GEOLOGICAL SURVEY OF THE TERRITORIES. 211 



licid i)eeu mainly eroded before the ponding of the waters and the vol- 

 canic ernptions which these buttes record. 



The valley on the east of the buttes is flat and partly swami)y for a 

 considerable distance on either side of the stream, forming very valua- 

 ble stretches of farm-land, though the lower portions are liable to over- 

 flow during the spring-floods. Large quantities of hay are cut here for 

 use at the neighboring stage-stations, and two or three trappers com- 

 monly winter in the immediate neighborhood, since their stock here find 

 abundant feed all the season. The water of Jlenry's h'ork, having been 

 turned from its direct course by the volcanic eruptions, now flows for 

 some miles np a channel i^arallel with the Snake before being able to 

 find exit into that river. 



The surface of the basalt is exposed here in places, and shows a 

 striated structure, with lines of elongated bubble-like cavities, such as 

 we might expect to find on the surface of a viscid fluid inflated with 

 gas, and running slowly down a very gentle incline. The course of this 

 structure, is about north 38^ east, nearly corresponding with the direc- 

 tion of the lower half of the valley of Henry's Fork, and indicates a 

 probable source of volcanic outflow toward the head of that stream. 

 Here we caught the first of the fine trout which fill these mountain- 

 streams, and which afterward so frequently eked out our scanty camp- 

 fare. 



We here left the valley of the main stream, which now bends ofl'to the 

 southeast, and turned up that of Henry's Fork. This stream was named 

 after one of the partners of the Missouri Fur Company, who, in 1810, 

 budt a fort on the banks of this stream about twenty-five miles from its 

 mouth. 



About ten miles above the Crater Buttes, a low double-crested knob 

 makes a prominent appearance, though it rises only 75 or 80 feet above 

 the plain. It consists principally of laminated trachyte, with much 

 scattered brown and brick-red scoria. This was probably an old vol- 

 canic vent, though so much material has been worn away that nothing- 

 like a crater can now be traced. 



Four or five miles due north from here stand the so-called Sand Hills, 

 more properly the Sand Hill Mountains. As we approach we find them 

 surrounded hy a belt of sand-dunes, from a half mile to a mile in width, 

 and reaching, on the south side of the mountains, elevations of 250 and 

 300 feet above the plain, though not more than 100 or 150 feet high at 

 the eastern end. The central portion of this belt is almost entirely 

 barren, consisting of fine, drifting, gray, and ferruginous quartz-sand, 

 with some comminuted lava ; only here and there a few straggling shoots 

 of a long-rooted grass appear. The glare of these uniform suriaces was 

 almost as dazzling beneath the mid-day sun as if they had been of snow. 

 From the hollows the sand has been mostly blown away, while the 

 coarser materials are left, forming a sort of pavement of sometimes 

 large masses of porous, gray trachyte, and again small bits of the same 

 mingled with fragments of brick red scoria, chalcedony, agate, and silici- 

 fied wood. The first impression was that these vast accumulations of 

 sand marked roughly an o:d lake-level against this island like mountain ; 

 but I afterward became satistied that the sole agent in the case had been 

 the vehement southwest winds, which sweep over the broad plain and 

 break against these isolated points, piling up the sand in these huge 

 drifts. The surfaces of the drifts are everywhere beautifully marked 

 with wind-ripples, except at the very summits of the various little 

 ridges, where the winds pile it into steep crests, on the lee side of which 

 the sand lies at so sharjD an angle as to slide at the least touch, and so 



