6 



CRANDALL— SAN FRANCISCO PENINSULA. 



[January 4, 



as showing, microscopically, the presence of many foraminifera and 

 is called by him foraminiferal limestone/ 



4. San Bruno Sandstojic. — Above the limestone come the most 

 characteristic beds of the series. These are shales, and sandstones, 

 which occasionally have coarse conglomerates interbedded with them. 

 With the shales are also large accumulations of pyro-clastic mater- 

 ials which are confined to a fairly well defined horizon just below 



Fig. 2. 



San Pedro shale beds dipping southwest ; 



Pedro Point. 



a point just north of San 



the jasper beds. In mapping, these volcanic tufifs have not been 

 differentiated from the rest of the series. It is the weathering of 

 these tuffs that gives rise to the most of the numerous and varied 

 forms of metamorphic rocks, which, in the field, have been called 

 greenstones. 



The larger part of this horizon is made up of sandstone proper, 

 which, when fresh, is a hard, blue gray rock, generally showing but 

 few bedding planes, the dip being only discernible where thin beds 

 of shale are present. 



^ 15th Ann. Rept. U. S. Geological Survey, p. 405. 



