Basalt from Williwaw Cove formation. Color: light reddish gray (5R 6/l). Texture: holocrystalline, porphyritic, with large, diameter 15 mm phenocrysts of greenish black augite (5 GY 2/1). Plagioclase phenocrysts, 10-15 percent, range from An45 to An65 in individual and separate crystals and average An57. Augite (nZ: 1.705, nX : 1.676) occurs as remarkably inclusion-free megaphenocrysts and as groundmass dust, making 40 to 45 percent of the rock. Subhedral olivine, 2 percent, and magnetite-ilmenite, 1 percent, are accessory minerals. The groundmass is a microcrystalline aggregate of all the above minerals.
Geology of Little Sitkin Island, Alaska