HOLMES.] DISPLACEMENT, YELLOWSTONE VALLEY. 5 



canic ranges about the headwaters of the Upper Yellowstone. The 

 general elevation of the Park Plateau is 7,o00 to 8,500 feet. Its geologi- 

 cal formations are entirely volcanic, and are composed for the most part 

 of light colored rhyolites, although dark pitch-stones occur on some of 

 the higher levels. 



The Eed Mountain and Washburne Eanges are the only considerable 

 mountain masses within the limits of the plateau. Both of these ranges 

 have steep but rounded outlines, with rudely conical summits, which 

 rise from 3,000 to 4,000 feet above the plateau. They are of small ex- 

 tent, comprising areas of less than one hundred square miles. In geo- 

 logic structure they are quite distinct, the Eed Mountains being entirely 

 rhyolites, while the Washburne Eange is composed greatly of fragmen- 

 tary materials in which the andesites and hornblendic trachytes pre- 

 dominate. 



The great range bordering the plateau on the east — which is known 

 as the Yellowstone Eange — is also igneous, the rocks, so far as known, 

 consisting of andesites and hornblendic trachytes. The slopes are com- 

 paratively gentle below, and are covered with heavy forests, but the 

 summit regions, which are from 10,000 to 11,000 feet in height, abound 

 in picturesque and striking scenery. This range connects around the 

 head of the valley of the East Fork with the remarkable range or mass 

 of mountains that borders the plateau on the north, and which termi- 

 nates in the lofty peaks that overlook the lower valley of the Yellow- 

 stone. A great part of this range is also igneous, and consists chiefly 

 of volcanic conglomerates, the paleozoic and metamorphic rocks appear- 

 ing in many places beneath them. 



The West Gallatin Eange, which also comes within the Park, lies in 

 the extreme northwest corner, and is almost exclusively sedimentary. 

 The range is quite rugged, and its chief summit, Electric Peak, is the 

 highest in the Park. 



THE GREAT DISPLACEMENT OF THE YELLOWSTONE VALLEY. 



The exj)osures of pre-Tertiary strata within the Park area are very 

 limited, and but for the erosion of Yellowstone Valley no parts of the 

 sedimentary strata would appear, excepting those which now form the 

 higher summits of the East Gallatin Mountains. To the erosion of this 

 valley, therefore, we owe almost all the knowledge we possess of the his- 

 tory of the pre-Tertiary movements. At the present stage of erosion we 

 are able to see that at a period synchronous, probably, with the general 

 Eocky Mountain uplift, a great displacement was produced"" along the 

 line now occupied by the Yellowstone Eiver. 



How much this displacement had to do with the great events of the 

 succeeding period it is by no means easy to determine. That it gave a 

 natural depression which is now occupied by the valley, and that the 

 great range of the Yellowstone rose along its northern border, are evi- 

 dent. Just what part of the extraordinary events that have since oc- 

 curred on the thrown or depressed side, or what part of the enormous 

 volcanic activity displayed is associated with the weak line then lu'o- 

 duced, I am as yet unable to make out. The multitude of changes that 

 have followed, and the enormous deposition of volcanic material com- 

 plicate and obscure all phenomena connected with this displacement. 

 The only locality in which the strata involved are exposed lies along the 

 Yellowstone Valley between Cinnabar Mountain and the valley of Black- 

 tail-deer Creek. The extension of this disj)lacemcnt beyond the limits 

 of this vullej', or its connections with other displacements in neighbor- 



