100 GEOLOGY OF THE EUEEKA DISTRICT. 



Along the east side of the Hoosac fault no sedimentary rocks are 

 known other than those belonging to the Lower Coal-measures, while on 

 the west side of the other three faults only Silurian and Devonian beds are 

 brought up against the fault line. In this uplifted mountain mass lying 

 between these great lines of faulting occur all the Cambrian rocks exposed 

 in the district, with the exception of two small patches of limestone, one of 

 Prospect Mountain limestone and one of Hamburg limestone, found on the 

 west side of Surprise Peak, one-half mile to the westward of the line of 

 the Sierra fault. They occur in a region of much local disturbance, not far 

 from the body of hornblende-andesite which occupies the bottom of Sierra 

 Valley, and are of no special geological interest otherwise than indicating 

 great displacement of strata. This uplifted mass of Prospect Ridge meas- 

 ures 10 miles in length by about 2 4 miles in width across its broadest expan- 

 sion, in the region of Prospect Peak, in places narrowing to one-half that 

 distance. Within this block, evidence of minor fractures and dislocations 

 are everywhere to be seen, influencing in a greater or less degree the geo- 

 logical structure of the country. 



Jackson Fauit.-Two faults, designated as the Jackson and the Ruby 

 Hill, profound in their displacement and of great economic importance, 

 deserve special mention ; both of them, however, lie within the limits of the 

 Prospect Ridge uplift. The Jackson fault starts in just north of the Eureka 

 tunnel, in Goodwin Canyon, on the east side of the ridge, and may be 

 traced northward along the line of contact between Prospect Cjuartzite and 

 the Hamburg limestone (atlas sheet viii). It follows down the narrow ravine 

 past the Jackson mine to the east of Ruby Hill and Adams Hill, and is lost 

 near the body of quartz-porphyiy just beyond the Wide West ravine. This 

 fault brings up the Pogonip limestone of the Silurian against the entire 

 series of Cambrian strata of Rub}' Hill, from the lower quartzite to the 

 Hamburg shales inclusive. On the east side of this fault exploration has 

 failed to bring to light any large and permanent bodies of ore, if we except 

 that of the Williamsburg mine ; the more valuable mining properties in the 

 inunediate neighborhood of Ruby Hill being, for the most part, on the west 

 side of the fault in the Cambrian rocks. 



