362 University of California Publications in Geology [Vol- 10 



CHAEACTER AND PURPOSE OP THE WORK 



During the fall of 1915 and part of the winter of the year 1916 

 the writer spent considerable time in the study of the Leona rhyolite 

 as to its extent, its petrographical character, the nature of the surface 

 on which it rests, its structural features, its age and the ore deposits 

 in it. 



Professor A. C. Lawson 1 in the San Francisco Polio has given a 

 brief account of the rhyolite and its relations and mapped its distri- 

 bution. He also devotes a paragraph to the description of the ore 

 deposits. It is the purpose of this paper to give a more detailed 

 description of this very interesting feature of Coast Range geology. 2 



PHYSICAL FEATURES OP THE AREA 



The area occupied by the Leona rhyolite is confined to a portion 

 of the front of the Berkeley Hills, which form the steep northeast 

 boundary of the valley of San Francisco Bay. The cities of Berkeley 

 and Oakland are in part located on the foothill slopes, but for the most 

 part they are situated on the alluvial embankments, in part terraced, 

 at the base of the hills. These embankments are now being dissected 

 by the streams which formed them. The front of the hills has a 

 straight northwest-southeast trend and is continuous in its abrupt 

 elevation above the bay plain from San Pablo to the town of Hay- 

 wards, except for a few canons which dissect it. A remarkable break, 

 however, occurs at Haywards, where Castro Valley forms a flask- 

 shaped einbayment connecting by a narrow gap with the Valley of 

 San Francisco Bay. 



The rhyolite, being more resistant to erosion than the surrounding 

 formations, stands out in relief along the front of the hills from its 

 northern end at Berkeley to the vicinity of Haywards. But south 

 of this point it is less prominent and conforms more to the aspect of 

 the topography of the adjacent rocks. The rhyolite is everywhere 

 studded with crystals of pyrite, the oxidation of which has given the 

 characteristic reddish color to the formation. In certain sections 

 where the outcrop of the rhyolite is obscure, the color of the soil 

 derived from it was of material aid in determining its boundary. 



i U. S. GeoL Surv., San Francisco Folio, no. 193, 1914. 



- The writer wishes to express his thanks to Professor A. C. Lawson for 

 helpful suggestions and criticism in the preparation of this report, and also to 

 Mr. D. A. McDonell, General Manager of the Leona Chemical Mine, and to Mr. 

 J. M. Conlin, Mine Superintendent, for many courtesies extended during the 

 examination, of the mine. 



