230 J. E. SPURR — ORIGIN AND STRUCTURE OF THE BASIN RANGES 



abl}^ persistent across Las Vegas valle3^ Transverse to this is a gentle 

 anticline, from whose apex the rocks in the center of the sj^ncline, which 

 should lie fiat, dip north and south. In the northern portion of the 

 range two heavy east-and-west faults were noted. One of these has an 

 estimated displacement of 1,000 feet and the other of several thousand 

 feet. The first is accompanied by a slight normal scarp, which, however, 

 is only an exceedingly small fraction of the vertical displacement, and 

 is indeed caused by the superior hardness of the upthrust Cambrian 

 quartzite. It is, therefore, a simple erosion ftiultscarp. The second 

 fault has left no mark on the topograph}^ since tlie Middle Cambrian 

 limestone to the north has about the same resistance to erosion as the 

 Upper Carboniferous on the south (see plate 25, figures 2 and 3). 



The general synclinal structure, and the powerful faults without at- 

 tendant displacement of the surface, bear evidence to long continued 

 erosion, to which the range seems to be due. The transverse anti- 

 clinal fold recalls the anticlinal structure of the Kingston range, which 

 lies next west and which is probably of more recent origin than the 

 synclinal ridges just described. The anticlinal swell in the Spring 

 Mountain range, therefore, is perhaps of a later origin than the syncline, 

 and the arching of the mountains along it may be directly due to de- 

 formation and not to differential erosion. This last peculiarity is the 

 ruling feature o^the district which lies west of here, where many of the 

 mountains are very likely due to uplift, so that this range ap})arently 

 lies on the boundary between a region of older and a region of newer 

 folding, and may have experienced both. According to this the range 

 is due to (1) erosion plus (2) simple deformation. 



Pdhroc, Hyko, and Pdhranagai ranges. — The Pah roc range, as seen from 

 the north by the writer, seems to be anticlinal. 



The H3'ko and Pahranagat ranges, according to Mr Gilbert, show 

 no folding, but a series of north-and-south faults and an east-and-west 

 fault. The toi)ography of these ranges and of the neighboring Timpa- 

 hute range as well, is explained as chiefl3^ due to this system of faulting, 

 which has produced i)arallel ridges by the downthrow of successive 

 blocks. 



Mr Gilbert has drawn sections of the Pahranagat and Timpahute 

 ranges.* In the case of the latter range he shows a number of faults 

 outcropi)ing in the bottom of a wide and shallow interior valley, which 

 he states has been eroded out of soft shales lying between more resistant 

 quartzite on the west and limestone on the east. The section shows 

 nowhere an example of toj)ography derived directly from structure ; 



♦Surveys West of the Hundredth Meridian, vol. iii, Geology, p. 38. 



