RHYOLITES AND OKE DEPOSITS. 293 



30,000 feet. Between the close of the Carboniferous and the close of the 

 Jurassic period dynamic action folded and faulted the strata, producing the 

 present complex structure and outlining the configuration of the mountains 

 much as they are found to exist to-day except such changes as have 

 been produced by denudation. Soon after the coming in of Tertiary 

 time the volcanic period began in the Great Basin, and probably not 

 long after it volcanic energy manifested itself in the Eureka Mountains. 

 Evidence seems to show that the profound displacements were augmented 

 by intrusive rocks, and in many instances fissures were formed along the 

 fault planes. Accompanying the fissuring of these faults by volcanic lavas 

 was the forming of lateral and oblique secondary faults, cross fissures, and 

 fractures, complicating the already disturbed sedimentary beds. 



After the outbursts of andesites and rhyolites, and possibly in part 

 subsequent to the basalts, the deposition of the ores took place. The basalt 

 is known to follow the rhyolite. As regards the relative age of the ores and 

 basalt, there is no direct evidence other than that in the region of ore 

 bodies the audesites and rhyolites show the action of steam and solfataric 

 agents, whereas the basalts are for the most part comparatively fresh and 

 unaltered. In a number of instances it is clearly evident that the ores 

 followed the rhyolite intrusions, the former being found to lie wholly undis- 

 turbed upon the latter rock. It is true that over the greater part of the 

 region the ores do not come in direct contact with the rhyolites, but, on the 

 other hand, all evidences of pre-rhyolite ore deposits are wholly wanting, 

 and it is hardly conceivable that there could have been such deposits with- 

 out some evidence of movement at a time when the region was undergoing 

 strain and dislocation on all sides. Furthermore, nowhere, so far as known, 

 does the network of dikes on Prospect Ridge cut any earlier ore body. 



Since the intrusion of the innumerable rhyolite dikes there is no 

 evidence of any orographic movement of sufficient intensity to disturb or 

 dislocate them by faulting of the strata, and the same may be said of 

 the ore deposits. This gives both to the dikes and ores a comparatively 

 recent origin in the geological history of the region. As regards the ores 

 it should be stated that they have undergone alteration and oxidation since 

 their deposition, and, as much of the loose, friable material occurs in lime- 



