ing, which show clearly that there was originally a cousidcrable thick- 

 ness of them. At the upper eud, broad sheets of basrdt have flowed 

 over it, protecting the lake-deposits from erosion. This basin has been 

 eT'tirely scooped out of the mountains, which originally extended across 

 much as we now see them on either side. All over the valleys are rem- 

 nants 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 Eiver. The channel which 

 the river has worn through these rocks is called the Lower Canon, or the 

 Gate of the Mountains. The calion is about three miles in length, lead- 

 ing 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 Lignitic 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 

 iivent. There is also evidence for the statement that the unchanged 

 rock, prior to this time, extended uniformly over nearly or quite the 

 entire area of Montana. There is no jiositive proof that the meta- 

 morphic rocks were anywhere exposed. The members of the survey 

 searched in vain for any proof of unconfoimabiiity 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 drain- 

 age from the higher lands around, and that the waters thus accu- 

 mulating 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 disturb- 

 ance of the strata; but it is probable that in most cases it is due 

 to the latter cause. We may state, however, in this connection, that 

 the channels of any 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 mount- 

 ain-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-ranges in the vicinity. The Yellow- 

 stone has woru out the enormous second canon with walls 1,000 to 1,500 

 feet from the granite nucleus. A little to the northwest, the Gal- 

 latin flows between walls of granite 2,000 feet high on either side. 

 Still we must admit that in order to produce si»ch 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 sub- 

 sequently, the aqueous forces were much more powerful and Avidely ex- 

 tended 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. AYe may suppose, therefore, 

 that there were really three important periods which should be consid- 

 ered : First, when the channels of the rivers and the lake-basins were 



