GEOLOGICAL SURVEY OF THE TERRITORIES. 29 



ridges, forming several anticlinals. The long walls of sandstone extend 

 across the country northwest and southeast, inclining 30° to 50°. The 

 sandstones are mostly of the age of the lignite tertiary, gray and rusty 

 brown color, with all kinds of texture and modes of deposition. 



About five miles west of the buttes a new formation appears of later 

 date, the latter dipping 5° to 10° in the same direction, but not con- 

 forming to the older beds. Long ridges and benches extend down nearly 

 parallel with the course of the Sweetwater, composed mostly of in- 

 durated argillaceous sands of a lighter color and less variable materi- 

 als than the lignite beds. The high ridges around our camp are capped 

 with a thick bed of quartzite sandstones, sometimes approaching a 

 pudding-stone. West of Willow Springs we ascend a high hill covered 

 with a thick bed of quartzitic sandstones in drift, and capped with the 

 coarse sandstones. It is quite probable that we have been passing over 

 the eastern rim of an ancient fresh-water lake, the rock materials of which 

 are incidental with the trend of their deposits. Long high benches, com- 

 posed of these beds, extend far southward as the eye can reach, parallel 

 with the North Platte and the Sweetwater. It is plain that they jut up 

 close against the sides of the ridge that borders the north side of the 

 Platte, near the junction of the Sweetwater. Here and there the creta- 

 ceous or lignite ridges rise above the more modern deposits, always in- 

 clining at a large angle, showing unmistakable discordancy. Ascending 

 an elevation of about four hundred feet, west of Willow Springs, we de- 

 scend a long slope, into the valley of the Sweetwater, which, in some 

 respects, is one of the most interesting geological districts I have ever 

 examined in the west. There is a high ridge or divide between the drainage 

 of Wind Eiver, North Platte, and Sweetwater, three hundred to four 

 hundred feet above the channels of these streams, which is composed of 

 the tertiary beds. The Sweetwater forms a distinct concavity, with this 

 high divide on the north and east, and the valley has been scooped out 

 so that until we reach the Sweetwater Canon, near the South Pass, only 

 the massive granite ridges rise up among the modern tertiary beds which 

 jut close up against their base. This is most emphatically a valley of 

 denudation, over a space of at least thirty to fifty miles in width. All 

 the unchanged formations, from the lignite tertiary down to the massive 

 feldspathic granites, have been worn away, leaving the granites scat- 

 tered over the valley in the isolated ridges. At that time there was a 

 fresh-water lake which occupied the entire valley, much as Salt Lake once 

 occupied the great basin, concealing most of the granite ridges, while 

 others rose above the waters like islands. Then was deposited what 

 might be called the Sweetwater group, or perhaps a series of beds identical 

 with the upper portion of the Wind Eiver deposits. These were scooped 

 out again in time, and the pliocene marls and sands were deposited ; and 

 then again there was an other scooping out of the valley, and finally a cov- 

 ering the hills with drift. In the pliocene marls andsandsare quite abund- 

 ant remains of mammals, similar to those which are found on the Niobrara 

 Eiver, in Nebraska. The Seminole and Sweetwater mountains, although 

 covered all about their flanks with tertiary beds, show, higher* up, the 

 elevated ridges of Potsdam sandstone and carboniferous limestones. The 

 whole range is comparatively smooth and grassed over, as if it had been 

 too high above the waters to have been affected to any extent by the 

 later forces that scooped out the valley ; so, too, the Sweetwater Moun- 

 tains, or hills as they should be called, simply expose the Potsdam sand- 

 stone, carboniferous, red beds, with very small areas of the Jurassic, cre- 

 taceous, and granites. Indeed, the Sweetwater valley is a sort of anticlinal, 

 with the Seminole and Sweetwater hills on the south side, and the divide 



