I907.] CRANDALL-SAN FRANCISCO PENINSULA. 31 



is overlain in some places by the upper jasper bed, notably at Blue 

 Mountain, Twin Peaks, and at Bernal Heights. It occurs in sev- 

 eral places at the Presidio quarry, near Masonic Cemetery, on the 

 extreme end of Hunter's Point. 



Telegraph Hill Sandstone. — A sandstone called Telegraph Hill 

 sandstone composes all the hills in the business portion of the city, 

 Russian Hill, Telegraph Hill, Rincon Hill, Lone Mountain, Lafay- 

 ette Hill, and is exposed beneath the serpentine at the Presidio, and 

 both under and over the serpentine at the Potrero. It does not 

 appear on top of the jasper anywhere in the city except at the 

 Potrero, and nowhere else is its relations to the Franciscan series 

 shown. Elsewhere in California, so far as the writer's observations 

 go, the Cretaceous overlies the jaspers, but there have been no 

 fossils found here to show that this Telegraph Hill sandstone be- 

 longs to the Cretaceous, and as it resembles the San Bruno sand- 

 stone petrographically and macroscopically, it is regarded as a sand- 

 stone belonging to the Franciscan series and overlying the jaspers. 



Serpentines. — Across the city in a northwest-southeast direction 

 from the Presidio to the Potrero and Hunter's Point is a line of 

 irregular areas of serpentine. This line of serpentine is geologically 

 continuous, though it does not appear so on account of the inter- 

 vening sandstones and aeolian sands. ^ The proof of this continuity 

 rests upon the field evidence and the petrographic similarity of the 

 serpentine. That from the Potrero was found to be a Ihezolite 

 serpentine, while that from the Presidio was found to be a pyroxe- 

 nite, but, as remarked in the previous discussion of the rocks, the 

 chemical analysis shows that olivine must be present. The Pre- 

 sidio rock must also be a Ihezolite serpentine as is that from the 

 Potrero. Chemical analysis would prove this point beyond a doubt. 

 There are a few small areas of serpentine in other places, namely, 

 in the hills at South San Francisco, Buena Vista Park, Castro 

 street hill, Islais Creek near Holly Park, University Mound, and at 

 several other localities. 



Igneous Dikes. — The igneous dikes are present as intrusions in 

 the serpentines of the Potrero and Hunter's Point, in the Silver Ter- 



^ Lawson, A. C, Geology of the San Francisco Peninsula. 15th Ann. Rept. 

 U. S. Geological Surve\', p. 445. 



