58 Hyalopilitic Types of Andesites

The Geology and Petrography of Crater Lake National Park, 1902





The following specimens are included here: Nos. 1, 2, 3, 4, 5, 6. 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, and 21.

This variety of andesite has mainly a very dark-gray to black, dense groundmass, and when it is not porous is apt to break in smooth, slightly lustrous surfaces. The specimens that contain less glass, as well as the more porous ones, have a much lighter color and rougher fracture. On account of the dark groundmass the plagioclase phenocrysts are usually prominent, and for the same reason the pyroxenes are hardly noticeable. Most of the hyalopilitic andesites are characterized in the sections by the presence of light to deep brown glass, which is usually very abundant and perfectly clear and unaltered (6, 9, 11, 13, 16, 17), or else this brown glass is more or less clouded by the presence of countless hosts of minute brown- to black-looking globulites (brown in 8, 15, 19, 1, 5; black in 14, 21). In case these globulites are thicker than usual, the glass base between the microlites becomes lighter or even almost colorless, but still the effect of the globulites in such cases is to lend a brownish color to the whole. In a few cases a decided change of color in the glass is to be noted, in that blackish spots appear locally in a glass otherwise brownish (12, 20). These black spots have the globulites very thick and somewhat larger, while the inclosing glass base is colorless. It is more than likely that magnetite dust is developed in addition to the customary globulites, which would explain the black color. These darker spots are similar to the whole groundmass in some of the hypocrystalline andesites.

This all-pervading glass base invariably incloses myriads of perfectly sharp and straight augite microlites; also plagioclase feldspars in slender strips of microlitic size or in larger laths and in rectangular individuals. The size of the augite microlites may vary considerably in the same thin section. Thus in No. 2, where they are uncommonly small, they vary from 0.001 to 0.003 millimeter in width and from 0.01 to 0.03 millimeter in length. In general, the width may be said to average about 0.005 millimeter, with extremes of 0.001 and 0.01 millimeter, and the length to average about 0.02 millimeter, with extremes of 0.01 and 0.08 millimeter. It will be seen that they are about five to ten times as long as wide. These microlites are very commonly free from inclosures, but perhaps more frequently inclose minute magnetite grains, or octahedral crystals of the same. The plagioclase microlites and laths vary extremely in size and form. In the majority of cases their form is either distinctly microlitic or, at least, slender lath shaped and minute (5, 14). When very small, the customary twinning is not easily distinguishable, but in the larger and broader individuals polysynthetic twinning is strongly in evidence. Interspersed with these slender plagioclases are others with short, rectangular forms that sometimes show twinning and sometimes do not. In a few rocks these rectangular forms are very numerous, and partially or almost completely supplant the lath form (11, 19, 21). Still larger plagioclases of rectangular form connect these rectangular plagioclases of the groundmass with the phenocrysts, so that a sharp discrimination is not always possible. Fluidal arrangement of all these groundmass ingredients is more or less conspicuous in all thin sections.

Intermediate stages between andesites of this type and the next are very common, and are mostly included in those listed under the following type. In one specimen, however, No. 18, collected about one mile southwest of the base of Red Cone, we have what is common, namely, a transition to the holocrystalline type. In the thin section from this rock there appears to be very little glass, but the appearance of dust-like globulitic material between the abundant feldspars of the groundmass indicates the presence of more glass than is at first apparent.

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