260 J. E. SPURR — ORIGIN AND STRUCTURE OF THR BASIN RANGRS 



north -south ones, and they display more openl}^ their relations. In 

 general they either have no effect on the topography or have induced 

 the formation of gulches. Occasionally they are accompanied by cliffs, 

 which are usually of trifling height as compared with the displacements. 

 These can often be proved to be erosion fault-scarps, not unfrequently 

 reversed. Finally there are rare faults, generally of relatively small dis- 

 placement, which have a direct effect on the topograpliy and have pro- 

 duced simple fault-scarps. 



History of tiif Development of the Fault Theory as Applied to 



THE Basin Ranges* 



In the Colorado })lateau there is a series of north-and-south disj^lace- 

 ments which have })roduced simple f;iult-scarps. The coincidence of 

 the direction of these scarps with the trend of the ranges of the Basin 

 region, whicli adjoins the i)lateau on the west and northwest, suggests 

 to the observer acquainted with the plateau that the ranges are the 

 result of more powerful faulting. Thus Mr G. K. Gilbert in 1873, after 

 reconnaissance in the Basin region, concluded that the Basin ranges 

 probably owed their definition and relief to faulting. He classified the 

 sections accumulated b}^ the geologists of the Wheeler survey as fol- 

 lows : t 



1. " Faulted monoclinals occur in which the strata on one side of the fault have 

 heen lifted, while those on the opposite side either do not appear (a) or (less fre- 

 quently) have been elevated a less amount [b). Two-thirds of the mountain 

 ridges can be referred to this class." 



2. " Other ridges are uplifts limited l)y parallel faults (r), and to tliese may be 

 assigned a few instances of isolateil synclinals (d), occurring under circumstances 

 that preclude the idea that they are remnants omitted by denudation." 



3. " True anticlinals (e) are very rare, except as local, subsidiary features, but 

 many ranges are built of faulted and dislocated rock-masses (/) with an imperfect 

 anticlinal arrangement." 



" Not only is it impossible to formulate these features by the aid of any hypo- 

 thetical denudation in such a system of undulations and foldings as the Messrs 

 Rogers have so thoroughly demonstrated in Pennsylvania and Virginia, but the 

 structure of the western Cordillera system stands in strong contrast to that of the 

 Appalachians. In the latter corrugation has been produced commonly by folding, 

 exceptionally by faulting; in the former commonly by faulting, exceptionally by 

 flexure. In the latter few eruptive rocks occur ; in the former volcanic phenomena 

 abound, and are intimately associated with ridges of upheaval. The regular alter- 

 nations of curved anticlinals and synclinals of the Appalachians demand the as- 



*Tlns outline has purposely been made compact, witli minor features omitted, 

 t Progress Rept. Geog. and Gaol. Survey West of the Hundredtli Meridian, 1872, published in 

 1874, p. 48. 



