16 GEOLOGY OF THE EUEEKA JHSTKICT. 



cumulation of these lavas poui-ed out along the line of displacement, in 

 places concealing' the underlying rocks for considerable distances on both 

 sides. Within certain limits, however, there is no great difficulty in de- 

 termining its main com'se, as on the one side only Silurian rocks occur, 

 while on the other all the beds known to be in their true structural position 

 belong to the Lower Coal-measures. At the southern end of the moun- 

 tains, where the sedimentary beds emerge from beneath the Quaternary, 

 the fault is completely obscured by rhyolite flows that flank the slopes of a 

 long ridge of Eureka quartzite, the uppermost member of the Prospect 

 Ridge series just to the westward. Opposite Pinto Peak, where the rhyo- 

 lite flows are of exceptional width and of great thickness, no indications of 

 its trend are visible, and not until east of Hoosac Mountain do the sedi- 

 mentary rocks rise above the rhyolite. At Hoosac Mountain occurs the 

 only case of Silurian beds found on the east side of the fault line, and this 

 is more apparent than real, as it is rather an instance where a body of 

 quartzite has been thrust eastward by powerful volcanic forces and lies 

 superimposed eitlier upon igneous rocks or a body of Carboniferous lime- 

 stone. It is probably only a thin capping of quartzite, and evidently out 

 of place, as just eastward of it the limestones may be seen in their true 

 position. 



Proceeding northward the Eureka quartzite, at the base of Hamburg 

 Ridge, marks the faidt on the west, and in direct contact with it lies the 

 Lower Coal-measures of Spring Hill Ridge, a contact which is maintained 

 nearly to New York Canyon, onlv here and there slightly obscured by 

 Quaternary accumulations. At New York Canyon the fault l:)ifurcates, one 

 branch turning to the northeast and the other to the northwest, the easterly 

 branch being the main one and retaining tlie name, Hoosac fault. The 

 fault trending to the northeast still continues to mark the boundar}- between 

 the Silurian and Carboniferous, following the course of New York Canyon, 

 and from here northward the contact is nowhere obscured Ijy outbursts of 

 lava, the Lone Mountain Silurian of McCoy's Ridge being found on the 

 northwest side of the displacement, with the Lower Coal-measures on the 

 southeast. A short distance beyond the entrance to New York Canyon, 

 near the Richmond siui-ltini;- works, the fault ceases to be traceable toward 



