86 Dacitic Ejectamenta

The Geology and Petrography of Crater Lake National Park, 1902




The large amount of volcanic ejectamenta that covers the surface of the crater rim in various places, especially on the northern and western sides, and that forms extensive deposits over the so-called Pumice Desert, appears to be almost entirely dacitic in character.

Some dark-colored sand, No. 134, collected on the Pumice Desert is composed of the light- and dark-colored minerals occurring as phenocrysts in the dacites, as well as of brown glass. These are plagioclase, hypersthene, hornblende, augite, and magnetite. The size of these crystals varies from about 2 millimeters downward. Plagioclase forms the most abundant of the sand particles, with forms that are usually roundish but that also may be well crystallized. Hypersthene, on the other hand, is very common in the characteristic prismatic forms with the unit prism and two pinacoids. When finely pulverized and examined under the microscope it is seen that both hypersthene and dark greenish-brown hornblende are very abundant and present in about equal amounts. Augite and magnetite are rather scarce. The latter can readily be separated from the sand by a magnet. This sand exactly resembles the coarser parts of the tuft found in the bottom of Sand Creek Canyon several miles below the crater rim (128) and in Anna Creek Canyon (129). The accumulation of the pyroxenes and of hornblende to a much greater extent than in the dacites is very noticeable in both the tuff and sand deposits.

A sediment brought up from the bottom of the lake on the west side, No. 1357 appears to be composed of somewhat similar material as the above, but as it is very fine—the largest grains being not much over 0.1 millimeter—and entirely composed of angular fragments, the percentage of the dark-colored phenocrysts, or rather of fragments of phenocrysts, is comparatively small. This is particularly true of hornblende, which is very scarce. On the other hand, glass fragments are very abundant. The nature of this sediment is not so well characterized as to make certain whether or not andesitic material is largely mixed with the dacitic ash.

The coarser ejectamenta which have been collected and submitted to the writer for investigation may be divided into three classes, viz, pumice, dark-colored secretions, light-colored granophyric secretions.

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