122 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



bottom breccia, There appears to have been a distinct time break between 

 the ejection of the lower breccia and that of the upper breccia. The occur- 

 rence of fragments of crystalline schists in the bottom light-colored breccia 

 and their absence from the overlying dark-colored breccia is a characteristic 

 difference between these two breccias wherever they have been observed 

 along the northern boundary of the Yellowstone Park. 



The andesites from the lower breccia at Sepulchre Mountain vary 

 somewhat in mineral composition, color, and microscopical habit. They are 

 mostly light colored — gray, white, and red; in places dark colored. Some 

 fragments have comparatively large phenocry sts ; others are crowded with 

 small ones. The greater number of fragments are hornblende-mica-andesite ; 

 some have little mica, and grade into hornblende-andesite. Others are 

 dacite, having quartz phenocrysts. The microstructure of the ground- 

 masses of these rocks ranges from glassy and microlitic to microcrystalline. 

 The characters of the minerals and the microstructures are the same as 

 those of the light-colored acid breccias on the Yellowstone River in the 

 neighborhood of Crescent Hill, and also those in the vicinity of Cook 

 City. 



It is important to note that there is associated with the lower acid 

 breccia of Sepulcher Mountain an obscure body of massive, vesicular basalt 

 with porphyritical augites and decomposed olivines. Its exposure is of 

 small extent, and its exact relation to the breccia was not seen. It is 

 amygdaloidal with quartz, agate, and calcite. It does not resemble the 

 recent basalts in the neighborhood, but is similar to basalt associated with 

 the bottom acid breccia at Crescent Hill and in the valley of Cache Creek. 

 Its petrographical character is more fully discussed in Chapter IX, where 

 it is classed with shoshonites. 



THE UPPER BRECCIA. 



The upper breccia, overlying that just described, is dark colored at its 

 base and lighter colored in the upper portion. It is at present 3,000 feet 

 thick through the summit of the mountain. The lower portion consists 

 almost wholly of pyroxene-andesites, with little or no hornblende. Many 

 fragments are vesicular and basaltic in habit, without megascopic pheno- 

 crysts. At the south base of the mountain there are vesicular massive 

 bodies of pyroxene-andesite, with large phenocrysts of pyroxene and 



