350 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



draining' south into Bear Eiver and northwest into th6 Snake tlirongh 

 Willow Creek, a considerable area lying west of Day's Lake belonging 

 to the npi)er basin of the Blackfoot. That portion of this basin within 

 the ])resent district, with the exception of the few square miles i^ertain- 

 iug to the Blackfoot, opens out into the great plain of the Snake to the 

 northwest, the extreme sources of AVillow Creek originating in the water- 

 shed on the south boundary of the district, where John Day's Ci'eek 

 rises in a shallow lake of uncertain or variable extent, according to the 

 season, its borders being occupied by extensive levels of marsh, mar- 

 gined by fields of tule and treacherous bog. This portion of the basin 

 reaches an altitude of 6,400 feet ; the descent to the northwest to the 

 junction of Day's Creek with the main channel of the Willow being some 

 400 feet in a distance of about 30 miles, and thence to the lower level of 

 the great plain a further descent of 1,000 feet, mainly over the great 

 volcanic upland bench which everywhere borders the Snake plains north 

 of Eoss Fork. 



The upper courses of the streams wind through shallow, meadow-like 

 depressions between the gently undulating basaltic slopes and low 

 plateaus, in which they begin their canoned course lower down, and 

 thence across the intervening broad stretch of volcanic upland. Tlieir 

 beds are hidden in the depths of the sombre rock until they emerge into 

 the plain. It seems almost incredible that these deep caiions could have 

 been excavated by the little streams which to-day occupy their beds; 

 yet such is doubtless the fact, though the work was performed when 

 their volume greatly exceeded its present amount. The results of erosive 

 action in these rocks are well known. In the high, level basins of this 

 region the waters are collected into a main channel which flows over the 

 surface, a large proportion of its spring-fed tributaries sinking in the 

 cavernous rock ere they reach the main stream, along which they some- 

 times reappear as powerful springs. But soon as the general surface 

 begins a more rapid inclination, then the collected force of the streams 

 is concentrated, and the work of deepening their beds is facilitated both 

 by the lay of the country and the nature of the rocks ; the one precii)i- 

 tating the water down rapid chutes, while the hardness of the other 

 prevents the lateral wear beyond certain narrow bounds, and the sub- 

 sequent and gentler acting atmospheric agencies which mould the flaring 

 valley-sides in softer materials, here meet with stubborn resistance in 

 the intractable basaltic ledges which usually form a mural cornice along 

 the brink of the gorge. 



The northeast and southwest margins of the basin within our territory 

 are bounded respectively by the Caribou and Blackfoot Eanges. It is 

 traversed centrally, in its longer direction, by a low highland belt, which 

 erosion has fashioned into isolated ridges which have a parallel arrange- 

 ment both in relation to one another, as also to the higher mountain 

 borders on either hand. These mid-basin ridges attain throughout quite 

 uniform relative elevations of 600 to 1,000 feet, and exhibit in their 

 geologic components interesting variety. The lower levels are floored 

 by basalt, which evidently inundated the valley in successive flows 

 during the later epoch of volcanic acti\dty. But besides the latter 

 deposits, others of presumably earlier origin are met with in the shape 

 of dikes of hornblendic trachyte, whose extrusion has tilted the sedi- 

 mentary deposits through which they pass, and which latter, it should 

 be mentioned, form the nuclei of the basin ridges. Elsewhere the crests 

 of these disturbed sedimentary elevations are crowned by heavy flows 

 of trachytic lava, while other elevations culminate in irregiUar rounded 

 domes comi)osed of vast quantities of scoriaceous lava. At various 



