109 Andesitic Basalts Type D

The Geology and Petrography of Crater Lake National Park, 1902

 PART II.

BASALTS.

ANDESITIC BASALTS.

ANDESITIC BASALTS, TYPE D.

This type is preeminently andesitic, either through the development of plagioclase phenocrysts or through the presence of an abundant glassy base. The rocks here represented are confined to the lava flows to the northwest of the lake. Nos. 188 and 189 come from the eastern slope of the second summit north of Desert Cone, No. 190 comes from the summit of the first hill north of Desert Cone, and No. 191 about half a mile south of this same summit and not far from the saddle between that hill and Desert Cone. Nos. 192, 193, and 194 were collected at, or not far from, the summit of Bald Crater, and Nos. 195 and 196 from the nearly flat region west of Red Cone.

These basalts are dark to black, dense and partly vesicular rocks with a more liberal development of minute feldspar phenocrysts than is the case with most of the other andesitic basalts. In spite, however, of the distinctly andesitic type, these rocks when examined under the microscope are seen to be almost entirely free from the larger, stout plagioclase phenocrysts that are so characteristic a feature in most of the andesites, and that may also be seen occasionally in some of the above-described basalts. Certainly they are free from all resorption phenomena and from the intermediate clouded zone characteristic of the larger plagioclase phenocrysts of Nos. 179, 180, and 181 of type A. In form they are usually rectangular or broad lath shaped, and they vary in size from one-half millimeter or even one millimeter down to the microscopic dimensions of the groundmass laths. There is, in fact, no sharp line to be drawn between the plagioclases of the groundmass and of the phenocrysts. They vary greatly in the character and amount of inclosures. At times the centers are thickly crowded with irregular glass inclusions; at times these glass inclusions are scattered or even altogether missing. Less commonly one may note inclusions of hypersthene or of augite.

While plagioclase is by far the most abundant phenocryst, the pyroxenes are also abundant. These are usually well developed, especially in No. 196, and have the customary properties already frequently described. With the exception of the three specimens from Bald Crater (192, 193, 194) both augite and hypersthene are present. In these three specimens hypersthene could not be found. On the other hand, olivine, as is usually the case where hypersthene is absent or scarce, is much more abundant than is the case with the other basalts of type D. The pyroxenes, however, fluctuate greatly in amount and in definition. In Nos. 188, 189, 190, and 191 hypersthene is the more abundant, the augite being almost confined to the groundmass. Olivine is never entirely wanting, but it is conspicuous only in the three rocks just referred to. In these it is not always easy to distinguish between the olivine and the very light green augite, as the color is almost alike in the two. This difficulty is greater in sections of olivine that are cut nearly perpendicular to one of the bisectrices, as in this case the interference colors are no higher than in augite. The most conspicuous form developed is the brachydome (021), in addition to which there occurs either a prism or a pinacoid. One section cut perpendicular to this brachydome gave in the center of the field a positive bisectrix with large optical axial angle. The trace of the sides measured 82°, which is about one degree larger than is given for the angle of the brachydome (021).

The groundmass of these basalts contains not a little glass which is either brown or is colorless, but rendered brown by the presence of dust-like globulitic material. This brownish glass is thickly crowded with minute augite prisms and, to a much less extent, with magnetite and with plagioclase microlites, the last named being, in fact, almost absent from the Bald Crater basalt.

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