Clark: The San Lorenzo Series of Middle California 



87 



thin layer of conglomerate, the basal bed of the Kirker formation, is 

 underlain by ten or fifteen feet of coarse, poorly assorted conglomer- 

 ate and sandstone which apparently are fluviatile in origin, and which 

 overlie the upper micaceous sandstone and shale of the Markley 

 formation. Less than a quarter of a mile east of this locality, the 

 basal conglomerate of the Kirker formation rests directly upon these 

 micaceous sandstones and shales. Some of the important evidences 

 of erosion found at the contact, as observed at the last locality, are 

 as follows : 



1. The contact is irregular ; in places, the conglomerate of the 

 Kirker formation rests upon the micaceous sandstone, and in places 

 upon the shale of the Markley formation. 



2. Large boulders of micaceous sandstone and clay-shale, derived 

 from the beds immediately below, are found in the basal conglomerate 

 of the Kirker formation. Some of the sandstone boulders are as much 

 as three feet and a half in length and over two feet both in width 

 and thickness. The shale boulders are elongate, flat slabs, some of 

 which are over three feet in length. 



3. A zone of weathering is found around the edges of most of the 

 sandstone and shale boulders ; this is especially noticeable on the 

 latter. 



4. Pholad borings of great abundance extend from the upper into 

 the lower beds. 



Litkology. — The basal conglomerate of the Kirker formation is 

 thin, varying from a foot or two in thickness to only a few inches ; at 

 some localities it is absent altogether. Besides the sandstone and shale 

 boulders, described above, boulders of quartzite, chert, and various 

 kinds of igneous rock are found in the conglomerate ; the largest of 

 these are about five or six inches in diameter, and as a rule are well 

 rounded. 



Immediately above the basal conglomerate is about fifty feet of 

 light-gray sandstone, which becomes more and more tuffaceous toward 

 the top. This is followed by about three hundred and fifty feet of 

 fine white tuff and tuffaceous sandstones, the tuff being by far the 

 more predominant. 



W. H. Turner" 7 and C. E. Weaver" 8 included the tuffs of this 



(J 7 Turner, W. II., The rocks of Mount Diablo Range of California, Jour. Geol., 

 vol. 6, pp. 488-499, 1898. 



,; » Weaver, C. E., Stratigraphy and palaeontology of the San Pablo formation 

 of Middle California, Univ. Calif. Publ., Bull. Dept. Geol., vol. 5, no. 16, pp. 258- 

 259, 1909. 



