20 SILVER-LEAD DEPOSITS OF EUREKA, NEVADA. 



Quartz-porphyry eruption. — About a mile and a quarter to the north of Ruby 

 Hill, and beyond Adams Hill proper, there has been an eruption of quartz- 

 porphyry which covers many acres. If this eruption took place at the time 

 of the folding and upheaval to which Prospect Mountain and Ruby Hill 

 owe their origin, it would account for the deflection to the northwest of the 

 different formations found on Ruby Hill. Whether the eruption of this vol- 

 canic mass actually caused the bending and twisting before mentioned or 

 not, the fact remains that these formations were so deflected during the up- 

 heaval, or subsequent to it, that they lie nearly at right angles to the posi- 

 tion they would have occupied had they not been subjected to some other 

 force than that of simple upheaval along their axis of fold. That pressure 

 was exerted from some point to the north of Ruby Hill is clearly proved 

 by the marks of striation observable at various points on the walls of the 

 cross-faults, or those faults which in many places traverse the limestone of 

 Ruby Hill in a northerly or northeasterly direction. These striation marks 

 usually dip to the northeast, which would indicate that the lateral force had 

 been applied from that direction. 



Faults. — It cannot be said that, all the fault-fissures occurring on Ruby 

 Hill have one general course, but they can be divided into two general sys- 

 tems; the first consisting of those which are approximately parallel to the 

 strike of the formations and which were produced entirely by the folding 

 and upheaval, and the second made up of those which were caused by the 

 same forces supplemented by a strong lateral pressure. These two systems 

 of fissures are mostly to be found in the limestone. To what extent they 

 occur in the quartzite cannot be determined, as the workings in that rock 

 are not very extensive, but it is probable from the nature of that formation 

 that they are not so frequent. There are numerous instances, however, of 

 cross-fissures faulting the quartzite, and this is particularly the case in the 

 Richmond, and will be more fully discussed when the quartzite in that mine 

 is examined. Cross-faults have been noticed in the shale, but they are not 

 so easily detected there, owing to its tendency to bend rather than to break. 

 There are several examples of fissures of the first kind which fault both 

 limestone and shale. 



