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



fault-scarp, it seems likely that the fold may also be comparatively 

 recent. Erosion has undoubtedly been active, but, since it apparently 

 has not yet overcome deformation, we may consider the range as being- 

 due primarily to the latter, with the minor topography determined by 

 erosion. 



Mormon range. — The Mormon range is anticlinal in structure. Along 

 the crest of the anticline, in the Carboniferous limestones, a notch has 

 been eroded, which is, however, not yet deep enough to form an interior 

 valley. The slight development of this valley is proof that erosion has 

 not yet been able to complete the work which it has undertaken, that 

 of overcoming the direct effects of deformation on the topography. The 

 folding must then be comparatively recent. The earlier Tertiaries which 

 lie at the foot of the mountains are conformable with the Paleozoic rocks 

 and have been upturned with them. The date of the general disturb- 

 ance, therefore, can hardly be earlier than Middle Tertiary. Moreover, 

 the late tertiary sediments also, which in some cases may even be early 

 Pleistocene, have been in })laces folded and faulted. These faults are 

 comparatively slight and of different ages, as is shown by the fact that 

 some are not expressed in the topography, erosion having overcome 

 their effects, while others, more recent, have faulted the present surface 

 equally with the underlying rocks (see i)late 25, figure 4). 



We may conclude that the origin of tliese mountains is like that of 

 tlie Virgin range; that tliey are primarily due to coniparativel}^ late 

 folding, and probably in a less degree to faulting, while in the minor 

 features of topogra[)hy erosion has already accomj)lished considerable 

 work. 



Meadow Vallei/ range. — The north end of this range is largely volcanic, 

 but in })laces, as at Delamar,'^ the heavy Cambrian c^uartzites and lime- 

 stones, which extend south from the Highland range, are exposed. Soutli 

 of Delamar stratified rocks are the rule, and at the southern end the 

 range is entirely made up of Carboniferous limestones. In a section 

 across the range at this point, following the valley of Muddy creek, no 

 less than six parallel and adjacent oi)en folds were observed. The syn- 

 clines generall}' form ridges, the anticlines longitudinal depressions or 

 valleys. North of Muddy creek the central ridge is synclinal, while 

 south from it there are two principal synclinal ridges, with an interven- 

 ing anticlinal valley. The faces of the ridges are often steep, and might 

 be considered by casual observers to be fault-scarps, yet there is no 

 known evidence of faulting along them. 



At Delamar Mr Emmons has observed a heavy east-west fault in the 

 Cambrian strata, having a downthrow on the north of probably several 



* Verbal communication by Mr Emmons. 



