OF THE UPPER MISSOURI. 119 
hydrographical basins. ‘The largest stream in this region, and one which gives name to 
the mountains, rises in the Wind river range, passes through the Big Horn mountains, and 
unites with the Yellowstone about seventy 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 river, which rises in this range by numerous branches, drains a large 
area, mostly Lignite Tertiary, and pours a considerable volume of water into the Yellow- 
stone, near lon. 105° and lat. 462°. Tongue river is the next most important stream, which, 
though not draining so great an area as Powder river, empties into the Yellowstone a much 
larger bulk of water. 
The Medicine Bow and Sweet Water mountains appear to be of the same character, for 
the most part ; but on the east side of the Sweet Water river the evidence of igneous action 
is shown on a large scale. The ancient volcanic material would seem to have been elevated 
toa 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 Popo Agie with Wind river, we come in full view of the Wind 
River 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 river, the mountains 
appear to be composed entirely of eruptive rocks. Even the three Tetons, which raise 
their summits eleven thousand feet above the ocean level are formed of very compact 
basaltic rock. The Wasatch and Green River 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 thick- 
nesses of the rocks the appearance of stratified beds. 
The mountains about the sources of the Missouri and Yellowstone rivers are of eruptive 
origin and in the valley of the Madison fork of the Missouri are vertical walls of these 
ancient volcanic rocks one thousand to fifteen hundred feet in height, exhibiting the ap- 
pearance of regularly stratified deposits dipping at a considerable angle. As we pass 
down the Madison we find some beds of feldspathic rocks and mica and clay slates beneath 
the eruptive layers, dipping at the same angle. After passing the divide below the three 
forks of the Missouri we see a number of partially detached ranges, which appear to be of 
the same igneous character. In the Belt, Highwood mountains, and indeed all along the 
eastern slope in this region, we find continual evidence of the outpouring of the fluid ma- 
