CHAPTER X. 

 THE RHYOLITES. 



By Joseph Paxson Iddings. 



INTRODUCTION. 



The rhyolite of the Yellowstone National Park occurs almost wholly 

 as extrusive surficial lava flows in the form of nearly horizontal sheets, 

 some having enormous proportions. In only one locality does it assume 

 the character of a volcanic mountain, in which place it occurs as breccia and 

 also as intrusions and surface flows. It constitutes the great plateau of the 

 Park, and sends out arms into valleys in the surrounding ranges of moun- 

 tains, and is found in isolated remnants upon their slopes. Owing to its 

 great areal extent, and also to the fact that it is in places more than 2,000 

 feet thick, its volume is very large. It is exposed to view in many cliffs 

 and bare slopes throughout the region, and has been studied in detail in 

 many places. 



Its two most striking penological characteristics are the uniformity of 

 its composition chemically and mineralogically and the multiformity of its 

 physical aspect. With a range of only 5 per cent in the silica, and of much 

 less in the other constituents throughout miles of material, there is the 

 Greatest diversity in the appearance and texture of the rock, even within 

 the limits of a few feet. Its color may be white, black, yellow, red, brown, 

 or grays of various tones, which may be uniform for broad areas, or mingled 

 in blotches, streaks, and layers, or finely speckled in small spots. Its 

 luster may be dull and stony or vitreous and brilliant, and its texture may 

 be rough or smooth as the rock is porous, vesicular, and pumiceous, or 

 dense and compact. In some localities the characters are quite uniform for 

 a large extent of rock; in others they are highly diversified. In order to 

 convey a proper idea of the relations of these various modifications to one 



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