114 GEOLOGY OF THE EUEEKA DISTRICT. 



far as New York Canyon, its uniform and simple structure, while the 

 Eureka quartzite, on the other hand, occurs only here and there in irregular 

 patches cropping out from beneath heavy flows of hornblende-andesite, 

 which come to the surface along the line of the Hoosac fault. This profound 

 fault coming up from the south may be said to bifurcate at New York Can- 

 yon, the main branch swerving off to the northeast, retaining the name of 

 Hoosac fault, the other, trending to the northwest, being designated as 

 the Ruby Hill fault. Between these two lines of faulting lies a block of 

 uplifted beds, which in structure is in some respects quite independent of 

 the Prospect Mountain Ridge, a result probably brought about by the 

 dynamic forces which produced both the Ruby Hill and Jackson faults 

 and the rhyolite outbursts of Purple Mountain. This block is wholly made 

 up of Silurian strata, all three periods being represented. The Ruby Hill 

 fault may be traced on the surface from New York Canyon to its junction 

 with the Jackson fault by the numerous outbursts of rhyolite, whereas 

 northward along the Jackson fault no rhyolite has anywhere been observed. 

 As far north as Shadow Canyon the strata incline southwest toward 

 McCoy's Ridge, but beyond this canyon the dip and strike of the beds is 

 most irregular, in general dipping away from the Jackson fault and under 

 Purple Mountain and Caribou Hill. The greatest thickness of limestones 

 anywhere represented in this belt is about 2,700 feet, measured across the 

 strata from Shadow Canyon to McCoy's Ridge. The age of the limestone 

 underlying the quartzite of McCoy's Ridge is determined by the presence 

 of a Pogonip fauna characteristic of the upper horizons, serving also to 

 identify the quartzite which here forms such a persistent ridge along the 

 north side of New York Canyon. The trend of the ridge is determined in 

 part by the direction of the Hoosac fault and in part by the outbursts of 

 the lavas of Purple Mountain. The limestones overlying the quartzites can 

 be no other than the Lone Mountain beds. Although they cany no organic 

 remains, their geological position and lithological habit, quite like the Lone 

 Mountain strata immediately over the Eureka quartzite elsewhere, leave no 

 doubt as to their true correlation. It is the only exposure of Lone 

 Mountain limestone found in the uplift of Prospect Mountain Ridge, but 

 owing to the want of well denned lines of stratification no reliable estimate 



