57 Classification of Andesites

The Geology and Petrography of Crater Lake National Park, 1902




While the phenocrysts fluctuate greatly in abundance and relative importance, they do not serve as well as does the groundmass as a ready means for classifying the widely different appearing varieties of the andesitic rocks. Certain structural types in the groundmass, on the other hand, recur with great frequency and are worthy of detailed description. The andesites of Crater Lake, therefore, may be divided into four main types, based upon the character of the groundmass, namely, hyalopilitic type; hypocrystalline type, with two subtypes (A and B); holocrystalline type; dacitic type.

Although each of these four types is quite distinctive, it is not to be supposed that all the examples cited in each case are equally so. Rather it may be said that they all grade into each other in such a way as to make the separation not always easy. Neither can the rocks of any one lava stream or of one locality be said to be limited to any one type.

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