EARLY ACID BRECCIA. 237 



unless we conceive the last act of vnlcanism to have been the greatest and 

 imagine a gigantic explosion to have blown the upper part of the volcano 

 into the air and to have left no evidence of such a culmination of events. 

 On the contrary, the evidence furnished by the structure of the granular 

 core indicates that the last eruptions were feeble, injecting narrow veins of 

 rock into the body of the core. 



It is to be remembered that the erosion which has thus laid bare a basal 

 section of so great a volcano was accomplished after the accumulation of 

 this vast pile of Miocene ejections and before the extrusion of the immense 

 flood of rhyolitic lava forming the plateau of the Yellowstone Park. In 

 the region of Electric Peak and Sepulchre Mountain there are evidences of 

 orographic movement accompanying this period of denudation, shown in the 

 profound faulting which cut in two that andesitic volcano ; but the region 

 of Crandall Basin seems to have escaped serious orographic disturbance. 



PETROGRAPHY OF THE ROCKS OF THE DISTRICT. 



The extrusive rocks of the volcano of Crandall Basin are in the main 

 the same as those found in various parts of the Yellowstone Park, those 

 of Sepulchre Mountain having been described in Chapter III. It will not 

 be necessary to repeat in detail the characteristics of most of the rocks, but 

 the petrographical features that are distinctive of this volcano will be fully 

 described. 



EARLY ACID BRECCIA. 



The early acid breccia consists of small fragments and dust of 

 hornblende-mica-andesite, hornblende-andesite, and hornblende-pyroxene- 

 andesite. The microscopical characters are cpiite normal. The andesites are 

 partly holocrystalline and partly glassy. The structures of the groundmass 

 are typical of andesites, and the phenocrysts of plagioclase, hornblende, 

 biotite, hypersthene, and augite exhibit the usual characteristics. The color 

 of the hornblende varies from reddish brown and brown to brownish green, 

 green, and bluish green, and is often strongly pleochroic. In one occur- 

 rence there is a little quartz in microscopic phenocrysts. There is a consid- 

 erable range of composition, and the rocks grade into varieties which are 

 like the more siliceous andesites of the overlying breccias, a small portion of 

 which is hornblende-pyroxene-andesite. They also appear to pass upward 

 into the basic breccias in certain localities, though in others there are evi- 

 dences of an intermission accompanied by erosion. 



