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Show 200 ain- ranges, and continued periodically, even up to the present tim nearly so. The drainage was undoubtedly marked out at an early period. Yellowstone and Madison Hi vers rise in the main divide of the R Mountains, while crowded in between them, as it were, is the Gal River, which fails to reach the divitle with auy of its sources. The latin flows through a narrow channel, or canon, most of the way fror source to its entrance into the Gallatin Valley, a distance of about miles. The drainage seems to have originated in a sort of depressic sag, in the sedimentary crust; for the gorge has there worn through 1 rocks for nearly the entire distance, and the inclination of the stral either side is toward the canon. Nothing seems to have even defh the river from its course, but it has worn its narrow way directly thr highest mountains, as is shown in the six consecutive sections in P IV, V, and VI. The erosion of the channel, or canon, must have menced with the elevation of the crust, and continued on, keeping with the elevating forces. Obstructions from time to time have occu which produced in part the numerous lake- basins which we find a present time in the valleys of all the mountain- streams. In the previous reports of the Suivey, I have treated much in <; the curious old lake- basins that are found so extensively all ove West. There is a certaiu group of them that might be classed unde head. They have been called Modern Lake- deposits, Lacustrine Their age is probably Pleiocene, but they undoubtedly overlap wh* have usually understood as the Post- Pleiocene period, reaching al up to the present time. There is, however, a subsequent dej which, ou the geological map, we have regarded as fluviatile, whic account of its extent must be noted. This oftentimes conceals the custrine deposits. The fluviatile deposits are entirely local, and con to the drainage areas iu which they are found. They assume im ance from the fact that they date back to a period when there much more water in the streams than at the present time, and in c quence the . results of the aqueous forces were much more ma than they are now. There is another important feature, that al placer- mining is carried on in these deposits. They undoubtedly baclf so as to include what is usually understood as the Glacial pi in the West. In the various mining- gulches in Montana, these fluvi deposits are oftentimes of great thickness, made up mostly of much-bowlders. In Alder Gulch, for example, which is the valley of a t branch of the Stinkingwater which flows into the Jefferson, $ 30,001 of gold have been taken out. Near the source of the gulch, the bow are of great size, but growing gradually smaller toward its junctiou the main branch until the fluviatile deposits are made up of rathei sediments, with perhaps layers of gravel. The vast extent of this de conveysa dim conception of the tremendous erosion the surface has ui gone in past times. There is comparatively little snow in these regio the present time, and the streams are never so high that they pro any very marked effect on these deposits, and scarcely nothing 8 as erosion is concerned, so that we are led easily to the conclusion there was a period when the aqueous and most probably the a< glacial forces acted with great power. As we have previously stated, the Lacustrine deposits beloni part at least, to a prior period. Perhaps the most conspicuous exa of one of these lakes is found at and near the junction of the three 1 of the Missouri. The junction of the three branches seems to b< north end of this basin, though immediately below the junction I |