108 Andesitic Basalts Type C

The Geology and Petrography of Crater Lake National Park, 1902





This is, perhaps, the least andesitic in appearance and the least deserving of being classified as a distinct type of all the basalts of this region. The two specimens that belong here (186 and 187) were collected from the basaltic rocks in the extreme southwestern part of the mapped area, about 1 mile from No. 182. As these two specimens bear a closer resemblance to this rock, perhaps, than to any other one, they may be looked upon as a facial development of No. 182. These are blackish-gray, very dense rocks with a basaltic rather than andesitic appearance, being apparently free from phenocrysts. In thin section, however, they are seen to contain numerous but small phenocrysts of plagioclase with broad lath-form, suggestive of a basaltic rather than an andesitic type. In addition to these there are only a few olivine and augite (but no hypersthene) phenocrysts in roughly idiomorphic or in granular form. The groundmass consists of a feldspathic paste thickly crowded with roundish, roughly prismatic or irregular augite grains, and with magnetite. Glass appears to be wholly wanting.

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