HA(;r „., TIIIC YELLOWSTONE PARK. 341 



both sides of the Gallatin range the rhyolites encircle sedimentary beds 



at about the same level. From Electric Peak to the Tetons, one may 

 travel the entire distance of 60 miles without leaving- the rhyolites. 

 Nowhere, except in limited outbursts along the base of the Absarokas, 

 do the rhyolites penetrate the mountains. 



Although the rocks of the plateau for the most part belong to one 

 group of acidic lavas, they by no means present the great uniformity 

 and monotony in held appearance that might be expected. These 

 2,000 square miles offer as grand a field for the study of structural 

 forms, development of crystallization, and mode of occurrence of acidic 

 lavas as could well be found anywhere in the world. They vary from 

 a nearly noncrystalline rock to one of pure volcanic glass. Obsidian, 

 pumice, pitchstone, ash, breccia, and an endless development of transi- 

 tion forms alternate with the more compact lithoidal lavas which make 

 Up the great mass of the rhyolite, and which in color, texture, and struc- 

 tural development present an equally varied aspect. In mineral com- 

 position these rocks are simple enough. The essential minerals are 

 orthoclase and quartz, with more or less plagioclase. Sanidine is the 

 prevailing feldspar, although in many cases plagioclase forms occur 

 nearly as abundantly as ortlioclase. Chemical analyses, whether we 

 consider the rocks from the crater of Mount Sheridan, the summit of 

 the plateau, or the volcanic glass of the world-renowned Obsidian Cliff, 

 present comparatively slight differences in ultimate composition. 



Occasional thin sheets of basalt reached the surface before the com- 

 pletion of rhyolitic outbursts, flows of acidic lava overlying basic 

 ones at several localities. In general, however, basaltic lava followed 

 rhyolite. The basalt occurs near the outer edge of the rhyolitic mass, 

 and in no single instance is there an extrusive flow of basalt in the 

 central portion of the plateau. 



After the close of the basaltic eruptions and the dying out of vol- 

 canic energy came the period of giaciation of the entire region. In the 

 Teton range several characteristic glaciers still exist upon the slopes 

 of Mount llayden and Mount Moran, remnants of a much larger sys- 

 tem of glaciers. Broad neve tields may be seen in the Bear Tooth 

 mountains northwest of the Park. The Park region presents so broad 

 a mass of elevated country that not only the surrounding mountains 

 but the entire plateau was in glacial times covered with ice. Glacial 

 lakes, kames, terminal moraines, and nearly all the phenomena of 

 ice action usually seen in glaciated regions may be found here. A 

 remarkable and exceptional feature, which is shown here on a grand 

 scale, is the action of thermal siliceous waters on glacial drift. 



Over the Absaroka range glaciers were forced down into the Lamar 

 and Yellowstone valleys, thence westward over the top of Mount Kvarts 

 to the Mammoth Hot Springs basin. On the opposite side of the Park, 



