GEOLOGICAL HISTORY OF IHK VELLOWSTOjS'E PAK'K. 



139 



only across the itaik. hut across the Madison Plateau, and out on to the 

 .liicat plains JSnalcc ot Ri\('r, str<'t('liin,ii far westward almost without a 

 break in tlie eontinnity ottlieeru])tive Hows. ()\-er the central i»ortionof 

 the park,whe. e the ryholites are thickest, erosion has tailed to penetrate to 

 thennderlyin<; rock. Even snch dee]) lioriicsas the Yellowstone, (Til)hou, 

 and JNIadison Cafions liave nowhere woi-n throngh these rhyolite tlows. 

 In the Uiand C'anon of the Yellowstone the andesitic breccias are found 

 beneath the rliyolites, but the deepest cuts fail to reveal the underly- 

 ing sedimentary beds. Although the rocks of the jdatean for the most 

 ])art l)elong to one group of acidic Ia\as, they by no nn'ans present tlie 

 great uniformity and monotony in Held appearance that might be ex- 

 pected. These 2,0(K> scpiare miles ofler as grand a iield for the study of 

 structural forms, develoi)ment of crystallization, and mode of occurrence 

 of acidic lavas as could well be found aiiywliere in tlie world. They 

 vary from a nearly holocrystalline rock to one of pure volcanic glass. 

 Obsidian, pumice, pitchstone, ash, ])reccia, and an endless development 

 of transition forms alternate with the more conii)act lithoidal lavas 

 which make up the great mass of the rhyolite, and wiiich in colors, 

 texture, and structural developments i)i'esent an equally varied aspect. 

 In mineral conii)ositi(ui these rock are simple enough. The essential 

 nunerals are orthoclase and (inartz, with more or less plagioclase. 

 Sanidine is the prevailing feldspar, although in many cases plagioclase 

 forms occur nearly as abundantly as ortlioclase. Ohemical analyses, 

 whether we consider the rocks from the crater of Mount Sheridan, the 

 summit of the plateau, oi- the volcanic glass of the woild-renowned 

 Obsidian Cliff, present comparatively slight differences in ultimate com- 

 position. 



Tlie following analyses of two locks, representing extreme forms in 

 physical habit, show how closely they approach each other in composi- 

 tion of the origiiud magnm : 



No. 1. \... ■-'. 



Miidisoii Plateau. Obsidian Clili'. 



Silica 



Titanic acid 



Plio.splioric acid . 



Alumina 



Ferric oxide 



Ferrous oxide . . . 

 Ferric sulphide . . 

 Manganese oxide 



Lime 



Masnosia 



Litliia 



Potasli 



Soda 



Sulphuric acid . . . 



Water 



Ignition 



Total 



