34 GEOLOGICAL SURVEY OF THE TERRITORIES. 



and worn crystals of quartz, with a thickness of from four hundred to 

 six hundred feet. Here indurated quartzose sands have been weathered 

 into most fantastic columns fifty to sixty feet high, giving to the group 

 in the distance the appearance of white marble monuments in a church- 

 yard. Much of the rock looks like the decomposed gangue of silver and 

 gold lodes as seen in Colorado, that is, small fragments of quartz in a 

 feldspar paste. On the side of this almost perpendicular hill, the summit 

 of which is eight hundred to one thousand feet above the Sweetwater 

 Valley, are enormous granite boulders of all kinds, gray and red feld- 

 spathic massive rocks. Not only the granites but also the sandstones de- 

 compose by exfoliation. In the brown beds are seams of rust-brown 

 pudding-stone, which disintegrates on the low hills and in the valleys, 

 covering the surface with small smooth pebbles and fragments of clay. 

 This entire range of hills has the north side very abrupt and high 

 toward Sweetwater valley, but the south side slopes gently down 

 into the plains. I have no doubt that the nucleus of this range of hills 

 is composed of the older rocks, as Silurian, carboniferous, or red beds, 

 &c. ; but, so far as we can see at this time, the modern tertiary beds 

 seem to conceal them from view. Some of the valleys of erosion parallel 

 to the hills show by the high walls a great thickness of tertiary beds. 

 From the summits the eye extends far southward, fifty miles or more, over 

 a most desolate, barren plain, with here and there a table-top butte to 

 show that the surface was once much higher than at present. It is cut 

 up into innumerable valleys, which give to the surface an irregular, wavy 

 appearance. Not a tree or shrub greets the vision over this vast desert 

 waste. The immense quantities of granite boulders, red and gray, which 

 literally cover the tops and sides of this range of hills, must have been 

 swept down from the Wind River Mountains. Some of these granite 

 masses are ten to fifteen feet in diameter; others are sunk so deep in the 

 earth that they appear to be in place. Across the plains, at least one hun- 

 dred miles distant to the southwest, two or three low ranges of mountains 

 or hills are visible. Toward the west end and on the north side of the 

 range of hills I noticed a peculiar semicircular depression, about twenty- 

 five feet below the summit, which affords an example of a land-slide on 

 a large scale. This slide covers an area of about a mile in length and 

 a fourth of a mile .in width, and is covered with groves of the aspen. 



Above the Three Crossings, on the north side of the Sweetwater, are 

 several quite conspicuous granite ridges, but they soon disappear. 

 Soon the beds of Potsdam sandstone, with carboniferous limestones and 

 portions of the triassic, make their appearance, inclining a,t an angle of 

 20° to 30° northwest, and, m a short distance, pass beneath the Wind 

 River deposits. We could not ascertain that any rocks older than the 

 miocene tertiary beds occur along the northern rim of the valley from 

 Willow Springs up to this point. 



From the Three Crossings to St. Mary's Station the valley bottom of 

 the Sweetwater is about half a mile in width and looks like a meadow, as 

 usual. Our camp on the night of the 30th was near the point where the 

 stream comes out of the first ridge. Hitherto it has flowed through an 

 anticlinal valley, with the Sweetwater mountains or hills on the south 

 side, and the granite ridges, capped with Potsdam sandstone ridges near 

 South Pass. One of the most conspicuous features are the long benches 

 that come down from the Sweetwater hills so regular, so high, and 

 evenly rounded that they strike the eye at once. They are composed of 

 modern tertiary marls, probably pliocene. 



About four miles below St. Mary's Station the Sweetwater flows 

 through a ridge of Potsdam sandstone with a trend nearly east and 



