Fbbeuaky 7, 1902.] 



SCIENCE. 



209 



ments and blown out to fall around the 

 caldera and form a rim of fragmental ma- 

 terial. From the size of the lake and the 

 remaining portion of Mt. Mazama it is pos- 

 sible to compute approximately what the 

 size of the rim formed in this way would 

 be. But before we can do this it is neces- 

 sary to consider the size and shape of the 

 caldera, especially that part which lies be- 

 neath the lake. 



THE BOTTOM OF CEATEE LAKE. 



To determine the configuration of the 

 bottom of Crater Lake a large number 

 (168) of soundings were made under the 

 direction of Major Button. His results 

 were published by the U. S. Geological 

 Survey upon a special map of the lake, 

 scale 1: 62,500 with a contour interval of 

 100 feet. The principal lines of soundings 

 are noted, including 96 of the 168 meas- 

 ured depths. From these data, together 

 with information from Mr. W. G. Steel, 

 who was present when the soundings were 

 made, the bottom has been roughly con- 

 toured upon the large scale map with a 

 vertical interval of 500 feet. The positions 

 of the two sublacustrine cones were indi- 

 cated, and it is clear from the soundings 

 that a large mass of lava spread from the 

 Wizard Island vent over the lake floor. 

 The great deep toward the eastern margin 

 of the lake may not have been filled up any 

 after the caldera was formed, but it is evi- 

 dent that the depth of the western portion 

 has been greatly reduced by the material 

 erupted from the three small vents upon 

 its floor. It appears well within the bound 

 of reason to assume that 1,500 feet is not 

 greater than the average depth of the 

 original caldera below the present level 

 of the lake. 



ESTIMATED SIZE OP FEAGMENTAL EIM. 



The area of the caldera, as marked out 

 by the crest of the rim, is over 27 square 

 miles, and its original volume, making 



some allowance for the subsequent refilling 

 from the craters on its floor, is about 12 

 cubic miles. If to this we add 5 cubic 

 miles for the part of the mountain above 

 the caldera, and this is a conservative esti- 

 mate, we get 17 cubic miles of material for 

 whose disappearance we have to account. 

 If this material were blown out by a great 

 explosion and fell equally distributed upon 

 the outer slope of the rim, within three 

 miles of the crest it would make a layer 

 over 1,000 feet in thickness. This mass 

 would be so conspicuous and composed of 

 such fragmental material that its presence 

 could not be a matter of doubt. There can 

 be no question concerning its complete 

 absence, for the surface of the outer slope 

 of the rim exposes everywhere either 

 glaciated rock, glacial moraine or pumice, 

 all of which are features which belonged 

 to Mt. Mazama before its destruction, and 

 no trace of a fragmental rim, such as is 

 referred to above, was found anywhere. 



The evidence of the outer slope of the 

 rim lends no support to the view that Mt. 

 Mazama was blown away and the caldera 

 produced by a great volcanic explosion. 

 In fact, it completely negatives such a 

 view, and we are practically driven to the 

 opinion that Mt. Mazama has been en- 

 gulfed. Major Button, who studied the 

 rim of Crater Lake with a training gained 

 from among the active volcanoes of the 

 Hawaiian Islands, recognized the wide dis- 

 tribution of the pumice, but the absence 

 of a well-defined fragmental rim kept him 

 from attributing the origin of the caldera 

 to an explosion. On the other hand, he 

 fully appreciated the difficulty of proving 

 that it originated in a subsidence.* 



The present inner slope of the rim may 

 not in all cases, or even generally, be the 

 one formed at the time of the collapse. In 

 some cases, however, the inner slope was 



* U. S. Geological Survey, 8th Ann. Kept., Part 

 I., p. 157. 



