63 Dacitic Type of Andesite

The Geology and Petrography of Crater Lake National Park, 1902





This fourth type of andesite is intermediate between the andesites and dacites, but seems to possess characteristics that ally the rocks here included more closely with the andesites than with the dacites. It is possible that a chemical analysis would place one or two of them in the list of dacites.

Of the six specimens placed in this group, three (75, 76, 77) have the dacitic features particularly prominent. The groundmass in these three consists very largely of a nearly colorless glass in which lie myriads of the extremely minute rod-like augite microlites characteristic of the glassy dacites (77). In two (75, 76) the groundmass consists of somewhat larger microlites of the same mineral inclosing minute black specks of magnetite, or possibly of globulites, suggestive rather of the andesites. In addition to these augite microlites very slender feldspar microlites, as well as a few other larger and broader plagioclase crystals, are sparingly developed. The glass base is rendered dark looking, especially in No. 76, by globulitic inclusions and by the presence of a very little magnetite. In Nos. 76 and 77 occur deep rusty-brown and somewhat granulated spots that tend to form around the pores and also to some extent around the plagioclase phenocrysts. The pores are apt to contain a little tridymite, partially or entirely filling them. These darker, iron-rusted spots bear a slight resemblance to the spherulitic growths in the dacites, but do not appear to possess radiated structure.

The three other specimens (78, 79, 80) are similar, so far as the augite microlites are concerned; but they contain little or no glass base, instead of which there is to be seen much more abundant plagioclase laths, as well as squarish or short rectangular feldspars with undulous extinction, or, rather, with zonal structure but no twinning; likewise small irregular or allotriomorphic colorless shreds of feldspar or, perhaps, of quartz.

The phenocrysts in these six rocks are not essentially different from those in other andesites (brown glass inclosures, abundant in plagioclase of 75). Hypersthene in two generations—the older in short, stout, and relatively large crystals, the younger in small, long, and slender prisms—is clearly developed in No. 78.

The last three rocks, without perceptible glass, are very light colored in the hand specimens while the glassy varieties are much darker.

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