REVIEWS 421 



dominantly Tertiary lavas, with interbedded sandstones. Faulting is 

 extensive, and the fault fissures have been mineralized. In Quaternary 

 times erosion was great and thick gravels accumulated ; the gravel depo- 

 sition was followed by more faulting, the fault plane paralleling the 

 present front of the range, with downthrow on the west. Renewed 

 erosion followed this last orogeny. The Tertiary formations, chiefly 

 igneous, reach 8,000 feet in thickness. The flows are chiefly andesites 

 and rhyolites, though there are a few basaltic lavas. Of the total 

 thickness mentioned, 6,400 feet are pyroclastics and flows; the remainder 

 are stream-laid sediments. Erosional unconformities within the se- 

 quence are common. Intrusives are represented by dikes of rhyolite 

 and basalt. 



Structurally the area is affected by complex normal faults; two 

 periods of faulting are recognized — one before, the other after, the deposi- 

 tion of the rival gravels. The former was far more widespread, and 

 nearly all the fault fissures of this period are now the site of fissure veins 

 trending roughly north-south or northwest-southeast; one of these — 

 the most prominent — can be traced for a length of seven miles, and the 

 fault involves a displacement of a thousand feet or so. The first faulting 

 resulted in increase erosion and the development of mature topography; 

 on the west slope of the region there was at this time a broad valley, 

 which later became filled with gravel to a depth of several hundred feet. 

 Then came the great fault that now defines the front of the range; this 

 renewed erosion and initiated the present cycle with its higher flat 

 benches and lower steep canyons. 



The ore deposits of the region are all in veins closely connected with 

 the faults. The richest are the small ones lying along the fault fissures 

 that are distributive from the great seven-mile fault (Queen fault) 

 mentioned above. Locally the mineralization shifts from the main 

 fissure to minor fissures in the wall, indicating that mineralization was 

 not contemporaneous with the faulting. The mineral content of the 

 veins includes quartz, calcite, a little adularia, and flourite (locally 

 plentiful). Argentite, with small amounts of associated pyrite, chal- 

 copyrite, bornite, galena, and sphalerite are the metaliferous primary 

 minerals; in a few veins copper minerals — bornite, chalcopyrite, chalco- 

 cite, and tetrahedrite — predominate. Quartz and calcite are the chief 

 gangue constituents. Oxidation is shallow and irregular. Sulphide 

 enrichment appears to have been somewhat effective. The oxidized ore 

 bears cerargyrite, native silver and gold, basic copper carbonates, 

 limonite, copper pitch ore, cuprite, and manganese oxides. Chalcocite, 



