76 Lithoidal Dacite

The Geology and Petrography of Crater Lake National Park, 1902






This rock (103) comes from the summit of Llao Rock. It consists of a light gray, dense, and somewhat porous groundmass with thickly scattered, small, white feldspar phenocrysts, similar to those in the vitrophyric variety: it consists also of a few almost unnoticeable darker phenocrysts. It breaks with a decidedly rough fracture and does not show any fluidal structure.

In thin section the groundmass is seen to contain small plagioclase laths, similar to, but much more abundant than those mentioned as occurring in No. 101; also a few feldspar crystallites with rectangular outlines and much more feldspathic matter that shows no distinct forms and that is not capable of absolute identification. There is undoubtedly much glass present but, being colorless, it is almost concealed by the abundant crystalline matter inclosed. In white light with weak magnifying powers the thin section has a somewhat dusty appearance. When strongly magnified this dusty matter resolves itself into minute, colorless, straight angite microlites, like those described as characteristic of the vitrophyric rhyolite, together with somewhat larger microlites of the same mineral that inclose a few very minute, black, and opaque grains—presumably of magnetite. Probably the dusty appearance is intensified by the presence of small air bubbles in the glass base. Phenocrysts occur about as in Nos. 102 and 101, with the addition of a very little augite in sharply defined and almost colorless crystals.

This can not be said to be a typical lithoidal dacite. The abundance of plagioclase microlites and the scarcity of the untwinned groundmass feldspar with square or rectangular forms cause this rock to resemble some of the more dacitic andesites of this region.

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