246 GEOLOGY OF THE EUREKA DISTRICT. 



by recent Quaternary aceuuiulations or else buried beneath Inroad masses 

 of lava. These latter overflows of lava, breaking out along- the base of 

 the escarpments, follow a somewhat sinuous course, yet cling most per- 

 sistently to the border line of the uplifted area, and xnrj greatl}' in the 

 amount of extravasated material and the manner in which they pile up at 

 the surface along lines of fracture. 



The mo.st striking illustration of this mode of occurrence may be 

 seen in the lavas surrounding the County Peak and Silverado block. 

 Along the Pinto fault, which deflnes these mountains on the west, the lavas 

 closely follow the fault, except for the short distance, already mentioned, 

 northeast of Dome Mountain. Richmond Mountain encircles the block on 

 the north, followed on the northeast by the broad basaltic flows of Basalt 

 Peak and the Strahlenberg, which lie between County Peak and the Dia- 

 mond Range. To the east every indication points to the occurrence of la\-as 

 beneath the alluvial deposits of Newark Valley, while the Rescue Canyon 

 overflows continue southward to the open valley, completing the circuit in 

 this direction. Facing Fish Creek Valley, bosses and knolls of both acid 

 and basic lava stretch westward in suflicient number to plainly suggest a 

 continuoiTs outbreak of igneous inaterial along the southern base of the 

 uplifted block of Silurian and Devonian limestones. 



In the case of the depressed Carboniferous area the bordering lines 

 of lava following the Hoosac and Pinto faults are clearly made out. 

 Between the Spring Hill and the Carbon Ridge bodies there is a sharp 

 break in the Carboniferous strata, along which acidic lavas have broken 

 out, 'crossing obliquely from the one great fault to the other. On the north 

 side of the Spring Hill body, Richmond Mountain cuts ofl" everything to 

 the northeast. The volcanic pumices and tuft" stretch westward under the 

 town of Eureka and terminate finally in the rhyolites of Purple Hill. To 

 the south of the Carboniferous area pumices and tuff's abut against the base 

 of Carbon Ridge, skirting the foothills, unless concealed beneath recent 

 deposits. South of Gray Fox Peak and Carbon Ridge there is a long line 

 of secondary ridges, now completely covered by Quaternary deposits. It 

 is easily seen that their trend is wholly out of accord with the line of the 

 Paleozoic uplifts, but is precisely what we might expect to find if the lavas 



