On the southern slope of Mount Wrangell, near the head of Cheshnina Glacier, at an elevation of between 7,500 and 8,000 feet above tide, is a vesicular augite-bearing hypersthene-andesite, which is presumably a recent flow. In the hand specimen the rock is seen to be very porous, with abundant square and lath-shaped feldspar sections of all sizes up to 5 mm in diameter imbedded in a jet-black glassy base. Under the microscope the larger plagioclase phenocrysts prove to be basic labradorite, Ab3An4 usually. One test gave the more acidic variety, and the presence of one or two zoned sections indicates some range in species. From the largest phenocrysts there is a gradation in size to laths too small for determination. The hypersthene is very subordinate to the feldspar in amount, and occurs usually in grains and imperfect crystals. Only one or two fragments of augite were observed. The groundmass is an opaque brown glass containing an abundance of minute crystals.
Geology of the central Copper River region, Alaska