits. The explanation for the 

 fact that the Markley formation is thicker than the San Ramon may 

 be that sedimentation began first in the middle of the trough, the 

 location of which now would be in the area to the north of Mount 

 Diablo, and then spread to the west. When the sea came in, probably 

 from the south, the trough was already partly filled with the conti- 

 nental deposits, which had not extended as far west as the area in 

 which the outcrops of the San Ramon formation are found ; deposition 

 extended over a wider area and sedimentation took place for the first 

 time in the western area. 



The possibility that the lower sandstones of the Kirker formation 

 on the north side of the mountain are the equivalent of the San Ramon 

 formation has been considered. This seems very improbable. In 

 the first place, the diatomaceous shales of the Markley formation con- 

 tain typical Oligocene species. Further, if the basal sands of the 

 Kirker formation are the equivalent of the San Ramon formation, 

 then the great thickness of the beds of the Markley formation is 

 entirely absent to the west of the mountain ; because of the nearness 

 of the two sections to each other this would imply a time-break of 

 considerable importance between the deposition of the beds of the two 

 formations, and of such a time-break the fossils give no evidence. 

 Finally, the fauna found in the basal beds of the Kirker formation 

 to the north of Mount Diablo is somewhat different from that found 

 in the San Ramon beds. Some of the species which are very common 

 in the basal Kirker sandstones and which have not been found in the 

 San Ramon beds are Acila shumardi, Macrocallista pittsburgensis, 

 Pitaria sp., Yoldia cf. oregona, and Solarium lorenzanum. Two of 

 these species, Acila shumardi and solarium lorenzanum, have been 

 found in the Concord formation to the west of the mountain. 



There is very good evidence for the correlation of the Kirker 

 formation to the north of Mount Diablo with the Kirker tuffs and the 

 Concord formation of the Sobrante anticline. As has already been 

 stated, there are very few species found in the Concord formation 

 that are not also found in the Kirker tuffs immediately below. It 

 is possible that the Kirker formation on the north side of the moun- 

 tain represents both the Kirker tuffs and the Concord formation of 

 the Sobrante anticline. The reason for giving a separate formational 

 name to these upper sandstones of the Sobrante section is the presence 

 of the disconformity which separates these beds from the tuff member 

 below. 



