138 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



all of these localities the tuff-breccias have the same general character as 

 the tuff-breccia of Sepulchre Mountain, just described. They vary some- 

 what in compactness. In all, the fragments are small. The general color 

 is dark, but that of the individual fragments is varied — dark and light 

 grays, with tones of red. There are abundant small phenocrysts in most 

 instances; some fragments are almost free from them, while others carry 

 larger and more prominent ones. The mineral composition varies slightly 

 among the fragments in any considerable mass. The greater part are 

 hornblende-pyroxene-andesites and pyroxene-andesites. Hornblende-and es- 

 ites without pyroxene are less abundant. Still less frequent are hornblende- 

 mica-andesites, representing the most siliceous rocks, while the least siliceous 

 are olivine-bearing pyroxene-andesites or andesitic basalts, which are much 

 rarer. These varieties naturally grade into one another and are inter- 

 mingled in the tuff-breccias. 



The microscopical characters of the various andesites from these local- 

 ities correspond to those of the Sepulchre Mountain andesites. The ground- 

 masses range from glassy to microcrystalline and microlitic. Some of the 

 glasses are colorless, others brown and globulitic. The hornblendes in 

 most cases are brown and reddish brown, seldom green (543), which con- 

 trasts them with those in the andesite-porphyries. They often have black 

 borders, especially in the more basic rocks, where they are sometimes 

 paramorphosed to magnetite and pyroxene. The feldspars are plagioclases, 

 more calcic in the less siliceous rocks. They are characterized by 

 numerous glass inclusions and marked zonal structure. The pyroxenes 

 are hypersthene and augite. The olivine in two rocks where it was 

 observed is serpentinized. 



COMPARISON OF THE ROCKS FROM ELECTRIC PEAK AND SEPULCHRE 



MOUNTAIN. 



The geological structure of Electric Peak and of Sepulchre Mountain 

 and the occurrence and character of the igneous rocks in each locality 

 having been desci-ibed, it remains to point out the relationship of the two 

 groups of rocks to each other and the penological deductions which may be 

 drawn from their investigation. 



To arrive at the relationship of the volcanic rocks of Sepulchre Moun- 

 tain to the intrusive rocks of Electric Peak, it is necessary to observe, in 

 review of the facts already presented, that the latter cut through Cretaceous 



