GEOLOGICAL SUKVEY OF THE TERRITORIES. 33 



valley at its base, will at no distant period be visited by multitudes of 

 tourists, and afford many a subject for correspondence for the secular 

 press. 



Glancing at the map, it will be seen that numerous little streams flow 

 down from the base of the mountains and eonpty their waters into the 

 Yellowstone. Each one of these streams in the mountains spreads out 

 into a great number of branches hve to twenty miles in length. It is 

 to these little streams that we are indebted for the inner history of these 

 grand mountains. We may say in general terms that the nucleus of 

 the Yellowstone Eange is composed of granitic rocks, and tha^ the 

 greater portion is made up of these, while far to the south and 

 southeast the summits are covered to a greater or less extent with vol- 

 canic rocks. The greater portion of Emigrant Peak is made up of vol- 

 canic material. There is no doubt that at some prior period the volcanic 

 rocks and breccia or conglomerate extended over a much larger area 

 and with a greater thickness than at the present time. Many of the 

 high, bald, rounded granite mountains bear all over them the marks of 

 terrific erosion. 



In the report for 1871, 1 described somewhat briefly the interest- 

 ing lake-basin which now forms the valley of the Yellowstone be- 

 tween the First and Second Canons. At the risk of some repeti- 

 tion, I may be permitted to take this as the type of these lake-basins, 

 and describe it somewhat in detail, although each one has some features 

 not common to the rest. It seems to me, however, that they must all 

 have one common origin, whatever that may be. Many of the basins 

 have been formed by erosion, but not altogether so. Although the lake- 

 basin which we are now describing is largely due to the action of the 

 erosive forces, yet I am of the opinion that its outline w^as marked out in 

 the process of upheaval. On the east side is the remarkable range of 

 mountains which I have called the Yellowstone, constituting the nucleus 

 or central portion of a distinct anticlinal ; while on the opposite or west 

 side there is a chaotic mass of volcanic peaks and ridges, which have no 

 necessary connection with the Yellowstone Eange. At the lower end 

 of the valley, however, are a number of isolated hills of limestone, with 

 strata inclining in the same direction with the main ridge, which forms 

 the cafion below, and these can easily be traced across the valley as 

 remnants of what were once high ridges extending directly across. 

 Other remnants may be observed farther up the valley, which seem to 

 convey a pretty clear conception of the immensity of the erosive action 

 in the past. In noticing this lake-basin as typical of a series or system 

 of lakes in the West, I do not refer to those great lake-basins of the 

 earlier Tertiary period, in which were entombed such vast numbersiof 

 animal remains in Wyoming, Nebraska, &c. Those which I am now 

 describing belong to a type of more modern date, which probably com- 

 menced in the Pliocene period, and extended up very nearly to our pres- 

 ent era. 



We shall not go back beyond the time of the existence of this 

 lake-basin and endeavor to indicate the condition of the surface or 

 the climate at that time, but simply remark that we believe that all 

 these valleys were the reservoirs for the accumulated waters from the 

 drainage of the mountains in the vicinity. When they were full, so as 

 to overflow the barriers which were raised in the uplifting of the mount- 

 ain-ranges, the waters, following the law of gravitation, gradually wore 

 a channel through these barriers, as, for example, at the Lower Caiion, 

 where they have carved out a channel 800 to 1,000 ieet deep, directly 

 through the massive limestone, at right angles to the direction of uplift. 

 3 G s 



