Palache. I 



Lherzolite- Serpentine. 



1 79 



Its relations to the San Francisco sandstone are unknown, as it 

 does not appear to intersect that formation anywhere within the 

 limits of the area here discussed. 



THE SAN FRANCISCO SANDSTONE. 



The San Francisco sandstone is represented in the Potrero by 

 several small patches lying- on the flanks of the serpentine mass. 

 It embraces shale, sandstones, and grits of the types common to 

 this formation, and well described by Blake and Newberry, by whom 

 it was named. Associated with the sandstones are two limited areas 

 of the green and red thinly-bedded jaspery rocks, the phthanites of 

 Becker, which are characteristic of this formation in the vicinity of 

 the Bay of San Francisco. 



No attempt was made to investigate the stratigraphy of these 

 sedimentary beds, their interrupted and fragmentary character and 

 imperfect exposure rendering such a study almost impossible within 

 the limits of the small area in question. Enough was seen, how- 

 ever, to indicate that they have been folded, and that they do not 

 appear to have undergone extensive metamorphism. 



No evidence was discovered bearing on the age of this forma- 

 tion, which was assigned to the Cretaceous by the State Geological 

 Survey on imperfect evidence, and has been since much in dispute. 



Geo/ ogical Laboratory \ 



University of California, April, i8gj.. 



