32 GEOLOGICAL SURVEY OF THE TERRITORIES. 



adding mucli to the picturesqueness of the scenery. These dikes are 

 quite common in some of the ranges, and all have a peculiar black ap- 

 pearance in the distance ; but the difference in texture is shown by the 

 influence of the atmosphere upon them. Some of them stand up as 

 sharp and angular as ever, but most of them have been so rounded off 

 that the loose masses are nearly spherical, and the thin coats are fall- 

 ing off like the layers of an onion. Sometimes the materials are coarse, 

 and the rock falls in pieces; again the mass is round and smooth, 

 glistening like black opaque quartz. There is no evidence at the pres- 

 ent time of the age of these dikes, except that the material was thrust 

 up through the fissures in the granite. They may be of different ages 

 or of the same age. We find they have been subject to the same 

 erosion as the granite, and extend across the country in regular bands. 

 From the Devil's Gate we traveled westward along the bottoms of 

 the Sweetwater to the " Cloven Peak," a conspicuous landmark for the 

 traveler. On our right hand we were walled in most effectually by the 

 granite ridges ; on our left, in the distance, about fifteen miles, was the 

 Seminole range, or hills, as I have regarded them in this report. They 

 rise high above the surrounding country, but seem to have formed the 

 south side of the tremendous erosive forces which once swept through 

 this valley. The outlines of these hills are so different from any others 

 that are within the range of vision, that they excite the attention of 

 the observer at once. There are no ridges of upheaval along the flanks, 

 as in the case of the Laramie range, but the grassy plains jut up close 

 to them, and the very summits are, for the most part, rounded and 

 grassed over. No timber clothes them, except now and then a group 

 of poplars. The north side is very abrupt, while the south side 

 slopes off into the plains. Near the Three Crossings there is a low 

 point or pass in the hills which extends for about ten or fifteen miles, 

 where a second range commences, called the Sweetwater Hills, and 

 continues to the South Pass. The Seminole Hills rise about fifteen hun- 

 dred feet above the Sweetwater Valley, while the Sweetwater Hills vary 

 from eight hundred to twelve hundred feet. Near Cloven Peak, fifteen 

 miles west of Devil's Gate, there are some high bluff banks on the south 

 side of the Sweetwater, about one hundred feet high, which indicate 

 the existence of quite modern tertiary beds, like those on the Niobrara 

 Eiver. They are composed of indurated sands and marls of a light- 

 gray or cream color, and are in appearance precisely like those seen on 

 the Laramie Eiver, and many other places, which I have usually re- 

 garded as of the pliocene age. Still farther to the westward are numer- 

 ous exposures of these beds, which are weathered into the usual fortifi- 

 cation-like forms, and scattered around their base are large numbers of 

 remains of extinct mammals and turtles, apparently identical with those 

 found on the Niobrara. They occur in the same beautiful state of pre- 

 servation. These beds are full also of oddly-shaped concretions of sand- 

 stones of all sizes. The sandy beds disintegrate very readily, and the 

 bottoms, as well as the road, are made up of loose sand, which is readily 

 moved by the wind, rendering traveling difficult. Indeed, the entire 

 valley of the Sweetwater, below St. Mary's Station, is more or less 

 covered with moving ajands, the result of the disintegration of these 

 pliocene beds. The appearance of the surface is similar to that seen on 

 the Niobrara Eiver and the head of the Loup Fork in Nebraska. I am 

 inclined to believe that these pliocene beds are a deposit made subse- 

 quent to the somber brown indurated sands, and the conglomerate sand- 

 stone which forms the outer rim of the Sweetwater basin. The pliocene 

 beds are best shown in the valley itself near the stream, while no traces 



