408 REPORT UNITED STATES GEOLOGICAL SURVEY. 



the distribution of the scant forest-growth over its rugged valley aspect. 

 The arborescent vegetation extends two-thirds the way from the base 

 toward the summit, where it is terminated by a shari)ly-drawn hori- 

 zontal line, extending quite around the visible face of the mountain, 

 above which the bare, rocky crown rises into the massive cone of the 

 summit. The mountain attains an actual altitude of 9,900 feet, or 4,500 

 feet above the valley, so that the limit of the forest growth is consider- 

 ably below the general altitude of timber-line in the neighboring mount- 

 ains. But it may, with some reason, be attributed to meteorological 

 influences, and to which the universally sparse-clad southvfest slopes 

 throughout the lower mountain-ranges bordering the Snake plains are 

 ahke attributable. The prevalent winds during the term of our sojourn 

 were from the southwest, hundreds of observations, such as the bent 

 boughs and unsymmetrical tops of the trees exposed to their force, every- 

 where attesting the violence of the winds, while the opposite slopes and 

 deeper ravines, which are sheltered from the fierce blast, are quite gen- 

 erally clothed with valuable forests. Almost the same observations hold 

 equally true in the distribution or maintenance of the drifts of snow, 

 the drifts on the southwest-facing slopes being hcked up or evaporated 

 by the searching winds before the season is far advanced, while couloirs 

 and snoAV-cornices may remain the whole summer just over the crests. 

 It is a singular fact, though not a universal one, that the majority of the 

 ami>hitheatres and abrui)ter breaks in the mountainous districts face the 

 east or north, as though the frost-bound side had the major share in the 

 slow process of mountain sculpturing. In this way, very many of the 

 loftier peaks bordering the Snalie Valley present amphitheatre excava- 

 tions in their northeast faces, while that toward the valley rarely shows 

 so extensively eroded depressions, however rugged theii' aspect may be, 

 and, indeed, usually is. 



Above the latter locality low bluffs of dark basaltic rock approach the 

 southwest margin of the river, along which narrow tracts of alluvial 

 bottom land border this side of the stream, Avhich on the opposite side 

 expand into the gravelly sage-covei-ed i)lain of the lower basin. Higher 

 up the valley the sedimentary mountains on either side approach nearer, 

 the volcanic benches becoming higher and crowded into a narrow space, 

 where they form a line of quite prominent bluffs on the northeast side, 

 the bottoms being crowded to the opposite side, where they were found 

 to be composed of coarse, water- worn materials, built up into low, well- 

 defined terraces 10 to 25 feet above the i^resent level of the stream. At 

 the upper end of this bottom tract, a prominent pyramidal hill chokes 

 the valley ; it was found to be composed of flesh-colored trachyte with 

 darker volcanic ledges, and which doubtless is a remnant of the volcanics 

 which formerly flooded the entii'e valley. Above this conspicuous land- 

 mark, to which the name Pyramid Butte was given, the A'olcanics cease 

 on the southwest side of tlie valley, which oi)ens out in a fine terraced 

 tract filling the deboucliure of Pyramid Creek. There are at least four 

 distinct terrace levels, of which the two upper and more prominent reach 

 a height respectively 40 and 90 feet above the level of the river. On the 

 opposite side the volcanic bench terminates in high bluffs, and at the nar- 

 rows, a short distance above, it breaks down vertically, in picturesque 

 l^alisades several hundred feet in height, whose base is swept by the 

 powerful current of the river. 



A marked change at once talvcs place on entering the upper basin of 

 the valley. If ever the volcanics extended farther than the narrows, 

 they have been entirely swept out of the upper basin, which is occupied 

 by a fine wide bottom tract based upon a deposit of water- worn pebbles 



