1907.] 



CRANDALL— SAN FRANCISCO PENINSULA. 



39 



are besides these two small synclines, one of which is exposed at 

 Lombard and Larkin streets, the other on Jones street near Vallejo 

 street. 



There are other minor lines of faulting but they are not of 

 enough importance to be mentioned here. 



A noticeable feature of the sandstone of the city is their joint- 

 ing. These joint planes cut the rocks in many directions, without 

 any apparent regularity, giving in exposed places such sharp, pro- 



.^- ;s^*- 



Fig. 17. 

 Serpentine of the Presidio sill resting upon a sandstone floor. 



jecting, triangular faces, as to make it difficult or impossible to 

 distinguish the original bedding planes. The pressure that pro- 

 duced these results in sandstone has not broken the jasper but 

 merely thrown it into complicated folds. 



Structure of Serpentine Intrusioiis.— AW the serpentine areas of 

 San Francisco peninsula are in the form of sills. The largest of 

 these is the Presidio, and next are two nearly as large at the Potrero 

 and at Hunter's Point. In the last two areas the hypersthene dia- 

 bases have been intruded and have been broken up by the move- 

 ments within the serpentine itself, during the process of hydration. 



