105 Andesitic Basalts

The Geology and Petrography of Crater Lake National Park, 1902




In nearly all of the basalts thus far described in these pages the structures have been distinctly different from what has been described as characteristic of the andesites. To a certain extent this may also be said of the composition. But in the rocks here designated as andesitic basalts these differences in structure and also in composition to a large extent disappear, so much so that were it not for close association with more distinctly basaltic types, some of those here included would certainty be classed with the andesites. In the absence of chemical analyses the correctness of these determinations must be taken with some reserve. A general statement of the differences noted between the andesitic basalts and the andesites will be given at the close of the more detailed description. As compared with the foregoing basalts, these may be said to be distinguished, with one or two exceptions, by the presence of a hypocrystalline groundmass or of one with an evident glass base, as well as by the presence of well-developed phenocrysts of plagioclase.

Four somewhat distinct structural types may be recognized. They are designated as types A, B, C, and D.

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