75 Vitrophyric Dacite

The Geology and Petrography of Crater Lake National Park, 1902






This variety was collected from the extreme southern edge of the lava flow, where it is necessarily very thin. It is represented by Nos. 101 and 102. The hand specimens present the appearance of a perfectly fresh vitrophyre in which the glass base, which constitutes at least four-fifths of the entire mass, has a dark, greenish-gray color. In this glass base are inclosed numerous phenocrysts of plagioclase that measure not over 1 to 2 millimeters, and that are white and glassy and show hardly a trace of cleavage. In addition to these are a very few minute, deep-brown to green and black, resinous-lustered crystals that are hardly to be seen without a magnifying glass, and that, in thin section, prove to be usually hypersthene or hornblende. There are also to be noted a few angular fragments, compact, grayish brown and dull lustered, measuring one-quarter inch to one inch or more in diameter. These are referred to later under the head of nodular secretions. This rock is more or less cracked in different directions owing to shrinkage in cooling. The rock parting along these cracks, which sometimes gap, breaks into very smooth flat surfaces that have a distinct gloss. Otherwise the fracture is rather rough or small-concoidal, owing to the presence of the abundant plagioclase phenocrysts. This vitrophyric dacite may also be streaked by more or less parallel bands of grayish-looking pumice, as may be seen in a specimen collected by the writer on the same spot. (No. 2013.1 of collection of H. B. Patton.)

In thin section the glass base appears to be clear and colorless and to be crowded with very sharp and straight and remarkably even-sized microlites of augite that measure 0.003 millimeter wide by 0.02 to 0.04 millimeter long. (See fig. A of Pl. XVIII, p. 132.) These microlites are too small to show any color, but the strong refraction is evident, as well as double refraction and an extinction angle up to 45° are also to be seen a very few opaque curved black trichites. The straight, colorless augite microlites appear to be the same as are to be seen sparingly developed in a thin section prepared by the United States Geological Survey from the rhyolitic perlite of the Yellowstone National Park.aThey still more closely resemble in size, appearance, and numbers the straight colorless microlites in the dacite from Lassen Peak, California, No. 82 of the above-mentioned Educational Series.b The close resemblance between the Llao Rock dacite and this dacite from Lassen Peak will be again referred to in these pages.

aNo. 61 of the Educational series of Rocks, described by J. P. Iddings in Bull. U. S. Geol. Survey No. 150, p. 151.bJ. S. Diller, op. cit., p. 217.

A few feldspar microlites occur in No. 101, but almost none in No. 102. These are in part lath-shaped plagioclase with very small extinction angles, but mainly short rectangular to square, untwinned feldspar that usually extinguishes nearly parallel to the sides. In No. 101 was seen a nearly rectangular section of feldspar that was not larger than nor even as large as many of the feldspar microlites (0.05 millimeter long), in the center of which was a brown-glass inclusion with sides parallel to the crystal edges and occupying at least one-third of the whole crystal, itself inclosing a comparatively large air bubble. The extinction angle measured to the longest side was 18° twinning was apparent. A reproduction of this crystal may be seen in fig. G of Pl. XIV, p. 76.

Fluidal structure, which is hardly to be seen in the hand specimen, is very conspicuous in the thin section, owing to the more or less parallel arrangement of the microlites which occur in flowing lines lapping around the phenocrysts of plagioclase, hypersthene and hornblende, and of accessory magnetite and apatite. Hypersthene is scarce and hornblende still more so. The latter occurs mainly in very small needles and only occasionally in crystals comparable in size with the hypersthene. In one instance a small crystal of hornblende with sharp crystal forms was observed clearly inclosed in plagioclase. This is a decided exception to the rule that hornblende in these dacites is the youngest of the phenocrysts with the possible exception of augite.

The chemical analysis of No. 102 is given on page 140.

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