74 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



wise abundantly verified), and that the dikes running out from the volcanic centers 

 occupied at times fault zones. These dikes were then traced, and when they were 

 conspicuously straight and narrow their course was critically examined to deter- 

 mine whether it could possibly be a fault plane. Often such dikes are intermit- 

 tent, appearing only in small outcrops here and there along the line, with no 

 visible connection. Such a condition was still more strongly suggestive of a 

 fracture zone. Frequently the examination of the rocks on both sides of such a 

 line confirmed the suspicion of faulting, and important faults were discovered in 

 this way. 



BOUNDARIES OF LAVAS. 



A.S the knowledge of the different formations increased it became possible to 

 draw their boundaries with frequently great accuracy. Where these were recti- 

 linear, as in the case of the tuft's, and could not reasonably be interpreted as 

 normal contacts caused by the outcropping of inclined formations, and one for- 

 mation could not have been intruded into the other, faults were considered to be 

 indicated. Even in the case of two volcanic rocks, like the earlier and later 

 andesite on Mizpah Hill, the boundaries, though obscure and traceable with 

 difficulty on the surface, could finally be determined to be rectilinear, intersecting, 

 and probably due to faulting. In this case the veins afforded valuable evidence, 

 for their outcrops were cut clean off along the fault planes. 



EROSION FAULT SCARPS. 



As the perception of the real connection between the strati graph}' and structure 

 and the topography grew, the latter often became an efficient guide. The underlying 

 rocks have exercised a remarkably efficient control over the surface forms. Where 

 two rocks of unequal hardness are brought together by faults, the harder rock will 

 rise above the softer in a more or less perceptible scarp. With the exception of the 

 rhvolites and the Brougher dacite, and to a less degree of the silicified earlier 

 andesite, however, the difference in resistance of the rocks is not great. The 

 Fraction dacite breccia and the glassy Tonopah rhyolite-dacite in the southern part 

 of the area mapped are chiefly friable fragmental surface deposits, while the later 

 andesite disintegrates rapidly. The Siebert tuff is softer than the others, and 

 when sufficiently removed from the influence of a protecting harder rock, forms 

 flat, smooth areas, on whose boundaries fault contacts are apt to be marked by 

 slight but pronounced scarps, usually only a few feet high, since the adjacent 

 rock is apt to be very little harder. These slight scarps afford strong preliminary 

 evidence, and invite the closest searching after stratigraphic corroboration. 



Nearly every topographic feature in the Tonopah district, however small, is 

 due to the nature of the underlying rock; thus many straight depressions or 

 slight valleys are probably due to the easier erosion of a fractured or faulted 



