72 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



Erosion. 



Siebert tuffs (lake beds) deposited, with an occasional thin dacite flow. 



Elevation of tuffs. 



Tilting. 



Basalt. 



Chief faulting. Affects eyerything preceding. 



Rhyolite intrusion (Ararat, Oddie, Rushton hills). 



Vein formation. Primary minerals, quartz, chalcedony, calcite, siderite, pyrite, etc. Values 



low; gold and silver, gold apt to predominate. 

 Brougher dacite intrusion (Butler, Brougher, Golden, Siebert mountains). 



Mineralization (chalcedony, manganese). Values slight to insignificant. Mud veins. 

 Erosion. 



Latest rhyolite-dacite flow (slopes of Oddie and Brougher). 

 Erosion. 



PRINCIPLES OF FAULTING. 



The chief recognized faulting of the district has already been described (p. 47) 

 as attendant and consequent upon the Brougher dacite intrusion. The writer deems 

 it unnecessary to attempt to describe separately the evidence and effect of each 

 fault. Their locations and the general nature of their displacement are shown on 

 the areal geology map. Their underground courses and intersections are doubtless 

 complicated, and their study would constitute a geometrical problem in three 

 dimensions for the solution of which there are in most cases no sufficient data. On 

 account of the irregular thickness and extent of each of the volcanic formations 

 at Tonopah, projection far beyond actual observation can not safely be made; so 

 no general cross sections have been constructed. 



Valuable observations on faulting have been made underground, however, in 

 some of the mines, especially where veins have afforded measures of displacement. 

 It has been found impracticable to separate the account of such faulting from the 

 discussion of the veins which they affect, so the reader is referred to such 

 discussions, particularly to those concerning the Fraction, Wandering Boy, Valley 

 View, Mizpah, and Montana Tonopah workings (pp. 115-176). 



CRITERIA OF FAULTING. 



It is worth while to record the manner in which the structure has been 

 worked out in this complicated region. Although the region mapped embraces 

 only about 6 square miles, and outcrops are very nearly continuous, several months 

 of study were necessary to reach an approximately satisfactory solution of the 

 areal geology. Ideas concerning the structure were successively exchanged for 

 newer ones as fact after fact was brought to light. The existence of faulting was 

 strongly suspected, from topographic; evidence, from the time of arrival in the field, 



