82 Cloud Cap Flow

The Geology and Petrography of Crater Lake National Park, 1902





This is an extensive flow that starts at Cloud Cap and extends for about 3 miles in a northeasterly direction with a breadth of about a mile. The rock specimens collected from this flow and included in this description are Nos. 115, 116, 117, 118, 119, and 120. With the exception of one distinctly lithoidal specimen (119), these are mainly very glassy and also largely spherulitic dacites, closely resembling the Llao Rock mass (as seen in Nos. 101, 102, and 104). The glassy portion of these rocks has a light-gray color. Phenocrysts of white glassy plagioclase arc abundant, and phenocrysts of the ferromagnesian minerals are scarce. Spherulites are to be found in Nos. 116, 117, and 120, but they are conspicuously visible in the hand specimen only of the last mentioned number. Here they occur from one-fourth to one-half inch in diameter, and have a whitish to light-drab color. The larger spherulites are more or less hollow and show on the cracked surface a fluted or ribbed structure radiating from the center outward.

As this rock does not differ very materially from the spherulitic dacites already described, a brief description of the microscopic characters will suffice for present purposes. The glassy portions in the spherulitic varieties, as well as in the entirely vitrophyric groundmass of No. 118, contain characteristic streams of minute straight and colorless augite microlites, like those in Nos. 101 and 102 of the Llao Rock mass. Microlitic feldspars, in the form of lath-like plagioclase and rectangular and apparently untwinned feldspar, occur in varying amounts, both in the same and in different thin sections. Some streaks may be almost entirely devoid of such feldspar microlites, while others may be crowded thickly with them, while between these two extremes there appears every conceivable intermediate stage. In two or three thin sections but more noticeably in No. 117, the untwinned feldspar, or more properly the microlitic feldspar, that does not appear to have the albitic twinning, occurs in very thin leaves that show striking Carlsbad twinning. The two individuals that make up one of these twins have each of them simple quadrilateral form, and the leaves lie over each other joined by the clinopinacoid, which is also the plane parallel to which the little leaves are extended. Professor Rosenbusch refers to such twinned microlitic feldspars as occurring in rhyolite pitchstone and rhyolite obsidian. No. 116, which shows in the hand specimen very strongly developed fluidal structure, consists of alternating, parallel streaks of vitrophyric, spherulitic dacite, and of a more lithoidal dacite free from spherulitic inclusions. The lithoidal streaks have the plagioclase laths very strongly developed and at times bear a marked resemblance to some of the Crater Lake andesitic rocks. On the other hand, No. 115 has a brown glass groundmass, inclosing numberless somewhat granulated augite microlites, and is almost free from feldspar microlites of any description. Slender, straight, and curved opaque black trichites, as well as a very little black ore in minutest grains, may also be mentioned as occurring in some of these specimens.

The spherulites differ somewhat from those in No. 104, in that zones of growth are not so plainly developed. There is, however, often an inner, coarse-grained portion occupying the greater part of the spherulite and an outer, denser, and usually deeper stained zone. The inner coarser part is made up of radiating shreds that show both positive and negative extinctions; usually, however, positive. They also have a tendency to fork at small angles and do not extinguish always parallel.

The phenocrysts do not present any characteristics peculiarly different from those common to the other dacites with the possible exception of hornblende, which is rather more abundant than elsewhere. It has mostly a greenish-brown color, but also occurs reddish-brown in No. 116. It is absent only in No. 115. It is of interest to note in this connection that, although these rocks contain more hornblende than do the other dacites of Crater Lake, still the actual amount is very small indeed. As far as may be judged, a thin section from this flow does not contain one-fiftieth or perhaps one-hundredth part as much hornblende as does the dacite from Lassen Peak, to which reference is made above and with which these Crater Lake rhyolites are closely allied, it should further be stated that occasional nests of older secretions are to be seen consisting of plagioclase, hypersthene, hornblende, augite, and magnetite, in which hornblende is apt to be very abundant. Zircon, which is so often reported as occurring in such rocks, appears to be a rare accessory mineral in the Crater Lake rocks of all types. It was noticed in a single crystal in No. 117.

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