107 Andesitic Basalts Type B

The Geology and Petrography of Crater Lake National Park, 1902





This is represented by three specimens collected 2 or 3 miles west and northwest of Red Cone. They are distinctly andesitic in appearance, being gray, rather compact rocks with numerous but small and inconspicuous plagioclase phenocrysts. These might readily be taken for andesites, but, aside from certain differences in structure that alone might not be decisive, their association with distinctly basaltic rocks may justify their being classed under the basalts.

Under the microscope the groundmass is quite different from type A. One specimen (183) is apparently holocrystalline, one (184) contains a very little, and the third (185) quite a perceptible amount of glass. The holocrystalline specimen has a groundmass composed of minute prismatic crystals of augite, octahedral magnetite, and abundant plagioclase. The plagioclase consists in part of distinctly recognizable laths, much smaller and less well defined than in type A, also a sort of residual feldspathic paste that does not show distinct form and can not be clearly demonstrated to have polysynthetic twinning. It has, however, a more or less undulous extinction, and resembles the feldspathic residual paste of some of the andesites. In the more glassy variety the augite is not so abundant, and the plagioclase appears clouded and indistinct because of the dusty-looking glass that impregnates the whole, but still can not be plainly recognized.

Phenocrysts are abundant and well formed. They are plagioclase, hypersthene, augite, and, in the case of No. 183, olivine. The plagioclase is characteristically andesitic in type, showing zonal structure and containing inclosures of hypersthene, magnetite, and glass in its accustomed distribution. Hypersthene and augite are in sharply defined crystals, except where they occur in nests, which is commonly the case. They have the color and pleochroism common to these minerals in the andesites. They both contain inclosures of colorless glass with air cavities, also of magnetite. Parallel growths of augite around hypersthene allies two of these (184 and 185) to the basalts. The olivine of No. 183 is not abundant. It occurs mostly in grains and in rounded forms with magnetite rims. Magnetite occurs in the customary octahedral crystals.

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