Rhyodacite of Pratt Point member of Little Sitkin dacite. Color: yellowish gray (5 P 7/1) to medium light gray (N 6). Texture: hypocrystalline, vesicular with "hairy" pumice developed locally, porphyritic with hornblende phenocrysts as much as 7 mm long and feldspar phenocrysts as much as 4 mm long. Plagioclase, 15-20 percent, ranges in composition from An27 to An50 and averages An40; sometimes poikilitic with inclusions of hornblende. Accessory minerals: oxyhornblende, 3-4 percent; hypersthene, 2-3 percent; quartz, 2-3 percent; magnetite, 1 percent. The quartz is characterized by conchoidal fractures and is very susceptible to plucking during grinding. The groundmass is about half small bubbles and half clear pinkish glass (N=1.490). The few plagioclase microlites average An39( ?).
Geology of Little Sitkin Island, Alaska