RANGES OF NORTH WKSTKRN NEVADA 225 



West Humboldt rmiijc. — The West Huniholdt range/'^ which iri next 

 west of tlie Pah Ute, has a main anticlinal fold, wliich trends diago- 

 nally across the topographic axis. 



Mr Hague t mentions two faults in this range, one along the eastern 

 side of the range, the other crossing it in a northwest-southeast direc- 

 tion west of Buffalo peak. Tiie existence of the latter fault is evident 

 from an inspection of the map accompanying the Fortieth Parallel re- 

 port. Judging from this, the displacement seems to be a downthrow on 

 the south side, bringing the Star Peak Triassic down against the under- 

 lying Koipato Triassic — a movement amounting to several thousand 

 feet. The fault line lies in a valley, from which the mountains rise on 

 the southeast side about 2,700 feet ; on the northeast side about 4,200 

 feet. The southeast or downthrown side of this valley is decidedly the 

 steeper. 



In this case we have an actually determined heavy fault, which is 

 not marked by a scarp, but by a transverse valley, where erosion has 

 excavated at least 2,700 feet deeper than in the rocks on each side ; nor 

 is this niore than a fraction of the total erosion, for while the valley was 

 being formed the mountains have also' been steadily wearing down, only 

 more slowly, on account of the zone of greater weakness along the fault. 

 On the northeast all the Star Peak Triassic (which is now found on the 

 other side of the fault, and so must have been on this side, too, before the 

 dislocation) has been worn away, leaving bare the underlying Koipato. 

 As the Star Peak group has an estimated thickness of 10,000 feet,| the 

 total erosion since the faulting has at some points exceeded two vertical 

 miles. The present greater elevation of the mountains in the upthrown 

 or northeast side of the fault is probably due to the greater resistance to 

 erosion of the Koipato quartzites, as compared with the softer rocks on 

 the south. This same resistant formation has been noted as determin- 

 ing topographic eminences in the Havallah range. 



In the West Humboldt range, therefore, there is evidence of erosion 

 powerful enough to have determined the topography and the range 

 itself, and in the one case where we are sure of our premises, erosion 

 has long since overcome all direct effects of deformation on the surface, 

 if indeed there ever were any. 



Resume of structure of northwestern Nevada. — Our knowledge of the ranges 

 of northwestern Nevada is comparatively slight, since in this region the 

 volcanic rocks are so abundant that nearly everywhere they mask the 

 structure, which is itself rather complicated, wdth folds often diagonal 



*King: Op. cit., p. 742. 



t Op. cit., p. 736. 



X King : Op. cit., p. 277. 



XXXIII— Bolt-. Geol. Soc. Am,, Vol. 12. 1900 



