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Show 208 surface; but through the vast erosion which has taken place subsequently, 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 by erosion. So uniform are these conditions, that one would suspect an intimate connection between the movements of the vast masses of metamorphic rocks which usually form the nuclei of our mountain- ranges aud the production of the igneous matter. 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 t 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 from its commencement, which has been published from time to time in the annual reports, in regard to the channels of our rivers. We fiud that the channelaof our large rivers have not been determined by special Hues of depression or fractures, and that there is no necessary connects . between them. It is not an uncommon o rrence to find the channel of a river passing directly through a mo. ntainrange or a ridge, wheu by a slight flexure it could have occupied * special depression or valley. The West Gallatin River, as is shown in section No. 5, Plate VI, cuts a canon 1,500 to 2,000 feet in depth, through Sedimentary and Archaean rocks, for several miles, when by an easy flexure, as the surface now appears, it might have occupied areas of special or natural depression. Nearly every cauon through which 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 conclude 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 canon, as it passed over what are now the very summits of the mouutains, or ridges; that the erosion of the river- channels kept pace with the slow, uniform, longcoutiuued elevation, and thus these enormous gorges may be accounted for. ' j In my Annual Report for 1872, page 85, i called attention to this feature, and it is undoubtedly applicable to all the great rivers of the I 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 canon for more than two hundred miles from half a mile to a mile in depth, and to the Missouri aud 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 discnssion at a more favorable time h the formation of caftons and valleys of rivers, which enter into the scenery of the country as a most conspicnons feature. The fact that the streams seem to have cut |
