Pyrite-bearing dacite vitrophyre from the Unalaska formation 1mile east of Chernofski Harbor. It is black and contains plagioclase phenocrysts 1 to 4 millimeters long. Phenocrysts compose 5 percent of the rock: 2 percent calcic labradorite, which form embayed crystals, euhedral laths and stubby crystals, and fragments, and which have oscillatory zones and apatite inclusions ; 1 percent euhedral augite (?) ; 1 percent euhedral hypersthene; and a trace of euhedral to subhedral ilmenitic magnetite. Microlites of plagioclase are common and of apatite rare. The groundmass is a perlitic glass, which contains a few small chlorite(?) spherulites.
Geology of Unalaska Island and adjacent insular shelf, Aleutian Islands, Alaska