84 Dacite Tuff

The Geology and Petrography of Crater Lake National Park, 1902




The deposit of fine volcanic materials that is to be seen filling the bottom of the Sand Creek Canyon is represented by No. 128. This is a loose, friable, dark-gray tuff. Examined with a pocket magnifying glass it appears to be composed mainly of dust-covered white grains with an admixture of some black crystals. The white grains turn out to be plagioclase and the black ones hypersthene and hornblende, all of them exactly similar to the phenocrysts of the above-described dacites. In addition to these crystals and grains there occur small (3 to 5 millimeters) angular fragments of compact dacite, or perhaps also of andesite.

The plagioclase does not have the form so well preserved as do the other minerals. The hypersthene crystals show the customary unit prism and two pinacoids, while the hornblende has only the prism clearly shown. The hornblende has the optical properties similar to those given for the phenocrysts in the dacites. That this tuft is undoubtedly dacitic is demonstrated by the presence of these abundant hornblende crystals which are almost entirely confined to the dacites of Crater Lake.

An entirely analogous dacite tuft is to be seen in the bottom of the Anna Creek Canyon (No. 129). This tuft also contains hornblende in great abundance,

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