Basalt. A light brownish-gray (5YR 5/1) rock, with seriate texture and numerous flattish, irregular vesicles. The larger plagioclase crystals, ranging in size from 4 to 0.1 mm, have a median composition of about An80. The dark-green salite has a maximum size of about 3 mm with a maximum index of 1.690, suggesting a composition near Ca50Mg30Fe20. Small amounts of "basaltic" hornblende, pleochroic in red-brown and yellow colors and with wide resorption borders of magnetite, are also present. The groundmass consists of plagioclase laths, rods of augite, granules of magnetite or ilmenite, and interstitial glass in small amount. Small prisms of apatite and irregular grains of magnetite or ilmenite are the principal accessory minerals.
Magmatic differentiation in Tertiary and Quaternary volcanic rocks from Adak and Kanaga Islands, Aleutian Islands, Alaska