The ratio of crystals to glass varies between wide limits. Close to Crater Lake, where the deposits are crowded with large lumps of pumice, the volume percentage of discrete crystals commonly falls below 1. At greater distances the percentage may rise above 25, and in one sample, from Wikiup on the Deschutes River, it reaches 40. Surprising was the discovery that among the deposits bordering the Dalles-California highway, the content of crystals generally increases with the distance from Crater Lake (figure 21). In the region southeast of the Klamath Marsh, the volume percentage of crystals ranges from I to 11 (figure 20).
Because of the great lateral and vertical variations, and the difficulty of estimating the amount of crystals still embedded in glass, it is impossible to give an accurate figure for the total content of crystals in the fall. Moore estimated the volume percentage to be about 5, but this figure seems too low. Probably 10 to 15 per cent is nearer the truth.
As to the nature of the crystals, they consist of plagioclase, hypersthene, and smaller amounts of augite and hornblende. The ratios of heavy to light minerals are shown in some of the histograms (figures 19-22). One might expect that toward the source the proportion of heavy minerals would increase at the expense of the light, but no such regular variation was observed. The only generalization which can be made is that in the marginal parts of the pumice sheet the ratio of light to heavy crystals is usually higher than it is nearer the source. In some samples feldspar may be ten times as abundant as the dark minerals, but the usual ratio is between 2 and 3 to I. Being in general both stouter and larger, the feldspar crystals, despite their lower density, tended to fall at the same time as the heavier crystals. Changes in wind velocity, in the strength of the eruptions, and in the crystal content of the magma combine to explain the irregular variations noted above.
If our estimate of the average content of crystals, between 10 and 15 per cent, be correct, it follows that the percentage in the magma prior to explosion was much larger, for when the magma frothed into pumice its volume was greatly increased. In other words, just before the pumice eruptions the upper part of the magma chamber had crystallized to the extent of more than 30 per cent of its volume. Retrograde boiling associated with this advanced state of crystallization may well have brought about the first explosions.
Lithic content. In addition to pumice and crystals, the ejecta include fragments of old lava torn from the conduit walls and from the upper part of Mount Mazama. These are composed almost entirely of andesite. Bits of dacite may also be found, but ejecta torn from the pre-Mazama basement rocks are extremely rare.
Because of their greater density, the lithic fragments fell close to the source. Had the early explosions blasted away much of the upper part of the volcano, we should expect an abundance of coarse, blocky detritus near the rim of the caldera. No feature of the ejecta is more striking, however, than the paucity of large blocks of old lava dose to the source. Even on Timber Crater, lithic fragments as much as 2 inches across are extremely rare and by far the bulk of such fragments measure less than 1/4 inch in diameter. Still closer to the source, on the west slope of Mount Scott, few of the lithic fragments exceed 3 inches across. Most of the coarser detritus on the rim of the caldera is not of explosive, but of glacial origin.
On the sides of Mount Thielsen, lithic chips larger than 1/2 inch across are practically absent. Chips 1/2 inch across have been found as far as 30 miles from the caldera, at Windigo Pass. On Walker Rim and Skookum Butte, an equal distance northeast of the caldera, where the thickness of the pumice fall is 10 feet and many of the pumice lumps measure 4 1/2 inches across, the largest lithic fragments are less than 1/4 inch in diameter. The maximum size of the lithic fragments in other samples is shown on the histograms, figures 19-22. More than 50 miles from the caldera, chips of old lava exceeding 1/8 inch across are exceptional. The amount of lithic material that fell more than 20 miles from Crater Lake was therefore quite insignificant as compared with the bulk of the pumice and crystals.
