st.johx.] LOWER SNAKE RIVER VALLEY. 409 



and alluvial matter, arranged in a series of much more prominent ter- 

 races than any occurring in the lower basin, the highest of which do not 

 exceed perhaps 50 feet, while the lowest is only a few feet above the river 

 level ; the valley is narrower, but the alluvial terraces occupy the whole 

 space intervening between the foot of the abrupt mountain borders. 

 Numerous small streams debouching from the hills have swept down 

 quantities of debris, which is piled in low-spreading accumulations like the 

 tailings of a sluice-way ; their waters are often spread over the more 

 level bottoms, where they to-day form extensive areas of miry soil, but 

 which will one day be converted into invaluable meadow tracts. At 

 several places along this side of the valley, between the narrows and 

 McCoy Creek, extensive deposits of calcareous tufa occur in two or more 

 fiat terrace levels, which at one point reach the present margin of tlie 

 stream. The highest of these regular-shaped terraces is about 15 feet 

 above the adjacent bottoms, and is terminated abruptly in a miniature 

 escarpment which was once washed by the stream. At one place just 

 above this tufa terrace, low, dome-like deposits of white tufa were seen 

 at the foot of the mountain, evidently the accumulation of a still flowing 

 spring. The pools here and there found on the lower tufa flats, which 

 here form an extensive deposit reaching the river side and some 6 feet 

 above the water-level, were tepid and very perceptibly saline ; their beds 

 are covered with an ochreous-colored precipitate, and the unctuous soil 

 saturated with the mineral substances from the springs was devoured 

 with avidity by our animals. Professor Bradley describes a group of 

 warm springs occurring on the opposite side of the valley, which have 

 built up similar "calcareous, sulphurous, and saline deposits," their tem- 

 perature ranging from 88° to 144° F. The west side springs are, how- 

 ever, in an advanced stage of extinction compared with their former flow, 

 when they built up the extensive, ancient-looking terraces. But the 

 very modern date of even the more ancient of these spring deposits may 

 be inferred from the total absence of river drift upon their surfaces. In 

 crossing these old spring deposits, our animals' tread gave out a hollow, 

 resonant sound, which may be due to the imperfectly impacted layers 

 which are separated by minute cavernous or porous spaces. 



We now come upon an interesting series of deposits, which, though of 

 much more ancient origin than any hitherto mentioned, are apparently 

 equally restricted in their distribution within the limits of the valley. 

 These were first encountered ascending the valley a few miles below the 

 mouth of Salt Biver — the exposures only appearing in the higher river 

 banks and terraces cut by the inflowing tributaries, and extending south 

 as far as our examinations were carried, and beyond, on the one hand, 

 quite to the debouchure of the grand caiion, and on the other, up the 

 valley of Salt Biver. They consist of drab and light-brown sandy clays 

 and x>artially indurated buff and brownish sands, banded with various 

 shades of red in thin layers, giving to the exposures a very beautiful 

 variegated appearance. As before mentioned, these deposits outcrop 

 quite extensively in the higher terrace banks along the river, where they 

 may be well studied. They dip eastward or northeastward with great 

 uniformity, probably at angles of 25° to 40°, or inclining obliquely up- 

 stream. In the valley side, at the debouchure of McCoy Creek, they are 

 seen resting unconformably upon the southwest-dipping Mesozoic sand- 

 stones, and also along the west side of Salt Biver, above its mouth. But 

 the river-bank exposures afford the most interesting sections. Here the 

 beds appear planed off to various levels by the former action of the 

 stream, showing shallow channels with low-terraced margins, upon 

 which subsequent depositions of water-worn bowlders, gravel, and sands 



