FAULT BLOCK CHARACTERS OP STAR PEAK RANGE 319 



trusions. This also corresponds to the period of great granitic intrusions 

 throughout the Cordilleran region wherever definitely determined.* 



The superjacent volcanics. — The occurrence of members of the super- 

 jacent volcanic series appears to be greatly restricted in the Star Peak 

 range. Only one area of basalt has been observed, but this occurs in a 

 significant position. Near the south end on the east side a sheet of 

 basalt, sloping with the hill slope — that is, forming the surface layer — 

 rises from the broad valley east of the range and extends almost to the 

 summit at that point. No faults can be seen to break its slope. On the 

 west side no volcanics make their appearance. Passing still farther 

 west, we meet the east slope of the Humboldt Lake mountains with 

 their eastward dipping basalt passing down underneath the valley 

 detritus, while the west slope contains none. This is exactly what we 

 would expect if the Star Peak range, like the Humboldt Lake range, 

 had been elevated by faulting and tilting, the fault plane running along 

 the west basal margin and determining the transverse valley separating 

 the two mountain ranges. The west slope of the valley would then be 

 the natural slope of the tilted surface of the Humboldt Lake range; 

 the east slope the eroded fault scarp of the Star Peak range. 



Structural and physiographic discordance. — The unity of the range front 

 and ridge line and their indifference to rock structures is as marked in 

 this as in the southern range. A glance at the Fortieth Parallel Survey 

 map also makes it at once evident. 



Erosion features. — The erosion features are very similar to those of the 

 southern range, except that, as the range is considerably higher, the 

 forms are more rugged and the relief more sharp. Absolutely no trace 

 of lowland valley lobes or low, flat internal strike valleys can be found 

 and no mountain projections into the broad intermontane valleys. The 

 streams run in narrow canyons, especially sharply V-shaped at their 

 lower ends, and debouch on the valleys through bedrock channels at 

 several hundred feet above the valley floor, which they reach by great 

 alluvial cones. 



Conclusions. — We may conclude, then, that the Star Peak range of 

 mountains has experienced practically the same history as the Hum- 

 boldt Lake range ; that after the post- Jurassic folding a long period of 

 erosion ensued. Just what the character of the relief was throughout 

 the range at the time of the outpouring of the basalt has not been inves- 

 tigated, but where the basalt flowed it must have been very low. Fol- 

 lowing the period of volcanic activity, the range was uplifted by relative 



*See Lindgren : U. S. Geol. Survey, Geologic Atlas, folio 66, Colfax, California. 

 Ransome : U. S. Geol. Survey, Geologic Atlas, folio 63, Mother Lode district, California. 

 Lawson : Journal of Geology, vol. i, p. 579. 

 Also Turner, Fairbanks, Whitney, and others. 



