46 CRATER LAKE, OREGON 



moraines deposited by glaciers descending from the mountain 

 formed the surface around a large part of the rim, and as there 

 is no fragmental deposit on these moraines it is evident that 

 there is nothing whatever to indicate an}' explosive action in 

 connection with the development of the pit. 



We may be aided in understanding the })0ssible origin of the 

 pit by picturing the conditions that must have obtained during 

 an effusive eruption of Mount Mazama. At such a time the col- 

 umn of molten material rose in the interior of the mountain until 

 it overflowed at the summit or burst open the sides of the moun- 

 tain and escaped through fissures. Fissures formed in this way 

 usually occur high on the slopes ot 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 sum- 

 mit — ^and the molten material escaped, the mountain would be 

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

 moved, might cave in and disappear in the molten reservoir. 



Something of this sort is described by Professor Dana as occur- 

 ring 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 dis- 

 tant from the lake and over 4,000 feet below its level. The col- 

 umn 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 tbe 

 intervals between eruptions the molten column rises again to- 

 ward 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 pit 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 to the westward. That Mount Mazama was engulfed 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 of this phenomena seems to be 

 that soon after the final eruption of Mount Mazama, and before 



