GEOLOGICAL SURVEY OF THE TEREITORIES. 71 



■with perpetual snow. We tlimk that the evidence is quite conclusive 

 that, up to the time of the accumulation of a large portion of the Lig- 

 nite Tertiary beds, all these formations, from the Silurian to the true Lig- 

 nite strata inclusive, were in a horizontal position, extending continu- 

 ously over the whole area occupied by the mountains; but, as they 

 were slowly elevated, the central portions were removed by the erosive 

 action of water. The eruptive portion, which unites the Big Horn 

 range with the Wind Eiver Mountains, is exceedingly picturesque, pre- 

 senting the appearance of a connected series of basaltic cones, and so 

 rugged and inaccessible are they that the persevering trappers have 

 never been able to penetrate them in their hunting explorations. 



Like the Black Hills, the Big Horn range does not give rise to many 

 important sub-hydrographical basins. The largest stream in this re- 

 gion, and one which gives name to the mountains, rises in the Wind 

 Eiver range, passes through the Big Horn Mountains, and unites with 

 the Yellowstone about 70 miles to the southward. Before reaching the 

 mountains it takes the name of Wind River, and assumes the name of 

 Big Horn after emerging from them. This range, however, constitutes 

 quite an important feeder to the Yellowstone. Powder Eiver, which 

 rises in this range by numerous branches, drains a large area mostly 

 Lignite Tertiary, and pours a considerable volume of water into the 

 Yellowstone, near longitude 105^° and latitude 46^°. Tongue Eiver is 

 the next most important stream, which, though not draining so great 

 an area as Powder Eiver, empties into the Yellowstone a much larger 

 volume of water. 



The Medicine Bow and Sweetwater Mountains appear to be of the 

 same character for the most part; but on the east side of the Sweetwater 

 Eiver the evidence of igneous action is shown on a large scale. The 

 ancient volcanic material would seem to have been elevated to a great 

 height in but a partially fluid condition, and then to have gradually 

 cooled, affecting to a greater or less extent the fossiliferous strata in 

 contact. 



Near the junction of the Popoagie with Wind Eiver we come in 

 full view of the Wind Eiver Mountains, which form the dividing crest of 

 the continent, the streams on the one side flowing into the Atlantic and 

 those on the other into the Pacific. This range is also composed to a 

 large extent of red and gray feldspathic granite, with the fossiliferous 

 rocks inclining high upon its sides. After passing the sources of Wind 

 Eiver the mountaiDS appear to be composed entirely of eruptive rocks. 

 Even the three Tetons, which raise their summits 11,000 feet above the 

 ocean-bed, are formed of very compact basaltic rocks. The Wasatch 

 and Green Eiver ranges, where we observed them, have the same 

 igneous origin, and the mountains all along the sources of the different 

 branches of the Columbia exhibit these rocks in their full force. In 

 Pierre's Hole, Jackson's Hole, and other valleys surrounded by upheaved 

 ridges, these ancient volcanic rocks seem to have been poured out over 

 the country and to have cooled in layers, giving to vast thicknesses of 

 the rocks the appearance of stratified beds. 



The mountains about the sources of the Missouri and Yellowstone 

 Elvers are of eruptive origin, and in the valley of the Madison Fork of 

 the Missouri are vertical walls of these ancient volcanic rocks 1,000 to 

 1,500 feet in height, exhibiting the appearance of stratified deposits, 

 dipping at a considerable angle. As we pass down the Madison Fork 

 we find some beds of feldspathic rocks and mica and clay slates be- 

 neath the eruptive layers dipping at the same angle. After passing the 

 divide below the three forks of the Missouri, we see a number of par- 



