FAULTING. 81 



that the phenomena within this small, carefully studied area are typical of the 

 unstudied similar volcanic region beyond the limits of the map. 



The individual faults have been shown to have been minor, irregular 

 movements attending broader elevations or depressions; and the hypothesis has 

 been presented that at an earlier period the lake basin in which the Siebert 

 tuffs were laid down was formed by general subsidence of an area that was 

 occupied by earlier eruptive rocks (the earlier dacitic eruptions) and that this 

 basin was destroyed by a broad uplift which preceded the later dacitic outbursts. 

 There is little doubt that these earlier movements were attended by some faulting, 

 although such faults would be difficult of detection, especially in the presence of 

 the subsequent complicated faulting of the period of the later dacitic intrusions. 



SUGGESTED EXPLANATION OF GREAT BASIN TERTIARY DEFORMATIONS. 



The recognition (pp. 52, 70) of the facts that the lake in which the white tuffs 

 were laid down was a very large one, and that it very likely corresponds to the 

 great Miocene Pah-Ute Lake of King, gives a broader interest to this hypothesis 

 of its origin; and the hypothesis naturally extends itself to the other Tertiary 

 lake basins which preceded and followed the Pah-Ute. 



In the great interior province in which Tonopah is situated, and which lies 

 between the Wasatch and the Colorado Plateau on the east and the Sierra Nevada 

 on the west, a number of successive lake basins of van-ing extent formed during 

 the Tertiary, as was first shown by King. These changing basins, of varying 

 shape and extent, were due to uneasy continual warpings (elevations and depressions) 

 which continued through the Tertiary period down to the present day. This 

 warping has been contemporaneous with folding and faulting, and all of this 

 crustal disturbance has been accompanied by volcanism. 



"In general the period of deformation which lasted from the Mesozoic to the 

 present has been contemporaneous with volcanic activity. By far the most energetic 

 vulcanism, so far as we know, occurred in the Tertiary, beginning probably in late 

 Cretaceous or early Eocene and extending into the Pleistocene. Vulcanism and 

 deformation were, therefore, allied phenomena."" 



In the earlier recognition of this coextension of the two phenomena of deforma- 

 tion and volcanism the writer's conception was that they were both the result of a 

 single unknown cause. In the light of the Tonopah studies, however, it seems fair 

 to admit that the former may have been the result of the latter, the effect of the 

 repeated accumulation and eruption of vast bodies of molten material, and the sub- 

 sequent subsidences and local adjustments. 



aSpurr, J. E., Origin and structure of the Basin ranges: Bull. Geol. Soc. America, vol. 12, p. 248. 

 16843 No. 4205 6 



