92 PROCEEDINGS OF THE AMHERST MEETING 



ill the vicinity of the trench was studied from Elko, just north of the inter- 

 national boundary, to Golden, 180 miles to the north. Because of its location, 

 the valley offers an excellent opportunity to examine the structure of the 

 flanks of the two ranges and the structural relation of these two ranges to one 

 nil other. 



Two distinct zones were found to exist in the 180 miles. One of these ex- 

 tends 90 miles, from Golden to Canal Flats. In this section there are thrust- 

 faults on both sides of the trench and in some cases within the trench itself. 

 On the west side these faults have westward-dipping planes and on the east 

 mostly eastward-dipping planes. The thrust-faults from the west are more 

 pronounced than those from the east, resulting in an older series of rocks on 

 the west side of the trench. Slaty cleavage, which dips to the west on the 

 west side of the trench, becomes vertical toward the center. Where observable 

 on the east side, it dips to the east. These structures dipping into the range 

 are evidence in favor of R. T. Chamberlin's hypothesis of wedge-shaped de- 

 formation in mountain-building. These two ranges represent the intersection 

 of two wedges. This part of the valley has been located by the resulting line 

 of weakness. 



South of Canal Flats the line separating the structure of the Purcell Range 

 from that of the Rocky Mountains leaves the trench and swings to the east, 

 being as much as 15 miles from the valley in some cases. In this part of the 

 valley, which extends for 60 miles to the south, the trench, does not follow 

 structural lines. The great width of the valley and the evidence for a Cre- 

 taceous peneplain over the sight of the Purcell Range suggests that the valley 

 developed originally on an old erosion surface and persisted in its location, 

 even though the zone of weakness developed near at hand to the east during 

 the later deformative movements. Thus, in this portion of the valley the 

 structural separation of the ranges is not shown by the relief. 



Presented extemporaneously. 



Discussed by W. M. Davis, who raised a point as to the author's use of 

 the term "trench" as being unusual. 



FAULTS, VNDERDRAG, AND LANDSLIDES OF THE GREAT BASIS RANGES* 



BY W. M. DAVIS 



Gilbert first assumed that the faults on which the crustal blocks of the 

 Great Basin mountain ranges have been uplifted were essentially vertical. He 

 and others later described the aligned spur-end facets along the base of the 

 steeper side of certain ranges as representing parts of the fault-planes on 

 which the range blocks were raised. But the moderate slope of the facets 

 gives so large a hade to the fault-planes (55 or 60 degrees) as to imply a 

 greater horizontal than vertical movement at the time of displacement ; and 

 in that case it would seem better to describe the ranges as the result of hori- 

 zontal extension than of vertical upheaval. Gilbert's latest expression of opin- 

 ion in this matter was made in connection with a discussion of the depth at 

 which crustal underflow takes place in the reestablishment of isostatic equi- 



1 Manuscript received by the Secretary of the Society in January, 1022. 



