In order to determine as far as possible the percentage of rock fragments among the total ejecta of the pumice fall, numerous screen analyses were made in the field, and a few are here recorded in histograms. On Mount Scott and Timber Crater, 4 and 8 miles respectively from the center of Crater Lake, the volume percentage of lithic fragments may reach as high as 12, or fall as low as 4. On Mount Thielsen, the percentage usually varies between 2 and 4, as it does over most of the area south and east of Klamath Marsh. North of the junction of the Willamette and Dalles-California highways, the percentage rarely exceeds 1 and is generally so small that it has been omitted from the histograms, figure 21.
Taking the pumice fall as a whole, the content of old lava fragments averages between 3 and 4 per cent, and in general it is slightly higher in the older part of the fall than in the younger. A similar paucity of lithic detritus has been observed among the pumice deposits of Krakatau, Santorin, and other calderas.6 Clearly, the eruption of the pumice fall removed only an insignificant volume of old rock from the summit of Mount Mazama.
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