From Nadina River Glacier, flowing out of the heart of Mount Drum, an abundantly but not coarsely porphyritic lava was collected, which is gray in color, with prominent white feldspar phenocrysts and less numerous brown biotite and glossy black amphibole crystals in a gray groundmass. The short, thick feldspar prisms seldom exceed 5 mm in cross section, while the much less abundant biotite plates are occasionally 1 cm in diameter. The amphibole needles are smaller, being usually only about 1 mm in length. Under the microscope the broad prisms of plagioclase, usually twinned and frequently zoned, varying in size up to about 5 mm, are seen to make up 40 or 50 per cent of the rock mass. Tested optically, the majority of them prove to be labradorite, but the zoned species give the extinctions for andesine in their more acid bands. The most abundant mineral, except the plagioclase, is a greenish-brown hornblende, with a low extinction angle, which occurs in large and small phenocrysts. A red-brown biotite is less abundant, and there are numbers of small phenocrysts of hypersthene. Magnetite, apatite, and zircon are accessories. The base is a gray glass, which, under a high power, exhibits abundant microlitic forms. There is practically a complete gradation in size between the phenocrysts and the minute crystals of this groundmass.
Geology of the central Copper River region, Alaska