338 G. D. LOUDERBACK — STRUCTURE OF THE HUMBOLDT REGION 



After a brief quiet period a flow of basalt flooded many square miles 

 of the district investigated, some small areas receiving several successive 

 coats, and then all volcanic activity ceased. 



OROGENIC DISTURBANCES 



Whether the Great Basin region was being elevated as a whole or not 

 during the Tertiary period of volcanic activity is something which the 

 study of this district did not elucidate, but it is reasonably certain that 

 up to and including the time of the basalt outpouring but slight differ- 

 ential elevation within the limits of the district studied could have 

 taken place. 



But after the outpouring of the basalt the region was broken up along- 

 great fault lines, and a period of active differential elevation was inau- 

 gurated. The major fault planes for which evidence has been given lie 

 along the west side of the Humboldt Lake range, along the west side of 

 the Star Peak range and running through and determining the trans- 

 verse valley between these ranges, and along both east and west sides 

 of the East range. The East range was elevated as an inverted wedge, 

 practically without tilting. The Star Peak and Humboldt Lake ranges 

 were tilted to the east, the exposed fault scarp being on the west side 

 only. 



During this differential elevation, by which the present mountain 

 ranges and the broad valleys between the ranges were formed, the rela- 

 tively rising masses and the relatively sinking masses acted on the 

 whole as great blocks, rising and sinking as units ; but they were not 

 absolutely rigid units, for we have evidence of internal deformations of 

 two kinds — faulting and warping. 



Both mountain blocks and valley blocks are broken along lines of 

 minor faulting into a number of secondary blocks sufficiently well 

 shown in the ideal section, figure 3, not to require further description. 



Besides this, we note that the Humboldt Lake range, with its curved 

 fault trace concave to the west, rises from the south out of the plains 

 and gradually grows more elevated, then finally sinks toward the north 

 into the plains again. The fault-throw is greatest in the north central 

 portions and grows less as one goes north or south. In other words, the 

 block has a curvature from north to south, like the axis of an anticlinal 

 fold, pitching north to the north, and south to the south. Faults in 

 general are almost always of this character, each showing a maximum 

 throw, which decreases to zero toward its extremities. 



During the period of differential elevation or the period of volcanic 

 activity, perhaps both, the Great Basin region was elevated as a whole 

 from a low altitude to such a height that the bottoms of the great valleys 



