In brief, the basic scoria differs from the preceding dacite pumice in the much greater content of ferromagnesian minerals, particularly hornblende, the presence of considerable olivine, the more basic composition of the feldspar, and the dark-brown color of the glass. Exceptionally the hornblende has the reddish color and low extinction angles of oxyhornblende, but normally it is green or greenish brown and never shows marginal separation of magnetite.
The matrix of the scoria flows consists partly of the same materials as the bombs. This part was obviously produced by shattering of the larger lumps during transit. The remainder consists of small lithic fragments.
Study of the lithic fragments in thin sections and with the binocular reveals the following types: (a) hypersthene andesites similar to those exposed in the caldera walls, composing by far the bulk; (b) many opalized chips of andesite, possibly derived from solfataric areas about the former summit of Mazama; (c) intergranular and hyalopilitic olivine-bearing basaltic andesites like those of the Union Peak volcano and the pre-Mazama lavas in the northwest corner of the park; (d) various types of obsidian identical with those erupted from the Northern Arc of Vents; (e) devitrified, tridymite-rich dacites comparable with the southern dacites; (f) epidote(?)-bearing quartz keratophyres and chips of propylitized lavas similar to those of the Western Cascade volcanic series. Thus, with the exception of the last-named fragments, which are extremely rare, all the lithic inclusions were derived from shallow depths, that is, from the Pliocene basement beneath Mazama and from the cone of Mazama itself. At no High Cascade volcano known to the writer have fragments of the pre-Tertiary basement ever been discovered among the ejecta.
Some of the scoria flows were almost entirely composed of crystals. For instance, after the lithic-rich flows had deposited their load in the Pumice Desert they were followed by avalanches consisting almost exclusively of hornblende and plagioclase crystals with accessory pyroxenes, ore, and a little brown glass. These avalanches were presumably among the latest, and they suggest that at the close of the eruptive cycle deep-seated, crystal-rich levels in the magma chamber were beginning to be tapped.
After they had come to rest, the scoria deposits were subjected to long-continued fumarolic action. Gases, temporarily trapped in the quickly compacted ejecta, escaped along irregular, though more or less vertical, joints. The nature of these gases is not known, but probably they included much ferric chloride. Reacting with air and percolating ground waters, they deposited iron oxides close to the surface. The walls of the fumarole cracks and the upper layers of the scoria deposits were deeply reddened with hematite, most of which has subsequently been converted to limonite. In addition, the fumarole cracks are commonly bordered by decomposed scoria in which kaolin and opal are widespread. Finally, it may be recorded that a coating of chrysocolla was found on a slab of scoria in the talus banks below the main falls on Annie Creek. Presumably, therefore, copper was also transported among the fumarolic vapors.
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