138 GEOLOGICAL HISTORY OF THE YELLOWSTONE PARK. 



dcsigiuitioii. This old volcano of early Tertiary time occupies a prom- 

 inent place in the geolo<;ical deve]o])iiient of the Park, and dates back 

 to the earliest outbursts of lava which have in this region changed a 

 depressed basin into an elevated plateau. We have here a volcano 

 situated far inland, in an elevated region, in the heart of the Rocky 

 Mountains. It lies on the eastern side of the continent, only a few 

 . miles from the great continental divide which sends its waters to both 

 the Atlantic and Pacific. 



After the dying out of the andesitic lavas, followed by a period of 

 erosion, immense volumes of rhyolite were eruj)ted, which not only 

 threatened to fill uj) the crater but to bury the outer walls of the vol- 

 cano. On all sides the andesitic slopes were submerged beneath the 

 rhyolite to a lieight of from 8,0()() to S,500 feet. This enormous mass 

 of , rhyolite, poured out after the close of the andesitic period, did more 

 than anything else to bring about the present physical features of the 

 Park table-land, A tourist inaking the customary trip through the 

 I'ark, visiting all the prominent geyser basins, hot si:»rings, and the 

 Grand Canon and Falls of the Yellowstone, is not likely to come upon 

 any other rock than rhyolite, excepting, of course, deposits from the 

 hot springs. If he extended his journey to the lake region, taking in 

 Shoshone, Lewis, and Yellowstone lakes, and spending a week or ten 

 days going over the beaten routes of travel, he will not, unless he as- 

 cends Mount Washburne, leave the rhyolite lavas. A description of 

 the rhyolite region is essentially one of the Park plateau. Taking the 

 bottom of the basin at 6,500 feet a,bove sea level, these acidic lavas 

 were piled up until the accumulated mass measured 2,000 feet in thick- 

 ness. It completely encircled the Gallatin Range, burying its lower 

 slopes on both the east and west sides; it banked up all along the west 

 Hanks of the Absarokas, and buried the outlying spurs of the Teton 

 and Wind River ranges. 



The Park Plateau covers an area approximately 50 by 40 miles, with 

 a mean altitude of 8,000 feet. It is accidented by undulating basins of 

 ^•Hried outline and scored by deep canyons and gorges. Strictly speak- 

 ing it is not a i)lateau; at least it is by no means a level area, but a rug- 

 ged country, characterized by bold escarpments and abrupt edges of 

 mesa-like ridges. But few large vents or centers of volcanic activity 

 for the rhyolite have been recognized, the two princii)al sources being 

 the volcano to which reference has already been made, and- Mount 

 Sheridan in the southern end of the Park, Mount Sheridan is the 

 most commanding peak on the plateau, with an elevation 10,200 feet 

 above sea level and 2,000 feet above Heart Lake. From the summit of the 

 l)eak on a clear day one may overlook the entire plateau country and the 

 mountains which shut it in, while almost at the base of the peak lie the 

 magnificent lakes which add so much to the quiet beauty of the region, in 

 contrast with the rugged scenery of the mountains. From no point is the 

 magnitude and grandeur of the volcanic region so impressive. The lava- 

 flows — bounded on the east by the Absarokas — extend westward not 



