376 CRATER LAKE, OREGON. 



flows, all of wliicli escaped from the smaller adnate cones upon the 

 outer slope of the rim, are basalts. The eruptions began with lavas 

 containing a medium amount of silica (andesites), and after long- 

 continued activity lavas both richer (rhyolites) and poorer (basalts) in 

 silica follow, giving a completeness to the products of this great vol- 

 canic center that make it an interesting field of study. Furthermore, 

 the remarkable opportunity afforded by the dissected volcano for the 

 examination of its structure and succession of lavas is unsurpassed. 

 It should be stated, before dismissing the kinds of lava, that there are 

 some rhyolites in the Sun Creek Canyon south of the lake tbat appear 

 to be older than those upon the north side, and that the final lava of 

 the region on Wizard Island is andesite. 



The glaciation and structure of the rim clearly establish the former 

 existence of Mount Mazama, but there may well be doubt as to its 

 exact form and size. Judging from the fact that Mount Shasta and 

 the rim of Crater Lake have the same diameter at an altitude of 8,000 

 feet, and that their lavas are similar, it may with some reason be inferred 

 that Mount Mazama and Mount Shasta were nearly of equal height. 

 The slopes of Mount Shasta maybe somewhat steeper than those of 

 the rim of Crater Lake at an equal altitude, but the glaciation of the 

 rim is such as to require a large peak for its source. 



In figure 9 is given a section of Crater Lake and its rim, with the 

 probable outline of Mount Mazama. Wonderful as the lake, encircled 

 by cliffs, may be, it serves but to conceal in part the greatest wonder — 

 that is, the enormous pit or caldera which is half filled by the lake. 

 The caldera is 4,000 feet deep. An impressive illustration of it is seen 

 in figure 15 which was prepared from a photograph of a model of 

 Crater Lake now in the United States National Museum. The water 

 surface is represented by glass, so that one may see through to the 

 bottom and get the full impression of the depth of this tremendous 

 hole in the ground. It extends from the top of the rim, which is 

 the very summit of the Cascade Eange, halfway down to the sea level, 

 and nearly a square mile of its bottom is below the level of Upper 

 Klamath Lake at the eastern foot of the range. The volume of the 

 caldera is nearly a dozen cubic miles, and if we add the volume of the 

 lost Mount Mazama, that amount would be increased by at least one- 

 half. How was it possible to remove so large a mass and in the process 

 develop so great a depression ? 



The caldera is completely inclosed, so that it can not be regarded as 

 an effect of erosion. The volcanic origin of everything about the lake 

 would suggest in a general way that this great revolution must have 

 been wrought by volcanism, either blown out by a great volcanic explo- 

 sion or swallowed up by an equally great engulfment. It is well known 

 that pits have been produced by volcanic explosions, and some of them 

 are occupied by lakes^of the kind usually called crater lakes. Depres- 

 sions produced in this way, however, are, with rare exceptions, sur- 



