44 CRANDALL-SAN FRANCISCO PENINSULA. 



[January 4, 



evident, though it follows the general northeast trend of the other 

 streams. It is at present slowly filling the basin between the Potrero 

 and Hunter's Point. 



The hills through the center of San Francisco, at Golden Gate 

 Cemetery, Strawberry Hill, San Miguel Hills, Bernal Heights and 

 University Mound, owe their prominence to the large beds of jas- 

 pers that compose the major part of them. This jasper does not 

 weather like a sandstone, but only breaks down slowly by mechan- 

 ical means. The canyon through Almshouse tract, with its feeding 

 streams, and the gap through Twin Peaks and Sunnyside addition 

 are formed by the more rapid disintegration of the Silver Terrace 

 sandstone than of the jaspers. The topography of this area in the 

 heart of the city is controlled partly by the jaspers and serpentines 

 and partly by the structure. 



Lone Mountain Block. 



Effect of Jaspers. — There is a low divide caused by the Lone 

 Mountain fault crossing San Francisco from a point just northeast 

 of Lobos Creek past Lone Mountain, to Hunter's Point. Parallel to 

 this is a line of hills on which the cemeteries are located and Lone 

 Mountain w^hich have survived erosion because of their synclinal 

 structure and the serpentine areas that have helped resist rapid 

 weathering. 



Effect of Serpentines. — The Presidio, Potrero, and Hunter's 

 Point remain as noticeable features not because of the slowness 

 with which serpentine alters, but because of the slowness with which 

 it breaks down and is carried off by erosion. The reason the upper 

 beds of sandstones and jaspers do not remain in places over these 

 sills is because the large amount of movement that has taken place 

 has so broken and crushed the overlying sandstone that erosion 

 has been greatly facilitated. 



Two small hills, one at Silver Terrace, and the other one and 

 three quarter miles south of South San Francisco, owe their to- 

 pographic relief to the intrusion of igneous rocks that have meta- 

 morphosed the tufaceous materials present to hard resisting green- 

 stone. 



Effect of Structure. — The divide between Lafavette and Russian 



