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602 DESCEIPTIVE GEOLOGY. 



exposed at the coal-mines in the northern end of the valley, consisting 

 mostly of very thin shales, sometimes calcareous, sometimes made up 

 entirely of quite siliceous material, often bituminous, and in some cases 

 containing several thin seams of brown coal. The Pliocene beds which 

 overlie them non-conformably consist at the summit of the pass of white 

 volcanic ash, very porous, and containing a good deal of fine black mica 

 and hornblende. 



The northern end of the Elko Range is, as we have seen, made up 

 of red, porphyritic rhyolites, which extend continuously to Upriver 

 Peak. These rhy elite flows seem to be generally rather thin, but, with the 

 exception of points already mentioned at Osino Canon and south of the 

 Elko Pass, no underlying sedimentary rocks were found exposed in the 

 range. Along the western flanks of these rhyolite ridges are numerous hot 

 springs, the most remarkable of which is that two miles south of the town of 

 Elko, where a public bath has been established. Owing to the peculiar taste 

 of its waters, this spring has been designated the "Chicken Soup Spring". 



Seetoya Mountains. — The country represented on the northwest cor- 

 ner of the map, west of the River Range, is a region which has been the 

 scene of great volcanic activity in later Tertiary times, and in which the 

 older formations are generally concealed beneath extensive flows of rhyolite, 

 which now cover the greater part of the surface of the country. It is inter- 

 sected by broad valleys, whose waters run alternately to the south into the 

 Humboldt River or northward into the Snake, which have generally been 

 filled by Pliocene beds, composed largely of fine re- arranged volcanic 

 material. The depression occupied by the valley of Susan Creek and the 

 north and south tributaries of the North Fork of the Humboldt presents a 

 type of one of these Pliocene Valleys. The streams run in broad Qua- 

 ternary bottoms of fine mud and silt, resulting from the decomposition of 

 the soft Tertiaries, and support considerable growth of native grass, while 

 on either side are mesa-like benches of Tertiary strata, which extend up on 

 to the flanks of the bounding ranges to a height of 600 to SOO feet above 

 the valley-bottom, covered with a light gravelly soil, which only supports 

 a scattered growth of sage-brush. 



The southern portion of the Seetoya Range represents an uplift of sedi- 



