210 



SCIENCE. 



[N. S. Vol. XV. No. 371. 



formed at tliat time. Of this we have evi- 

 dence in the behavior of the flow at 

 Rugged Crest. It was one of the final 

 flows from the slope of Mt. Mazama. Be- 

 fore the central portion of the flow where 

 thickest had congealed within the solid 

 crust, Mt. Mazama sank away and the yet 

 viscous lava of the middle portion of the 

 stream flowed down over the inner slope of 

 the andesitic rim into the caldera. The 

 liquid interior of the flow having with- 

 drawn, the crust caved in and formed 

 Rugged Crest with its peculiar chaotic 

 valley of tumbled fragments, columns and 

 bluffs. Other explanations of the peculiar 

 reversed flow of Rugged Crest have been 

 sought, but without avail. The facts are so 

 simple and so direct that they appear to 

 preclude any other hypothesis. 



It would be apparent from the facts also 

 that the collapse of the mountain was at 

 least moderately sudden, for it is not at 

 all probable that the Rugged Crest flow 

 was long exposed before reaching the pres- 

 ent level of the lake and beyond into the 

 caldera. 



"We may be aided in understanding the 

 origin of the caldera by picturing the con- 

 dition that must have obtained during the 

 eruption of the Rugged Crest daeite from 

 the upper slope of Mt. Mazama. At that 

 time a column of molten material rose in 

 the interior of the mountain until it over- 

 flowed at the summit or burst open the 

 sides of the mountain and escaped through 

 the fissure. The rent of the mountain side 

 is formed in such cases by the pressure of 

 the column of molten material it encloses. 

 The molten lavas being heavy, the pres- 

 sure of the column within the mountain 

 is very great, and increases rapidly vnth 

 the height of the volcano. During the final 

 activity of Mt. Mazama there must have 

 been -within it a column of lava over 8,000 

 feet in height above the base of the Cas- 

 cade Range. It is possible that on ac- 



count of this great pressure, aided pos- 

 sibly by some other forces, an opening was 

 formed low down upon the mountain slope, 

 allowing the lava to escape. The sub- 

 sidence of the lava within the mountain 

 left it unsupported and caused its col- 

 lapse. Phenomena of this sort are well 

 known in connection with the Hawaiian 

 volcanoes. In 1840, according to Professor 

 J. D. Dana, there was an eruption from the 

 slopes of Kilauea, 27 miles distant and 

 over 3,000 feet below the level of its sum- 

 mit. At Kilauea the summit of the lava 

 column is well exposed in a lava lake. In 

 connection with the eruption of 1840 the 

 lava of the lake subsided to a depth of 

 385 feet, and the irregular walls surround- 

 ing it left without support broke off and 

 fell into the molten material below. Dur- 

 ing the intervals between the eruptions of 

 Kilauea the molten column rises towards 

 the surface only to be lowered by subse- 

 quent eruptions. The subsidences, how- 

 ever, are not always accompanied by an 

 outflow of lava upon the surface. At other 

 times it may gush forth as a great fountain 

 hundreds of feet or more in height, as if 

 due directly to hydrostatic pressure. 



That Mt. Mazama disappeared and the 

 caldera originated through subsidence 

 seems evident, but the corresponding effu- 

 sion up on the surface, if such ever occurred, 

 has not yet been found. It is hardly con- 

 ceivable that 17 cubic miles of material, 

 much of it solid lava, could collapse, be re- 

 fused and sink away into the earth with- 

 out a correlative effusion at some other 

 point. 



The bottom of the caldera is over 200 

 feet below the level of Klamath Marsh, 

 which lies at the eastern base of the Cas- 

 cade Range, and it is not to be expected 

 that the point of escape would occur at 

 any level above (4,200). This considera- 

 tion would indicate that the effused mass 

 should be sought on the western slope of 



