244 CKOLOGY OF THP] EUEEKA DISTRICT. 



the Cambrian and Silurian of Prospect Ridge on the one side, and the 

 Silurian and Devonian of County Peak and Silverado Mountain on the 

 other. These profound faults, as already described, show nearly 2 miles 

 of vertical displacement. Fissures accompany these faults and through 

 them vast masses of lavas have reached the surface and poured out along 

 both sides of the fault planes. In places the lavas are found only in narrow 

 belts without any great accumulation of material and in others they are 

 piled up in rounded hills and knolls of irregular outlines, concealing every- 

 thing beneath them for long distances. In general acid lavas accumulate 

 near their source of eruption while basic lavas, owing to their greater fluid- 

 ity, show a tendency to flow from their vents in broad masses. Along the 

 Hoosac fault from Fish Creek Valley to its junction with the Ruby Hill 

 fault, a continuous body of acid lavas, either rhyolites or hornblende-aude- 

 sites, follow the course of the fault, but beyond their junction no lavas come 

 to the surface along the Hoosac, although they persistently follow the course 

 of the Ruby Hill fault as described in detail in the chapter devoted to the 

 discussion of the ore deposits of the District. Along the latter fault the 

 rhyolites do not form a continuous surface overflow, but break out in 

 isolated knolls all the way from its junction with the Hoosac to the Jackson 

 fault, beyond which, on Ruby Hill, the lavas never reach the surface, 

 although their presence is shown by underground mining galleries. West- 

 ward from the Hoosac fault volcanic energy slowly died out. 



Following the trend of the Pinto fault from the southern end of the 

 mountains, rhyolitic pumices and tuffs define the general course of displace- 

 ment all the way to Dome Mountain. This light porous material has, by 

 gradually welling out along the fissure, heaped up bosses and knolls of vol- 

 canic products, but owing to their peculiar physical habit they have eroded 

 more easily than the denser rocks and present a more broken, undulating 

 surface. Isolated outbursts of pumices occur, penetrating the sedimentary 

 strata on both sides of the fault, and along Wood Valley extend far back 

 into the Devonian limestones. Northward of Dome Mountain the pumices 

 and tuffs again come in, but are finally lost beneath the imposing mass of 

 pyroxene-andesites of Richmond Mountain. Along the Pinto fault only 



