288 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



the larger number of rocks are hornblende-pyroxene-andesites. With these 

 are intimately mingled hornl dende-andesites and hornblende-mica-andesites, 

 and in a few instances dacites. In some places pyroxene-andesites are 

 abundant, so that thei'e is a transition into the overlying basic breccia. In 

 fact, a distinction between the two is hardly possible except in a broad way, 

 the upper breccia being prevailingly dark colored, and the predominant 

 kinds of rock being pyroxene-andesite and basaltic varieties, with basalt, 

 and much less hornblende-pyroxene-andesite. 



In the acid breccia there are places where fragments of basaltic andes- 

 ite occur sparingly. Such a mingling of varieties is natural because of the 

 occurrence of this breccia upon the flanks of volcanoes which are composed 

 of basic andesites and basalts. Curiously enough, there is less mingling 

 than might be expected, and the contrast between the underlying basalts 

 and the overlying acid breccia is well marked in many places. The 

 uncertainty of limits and the transitional character of the breccia are most 

 marked between this upper acid breccia and the overlying upper basic breccia. 



In describing the microscopical characters of these rocks, it seems best 

 to follow a mineralogical sequence, rather than to treat the rocks in the 

 order of their relative abundance. In the first case the order would be: 

 Dacite, hornblende-mica-andesite, hornblende-andesite, hornblende-pyrox- 

 ene-andesite, pyroxene-andesite (basaltic andesite). 



Dacite. — The only rocks belono-ino- to this terrane that are clearly dacite 

 occur in two localities some distance apart: Lake Butte, on the northeast 

 shore of Yellowstone Lake, and the north base of Mount Doane. At the 

 first locality the rock is massive, and is exposed for only a short distance, 

 so that the ffeolosncal character of the mass is not evident. It was 

 probably a lava flow of considerable thickness, for the sections examined 

 microscopically are holocrystalline. The rock is light colored, gray and 

 red, and extremely porous, but not pumiceous, and has abundant small 

 phenocrysts of quartz, feldspar, hornblende, and biotite. The quartzes are 

 first recognized under the microscope, when they are found to be very 

 abundant. They are partly idiomorphic, in double six-sided pyramids, 

 more or less rounded. They contain, as inclusions, lumps of glassy 

 groundmass, colorless glass in pyramidal cavities, crystals of biotite, and 

 the other mineral constituents of the rock. In some cases rhombohedral 

 cleavage is noticeable. 



