87 Pumice

The Geology and Petrography of Crater Lake National Park, 1902





No. 102, from the summit of Llao Rock, is an almost pure pumice of a pinkish white color and extremely light and porous. With a magnifying glass one can detect a very few glassy plagioclase crystals, and also still fewer dark-colored pyroxene or hornblende phenocrysts, similar to the phenocrysts in the Llao Rock dacite. Under the microscope this rock presents the customary porous, pumiceous structure, with long drawn out gas pores; but the glass differs from the vitrophyric portions of the Llao Rock flow, in that the very characteristic rod-like augite microlites are almost entirely missing. A few phenocrysts of plagioclase in crystals and crystal fragments are to be seen, also an occasional hypersthene and two or three minute brown hornblende crystals. One of the plagioclase crystals was seen to contain numerous inclosures of apatite and also several dark-brown glass inclusions, in one of which was an air bubble. No. 137 is a fragment of pumice collected from the same place, and differs only in being more discolored. It contains similar phenocrysts, and also brownish glass inclosures with gas bubbles in the plagioclase. This pumice evidently is of the same character as the dacite of Llao Rock.

No. 138 is a specimen collected from a large fragment in the Pumice Desert, the north of Crater Lake. It has a brownish-yellow color and looks as though it contained more or less clayey matter. It also has a fragmental appearance as though it were a brecciated rock composed mainly of pumice. It also contains a few small fragments that appear to be andesite. Plagioclase grains are very abundant in the hand specimen, but the thin section contains hardly any, as they have probably disappeared in the grinding. A very few minute hypersthene and augite crystals and one small fragment of reddish-brown hornblende were noted.

In addition to this ordinary pumice are also fragments of a distinctly pumiceous rock of light color in which are readily seen numerous black crystals, mostly hornblende. These hornblende crystals vary in size from one-fourth of an inch downward. Probably one-eighth of an inch in length is a fair average size. One of these fragments was collected on the northeast rim of Crater Lake, east of Round Top, and another (146) from the summit of Red Cone, while two others were collected by the writer on the crater rim just south of Llao Rock (2011, a and 2013, 2 of the private collection of H. B. Patton). The ferromagnesian minerals which are very conspicuous in the hand specimen are still more apparent in the thin section, as, indeed, are also the colorless plagioclase phenocrysts. The groundmass of these specimens does not appear to differ materially from that of the more typical pumice. It is a nearly pure glass, full of elongated air cavities and more or less stained with yellowish to brownish ferritic matter. Only a few augite microlites and but little magnetite dust is to be seen. On the other hand all the minerals that have been mentioned as occurring among the phenocrysts in the Crater Lake dacites and that are there sparingly developed are unusually numerous in these specimens. The minerals here included are plagioclase, hornblende, hypersthene, and augite. All but the hornblende appear to occur in about the same sizes as in the dacites. In the case of plagioclase there is developed a very strong tendency to inclose comparatively large and irregularly lobed and more or less connected inclusions of very deep brown glass. These inclusions are not arranged around the margin in a well-defined zone, as is so frequently the case with the plagioclase phenocrysts of the andesites, but they are more evenly distributed throughout the crystal and produce a structure that may fairly be characterized as spongiform. These spongiform plagioclases are by no means as common or characteristic as they are in the dark-colored secretions whose descriptions follow; neither do all or even most of the plagioclase phenocrysts have these inclusions, but their appearance here is important as forming a link between the dacites and the inclusions in the same. Fracturing of the feldspars is very noticeable.

Hypersthene and augite occur in the same forms and with the same general properties as in the dacites. Perhaps a stronger tendency is to be noted toward rounding of the edges, particularly at the ends. Likewise inclusions of glass and especially of brown glass are more abundant. The most interesting mineral is hornblende, whose abundance is increased relatively much more than is the case with the other phenocrysts. It also occurs in very much larger crystals than are to be seen in the dacites. Except for abundance and size, however, these hornblende crystals do not present any marked peculiarities. They show a strong tendency to develop the unit prism and the clinopinacoid, but they are usually broken into fragments like the plagioclase. In color they are similar to the hornblendes of the dacites. Greenish brown predominates, but the more reddish colors are also to be seen, notably in No. 139. Magnetite occurs rather sparingly in these rocks in the form of distinct grains inclosed in the phenocrysts as well as scattered through the glass groundmass.

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