340 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



studying the outline of the old crater, together with the composition of 



its lavas, its true origin and history may readily be made out. This 

 older crater has, as yet, received no special designation, bid when our 

 maps and reports are finally published, this ancient geological ruin 

 will receive an appropriate designation. This old volcano occupies a 

 prominent place in the geological development 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 Pocky 

 .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 long period 

 of erosion, immense volumes of rhyolite were erupted which not only 

 threatened to till up the crater but to bury the outer walls of the 

 volcano. On all sides the basic lava slopes were submerged beneath 

 the rhyolite to a height of from 8,000 to 8,500 feet. These great Hows 

 of rhyolite did more than anything else to bring about the present 

 physical features of the Park table land. But few large vents or cen- 

 ters of eruption for the rhyolite have been recognized, the two prin- 

 cipal sources being the volcano to which reference has already been 

 made, and Mount Sheridan, a volcano in the southern end of the park. 



Mount Sheridan stands unsurpassed as a commanding peak, rising 

 grandly above the general level of the plateau, with an elevation of 

 10,200 feet above sea level and 2,000 feet above Heart bake at its 

 eastern base. From the summit of this peak on a (dear day one may 

 overlook the entire plateau country and the mountains which shut it 

 in, while almost at its base lie the magnificent lakes which add so 

 much to the (pact beauty of the region, in contrast with the rugged 

 scenery of the mountains. From no point is the magnitude and gran- 

 deur of the volcanic region so impressive. 



Taking the bottom of the basin at G,500 feet above sea level, these 

 acidic lavas piled up until the accumulated mass measured 2,000 feet 

 in thickness. In none of the deep gorges like the Yellowstone, Gibbon, 

 and Madison canyons, are the underlying sedimentary rocks exposed. 



The Park plateau, built up of rhyolite flows, embraces an area 50 by 

 40 miles, with a mean altitude of 8,000 feet. Strictly speaking, in the 

 common acceptation of the word, it is not a plateau ; at least, it is by no 

 means a level country, but an undulating region characterized by bold 

 escarpments and abrupt edges of mesadike ridges. It is accidented by 

 shallow basins of varied outline and scored by deep canyons and 

 gorges. The rhyolites rest against the steep slopes of the Absarokas 

 and bury the northerly spurs of the Wind River and Teton ranges. On 



