348 University of California Publications. [Geology 



posed in artificial and natural cuttings, and is there shown to 

 be well stratified and to have a thickness several times greater 

 than the estimate given above. The tuff has been quarried to 

 some extent at various croppings for the building of fire boxes 

 for steam boilers at the various mines of the district. 



Above the tuff lies a sheet of black glassy porphyritic lava 

 having the greasy luster of a pitchstone. This lava is best ex- 

 posed on the north slope of the hill but its thickness could not 

 be satisfactorily determined. It may be 40 feet. Above this the 

 bulk of the hill is composed of a purplish red lava of lithoidal 

 aspect with abundant phenocrysts of feldspar and black quartz. 

 This rock extends to the summit of the hill where a small quarry 

 has been worked which gives its name to the hill. This purple 

 lava is traversed by structural planes of parting which appear 

 to be parallel flow planes of the lava and dip to the northeast 

 at angles of about 23°, indicating a tilting of the formation 

 since its solidification. Traversing these flow and parting planes 

 are several well marked jointages. The best defined system 

 strikes N. 65° E. with a dip of 70° to the south-southeast. The 

 next best defined system has a similarly steep dip to the south- 

 west. In the absence of all suggestion or evidence of compres- 

 sive stresses, these joints are most simply explained as due to 

 relief from tensile stresses. There is no columnar structure in 

 the rock. 



At Copper Flat village and a little to the north of it there 

 are three outlying patches of the purple variety of the lava 

 lying partly on the Arcturus limestone and partly on the por- 

 phyry. The largest of these lies on both sides of the main street 

 of Copper Flat but chiefly on the east side. It has a length from 

 north to south of about 1,000 feet and a width of about 500 feet. 

 Neither the black pitchstone nor the white tuff appear to be here 

 present beneath the purple rhyolite. 



The second important area of rhyolite is found on White Hill, 

 the summit of which is about a mile and a half from Copper 

 Flat to the southwest. This is a roughly triangular area, the 

 longest side of which is to the north and is determined by a 

 fault which has dropped the rhyolite against the porphyry, the 

 Ely limestone and the Arcturus shaly limestone. It is nearly 



