The Geology and Petrography of Crater Lake National Park, 1902
DISTRIBUTION AND DESCRIPTION OF DACITE MASSES.
CLEETWOOD COVE FLOW.
This mass is one of the latest lava flows to be found on the rim of the crater. According to Mr. Diller a part of this stream was fluent at the time the summit of Mount Mazama subsided, and a portion of the stream that ran back into the pit formed by the subsiding cone may be seen at the head of the cove at the water’s edge. The lava of this mass, as far as may be judged from the five specimens collected, is of two varieties, a vitrophyric and slightly spherulitic and a lithoidal variety.
The vitrophyric type is seen in Nos. 109, 110, and 111, which were collected on the crest of the rim. These appear in the hand specimens to be very glassy and to consist of mixtures of black to grayish-black and of reddish-brown glass. The reddish-brown portion is a little more abundant than the black and forms more or less continuous streaks, while the black is more frequently in the form of small, angular patches, inclosed in the former. When examined with a pocket magnifying glass the brown portions appear often porous and possess a somewhat ropy structure, while the black portions are solid. Small phenocrysts of white glassy-looking plagioclase abound as usual.
In thin section it is evident that the brownish portions result from the alteration of the black glass simply by a process of hydration of the iron contained in the glass. The fresh and unaltered parts appear in a thin section as a light-brown glass, perfectly fresh and clear, but irregularly dotted with very small colorless glass spots. This light-brown glass contains countless minute and colorless augite microlites which, in No. 109, are identically like those in the glassy dacite from the southern end of Llao Rock. In No. 111 the groundmass contains, in addition to these augite microlites, also a few plagioclase microlites. In No. 110 the augite microlites are more slender and longer, as well as more numerous, and the feldspar microlites occur in lath-shaped plagioclase, and still more conspicuously in the squarish or rectangular untwinned forms that are more especially characteristic of dacites. In all three of these there is a partial development of spherulitic forms. These occur in small reddish to brownish or yellowish spheres that very faintly polarize light and have a not very distinct radial structure.
The brown color of the rock as seen in the hand specimen is due largely to the ferritic matter contained in the spherulites, but is also due to staining of the glass in the vicinity of cracks and pores and around the spherulites.
The phenocrysts, with the exception of plagioclase, are small and very scarce. They consist of hypersthene and a very few minute, reddish-brown hornblendes (not more than three or four in a thin section). Augite is entirely wanting.
The lithoidal variety of dacite from this lava flow is seen in Nos. 112 and 113. The former has a dark-gray, compact, and dull-lustered groundmass and strongly developed fluidal structure; the latter is light gray and lusterless, but not fluidally developed. Both of these probably contain considerable glass, but in No. 112 this is partially, and in No. 113 almost entirely, obscured by devitrification products common to lithoidal dacites. In both, the augite microlites abound, but in No. 113 these are notably larger than in the other dacites heretofore described in these pages, and they are granulated by means of adhering and inclosed magnetite grains. In this rock, too, the colorless crystalline material of the groundmass is coarser than usual. Very little of it can actually be identified as plagioclase microlites. It consists, rather, of allotriomorphic shreds that may be feldspar or quartz, and that have a general elongation parallel to the fluidal planes of the rock. Small plagioclase phenocrysts abound, but other phenocrysts are very scarce. Hornblende is nearly missing and augite entirely so.
A chemical analysis of No. 113 will be found on page 140.