_Forming of the crater._—The broken edges of rock layers seen on the crater slopes indicate widening of the crater in all directions. The edges of these rock layers inside the crater wall are clearly exposed because they have been sharply broken around the entire inner rim region. This fracturing occurred in the course of widening the crater. Increase in size of the opening at the summit of the mountain, which eventually formed the present crater, may have been caused by tremendous explosions, or by collapse of the peak, or by a combination of such activities. If the activity of a volcano diminishes slowly, growth of the mountain may end in forming a symmetrical cone. If activity continues by spasmodic outbursts, explosions may blow away a considerable part of the peak. Other conditions may bring about undermining of the walls in such manner as to produce a wide cauldronlike crater, but without tremendous explosions. Recent investigations by Howel Williams, under a grant from the National Academy of Sciences, have led to the conclusion that the crater owes its origin principally to collapse or engulfment of the mountain peak. Formation of the crater by collapse was first proposed by J. S. Diller of the United States Geological Survey. Diller’s explanation differs from that of Williams principally in the method by which the void beneath the crest of Mount Mazama was formed. Diller thought that great quantities of molten rock were drained away through subterranean passages, thus weakening the support of the mountain peak and causing ultimate collapse. In a report to be published by Williams, he describes great quantities of volcanic ash or pumice extending for a distance of more than 80 miles northeast of Mount Mazama. This ash is equivalent to more than 20 cubic miles of material and is thought to have been blown from the mountain in a catastrophic event, and carried northeastward by the prevailing winds. Analysis of this ash shows that it is new material derived from the magma within the volcano and not finely divided fragments of the original mountain walls. Following this explosion the mountain is thought to have literally boiled over, pouring out great quantities of frothy magma which flowed down the sides of the mountain and overflowed the lowlands below. The greater quantity extended to the south and southwest for distances up to 35 miles. This material poured out from the crater as a series of avalanches which must have flowed at a terrific speed for those on the south and west sides of the mountain did not begin to deposit their load until they reached a distance of 4 or 5 miles from the crater. Accompanying these explosions and the outpouring of this lava material, cracks developed in the flanks of the mountain and eventually the top collapsed and was engulfed within the void produced by the outpourings of ash and molten rock, thus forming the crater as we see it today.