June, 1920] The Great Mageik Lmidslide 335 



must owe their origin to glaciers; no other agencies could trans- 

 port such vast quantities of rock so far from their sources." 

 This same opinion was held by Chamberlin and Salisbury* 

 who say, "the loose debris on steep slopes sometimes assumes a 

 sort of flowing motion and descends the slope with some such 

 form and at some such rate as a glacier, " illustrating such "talus 

 glaciers " by a picture of one of the Silver Basin " Rock Streams." 

 This view is further strengthened by the strong resemblance 

 of the gouged out mountain side from which they originate 

 to a glacial cirque. But in the landslides of the Katmai 

 district any inclination to interpret the hollows at the heads 

 of the flows as glacial is at once disposed of by the obviously 

 fresh unweathered surface of the rock, which of itself shows 

 that they were never occupied by a glacier whose disappearance 

 would have involved a considerable lapse of time and the 

 consequent weathering of the surface of its cirque. 



After further study, however, Howe reached the same con- 

 clusion concerning his rock streams that was independently forced 

 upon the writer in his field examination of the Mageik Slide, 

 for in this final review of the situation, four years later, he 

 concludes :t "There can be little doubt that the detritus of 

 which the Rock Streams are composed flowed down the valley 

 sides or basin floors; not, however, 'at some such rate as a 

 gjlacier' but with a sudden violent rush that ended as quickly 

 as it started. A study of the San Juan Rock Streams carried on 

 through a number of field seasons has failed to show that they 

 are now in motion or that they have come to their present 

 position as a result of slow glacier-like movement. "| 



If the evidence was sufficient to justify Howe in deciding 

 that the comparatively old features he observed were a special 

 class of landslide, it is much clearer in the fresher Mageik 

 Landslide, for when glacial action is eliminated and the 

 destructive action on bushes is observed, little doubt of its 

 nature can remain. 



*Geology, Vol. L 2nd ed., 1905. pp. 232-233. 



tHowe, Ernest. Landslides of the San Juan Mountains, Colorado. U. S. 

 G. S., Professional Paper 67, p. 54. 1909, 



Jin this connection I would take occasion to express my obligation to several 

 of my geological friends, Dr. Whitman Cross, of the United States Geological 

 Survey; Dr. Rollin D. Salisbury, of the University of Chicago; Dr. R. A. Daly, 

 of Harvard; and Dr. J. E. Carman, of the Ohio State University, who by their 

 counsel have aided me in the preparation of this paper. 



