79 Grouse Hill Flow

The Geology and Petrography of Crater Lake National Park, 1902





The four specimens collected from this mass (105, 106, 107, and 108) are far from being a typical dacite. If this were the only example of this kind of rock in connection with the Crater Lake lavas the rock would naturally be classified as an andesite. As a matter of fact it partakes of the characteristics of both dacites and andesites to such an extent as to represent a thoroughly intermediate type. Taken in connection with both the andesites and the dacites of the surrounding region the dacitic characteristics predominate, so that the rock may well be called an andesitic dacite.

These four specimens are light gray or, in the case of No. 107, brown and gray streaked, rough-fracturing and slightly porous rocks with the usual small find not very conspicuous phenocrysts. Under the microscope the groundmass varies from glassy to hypocrystalline, the variations occurring in irregular streaks to be seen in the same thin section. In general the groundmass contains a good deal of crystalline matter, which consists mainly of small lath-like microlites of plagioclase, together with small, rectangular, untwinned feldspar crystallites, such as are to be found very abundantly in the more typical dacites. The plagioclase microlites are similar to those seen in the dacite of Llao Rock, but are here much more numerous and are the feature that most closely reminds one of the andesites. They are probably oligoclase, as the extinction angles are mostly very small. In addition to these feldspars may also be seen very slender and minute colorless microlites resembling the augite microlites of the Llao Rock flow, but differing in being not quite so sharp or straight, also in having a tendency to taper out at the end. They are undoubtedly augite. A small amount of black, opaque, iron oxide, distributed as minute, dust-like particles, assists the augite microlites in giving to the groundmass a clouded aspect. The brown color of No. 107 is produced by the presence of brownish and yellowish ferritic staining matter.

All four of these specimens contain plagioclase, hypersthene, augite, and reddish-brown hornblende as phenocrysts. The plagioclase has a stronger tendency than is customary in the Crater Lake dacites to become crowded with glass inclusions. Augite is more abundant than in the more typical dacites. It occurs both in sharp but small and often twinned crystals and in irregular grains. The hornblende, which belongs to the reddish-brown variety, is also somewhat more abundant than usual. In No. 106 it occurs in uncorroded crystals, on one of which the forms (110), (010), and (100) were noticed. In the others the hornblende is more or less corroded and has developed resorption rims of magnetite and augite. In Fig. H of Pl. XIV (p. 76) may be seen a reproduction of a crystal of hornblende in No. 105.

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