208 



surface ; but through the vast erosion which has taken place subse- 

 quently, the igneous rocks of different ages have been exposed to view. 



Over a considerable portion of Montana, Northwestern Wyoming, and 

 the greater part of Idaho, the igneous material, with the accompanying* 

 tuffs and breccias, conceal the sedimentary as well as the metamorphic 

 rocks ; and they are only exposed to view in the deep gorges of the 

 streams which are produced bj' erosion. So uoitbrm are these condi- 

 tions, that one would suspect an intimate connection between the move- 

 ments of the vast masses of metamorphic rocks which usually form the 

 nuclei of our mountain-ranges and the production of the igneous mat- 

 ter. Is it probable that the movements are so deep-seated as to reach 

 down to melted matter already in that state, and thus affording it access to 

 the surface, or was there sufficient heat generated by the friction of vast 

 masses of rocks upon each other to produce the igneous rocks, as well 

 as the force which has ejected such a vast amount of tuffs and breccia, 

 as we find about the sources of the Yellowstone f 



I am not now prepared to discuss this subject, but will simply state 

 that our observations all over the West tend to show a most intimate 

 relation between the eruption of igneous rocks and the elevations of the 

 regular mountain-ranges. Again, an interesting series of observations 

 has been made by the Survey tVom its commencement, which has been 

 published from time to time in the annual reports, in regard to the chan- 

 nels of our rivers. We find that the channels of our large rivers have 

 not been determined by special lines of depression or fractures, and that 

 there is no necessary connection between them. 



It is not an uncommon occurrence to find the channel of a river 

 passing directly through a mountain range or a ridge, when by a slight 

 flexure it could have occupied a special depression or valley. The West 

 Gallatin River, as is shown in section No. 5, Plate VI, cutsacaiion 1,500 

 to 2,000 feet in depth, through Sedimentary and Archisan rocks, for sev- 

 eral miles, when by an easy flexure, as the surface now appears, it might 

 have occupied areas of special or natural depression. 



Nearly every caiion through Avhich any of the rivers or smaller 

 streams of Montana pass, and they are very numerous, present the same 

 peculiarities. The Jefferson Fork presents striking examples of this 

 kind, while the Missouri River below the junction of the three forks, the 

 numerous gorges or canons are plain illustrations of this statement. 

 We must couclnde that since the channels of these streams were marked 

 out, the mountains have been elevated at least the amount of the present 

 height of their summits above the beds of the streams; that the surface 

 at that time was more favorable for the concentration of the drainage- 

 waters along the line of the present caiion, as it passed over what are 

 now the very summits of the mountains, or ridges ; that the erosion of 

 the river-channels kept pace with the slow, uniform, long-continued 

 elevation, and thus these enormous gorges may be accounted for. 



In ray Annual Report for 1872, page 85, I called attention to this 

 feature, and it is undoubtedly applicable to all the great rivers of the 

 West, to the Snake and Columbia Rivers flowing through the vast 

 basaltic walls to the Pacific Ocean, to the Colorado of the West, that has 

 worn out its caiion for more than two hundred miles from half a mile to 

 a mile in dei)th, and to the Missouri and Yellowstone Rivers from their 

 sources to their entrance into the plains. In the Annual Report for 1872, 

 I made the following remarks : 



Another interesting point I have reserved for discussion at a more favorable time is 

 the formation of canons and valleys of rivers, which enter into the scenery of the 

 country as a most conspicuous featiire. The fact that the streams saem to have cut 



