The Geology and Petrography of Crater Lake National Park, 1902
PART II.
HYPERSTHENE-ANDESITES.
MINERAL COMPONENTS.
PYROXENE.
Both orthorhombic and monoclinic pyroxenes are characteristic ingredients among the phenocrysts, the former being hypersthene and the latter augite. These two minerals are nearly always abundant, but are never large enough to become prominent in the hand specimen. In general, they are both of about the same size, sometimes one and sometimes the other being the larger. They are not often over 1 millimeter and never more than 2 millimeters in greatest diameter, and from this they may sink to microscopic dimensions.
In the hand specimen they can not be distinguished from each other. They have a greenish to brown and brownish-green color and distinctly resinous appearance. They are perfectly fresh, and break without apparent cleavage in roughly conchoidal fractures. Although frequently bunched together, so as to appear larger than they really are, they require a magnifying glass to be clearly seen. Although both of these pyroxenes are almost always present, they vary greatly in their relative abundance. For instance, in a series of six specimens collected along the path descending from the camp ground to the water’s level—a vertical distance of about 800 feet—we have hypersthene the more abundant in Nos. 8, 9, and 24, while the reverse holds for Nos. 7, 42, and 44. In general, however, hypersthene is the more abundant (2), although it may become scarce (16). Augite, on the other hand, is rarely more abundant than hypersthene, and is often either scarce (40) or even almost completely wanting, so as to be represented by only one or two roundish grains in a thin section (30, 56).
