- The smoke-gray scoria layer, averaging approximately 80 feet thick. This is made up of bombs of crystal-rich, basic scoria up to 2 feet across, set in a finer matrix of pulverized scoria. Fully 90 per cent of the fraction larger than 5 mm. consists of scoria; the remainder is made up of old lava fragments. Despite the large size of the scoria bombs, few of the lithic pieces exceed even an inch across. In the finer fraction of the deposit, the content of lithic detritus is considerably higher, making up 18 and 23 per cent of the volume of two samples examined. Most of the lithic chips lie in the size range 1 to 5 mm., few occurring in the fraction less than 0.5 mm. As compared with the underlying dacite pumice, the scoria layer is generally much richer in crystals, particularly in ferromagnesian minerals. Moreover, though pyroxene predominates among the dark minerals of the pumice, hornblende predominates in the scoria. In some samples, between one-third and one-fourth of the fraction below 5 mm. in diameter is made up of crystals. How thorough the comminution of the scoria must have been is amply shown by the fact that of the material measuring less than 5 mm. in diameter, more than half consists of dust less than 0.5 mm. across.
- The bottom layer of dacite pumice, also averaging approximately 80 feet in thickness. Lithic detritus makes up on an average approximately one-fourth of the volume of material less than 5 mm. in diameter, but in the entire deposit the content of such detritus is only about 10 per cent. By far the bulk of the lithic material lies in the size range 2 to 5 mm.
Of the fresh magmatic material in the deposit, bombs of pumice up to 2 feet across form between 5 and 10 per cent of the volume, the remainder consisting of pulverized pumice, much of it in the form of impalpable dust, and crystals, among which feldspar far predominates over pyroxene.
Beyond the park boundary, the smoke-gray scoria layer disappears, though many scattered bombs of scoria may be found close to the Dalles-California highway. Whereas the earlier pumice flow continued for many miles, the later scoria flow spent itself soon after reaching the flats at the base of the volcano.
From the histogram, figure 22, it will be seen that in the pumice flow near Sun Pass, approximately 40 per cent of the material less than 10 mm. in diameter measures less than 0.5 mm. Here the volume percentages of pumice, old rock, feldspar, and heavy minerals in the size range 0.25 to 10 mm. are respectively 65.7, 24.9, 8.2, and 1.2. When the coarser material is also considered, the total volume of lithic material is reduced to between 10 and 15 per cent by volume, for among the fragments more than an inch across fully 95 per cent consists of pumice. The largest lithic fragments measure 3 inches across.
We may now briefly follow the course of the Sand Creek pumice flows after they deployed onto the plateau east of Mazama. Such was their mobility and momentum that they rushed onward for another 12 miles. In their path lay Boundary Butte. Up the onset slope of this obstacle they surged for 200 feet, carrying lumps of pumice 2 feet in diameter. Swirling round this and the neighboring buttes, the flows then emptied into the canyon of Williamson River, near the present site of the village of Kirk. In doing so they must, for a time, have choked the outflow of water from the Klamath Marsh. When the channel was cleared, great quantities of pumice were carried downstream into Upper Klamath Lake.
In the vicinity of Boundary Butte, the pumice deposits are marked by flat-topped benches m to 30 feet above the general level. The road running northwest from Kirk passes through a hollow between such benches. How they were formed is not clear. Either the deposits were channeled by floodwaters in the wake of the flows, or, more, likely, the benches resulted from successive waves of pumice.
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