GEOLOGICAL SUEVEY OF THE TERRITOEIES. 139 



of those near tlie sources of the river. The granite rocks begin to 

 prevail, and the mountains have an older appearance. The valley is 

 full of immense, rounded, granite bowlders, which have been swept down 

 from the mountains by aqueous forces not now in existence. There are 

 also in this valley well-defined terraces 30 to 50 feet high, and above 

 the forks are rows of basaltic columns like those in the lower portion of 

 the Grand Canon. At the mouth of Hell-Eoaring Eiver the granitoid 

 rocks are displayed on a grand scale. As I have previously stated, the 

 basis rocks of the mountains are granite or gneissic granites; some- 

 times they are true granites, as exposed about the junction of the 

 East Fork and main branch of the Yellowstone, and at Hell-Eoariug 

 Mountain ; even these, perhaps, come under the head of stratified meta- 

 morphic rocks, from the fact that above and below these thick, massive 

 granites are groups of gneissic strata of various textures. On the east 

 fork I saw only the Carboniferous limestones. Although the Jurassic, 

 Cretaceous, and Tertiary formations occur in full force at Gardiner's 

 Eiver, over all has been poured the igneous material, which rapidly 

 increases in mass and importance as we ascend the valley, until, about 

 the sources, it entirely covers all other rocks, and sends its multiform 

 peaks high up among the perpetual snows. 



The bridge which has been constructed across the Yellowstone, near 

 the forks, was designed to accommodate the miners on their way to 

 the gold-mines on Clark's Fork, and is the first and only bridge ever 

 built on the Yellowstone. It may become a matter of some historical 

 importance to note this fact here. The gold-mines are all in the granit- 

 oid rocks, and, from what I can learn, all the streams that flow into 

 the Yellowstone from the east side of the range cut deep down into the 

 metamorphic group. The mines are reported to be excellent, and I 

 am inclined to the belief that the most important mining districts of the 

 Yellowstone drainage will be found eventually on the eastern slope of 

 the Heart and Snowy Eanges. 



CHAPTEE YIII. 



FORT ELLIS— THREE FORKS— JEFFERSON FORK— BEAVER HEAD GASTON— 

 MEDICINE LODGE CREEK. 



In this and the following chapter, I will endeavor to present a brief 

 summary of the geological features of the country along our homeward 

 journey, from Fort Ellis to Evanston, on the Union Pacific Eailroad. 

 In a former chapter I have alluded to the range of mountains which 

 extends along the east side of the Gallatin Fork. I also spoke of the 

 Pliocene or lake deposits which jutted up against the base of these 

 mountains, sometimes reaching a thickness of 600 or 800 feet. 



The beautiful valley of the Gallatin was undoubtedly one of the numer- 

 ous lake basins of the West of which so much has been written in my 

 reports for years past. The Pliocene hills opposite Fort Ellis and 

 Bozeman overlook the valley for a great distance, and at this season 

 of the year (September 6) hundreds of acres of golden grain can be seen. 

 There is a remarkable uniformity in the bright-yellow color of a field of 

 grain in this country, probably due to the uniformity of the climate ; 

 the sun shines without interruption for weeks in succession. The mount- 

 ains are composed mostly of rocks of Carboniferous age. They incline 



