80 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



ACCURACY OF FAULT MAPPING. 



In this volcanic region faults can very often not be distinguished at all. This 

 is the case if similar rocks lie on both sides of a fault and other signs fail. There- 

 fore on the map some faults have been projected a reasonable distance and probable 

 connections made across spaces intervening between different fragments of what is 

 probably a single fault line. While the structure as finally depicted is undoubtedly 

 not strictly accurate in many details, the general features are well shown, and the 

 error, were a closer study possible, would undoubtedly be found to be not that too 

 many faults are represented, but that many have escaped detection. 



FAULTING DUE TO VOLCANIC ACTION. 



The faulting in this district is of extraordinary interest, for the origin, 

 time, and cause are clearly understood. It is rare that any explanation other 

 than a general unsubstantiated hypothesis can be applied to any particular case 

 of faulting. Here, however, it is plain that the faulting was the result of 

 adjustments of the crust to suit violent migrations of volcanic rock; that it 

 originated with the swelling up of the crust and its forcible thrusting up and 

 aside to make way for the numerous columns of escaping lava; and that after 

 the cessation of the eruptions it was continued by the irregular sinking of the 

 crust into the unsolid depths from which the lavas had been ejected. It can 

 readily be seen that all sorts of pressure (from below upward, lateral, and 

 downward, by virtue of gravity) must have been concerned in such movements, 

 and that the first faults were due rather to upward and lateral irregular thrusts, 

 while the later ones (in many cases along the same planes as the first) were 

 due to gravity. So reversed and normal faults are equally natural, and both 

 occur frequently. 



APPLICATION OF PRINCIPLES TO REGIONS LYING BEYOND AREA MAPPED. 



These observations are probably not of slight and local significance. The 

 faulting is intense, and the faults have frequently very great displacements, 

 amounting to many hundred feet at least. Moreover, considerable areas are 

 affected by subsidence or elevation connected with and in part, at least, accom- 

 plished by faulting, as, for instance, the relative depression of the southern part 

 of the area mapped (near the dacite necks), as compared with the northern portion. 

 The cause of these larger movements is plainly the same as that of the individual 

 faults. Evidently such phenomena are not confined to the area mapped, but 

 extend indefinitely beyond it. The writer at first looked upon the faulting at 

 Tonopah as exceptional and local, and not to be connected with ordinary 

 faulting in the Great Basin; but there now appears no reason for doubting 



