626 IDAHO MINING DISTRICTS. 



ranges from 50 to 125 miles, its total area being about 34,000 square 

 miles. The grade of the river is moderate. At Blackfoot the eleva- 

 tion is 4,505 feet, and 350 miles lower down, at Weiser, it is 2,125 feet, 

 giving an average grade of less than 7 feet per mile between these 

 places. On both sides of this. valley rise higher ranges, chiefly of 

 granite in the lower valley, of granite and Paleozoic and Mesozoic 

 rocks in the upper valley. The lower slopes of these ranges are often 

 flanked by Tertiary lake deposits. The larger part of the valley is 

 occupied by vast flows of basalt, frequently resting upon and covered 

 by fluviatile and lacustrine accumulations contemporaneous with the 

 flows. The basalt of the Snake River Valley bears evidence of being 

 throughout of the same age approximately, though consisting of a great 

 number of individual flows, and has generally been regarded as Plio- 

 cene. The eruptions did not originate from large volcanoes. Ashes 

 and other fragmentary rocks are generally absent, and the basalt evi- 

 dently flowed out quietly and without explosions from numerous local 

 vents along the margin of the valley or within the valley itself. This 

 volcanic action is usually referred to as fissure eruption, but it must not 

 be inferred that these fissures were long or large. It appears rather as 

 if the vents had the character of rounded local orifices, hardly exten- 

 sive enough to be classed as fissures. The basalts often flowed down 

 from the foothills of older rocks, closely following the present can- 

 yons, though the streams have since then generally succeeded in 

 wearing through the filling in their bottoms. Thus it is, for instance, 

 along the Boise River. 



It will be shown that the Snake River Tertiaries consist of a thick 

 series of early Neocene (Miocene) lake beds, with which are associated 

 vast masses of eruptives distinct from the Snake" River basalts proper, 

 and another series of deposits of late Neocene age (Pliocene), consist- 

 ing of the Snake River basalts and associated sedimentary rocks. 

 These two terranes represent successive stages of the Neocene lake 

 and are often difficult to separate. 



LITERATURE. 



The upper Snake River Basin has been described by Messrs. 

 Hayden, 1 Bradley, 2 Peale, 3 and St. John, 4 in the reports of the United 

 States Geological and Geographical Survey of the Territories. 

 Hayden describes the basalt flow along the present line of the Utah 

 and Northern Railroad, mentioning that there were several flows of 

 basalt, or at least two, separated by somewhat tilted Pliocene depos- 

 its of slight depth. St. John and Peale describe the basalt flows east 

 of this, near the headwaters of the Snake River. Peale states that a 

 number of extinct craters exist, that there were several flows of 



1 U. S. Geol. and Geog. Surv. Terr., Kept. 1871, pp. 25-30. 



2 Ibid., Kept. 1872, p. 190. 



3 Ibid., Rept. 1877, p. 543. 

 4 Ibid.,Eept. 1877, p. 323. 



