PYROXENE ANDESITES OF ABSAROKA RANGE. 295 



of hornblende paramorphs. The groundmass is aphanitic and dark gray 

 or black, or light gray, occasionally red. 



In thin section the feldspar phenocrysts are mostly rectangular, with 

 polysynthetic twinning after both laws ; relatively strong double refraction 

 and high extinction angles, indicating labradorite-anorthite. Zonal struc- 

 ture is pronounced, and glass inclusions are abundant, the larger feldspars 

 being sometimes filled with them. The pyroxenes are hypersthene and 

 augite in variable proportions. In some varieties of the rock hypersthene is 

 greatly in excess of augite, and is distinctly pleochroic. It occasionally 

 forms small aggregations with labradorite, the crystals of feldspar project-- 

 ing radially from the margin of the aggregate. Similar aggregates some- 

 times surround hornblende phenocrysts. Inclusions of magnetite are 

 common, and those of glass less frequent. There are occasional evidences 

 of the nearly synchronous growth of the pyroxene and feldspar, ciystals of 

 the former being interrupted in their growth by those of feldspar. Rare 

 cases of actual synchronous crystallization resulting in mutual intergrowth 

 have already been mentioned. Augite is almost identical with hypersthene 

 in its modes of crystallization and association. They are sometimes inti- 

 mately intergrown, and are distinguishable only between crossed nicols. 

 Zonal structure is occasionally noticeable in the distribution of color, and 

 in the optical properties, the color varying from pale green to brown and 

 reddish brown. In most cases they are fresh and not decomposed. Rarely 

 they are partly altered into opal. The small individuals of hornblende, 

 usually irregularly outlined and with magnetite or pyroxene border, are 

 generally reddish brown. Magnetite and apatite are the same as in the 

 other andesites, but are somewhat more abundant. 



The groundmass structures of the pyroxene-andesites are almost identi- 

 cal with those of the hornblende-pyroxene-andesites. They are mostly 

 microlitic, with prisms of plagioclase and pyroxene and crystals of magnet- 

 ite. The more glassy modifications have colorless glass, and often brown 

 globulitic glass as the matrix. Not infrequently the glass inclosed in the 

 feldspar phenocrysts is brown while that in the surrounding groundmass is 

 colorless. Brown glass is rather more frequent than in the more siliceous 

 varieties of andesite. Only a few of the specimens examined were noncrys- 

 talline. 



