CRATER LAKE, OREGON. 377 



rounded by rims composed of the fragmental material blown out from 

 the depression. 



At first sight the rim about Crater Lake suggests that the caldera 

 was produced by an explosion, and the occurrence of much pumice in 

 that region lends support to this preliminary view; but on careful 

 examination we find, as already stated, that the rim is not made up of 

 fragments blown from the pit, but of layers of solid lava interbedded 

 with those of volcanic conglomerate erupted from Mount Mazama before 

 the caldera originated. The moraines deposited by glaciers descending 

 from the mountain formed the surface around a large part of the rim, 

 and as there is no fragmental deposits on these moraines, it is evident 

 that there is nothing whatever to indicate any explosive action in 

 connection with the formation of the caldera. 



We may be aided in understanding the possible origin of the caldera 

 by picturing the conditions that must have obtained during an effusive 

 eruption of Mount Mazama. At such a time the column of molten 

 material rose in the interior of the mountain until it overflowed at the 

 summit or burst open the sides of the mountain and escaped through 

 fissures. Fissures formed in this way usually occur high on the slopes 

 of the mountain. If instead, however, an opening were effected on the 

 mountain side at a much lower level — say some thousands of feet below 

 the summit — and the molten material escaped, the mountain would be 

 left hollow, and the summit, having so much of its support removed, 

 might cave in and disappear in the molten reservoir. 



Something of this sort is described by Professor Dana as occurring at 

 Kilauea, in Hawaii. The lake in that case is not water, but molten lava, 

 for Kilauea is yet an active volcano. In 1840 there was an eruption from 

 the slopes of Kilauea, 27 miles distant from the lake and over 4,000 feet 

 below its level. The column of lava represented by the lake of molten 

 material in Kilauea sank away in connection with this eruption to a 

 depth of 385 feet, and the floor of the region immediately surrounding 

 the lake, left without support, tumbled into the depression. In the 

 intervals between eruptions the molten column rises again toward the 

 surface, only to be lowered by subsequent eruptions, and the subsidence 

 is not always accompanied by an outflow of lava upon the surface. 

 Sometimes, however, it gushes forth as a great fountain a hundred feet 

 or more in height. 



The elevated position of the great caldera occupied by Crater Lake 

 makes its origin by subsidence seem the more probable. The level of 

 the lowest bed of the lake reaches the surface within 15 miles down the 

 western slope of the range. That Mount Mazama was ingulfed is 

 plainly suggested by the behavior of its final lava stream. The greater 

 portion of this last flow descended and spread over the outer slope of 

 the rim, but from the thickest part of the flow where it fills an old 

 valley at the head of Cleetwood Cove some of the same lava, as already 

 noted, poured down the inner slope. The only plausible explanation 



