LINDOREN.] SNAKE RIVER VALLEY. 627 



basalt, and that the basalts are generally horizontal in position and 

 fill the valleys and the more depressed portions of the basins. There 

 appear to have been two periods of basaltic flows, one at the close of 

 the Pliocene, the other at the beginning of the Pleistocene. The 

 Pleistocene age is inferred from exposures at Marsh Valley, near 

 Red Rock Pass, where Pleistocene beds were somewhat eroded before 

 the basaltic flow. 



According to Gilbert, however, this Pleistocene is older than the 

 highest stage of Lake Bonneville, during which the lake found an 

 outlet at Red Rock Pass. The river draining the lake at this time 

 appears to have flowed over the surface of the basalt. 



According to Hague, 1 the latest eruptions in the Yellowstone 

 National Park are of basalts, which stretch far into Idaho in somber, 

 monotonous beds. These basalts are pre-Glacial, and their eruption 

 is referred to the Pliocene. 



About 1869 Mr. Clarence King visited the lower part of the Snake 

 River basin and collected a number of fossils from beds beneath the 

 basalt at Castle and Sinker creeks, tributaries from the south, joining 

 the river about due south of Boise. The fossils have been described 

 in detail, while no description of the localities was ever published, a 

 fact which has led to some confusion. A few notes regarding this 

 occurrence are contained in King's Systematic Geology 2 and may be 

 quoted : 



In the basin of Snake River . . . there were basaltic eruptions in the middle 

 of the Pliocene period which overflowed the earlier lacustrine beds of the period, 

 and in turn were themselves overlaid ... by the main, later Pliocene series. 

 . . . Sections obtained along the plains between the Owyhee Mountains and Snake 

 River show that a considerable portion of the beds of the valley, which consist 

 chiefly of white sands and marls carrying numerous well-defined Pliocene forms, 

 were overlaid by large accumulations of basaltic flow, and that subsequently a 

 second period of lacustrine deposition took place, likewise characterized by Plio- 

 cene forms, the latter representing a more advanced stage of development and 

 more recent type than those beneath the basalt. 



King further states that near Shoshone Falls the basalt rests on 

 the eroded surface of a trachytic soft rock which he considers of pre- 

 Miocene age. 3 From the collections of King and the later collections 

 of Wortman, Cope has described an extensive fauna of fresh-water 

 fishes, and proposed for the sediments in which these are contained 

 the name Idaho formation.* 



The locations given are very vague, as "Catherine Creek," " Castle 

 Creek," or "Southern Idaho," and no description of the beds is 

 vouchsafed. The fauna consists of 22 species, all differing from 

 existing species so far as known. Professor Cope thinks that the 



1 Am. Jour. Sci., 4th series, June, 1896, Vol.1, p. 455. 

 U. S. Geol. Expl. Fortieth Par., Vol. 1, 1878, pp. 418, 440. 

 a Ibid., p. 593. 

 t Proc. Phila. Acad. Sci., 1883, pp. 153-166. 



