344 G. D. LOUDEHBACK — STRUCTURE OF THE HUMBOLDT REGION 



range, including the alluvial cones, indicates that the difference can not 

 be very great in the amount of material removed. It is possible, there- 

 fore, that the present west slope of the Humboldt Lake range and the 

 corresponding slopes of the other ranges may not differ widely in angle 

 from the dip of the fault planes which gave rise to them. In other 

 words, the fault planes may have been of rather moderate inclinations 

 to the horizontal. 



Use of volcanic Rocks in deciphering orogenic Movements 



The post- Jurassic volcanic rocks seem to have been the despair of 

 some geologists, and a paper already quoted says : 



" Our knowledge of the ranges of northwestern Nevada is comparative! y slight, 

 since in this region the volcanic rocks are so abundant that nearly everywhere 

 they mask the structure * * *." 



The fact is, that as the only post-Jurassic rocks of northwestern Nevada 

 are volcanics and lake sediments, and as the lake sediments are only 

 sparsely distributed on the mountains, the structure and attitude of the 

 volcanics are frequently the only available sources of Tertiary and Qua- 

 ternary orogenic and physiographic history, and at the least they almost 

 always offer suggestions and details which would otherwise escape notice. 

 Besides the cases already described in the various ranges and hills in 

 the Humboldt region, an example may be given from the Sierra Nevada. 

 The east slope of the Sierra Nevada has long been supposed to have 

 been determined by a great fault, but physiographic evidence alone can 

 not always determine whether such a fault be single or multiple. Sec- 

 tion 3, plate 16, represents the east slope * of the Sierra from the top of 

 mount Rose, 10,800 feet, to Washoe valley, 5,032 feet above sealevel. 

 The bedrock is granite, above which occurs a series of andesitic breccias 

 capped by solid andesitic lava. The identity of the series on mount 

 Rose and on the low shoulder is easily recognized and the existence of a 

 fault immediately evident. Such a fault is as near an " actually observed 

 fault" as one is likely to encounter. The throw is easily estimated, and 

 is 2,500 to 3,000 feet. Its presence might have been indeterminable or 

 have been a subject for controversy had the volcanics not been present. 



The writer desires to urge the value of the volcanics in the study of 

 the post-Jurassic history of Nevada. He considers them the key to the 

 succession of events in many of the western ranges. It is evident, how- 

 ever, that volcanic rocks must be used with the utmost care and reserve, 

 for it is difficult to correlate them if the outpourings have been irregular 



* Based on topography of Carson sheet, U. S. Geol. Survey Topographic Atlas. 



