GENERAL TECTONICS OF THE DESERT REGION 547 



Eliminating for the moment the mastering effects of general and local 

 desert-leveling and confining the attention to tectonics alone, we find 

 that there are discernihle throughout the entire arid region two general 

 and distinct systems of geologic structures. These are widely separated 

 from each other in point of time. The younger of them may be regarded 

 as mainly produced during the late Tertiary period, while the older may 

 be considered as having been impressed during Jurassic times. 



In general, it may be said, the expression of the younger system of tec- 

 tonics is mainly that of faulting ; that of the more ancient that of flexing. 

 That the two are widely separated in point of time is a fact which usually 

 does not appear to have been clearly distinguished in any part of the 

 desert region, and especially in the Great Basin province, where the tec- 

 tonics of the arid country has been chiefly studied. In this last named 

 province it is the older system of tectonics that is most conspicuous in 

 the bare mountains and that is most clearly open to observation. 



Nature of the Younger Tectonics 



As already noted, the most characteristic feature of the younger system 

 of tectonics is the fault. Usually it is assumed that there is only a single 

 line of dislocation blocking out each mountain ridge. In reality, the 

 total amount of displacement is attained by compound or repeated fault- 

 ing. The accompanying effects are imposed on all other older geologic 

 structures. Their immediate influence is to disguise more or less com- 

 pletely the older tectonics. 



When of very late occurrence faulting may make itself conspicuous in 

 the landscape; in all other cases the topographic effects are quite gener- 

 ally obscured by erosion. Complete mastery of the acquired geologic 

 structures by erosion appears to be especially true of the G-reat Basin 

 region. 



In the light of the newest observations it is somewhat difficult at the 

 present time to say with confidence just how far recent folding has im- 

 pressed itself upon the strata of the Great Basin province. So far as my 

 own investigations indicate, the effects appear to be very slight and local. 

 In those localities in which flexing has been regarded as best displayed 

 careful inquiry has in every case failed to substantiate the claims. It 

 is concluded that most of the Appalachian structures ascribed to the 

 ranges of this region will be found to belong mainly to the older period 

 of mountain building instead of to the younger period. 



There are several localities in Nevada in which gentle folding is re- 

 garded by some observers as having taken place during Late Tertiary or 



