EXPLANATION OF SECTIONS. 271 



stones, as a continuation of the lower plieet of Pyritiferous Porphyry which 

 forms the bed of California gulch above the Pilot fault. West of Mike 

 fault the contact has been carried back at the angle shown in the develop- 

 ments of the Oro La Plata mine, and the intrusive body of Gray Porphyry, 

 which cuts across it at the mouth of the Oro La Plata tunnel, is repre- 

 sented as probably thinning out to the etistward. It is possible, however, 

 that thei contact basins up toward the Mike fault, as it does on Iron Hill 

 and the Gray Porphyry sheet may have an underground connection with 

 the Printer Boy Porphyry, which it somewhat resembles, and both come 

 up through the same channel or vent. West of the Dome fault the west- 

 ward slope of the beds in the lifted-up block of ground between the Robert 

 Emmet and Iron faults is shown. Beyond the Iron fault the only data 

 obtained from shafts are the relative positions of the Wash, Lake beds, and 

 underlying porphyry. 



Section H. — Scction II is taken along a straight line running from the 

 bed of Iowa gulch, on the eastern border of the n]ap, through Printer Boy 

 Hill and down the bed of Georgia gulch. The three eastern blocks do not 

 differ sensibly from those of the preceding section, except that, as proved 

 by actual developments on Printer Boy Hill, the White Porphyry above 

 the Blue Limestone is exceedingly thick, for which reason its thickness in 

 the adjoining block to the eastward is proportionally increased over that of 

 Section G. The cross-cutting of the White Porphyry comes just west of 

 the Weston fault, and, though entirely below the surface, is not wholly theo- 

 retical here, but proved by developments of adjoining mines. The anticli- 

 nal structure of Printer Boy Hill, the intrusive sheets of porphyry, and the 

 three vertical dikes are also proved by actual observation. The conver- 

 gence of the planes of the Mike and Pilot faults is a theoretical deduction 

 founded on the theory of fault planes by observers in other parts of the 

 world. The existence of Lake beds at this height is proved by the data 

 afforded by the explorations of the Printer Boy mine. The depth shown 

 for the Blue Limestone here may possibly be too great, since it is obtained 

 by carrying back the angle of dip at the surface near the Dome fault, and 

 it is very possible that the beds turn iipwards towards the Mike and Pilot 

 faults, under the influence of the Printer Boy anticline. The thickness of 



