116 GEOLOGY OF THE EUEEKA DISTRICT. 



Granite.— North of the gTauite exposure at the end of Mineral Hill the 

 strata all dip nortliward, curving gently around the crystalline rock which 

 apparently has acted as a center of upheaving forces. The beds present a 

 broad anticlinal arch, less and less disturbedas they recede from the granite 

 and with a slightly decreasing angle of dip. The granite body occupies but a 

 small area on the steep slope of Mineral Hill. It is quite obscure in its sur- 

 face exposure, and might readily be overlooked but for its probable influ- 

 ence in producing the present geological features of the country. Fortu- 

 nately, it gives some clue to the peculiarities of stractiu-e. The age of 

 this granite is by no means easily determined," but the evidence seems to 

 show that it was a portion of an Archean island, around which the sedi- 

 ments were deposited. At some later period there was a movement of the 

 entire region, and the beds were uplifted and arched into their present 

 position around the granite. The exposure of the granite is wholly due to 

 erosion, and up to quite- a i-ecent date was covered with quartzite. There is 

 reason to believe that at the time the quartzite was deposited, a land surface 

 existed at no great distance, and this granite may have been connected with 

 it. Evidence in favor of such a supjiositiou was found near the bottom of 

 the Richmond shaft, 1,200 feet below the surface. The A'ertical shaft, after 

 passing through limestone as far as the seventh level of the mine, pene- 

 trates the quartzite for 500 feet. In a white, fine grained quartzite, small 

 pieces of rock were obtained, closely resembling granite. Although some- 

 what decomposed, the rock was found to be made up of quartz, mica, and 

 an altered highly kaolinized mineral, probably feldspar. 



Encircling the granite and resting directly upon it, occurs the Prospect 

 Mountain quartzite, followed in turn by the Prospect Mountain limestone, 

 Secret Canyon shale, Hamburg limestone, Hamburg shale, and Pogonip lime- 

 stone, the entire series of sedimentary beds exposed on Prospect Ridge. That 

 the Ruby Hill series of beds were once continuous with those of Prospect 

 Ridge there is no reason to doubt, ample evidence being found in the char- 

 acter of their sedimentation and the sequence of strata. The continuity 

 was broken only by profound faulting in much later times. As the quartzite 

 lies next the granite it is much broken up in the sharp turns which it is com- 

 pelled to make as the underlying rock of the arch. No dips or strikes can be 



