FAULTING. 75 



zone, as compared with the less fractured rock on each side. Such is probably 

 the case with the northeast depression at the southeast base of Brougher Mountain, 

 and with other creases in the surface. 



SCARP PHENOMENA WEST OF BROUGHER MOUNTAIN. 



Some especially interesting observations on the . surface configuration as an 

 indication of faulting were made in the comparatively flat area in the west part 

 of the district mapped, west and northwest of Brougher and Siebert mountains, 

 respectively. Here rhyolitic-dacitic breccias, chiefly detrital, are intermingled with 

 tuft's, so that they sometimes can be distinguished only with difficulty from the 

 main overlying Siebert tuff. Where the Siebert tuff is certainly distinguishable 

 the rectilinear intersecting boundaries show that complicated faulting has taken 

 place, but the mass of rhyolite-dacite breccias offered at first little suggestion as 

 to structural relations. 



When this area is viewed from an eminence, as from Brougher Mountain or 

 from the hill west of Siebert Mountain, just beyond the area mapped, there is 

 seen a significant series of parallel ridges which were at once surmised to indicate 

 the presence of faults. From the hill last referred to, these slight scarps are seen 

 to bound areas which have rectilinear outlines, and which are plainly distinguished 

 in tint from one another, one being purplish, another reddish, and so on. A minute 

 study strengthened the conjecture that in this region there are complicated and 

 numerous intersecting faults. It was concluded that these faults brought into 

 juxtaposition the Tonopah rhyolite-dacite breccia, the Fraction dacite breccia, the 

 Siebert tuff, or different parts of any one of these, and that the resulting erosion 

 brought out the harder blocks, which were thus bounded by straight scarps, usually 

 of slight relief. The Tonopah rhyolite-dacite breccia, being harder, nearly always 

 occupies the relatively elevated portions, while the soft, Fraction dacitic breccia and 

 the Siebert tuff lie in the depressions. These depressions are covered with a slight 

 thickness of detritus, but prospect holes show in almost every case that they are 

 floored with the softer breccias. The straight boundary lines are strongly con- 

 trasted with the irregular unfaulted contact of the glassy Tonopah rhyolite-dacite 

 in the north corner of the area mapped. 



DESCRIPTION OF ZIGZAG SCARPS. 



One or two of the most interesting occurrences of these slight scarps were 

 made the subjects of especial study. Between Siebert and Brougher mountains 

 the flat area floored by the dacitic breccias and by the Siebert tuffs reveals to 

 the close observer certain straight lines, which are apparently slight ridges and 

 depressions in the detritus, but which are really closely underlain by the soft 

 bed rocks, though these outcrop only occasionally. In this area the occurrence 

 of a number of faults was proved by stratigraphic evidence, chiefly by the 



