Moore,1 who was the first to emphasize these differences, termed the coarser typelump pumice, and the finer granular pumice. Here they are spoken of as pumice flowsand pumice fall, respectively, following a usage adopted by Kôzu2 in describing similar deposits laid down during the great eruption of the Japanese volcano Komagatake, in 1929. These terms seem preferable because they stress the different modes of origin of the two types of ejecta: the pumice flows were products of glowing avalanches (nuées ardentes) that truly flowed down the flanks of Mount Mazama, following the dictates of topography as faithfully as any lava might; the granular pumice fell from the air in showers, and its distribution and thickness were determined largely by the vagaries of the winds at the time of eruption.
Only close to the caldera rim is there any difficulty in distinguishing between the products of the pumice flows and pumice fall, This is to be expected, for close to the source pumice fell from the air in such large volume that it acquired little stratification and included lumps as large as those left by the glowing avalanches in the same vicinity.
Not all, but by far the greater part of the pumice which fell from the air did so before the pumice flows were erupted. Similarly, the main pumice falls at Krakatau, Santorin, and Komagatake preceded the pumice flows, and at each of these three volcanoes the flows were followed by weak, dying explosions of ash. Having at the outset insisted on the recognition of the two principal types of pumice surrounding Crater Lake, we may now proceed to discuss each of them in detail. If the discussion seems unduly long, it may be said in explanation that the pumice deposits provide the key to the origin of Crater Lake, one of the main themes of this report, and that hitherto they have not received the attention they merit.
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