50 Hypersthene – Andesites

The Geology and Petrography of Crater Lake National Park, 1902

 PART II.

HYPERSTHENE-ANDESITES.

The andesites are the oldest of the volcanic rocks of Crater Lake and form the great bulk of the exposed rocks. They are all hypersthene bearing, and may therefore be classified as hypersthene-andesites. But in spite of the fact that they may all be placed under this one head, there is a remarkable diversity in their external appearance, inasmuch as they vary in color from black to light gray or light drab and in texture from vitrophyric to holocrystalline. This diversity of texture and color is confined, however, to the groundmass; that is to say, the phenocrysts, although they may show many and varied distinctions, do not appear to be dependent in their variations upon the character of the groundmass to any very great extent. In the following description of these rocks the phenocrysts will be first considered, as they occur in all the andesitic varieties, while the groundmass will be described in connection with the discussion of the different varieties, which depend entirely on the variations of the groundmass.

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