Clark: The San Lorenzo Series of Middle California 75 



massive; here thin beds of micaceous, gray, medium-fine sandstone 

 alternate witli thinner layers of sandy shale and clay-shale. Very 

 good exposures of this part of the section may be seen on the side 

 of the road just south of the divide between Bear and Pinole creeks 

 on the north side of the anticline. Here the formation is somewhat 

 thicker than at the other locality, mentioned above. 



Kieker Tuffs 



Overlying the San Ramon formation are about one hundred feet 

 of fairly indurated white tuff beds, which contain a few minor layers 

 of tuffaceous sandstone. Taken as a whole, these beds are very fine 

 and homogeneous in texture. The separation between the Kirker 

 tuffs and the San Ramon formation is marked by a very sudden 

 change from the sandstone of the latter to the tuff of the former. 



The Kirker tuffs are correlated by the writer with the tuff of the 

 Kirker^ formation, the upper member of the Oligocene on the north 

 side of Mount Diablo, the beds being named after Kirker Creek of 

 that vicinity. 01 Evidence for the correlation of these beds with the 

 tuffs of the Sobrante anticline will be considered with the discussion 

 of the fauna, further on in the paper. 



Concord Formation 



Resting disconformably upon the Kirker tuffs is the Concord 

 formation, the beds of which consist chiefly of fine grayish sandstone. 

 The thickness of the formation is about two hundred and fifty feet. 

 At the base is a thin layer of conglomerate, the boulders of which are 

 composed mainly, if not entirely, of tuff, sandstone and shale, appar- 

 ently derived from the Oligocene beds immediately below. The close 

 lithologic similarity of the tuff boulders to the Kirker tuff is especially 

 convincing. At none of the localities studied was this conglomerate 

 over six inches thick. Some of the larger boulders are rather angular. 



A very interesting and important character of the basal sandstone 

 of the Concord formation, including the matrix between the boulders 

 of the conglomerate, is that it is white, due to the presence of tuff 

 found in between and coating the grains- of sand. This character 

 was undoubtedly the result of the reworking of the tuff beds by the 

 advancing sea as it encroached upon the Kirker beds, which had, just 

 previously, been subjected to erosion. The beds which contain this 

 reworked tuff have a thickness of from six inches to possibly one foot. 



81 The most important fossil locality in the Kirker tuffs of the Sobrante anti- 

 cline is University of California locality 3055. 



