61 Hypocrystalline Andesite Subtype B

The Geology and Petrography of Crater Lake National Park, 1902






This variety of hypocrystalline andesites is represented by the following specimens: Nos. 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, and 58. It is a more abundant type, therefore, than is subtype A. As above stated, these andesites do not differ materially in the hand specimen from the subtype just described. Under the microscope the chief distinction lies in the comparative suppression of the distinctly microlitic plagioclase and in the greater abundance of the larger and broader feldspars that are not always to be sharply distinguished from the phenocrystic plagioclases. In this subtype the rock also is strongly inclined to develop allotriomorphic feldspar, and in this way, as well as in the development of more and more lath-shaped and broad rectangular-shaped feldspars, to pass into the holocrystalline andesites.

Those specimens that are transitional to subtype A (40, 43, 47, 49, 53, 54) have plagioclase laths very abundantly developed, but these are usually so much larger than the microlitic feldspars above described that they may hardly be designated as microlites. At the same time the augite microlites become larger and lose their sharp outline, and glass is hardly distinguishable. Other specimens, transitional to the holocrystalline andesites of the third type, show the development of a small amount of allotriomorphic plagioclase or quartz and contain a great amount of the rectangular feldspar forms (50, 52). It even happens that both these tendencies are manifest in the same rock (54), so that numerous small plagioclase laths in fluidal arrangement occur with a rather vague paste of allotriomorphic material.

The more normal members of this type, although subject to considerable variation in size and relative abundance of the component minerals, as well as in their structural arrangement, may be said to contain numerous phenocrysts of hypersthene and augite, and especially of plagioclase, embedded in a hypocrystalline, or at most very finely crystalline groundmass of plagioclase, augite, and magnetite, with probably a little hardly distinguishable colorless glass base. The glass base may be presumed to be present on account of the presence of a little brownish globulitic matter in the interstices between the feldspars. The augite of the groundmass rarely has as sharp forms, and is also rarely as small as in the two already described types. In a number of the thin sections studied it has a granular or roughly prismatic granular habit. The sharpness of outline seems to disappear almost regularly with the increasing size of the grain of the groundmass. The plagioclase of the groundmass is the most fluctuating ingredient both as to amount and as to size and habit. In some the slender lath-form, in others the short-rectangular habit predominates. The color of the groundmass is in most cases lighter than in subtype A, owing to the greater coarseness of grain and to the scarcity of glass. These andesites even more than those just described possess the strongly felted structure characteristic of the so-called pilotaxitic groundmass of Rosenbusch.

Mr. Diller has briefly described a hypersthene-andesite from the later lavas of Mount Shasta, in California,a that is almost identical with several of the Crater Lake andesites included in subtype B. The resemblance extends both to the groundmass and to the phenocrysts of plagioclase, hypersthene, and augite. It would, in fact, be impossible to tell from the thin section alone from which of the two volcanic areas this rock came. This andesite from Mount Shasta is No. 87 of the Educational Series of Rock Specimens collected and distributed by the United States Geological Survey, and was collected at Horse Camp, near the timber line upon the western slope of the mountain.

aBull. U. S. Geol. Survey No. 150, 1898, p. 227.

Another well-known hypersthene-andesite that resembles this type as far as the groundmass is concerned is the rock from Buffalo Peak, Park County, Colo., described by Whitman Cross.b This rock is No. 86 of the above-mentioned series. It differs considerably from all the Crater Lake andesites, not only of this but of other types, in the much greater abundance of the hypersthene and augite phenocrysts.

bIdem, p. 224.
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