372 University of California Publications in Geology [Vol. 10 



The analyses cannot be considered to represent the composition of 

 the original rock, for all of the specimens collected have suffered 

 partial alteration. The high content of silica and low percentage of 

 lime, with a rather high content of Na to that of K, indicates that the 

 rock is a soda rhyolite. The rather constant percentage of A1,0 3 in 

 the three analyses given above is of interest because in all the sections 

 studied of this rock free kaolin was observed as an alteration product. 



Fig. 2. Plan of mine workings. 



PYRITE DEPOSIT 



Location. — Near Leona Heights, east of Oakland, deposits of pyrite 

 occur in the rhyolite which have been mined for a number of years. 

 At the present time two mines are in operation, one having just re- 

 cently started. One of these mines, once known as the Leona Heights 

 Mine, was described in 1911. 11 The larger mine, operated by the 

 Leona Chemical Company, is the principal source of information con- 

 tained in this report. 



ii Mace, Clement H., Min. and Eng. World, vol. 35, p. 1320, 1911. 



