Olivine-bearing feldspathic basalt porphyry from the Makushin volcanics 3 1/4 miles north-northeast of Sugarloaf Cone. It is dark gray to grayish black and contains plagioclase and pyroxene phenocrysts 1-3 millimeters long. Phenocrysts compose 25 percent of the rock: 15 percent sodic bytownite to sodic labradorite, which forms stubby euhedral crystals and fragments having faint oscillatory zones, narrow rims of normal zones, vermicular zones, zones of dusty inclusions, and apatite inclusions; 5 percent euhedral to subhedral augite (?); 4 percent subhedral olivine with pseudomorphs of serpentine (?); 1 percent hypersthene with augite reaction rims; and a trace of anhedral magnetite. The groundmass has intergranular texture and includes abundant subhedral laths and stubby crystals of sodic bytownite to sodic labradorite, and much granular augite(?) and a black opaque mineral. Alteration is sparse; 3 percent of the rock is chlorite(?), which replaces olivine.
Geology of Unalaska Island and adjacent insular shelf, Aleutian Islands, Alaska