20U 



ainranjiPS!, and continued periodically, even up to the present tinie, or 

 nearly so. 



The drainage was undoubtedly marked out at an early period. The 

 Yellowstone and Madison Kivers rise in the main divide of the Eocky 

 Mountains, while crowded in between them, as it were, is the Gallatin 

 Eiver, which fails to reach the divide with any of its sources. The Gal- 

 latin flows through a narrow channel, or canon, most of the way from the 

 source to its entrance into the Gallatin Valley, a distance of about forty 

 miles. The drainage seems to have originated in a sort of depression, or 

 sag, in thesedifnentary crust; for the gorgr has there worn throngb these 

 rocks for nearly the entire distance, and tlie inclination of the strata on 

 either side is toward the canon. Nothing seems to have even deflected 

 the river from its course, but it has worn its narrow way directly through 

 highest mountains, as is shown in the six consecutive sections iu 'Plates 

 IV, V, and VI. The erosion of the channel, or canon, mnst have com- 

 menced with the elevation of the crust, and continued on; keei)ing pace 

 with the elevating forces. Obstructions from time to time have occurred, 

 which produced in })art the numerous lake-basins which we find at the 

 present time in the valleys of all the mountain-streams. 



In the i)revious rei)orts of the Survey, I have treated much in detail 

 the curious old lake-basins that are-found so extensively all over the 

 West. There is a d-ertaiu gioup of them that might be classed under one 

 head. They have been called Modern Lake-deposits, Lacustrine, &c. 

 Their age is ])iobably Pieiocene, but they undoubtedly overlap what we 

 have usually understood as the Post-Pleiocene period, reaching almost 

 up to the present time. There is, however, a subsequent deposit, 

 which, on the geological map, we have regarded as fluviatile, which on 

 account of its extent must be noted. This oftentimes conceals the La- 

 custrine deposits. The fluviatile deposits are entirely local, and confined 

 to the drainage areas in which they are found. They assume import- 

 ance from the fact that they date back to a period when there was 

 much more water in the streams than at the present time, and in conse- 

 quence the results of the aqueous forces were much more marked 

 than they are now. There is another important feature, that all the 

 placer-mining is carried on in these deposits. They undoubtedly date 

 back so as to include what is usually understood as the Glacial period 

 in the West. In the various mining- gulches in Montana, these fluviatile 

 deposits are oftentimes of great thickness, made up mostly of much-worn 

 bowlders. In Alder Gulch, for example, which is the valley of a small 

 branch of the Stinkingwater which flows into the Jefi:erson, $30,000,000 

 of gold have been taken out. Near the source of the gulch, the bowlders 

 are of great size, but growing gradually smaller toward its junction with 

 the main branch until the fluviatile deposits are made up of rather fine 

 sediments, with perhaps layers of gravel. The vast extent of this deposit 

 conveys a dim conception of the tremeudouserosion the surface has under- 

 gone in past times. Thereis comparatively little snowin these regions at 

 the present time, and the streams are never so high that they i)roduce 

 any very marked effect on these deposits, and scarcely nothing so far 

 as erosion is concerned, so that we are led easily to the conclusion that 

 there was a period when the aqueous and most j)robably the aqueo- 

 glacial forces acted with great power. 



As we have previously stated, the Lacustrine deposits belong, in 

 part at least, to a prior period. Perhaps the most conspicuous example 

 of one of these lakes is found at and near the junction of the three forks 

 of the Missouri. The junction of the three branches seems to be the 

 north end of this basin, though immediately below the junction there 



