320 G. D. LOUDERBACK — STRUCTURE OF THE HUMBOLDT REGION 



elevation along a fault plane at its present west margin, with tilting to 

 the east. This fault plane extended south and passed through Black 

 Knob valley, curving to the east around the southern end of the range, 

 making of the Star Peak range a crust block separate from and inde- 

 pendent of the crust block which produced the Humboldt Lake range. 

 The nearness of approach of these two blocks is a most interesting fea- 

 ture. It has led to the single name Humboldt range, which has been 

 given to the two together, although the break is distinct and the ridge 

 lines are independent. Instead of being roughly continuous, the two 

 range lines overlap in a north and south direction, the Star Peak moun- 

 tain ridge line running south until, descending, it passes into the inter- 

 montane valley of the Carson sink some miles east of the Humboldt 

 Lake range, the Humboldt Lake mountain ridge line running north 

 until, descending, it passes into the intermontane valley of the Hum- 

 boldt some miles west of the Star Peak range. The Star Peak division 

 of the Humboldt range is geologically more closely related to the low 

 divide which separates the valley of Carson sink from Buena Vista valley, 

 and which will be described later, than to the Humboldt Lake division. 

 In this paper, therefore, the two divisions have been called the Star Peak 

 range and the Humboldt Lake range respectively. 



Mountain profile. — Viewed from the top of Chocolate butte, in the valley 

 to the south, the Star Peak mountains show a beautifully regular pro- 

 file, with a long, gentle slope to the east and a distinctly shorter, steeper 

 slope to the west. Such characteristic profiles have been used frequently, 

 it is believed, as indications, perhaps by some as proofs, of the tilted 

 fault-block character of various ranges. This characteristic has not been 

 used here as an element of the proof in any discussion because of its 

 imputed abuse. " Indeed," says Spurr,* '* as in so many other cases, the 

 existence of the fault seems to have been assumed from the presence of 

 a scarp. " The regularity and expressiveness of the profile in this case 

 has induced the writer to at least call attention to its existence. 



The great transverse valley. — An explanation has already been given of 

 the transverse valley which separates the Star Peak from the Humboldt 

 Lake mountains, but further discussion is desirable. Spurr,f speaking 

 of the valley, says : 



" The existence of the latter fault is evident from an inspection of the map ac- 

 companying the Fortieth Parallel report. Judging from this, the displacement 

 seems to be a downthrow on the south side, bringing the Star Peak Triassic down 

 against the underlying Koipato Triassic — a movement amounting to several thou- 



* Bull. Geol. Soc. Am., vol. 12, p. 226. 



fBull. Geol. Soc. Am., vol. 12, p. 225. His discussion is based on the Fortieth Parallel Survey 

 Report. 





