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Andesite tuff, probably representing caldera-forming eruption. From a layer of yellowish-gray (5Y8/1) pumice, 1 to 2 feet thick, composed of fragments of pumice up to 0.25 inch across; during shipment, most of the coarser material was crushed, so that after shipment only about 5% of the material was coarser than 10 mesh. Of the coarser material, about half was made up of fragments of accidental and accessory rocks, the rest of essential and cognate material, largely pumice and cognate crystalline rocks. Of the material finer than 10 mesh, about 3% of the total sample was thought to be accidental and accessory material, divided about as follows: 30% plagioclase, 30% rock and glass, 20% pyroxene (augite and hypersthene), 10% magnetite or ilmenite, and 10% basaltic hornblende. Essential and cognate material makes up about 92% of the total; it includes the 81% of glass, the 1% of magnetite, the 1% of green hornblende, and the 9% of plagioclase. The chemical analysis was made on the material finer than 10 mesh; it is probable that the elimination from analyzed material of the accidental and accessory material could not have differed very greatly from that of the essential material. The essential material of the eruption is a clear, pumiceous glass, with index ranging from 1.495 to 1.505, containing phenocrysts of plagioclase, green hornblende, magnetite, and apatite. The plagioclase is zoned, and has a mean composition of An[53?]. The hornblende is clear, with a slight development of magnetite along the cleavages of a few grains. The pleochroism formula is: Z= olive-green, Y= yellowish-green, X= greenish-yellow. The extinction angle on (010) is 14 degrees. The nZ is 1.676, nX is 1.653. The magnetite or ilmenite is in equant grains, about 0.2 mm in maximum width. Apatite is in clear, stubby prisms, with sharp prismatic terminations. It may be noted that the index of the glass is rather lower than that to be expected from the diagrams of George (1924). |