406 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



of 4 or 5 miles, when it expands into the beautiful and still more exten- 

 sive upper basin which extends to the mouth of Salt River, above which 

 it expands into the basin-like area through which that stream flows from 

 the south. Above the narrows the volcanics suddenly cease, and thence 

 southward the bared sedimentary formations appear in the abrupt slopes 

 of the border mountains between which the terraced bottoms stretch 

 from side to side. 



Our route entered the valley just above the north bend, where the 

 stream enters the lower canon in the basalt, and thence followed up along 

 the southwest side to the confluence of Salt Eiver. The character of 

 of the volcanic materials at this point was casually mentioned in connec- 

 tion with the description of the section through Station XXIV, in a 

 preceding page. They are seen to be made up of pinkish and drab tra- 

 chytic flows, associated with peculiar light, soft materials, recalling mod- 

 ern Pliocene deposits along the border of the Snake plains, and reclin- 

 ing directly upon the sedimentary deposits which compose the border 

 mountain flanks. These flows dip gently northward in the direction of 

 the lower level of the volcanic plain which fills the debouchure of the 

 valley ; and which has the dark, somber appearance of basalt. Just 

 above the little recess at Butte Creek, which is shut off by a low butte 

 capped by trachyte, we regain a, larger bottom tract which reaches a 

 mile or two along the stream on the south side, extending back to the 

 foot of the mountain, which is here denuded of the volcanics ; the north 

 side of the stream still showing a high bluff of volcanic rock, underlaid by 

 lighter colored deposits. This bottom is made up of gravel, which forms 

 a bar-like dry ridge between the stream and a low, marshy tract inland, 

 and is further molded into low terraces. The turbid, swift-flowing 

 river is filled with low islands, covered with willow and cottonwood, and 

 dense copses of wild rose. The volcanic bench again approaches the 

 stream, along which it forms bluffs a hundred feet or more in height, 

 gradually rising up on the neighboring mountain flank. Fall Creek, 

 after leaving its debouchure in the mountain border, cuts a narrow caiion 

 across this bench, and on reaching the left bank of the Snake its waters 

 are precipitated over a ledge of basalt, making a pretty fall of 30 to 50 

 feet in height. The section of the volcanics exposed in the canon wall 

 of Fall Creek is one of much interest, inasmuch as it exhibits the rela- 

 tive position of the trachyte to the overlying basalt, both of which incline 

 off the sedimentary border ridge. At this locality the trachyte was seen 

 at an elevation of 700 to 800 feet above the river, in close proximity to 

 exposures of Carboniferous limestone, the basalt not reaching the same 

 elevation, its limit being rather well defined by a bench-like break in 

 the sloping surface. The basalt is the usual dark gray or brown vesic- 

 ular variety, holding small globules of white quartz which have a fused 

 appearance. It occurs in heavy layers, broken into irregular cross-frac- 

 tured columnar fragments, and undulating as though the material flowed 

 over an uneven surface. The same ledges appear in the bluff-face of the 

 lower plateau bench that fills the opposite side of the valley, where, how- 

 ever, they appear to incline gently in the direction of the opposed mount- 

 ain border. This appearance is not readiiy accounted for, unless the 

 flows have been slightly tilted by forces acting in the mountain belt 

 along the southwest border of the valley. 



This high volcanic bench continues tbence to Pyramid Creek, every- 

 where showing the same sloping surface outlying the sedimentary 

 mountain border, across which numerous little streams have eroded deep, 

 narrow channels, on their way to the river. On the opposite side of the 

 valley, corresponding benches are seen along the farther border of the 



