CHARACTER OF THE OLDER TECTONICS 



553 



able but that several of the most conspicuous ranges have been formed 

 directly through thrust-action. At any rate, the present configuration 

 of the mountains gives no suggestion of the slightest genetic relationship 

 between profile and structure. The contours of the ranges are those of 

 general differential erosion under ordinary conditions of an arid climate 

 ( figure 5 ) . 



A simple synclinal structure is represented in the Spring Mountains, 

 at the extreme southern point of Nevada (figure 6). Here again the 

 earlier observations are at fault in not disclosing important dislocations. 



Figure 6. — Ancient Syncline of the Spring Mountain Range 



It is now known that there are in this range numerous and profound 

 faults! Existence of some of the more pronounced faults should have 

 been at least suspected long ago, as they are clearly visible from the town 

 of Las Vegas, a distance of 15 miles. It is quite manifest that the pro- 

 files of the range do not bear any relation whatever to the geologic 

 structure. 



Not very unlike the Spring Mountains in structural character is the 

 Grant range in central Nevada. There is here represented in the cross- 



FiGURE 7. — Old Arch and Trough of Grant Range 



section both an arch and a trough (figure 7). Were the eastern ridge of 

 the range alone considered, there might be some ground for regarding as 

 possible a close relationship between the mountain contour and its struc- 

 ture; but in the ridge on the west side the strata of the high parts are 

 disposed in a syncline. There are reasons for believing that profound 

 faulting exists near the eastern foot of the range. The configuration of 

 the mountain is easy of explanation on the h}^othesis of general desert- 

 leveling, but extremely difficult on any other theory. 



