60 Hypocrystalline Andesite Subtype A

The Geology and Petrography of Crater Lake National Park, 1902






This variety includes the following specimens: Nos. 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33.

The groundmass of these rocks, as seen under the microscope, is very dense. It is crowded with microlitic crystallizations similar to but more abundant than those in the hyalopilitic groundmass. Glass is probably always present, impregnating the whole, but, with one or two exceptions (26, 28), it is not at all conspicuous. In the two exceptions, which are rocks connecting this type with the preceding, the glass is readily recognizable and has a distinct brown color; otherwise the glass base appears to be almost colorless. As stated above, in discussing the plagioclase phenocrysts, these rocks, with a small amount of colorless glass base, are very apt to contain clear brown glass inclosures in the plagioclase. The marked feature of this subtype A, as distinguishing it from subtype B, is the presence of innumerable minute plagioclase microlites with their customary fluidal arrangement. These microlitic plagioclases are mostly very minute, being often no larger than the augite microlites (26). The larger, squarish, or short-rectangular plagioclase crystals, with and without twinning striae, that are to be seen commonly in the groundmass of the preceding type, are usually conspicuously absent. Hence the contrast between the plagioclases of the groundmass and of the phenocrysts is very pronounced, as connecting types are absent. In one or two specimens the appearance of faintly developed allotriomorphic feldspar (27, 29) connects this type with the holocrystalline andesites.

In the finest-grained varieties the plagioclase microlites are hardly discernible in white light. In such rocks the groundmass presents a dirty-brown or grayish-brown and minutely granular appearance, owing to the development of thickly crowded augite microlites, which are interspersed with minute grains and octahedral crystals of magnetite. The brown color is doubtless due in part to the glass base, and in part also to the presence of brownish globulites. Globulitic forms are, however, by no means as common or abundant as in the hyalopilitic type. On the other hand, both magnetite and augite in microlitic form are much more abundant in this type of andesite. The augite microlites are, as a general thing, not as sharply crystallized, although they do not differ greatly in size from the similar microlites in the hyalopilitic andesites. As the groundmass becomes more distinctly crystalline the size of these microlites tends to become larger and the form less sharp.

A few crystals of apatite of a brownish color measuring 0.1 millimeter in length and 0.015 millimeter in width were noticed. They are faintly pleochroic, with the customary absorption E>O.

Hypersthene in two generations—one in short, stout, and relatively large crystals, the other in slender prisms of much smaller size—are usually to be seen, but the distinction between the two is not as well defined as is the case mentioned in the hyalopilitic type.

In all but one case (33) these andesites appear to be very fresh, the alterations being hardly more than iron stains. In the one case referred to, the ground mass has undergone considerable alteration, which has attacked mainly the glass base and the augite microlites. The decomposition products are carbonates and a nearly homogeneous greenish substance which, in polarized light, appears to be formed of extremely minute scales that polarize light strongly. This green substance appears to be delessite.

The groundmass of subtype A corresponds closely to what Rosenbusch calls the pilotaxitic type of andesites, in that it may be described as a densely felted aggregate of plagioclase and augite microlites impregnated to a more or less extent with an inconspicuous glass base.

 << previousnext >>