330 KRAMM— SERPENTINES OF THE CENTRAL [June 6, 



near the Missouri Mine about six miles northwest of Pine Moun- 

 tain and is fairly well preserved. 



Serpentine is also found bordering Pope Valley. The extent of 

 these areas however has not been determined. Specimens from 

 these areas show a typical Iherzolite serpentine. 



IV. The San Francisco District Serpentines. 

 The San Francisco Peninsula Serpentines. 



Serpentine is found in the vicinity of San Francisco and at San 

 Francisco proper. According to Lawson^^ they are grouped into 

 three linear tracts which traverse the peninsula of San Francisco in 

 a northwest-southeast direction. The most northern one extends 

 from Fort Point on the Golden Gate to Hunters Point on the Bay 

 of San Francisco, a length of ten miles and with a maximum 

 width of 13^ miles. It is probably a continuous belt although sand 

 and alluvium cover portions of it and make it appear as four distinct 

 areas. These areas have the character of laccolites or dikes which 

 are intrusive into the Franciscan series of rocks. 



The character of the serpentine has been in detail investigated by 

 Palache." It is represented in two facies which are named the 

 slickensided and the massive facies. The slickensided is similar to 

 the one at Sulphur Creek described by the writer. The massive 

 facies is dark green in color, translucent on thin edges and has a 

 splintery fracture. The parent rock is a Iherzolite. 



Analysis of Serpentine from Presidio, San Francisco. 

 J. D. Easter, Analyst. 



Per Cent. 



Si02 39-60 



AUO3 1-94 



F^O I 8.45 



MnO j 



MgO 36.90 



Cr.03 20 



H2O 12-91 



100.00 



FeO in this case probably also includes FcsOa. 



"A. C. Lawson, " Geology of the San Francisco Penninsula," 15th Annual 



Report U. S. G. S., 445, Washington, 1893. 



" C. Palache, " The Lherzolite Serpentines and Associated Rocks of the 



Potrero, San Francisco," Bull. Geol. Dept. Univ. Cai, I., 164-169, 1894. 



