feldspathic basalt porphyry from a dike in the Unalaska formation near Summer Bay in Unalaska Bay. It is dark bluish gray and contains phenocrysts as large as 1 millimeter. Phenocrysts compose 10 percent of the rock: 9 percent calcic labradorite, which forms embayed crystals, euhedral laths, and grains with inconspicuous normal zoning ; 1 percent augite (?) , which forms altered euhedra and is replaced by penninite(?) ; and a trace of pyrite. The groundmass has a felty texture and consists of abundant euhedral and subhedral laths of calcic andesine and scattered granules of a black opaque mineral. Alteration is moderate, producing disseminated calcite and penninite and traces of uralite and chlorite.
Geology of Unalaska Island and adjacent insular shelf, Aleutian Islands, Alaska