32 CRANDALL— SAN FRANCISCO PENINSULA. [January 4. 



race sandstones and tuffs, and in the San Bruno sandstone in a 

 number of places. These dikes are on the north side of Visitation 

 Valley, near Railroad avenue, and near the entrance to the valley, 

 and also on the southwestern slope of San Bruno ]\lountain, and 

 in the little hills in the marsh, one and one half miles south of 

 South San Francisco. 



Schists. — The schists of San Francisco come mainly from one 

 area along Golden Gate Strait between Point Lobos and Lobos 

 Creek. The quartzose glaucophane schist described comes from 

 the San ^Miguel hills about one mile north of Ingleside Race Track. 

 The biotite schist is from just north of the entrance to Visitation 

 A^alley. 



I\^. Structure. 



Main Faults. — The structure of San Frsncisco peninsula is com- 

 plex but controlled in general by three main faults. The type of 

 structure is the same as that which characterizes other parts of the 

 Coast Ranges. These are dominant fault lines, with a general north- 

 west and southeast trend. Parallel to these faults are the major 

 lines of folding and minor lines of faulting. Making almost a right 

 angle with these lines are the minor lines of folding. 



Lone Mountain Fault. — Lone Mountain fault is the first of the 

 three main faults which control the geology and topography of the 

 San Francisco peninsula to be considered, as it crosses the city 

 from the Presidio to Hunter's Point in a straight line. This fault 

 may be seen only in the cliff near the Presidio. The line can be 

 traced across the city only by the topography and by the areal dis- 

 tribution of the rocks. It is this line which determines the northern 

 limits of the jaspers (excepting that at the Potrero), and the 

 southern limit of the Telegraph Hill sandstone. As the jasper does 

 not appear again north of this line, it is believed that the downthrow 

 of this fault is on the northeast. This is also substantiated by the 

 presence of a very closely compressed syncline, with its axes par- 

 allel to the fault and apparently developed by the slipping of the 

 fault block. The throw of this fault is not known, but it is judged 

 to be between 800 and 1,000 feet. 



San Bruno Fault. — The second fault is the San Bruno fault. 

 This is clearly a thrust, fault with the upthrow of perhaps 1,500 feet 



