THE TRANSVERSE VALLEY 321 



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

 southeast * side about 2,700 feet, on the northeast side about 4,200 feet. The south- 

 east or downthrown side of this valley is decidedly the steeper. 



" In this case we have an actually determined! 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 more 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 weak- 

 ness 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, be- 

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

 As the Star Peak group has an estimafed thickness of 10,000 feet, J the total erosion 

 since the faulting has at some points exceeded 2 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. 



"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." 



The southern part of the west slope of the transverse valley is covered 

 with the basalt and underlying volcanics before described. Evidence 

 has been presented to show that these volcanic rocks were formed at a 

 low angle § and have been tilted into their present attitude during the 

 upheaval of the range. In plate 18, figure 1, the snow-tipped range in the 

 distance is the southern part of the Star Peak range. The southern por- 

 tion of the transverse valley which separates the ranges can be seenwith 

 the basalts and their underlying tuffs dipping down at the valley margin 

 underneath the valley deposits. In other words, since the tilting of the 

 volcanic series there has been no corrasion along the southwestern valley 

 margin. The sense of the faulting indicated by the volcanics and by 

 physiographic evidence is the same as that given by Spurr; that is, the 

 northeast side has been elevated relatively to the southwest side. The 

 higher mountains on the northeast are not, however, due to rocks more 

 resistent to erosion, but to a greater throw on the Star Peak Mountains 

 fault than on the Humboldt Lake Mountains fault. This difference is 

 characteristic of the two ranges throughout, even though the Star Peak 

 and Koipato groups, including both hard and soft rocks, are found about 

 equally distributed in both. The very culminating point of the range — 

 Star Peak — the highest mountain in the vicinity, is made up of " Star 



*This should be southwest; probably a typographical error. 



fSpurr uses this expression, " actually determined fault," for one determined on the evidence 

 of stratigraphic discordance. 

 % It was not estimated in the vicinity of this point, however. 

 I See pages 303 and 305. 



XLII— Bull. Geol. Soc. Am., Vol. 15, 1903 



