59 Hypocrystaliline Type of Andesite

The Geology and Petrography of Crater Lake National Park, 1902





This may fairly be said to be the normal type of Crater Lake andesites; at least, it is more abundantly represented than any other. It may be divided into two subtypes, which, though united by numerous connecting links, are upon the whole readily distinguished. In the hand specimen, however, this distinction between the two subtypes can not be made. Neither can the two be readily distinguished from the hyalopilitic type except in thin section. Taken as a whole, they are decidedly dark-colored rocks with a basaltic look, but the darker varieties distinguish themselves by the relative prominence of the plagioclase phenocrysts. Not a few are as dark as the darkest of the hyalopilitic andesites, but they lack the glossy luster common to some of that type. The average color is naturally lighter than in the hyalopilitic type, as they are more decidedly crystalline. In fact, not a few included under both of the two subtypes have a gray or even a light gray or drab color, break with rough fracture, and approach to the holocrystalline andesites. The pyroxenes, as usual, are inconspicuous and have a slightly resinous luster with green to brown colors.

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