72 GEOLOGICAL SURVEY OF THE TERRITOEIES. 



tially 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 continued evidence of the outpouring of 

 the fluid material in the form of surface beds or in layers thrust between 

 the fossiliferous strata. These igneous beds thin out rapidly as we re- 

 cede from the point of effusion. A large number of these centers of 

 protrusion may be seen along the slope of the mountains west of the 

 Judith range. 



The erupted material sometimes presents a vertical wall 300 feet high, 

 then suddenly thins out and disappears. The Judith, Bear's Paw, and 

 Little Eocky Mountains seem to be composed for the most part of granite 

 and other rocks, with igneous protrusions here and there. I had sup- 

 posed from the observations made in my former explorations that the 

 central portions of our mountain-ranges were composed of feldspathic 

 granite, and to a certain extent this is true of the more eastern outliers; 

 but the observations during this expedition have convinced me that these 

 rocks, which I have classed as eruptive, composed by far the greater 

 portion of the mountain masses of the West. 



In this connection I have thought it best to remark more systematically 

 in regard to the x)rincipal rivers that drain this immense area of country. 

 The Missouri Eiver and its tributaries form one of the largest as well as 

 most important hydrographical basins in America. It drains an area 

 of nearly or quite 1,000,000 square miles. Taking its rise in the loftiest 

 portions of the Eocky Mountains, near latitude 44"^, longitude 113^, it 

 flows northward in three principal branches, Madison, Gallatin, and 

 Jeflerson Forks, to their junction, and then proceeds onward until it 

 emerges from the gate of the mountains, a distance of nearly 200 miles; 

 it then bends to the eastward, flowing in this direction to the entrance 

 of White Earth Eiver, a distance of nearly 500 miles; it then gradually 

 bends southward and southeastward to its junction with the Mississippi, 

 a distance of 1,500 to 2,000 miles. The branches which form the sources 

 of the Missouri rise in the central portions of the Eocky Mountain range, 

 flowing through granite, basaltic, and the older sedimentary rocks, until 

 it emerges from the gate of the mountains, when the Triassic and Jurassic 

 are shown. The falls of the Missouri, extending for a distance of 20 or 

 30 miles, cut their way through a great thickness of compact Triassic 

 rocks. Below the falls the channel makes its way through the soft yield- 

 ing clays and sands of the Cretaceous beds for about 250 miles, with the 

 exception of the Judith Tertiary basin, which is about 40 miles in length. 

 The Cretaceous beds continue, extending nearly to the mouth of Milk 

 Eiver, when the Lignite Tertiary formations commence. These are also 

 composed of sands, marls, and clays, as the character of the valley will 

 show. 



The river flows through these Tertiary rocks to the mouth of Heart 

 Eiver, below Fort Union, a distance of nearly 250 miles, when the Cre- 

 taceous rocks come to the surface again. These latter rocks extend 

 nearly to Council Blufifs, a distance of over 500 miles. I have estimated 

 the distance in a straight line as nearly as possible. Just above Council 

 Blulfs the Coal-Measure limestones commence, and the valley of the 

 Missouri becomes more restricted, though it is of moderate width even 

 below the mouth of the Kansas. 



The Yellowstone Eiver is by far the largest branch of the Missouri, and 

 for 400 miles from its mouth up it seems to be as large as the Missouri 

 itself from Fort Union to Fort Pierre. It is navigable for large steamers 

 during the spring and early summer for 300 to 400 miles above its 

 junction with the Missouri. This river also takes its rise in the main 



