Sample of a dense flow of chrystophyric hyalobasanite, cropping out over a distance of several miles along the west shore (and under Einahnuhto Bluffs) of St. Paul Island. The rock is gray with finely crystalline groundmass studded with clear, yellow phenocrysts of olivine from 1 mm to 10 mm across, some zoned; refractive index and 2V angle correspond to about Fa20. Very few phenocyrsts are clinopyroxene; they are always zoned and usually pitted and intergown with minerals of the groundmass. Refractive index and 2V angle correspond to He40. The glassy groundmass includes small grains of olivine and pyroxene. Small plagioclase laths form a mesh; indicies of refraction lie in the range 1.555-1.565, indicating about An55. The glass shows incipient crystallization, index, n=1.53-1.54.
Geology and petrology of the Pribilof Islands, Alaska