78 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



or jogs, although the detail is somewhat rounded by erosion. It runs chiefly in 

 two directions N. 60-70 E. and N. 30-40 W. This zigzag course, and the 

 absence of the scarp on the south side of the depression, as in the case of the 

 occurrence previously described, seem to indicate a complex fault fracture, and 

 the directions of the rectilinear components in each case are similar. In this 

 case also the indentations are not due to gulches, for there is usually not the 

 slightest depression at the top of the scarp, at the angles. The scarp continues 

 beyond the area mapped. The general trend (being the resultant of the two 

 directions noted) is almost exactly parallel to the similar scarp previously described. 



ORIGIN OF ZIGZAG FAULT SCARPS. 



From the general sum of knowledge concerning the relation of faulting to 

 topography in this district (see p. 114), it is inferred that probably these slight 

 scarps are due to differential erosion and mark the limits of fault blocks which are 

 slight^' harder than those contiguous. Their invariable slight relief strengthens 

 this idea. Similar scarps, which have been proved to have originated in this 

 manner, are characteristic of fault contacts in other portions of the area mapped. 

 The other possible hypothesis is that the faults are recent, and that the scarps 

 have formed as a result of direct displacement of the surface. In spite of the 

 fact that the probabilities seem to favor the first explanation, certain features 

 support the second. One of these is that scarps of this sort, like those just 

 described, sometimes have on each side material belonging to the same formation, 

 as the scarp marked B in PI. XII, which has tuff on both sides, or, as the scarp 

 last described, on the Tonopah-Sodaville road, which has the glassy Tonopah rhyo- 

 lite-dacite on both sides. If these surface features are due to erosion, the higher 

 block must be slightly harder than the lower and must represent a slightly more 

 resistant part of the formation. This indeed is true in the place last mentioned, 

 where the glassy Tonopah rhyolite-dacite in the area north of the road is the solid 

 intrusive lava, while the formation included under the same head in the region 

 south of the road is surface material, breccias and tuffs, and therefore more fragile 

 and more easily eroded. Another circumstance which also favors the idea of 

 direct displacement is that the two chief compound scarps just described both 

 face the south. It is known from independent evidence that the southern part of 

 the area mapped has been downthrown in respect to the northern part, so that a 

 slight continuation of the general movement into very recent times might result 

 in these south-facing scarps. 



