Andesite vitrophyre from an agglomerate from Three-quarter Cone (121,700' E, 291,900' N). This is a dark-gray rock, aphanitic, with small tabular plagioclase phenocrysts showing a parallel arrangement. About 5 percent of the rock is plagioclase phenocrysts (1 mm in diameter), having an average composition of An58. A few phenocrysts are hypersthene and olivine. The groundmass is mostly plagioclase microlites (0.05 mm long), minute granules of augite(?), and magnetite dust, in brown glass, having an index of 1.546.
Geologic reconnaissance of Semisopochnoi Island, western Aleutian Islands, Alaska