376 EEPOET UNITED STATES GEOLOGICAL SUEYEY. 



hills. To the southwest a belt of low grassy foot-hills sweeps round the 

 northern border into the debouchure of the Snake, in which highly tilted 

 reddish-brown strata appear, overlaid by volcanic Hows whose debris 

 forms bare rocky sMdes in the high declivities, like the disintegrated out- 

 crops of volcanic-filled fissures or dikes in the sedimentary formations. 

 The volcanics, indeed, sweep up on the higher eminences, crowning iso- 

 lated points on the watershed within a short distance north of Station 

 XXII, increasing' in consequence farther north, where they doubtless 

 still envelope quite extensive summit areas, and remnants like that on 

 the high point between the last two stations were met with on the north- 

 east flank of the range. These higher deposits are trachytic and rusty 

 vesicular lava or porphyritic trachyte, the latter often met with in bowl- 

 der-like masses. Crater Buttes and the Sand-Hills appear near at hand, 

 though 30 or 40 miles to the northwest, the mirage mimicking a lake 

 expanse at their base. The Three Buttes, huddled together from this 

 direction, are seen far away to the southwest. Haze fills the great plain 

 of the Snake, on the farther side of which the crests of the distant lofty 

 ranges of the Salmon and Boise appear like phantom mountains, their 

 bases buried in the evening gloom, and their summits lighted up with 

 indescribably delicate tints, only less brilliant than the glow of the west- 

 ern sky into which the more distant ranges dimly fade. But at a later 

 hour, when the light has faded, and the sky an even silvery blue, against 

 which the mountains are projected in sharp relief, the grandeur of the 

 effect is almost inconceivable. These early morning and evening studies 

 compensate a thousand fold the midday weariness of the shimmering 

 plain and shadowless mountains and low overarching brazen sky. 



Crossing around the narrow northern end of the range, the gray sand- 

 stones and red shaly deposits are met with along the western flank in 

 the shallow valleys hemmed in by the trachytic flows, and a feAV miles 

 north of XXII, these deposits pass out into low, rounded, grassy hills, 

 where they appear as variegated beds over a limited area, but are soon 

 hidden beneath the encircling volcanics. The sedimentaries appear in 

 a low, broken ridge at the north end of the watershed, from the foot of 

 which spreads a gently undulating plateau bench, to the north sinking 

 into the volcanic upland, which descends into the broad debouchure of 

 the Snake. The low ridge is composed of heavy ledges of gray and red- 

 dish laminated sandstone and shales, with drab and gray fragmentary 

 limestones, the southerly dip gradually lessening from an angle of 50° 

 on the southwest border of the ridge to 15° in the western border of the 

 plateau, in which latter quarter these deposits seem to form the roof 

 of a low-arched fold. To the east the plateau is floored by volcanic 

 rocks, consisting of the vesicular lava and trachytic debris, and extend- 

 ing across to the eastern border, where they show heavy ledges in the 

 coping of the high bluffs on Grouse Creek. The sedimentaries above al- 

 luded to are doubtless the series shown in the section through Station 

 XXI, belonging, however, to a fold on the north of that represented at 

 the latter locality. 



In the angle between Grouse Creek and the Snake the culminating 

 crest of the range terminates in the height selected for Station XXIII, 

 from which the view, looking northward out over the debouchure of 

 Snake River, gives at a glance the relations of this bay -like expanse to 

 the converging mountain barriers, which, a little higher up, define the 

 border limits of the lower valley of the Snake and its southern extension 

 up Salt River south of the grand canon. This indentation belongs prop- 

 erly to the upland volcanic region, which everywhere in this quarter 

 margins the great plain — a sort of transition from plain to highlands — 



