134 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



GENERAL CONSIDERATION OF THE MINERAL AND CHEMICAL COMPOSITION 

 OF THE ERUPTIVE ROCKS OF SEPULCHRE MOUNTAIN. 



MINERAL COMPOSITION. 



The mineral variations in the group of rocks forming Sepulchre 

 Mountain are much simpler and require much less discussion than those of 

 the intrusive rocks of Electric Peak. They have already been expressed 

 in Tables XI and XII. From these tables it is evident that the so-called 

 transitional forms of the rocks are as numerous and as important as those 

 forms which would be considered type rocks. There is no particular 

 mineralogical modification of the rocks at this place which from its greater 

 abundance or its special mode of occurrence renders it a type rock. On the 

 contrary, the whole accumulation of eruptive rocks which are subsequent 

 to the bottom breccia, with its admixture of Archean fragments, must be 

 considered as a series of volcanic rocks that vary in mineral composition 

 through gradual changes from pyroxene-andesite to dacite. 



Starting with those rocks which carry phenocrysts of pyroxene and 

 plagioclase, it is observed that as the hornblende makes its appearance and 

 increases in amount the pyroxene decreases. Biotite accompanies the 

 hornblende in the more acid varieties, and increases in amount with the 

 acidity of the rock. Quartz first appears in small quantities, and increases 

 with the acidity of the rock, the hornblende decreasing at the same time. 

 To this rule there are exceptions, which are indicated in the table. Biotite 

 is found to a slight extent in some of the hornblende-pyroxene-andesites, 

 and jjyroxene occurs in small amounts in some of the hornblende-mica- 

 andesites. It is, of course, understood that this relation between the 

 essential minerals may be different for groups of andesites in other regions. 



