Communications from the U. S. Geological Survey, Rocky Mountain Division; on hypersthene andesite

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1883
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Abstract
IN the coul'se of the investigation of some apparently normal augite-andesites, of the most typical variety, occurring at the Buffalo Peaks in South Park, Colorado, the writer found that a large part of the pyroxenic constituent possessed the crystalline form and chemical constitution of hypersthene rather than of augite.The comparative study of similaJ' andesites fl'om this countl'y and fl'om well-known European localiti.eshas forced him to the conclusion that in very many, if not in all of them, augite is decidedly subordinate to a rhombic pyroxene, which is presumably hypersthene..As this conclusion, if proven to be correct, affects materially the current classification of andesitic rocks, the grounds upon which it is based will be concisely stated. Hypersthene-andesite from Buffalo Peaks, Oolorado.The rock in question occurs in mass, associated with a number of other andesites, prominent among which is a normal hornblende-andesite, and too in fragments imbedded in an important series of volcanic tufas, also of andeflitic character.For further information concerning these rocks, the reader must be referred to the forthcoming" Report upon the Geology and.Mining Industry of Leadville, C.Jlomdo," by S. F. Emmons.The rock with which we al'e now occupied is very compact, almost black in color, showing macroscopically a large number of small glassy feldspal's, and a few dark, green grains.The ground•mass in which these crystals lie has a dull vitreous luster.When examined under the microscope in ordinary light the rock seems to be an augite-andesite of very typical composition and structure.Clear plagioclase crystals and pyroxene in small crystals and it'l'egular grains, with magnetite and apatite, are the only mineral constituents to be recognized.These larger individuals lie in a ground-mass composed of delicate staves of plagioclase, light green microlites of pyroxene and minute octahedra of magnetite, with a glass base between them, which is usuall'y clear, though sometimes devitrified by light brownish globulites.
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Authors W. CROSS
Journal american journal of science
Year 1883
DOI
10.2475/ajs.s3-25.146.139
URL
https://ajsonline.org/article/64012.pdf
Keywords Keywords not found

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