68 Hypersthene – Dacites

The Geology and Petrography of Crater Lake National Park, 1902



In all the descriptions of the rocks of Crater Lake heretofore published the rocks which are here designated as dacites have been called rhyolites. This is not to be wondered at, as these rocks present all the outward characteristics of rhyolites, and even a microscopic examination does not at first give strong ground for the change of name. Previous descriptions have been based largely upon field observations. But, in spite of the fact that comparison may frequently be made and is made in this paper between these dacites and rhyolites of well-known occurrence, a careful study of the mineralogical characteristics, supported by the chemical analyses, has led the writer to the conviction that the change of name is justified. As will appear later, the determination of these rocks as dacites is further strengthened by resemblance to other dacites in the adjacent regions of northern California.

As will be seen to be the case with the basalts the dacites of Crater Lake are somewhat local in their distribution. As they are chiefly confined to distinctive lava flows that, with one exception, represent the latest eruptions to be seen on the rim, and whose outlines can be definitely traced, it is possible and desirable to treat each of the dacite flows separately. Beginning with the most conspicuous flow, that of Llao Rock, and passing around the lake toward the east, the dacite flows will be described under the following heads: Llao Rock, Grouse Hill, Cleetwood Cove, Wineglass, Cloud Cap, Sun Creek. Outside of these six areas, as well as within the same, there occurs abundant dacitic material, in the form of bombs and tufaceous matter, that will also receive separate consideration.

As is usually the case with dacites, the groundmass of these rocks presents a most remarkable variety of structures, and upon the variations in the groundmass most of the distinctions are to be based. To a certain extent this is true of the phenocrysts, at least as far as their relative abundance is concerned. Upon the whole, however, the Crater Lake dacites, irrespective of the extreme variations in the groundmass, are characterized by the presence of well-defined phenocrysts. These minerals are labradorite, hypersthene, and brown hornblende, all of which are usually present, also augite and olivine, which may occasionally be seen.

 << previousnext >>