1917 ] Clark: Geology and Ore Deposits of the Leona Rhyolite 367 



Numerous faults of smaller dimensions and probably intimately associ- 

 ated with the Haywards fault have been observed in the area. These 

 minor faults are oblique, parallel, and transverse to the Haywards 

 fault. The northern exposure of rhyolite is cut off at Hamilton gulch 

 by a fault that is transverse to the major movement. This portion of 

 the rhyolite is also faulted against the Chico and Knoxville formations 

 along most of its eastern boundary. At Leona Heights two faults 

 more or less parallel to the Haywards fault and undetermined in 

 extent have aided in producing the geomorphic features of this locality. 

 South of Lake Chabot and again south of Haywards oblique faults 

 extend in a southeasterly direction from the main fault. These ob- 

 lique faults are probably connected with a main fault, approximately 

 parallel to the Haywards fault, that traverses the country from 

 Lake Chabot southeasterly. 



In general the relative movement along the fault plane appears to 

 have been horizontal rather than vertical. At Temescal Lake quite 

 extensive masses of gouge and breccia are exposed at the surface in a 

 railroad cut that traverses the fault for a short distance. It does not 

 seem likely that this amount of gouge was produced by any vertical 

 movement that could have taken place here. It is quite certain that 

 a considerable movement has occurred along the Haywards fault in 

 recent geological time, as is evidenced by the diversion in the direction 

 of the streams that flow transverse to the fault valley. Any statement 

 as to what effect the faulting has had in the geomorphogeny of the 

 region, the number of movements and the geologic epoch in which they 

 occurred would be largely conjectural. But there probably have been 

 several periods of movement along this fault zone in post-rhyolite time, 

 and there is a possibility that the rhyolite may have been extruded at 

 some point on the zone. 



GENERAL FEATURES OF THE RHYOLITE 



The Leona rhyolite reaches its maximum width of one and one-half 

 miles a little north of the F. C. Talbot ranch, east of Elmhurst. The 

 lava varies in thickness, but reaches a maximum of perhaps 600 feet. 

 Where fresh the rhyolite is light green to bluish in color and varies 

 in texture, being glassy and porphyritic in the northern part, while 

 towards the southern end it is less porphyritic and becomes more 

 felsitie. The rhyolite is everywhere studded with pyrite crystals and 

 contains very few ferromagnesian minerals in the fresh rock. Certain 



