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Show 203 ing, which show clearly that there was originally a considerable thickness of them. At the upper end, broad sheets of basalt have flowed over it, protecting the lake- deposits from erosion. This basin has been ertirely scooped out of the mountains, which originally extended across much as we now see them on either side. All over the valleys are remnants of the various rocks, changed and unchanged, just rising above the surface. At the right are seen a group of Carboniferous limestones, which extend directly across the Yellowstone River. The channel which the river has worn through these rocks is called the Lower Canon, or the Gate of the Mountains. The caiion is about three miles in length, leading into a comparatively small oval expansion, which was originally a lake, and thence into the plains. Inasmuch as all the sedimentary strata from the Lower Silurian to the Liguitic inclusive seem to have been alike involved in the movements of the crust that elevated them to their present position, we infer that these movements were a subsequent event. There is also evidence for the statement that the uuchanged rock, prior to this time, extended uniformly over nearly or quite the entire area of Montana. There is no positive proof that the roeta-morphic rocks were anywhere exposed. The members of the survey searched in vaiu for any proof of unconformability of position in any of the unchanged strata. Therefore, the vast erosion which has taken place in this region must have occurred during the crust- movements and subsequently. I have before stated that these lake- basins are largely due to erosion. We must conclude, therefore, that depressions were produced during the crust- movements, which received the drainage from the higher lands around, and that the waters thus accumulating gradually carved out these basins as we find them at the present time. These depressions may have been limited in area at first, and may or may not have been produced by the disturbance of the strata; bat it is probable that in most cases it is dae to the latter cause. We may state, however, in this connection, that the channels of auy of the rivers of the West do not necessarily lie along any fissures, anticlinal or synclinal depressions; but the rivers seem to have, in the majority of cases, cut their way directly across the line of fracture, thus carving out deep gorges through the loftiest mountain- ranges. There are numerous examples where rivers seem to have avoided natural depressions, where it would appear that the waters must flow, and cut narrow channels through the hardest rocks. . The Gallatin, as well as the Yellowstone, is a remarkable example of this statement. In their entire course, they run almost directly across the line of fracture of the mountain- rauges in the vicinity. The Yellowstone has worn out the enormous second canon with walls 1,000 to 1,500 feet from the granite nucleus. A little to the northwest, the Gallatin flows between walls of granite 2,000 feet high on either side. Still we must admit that in order to produce such results they must have had an original foothold, as it were, to commence their work. The evidence seems to be clear that, during the Tertiary period, and subsequently, the aqueous forces were much more powerful and widely extended than at the present time. All the watercourses and lake- basins, of the West indicate the existence then, of bodies of water compared with which our present rivers and lakes are but rivulets and ponds. We do not find the Miocene lake- deposits underlying the Pleiocene in the vicinity of the mountain- districts. They are found at a long distance from the principal centers of disturbance. We may suppose, therefore, that there were really three important periods which should be considered : First, when the channels of the rivers and the lake- basins were |
