THE VARIATIONS OF GLACIERS 55 



could be observed, no special characteristics, had experienced such 

 changes, whereas adjoining ones had not, makes it evident that these 

 changes were not due merely to general cHmatic conditions, but to 

 some special cause. Professor Tarr suggests that this cause was the 

 severe earthquakes which occurred in this region in September, 

 1899, and which brought about marked changes of level, in some 

 places amounting to 40 feet/ Professor Tarr thinks that these 

 earthquakes shook down enormous quantities of snow and ice from 

 the surrounding mountains and thus added so large a supply to the 

 reservoirs or to the upper parts of the dissipators of some of the glaciers 

 as to cause a sudden and great increase in the velocity, resulting in a 

 strong thrust, which produced abundant crevasses. Some glaciers, 

 on account of the forms of the surrounding mountains, may not have 

 received such great additions to their masses; and others, on account 

 of greater length, may require several years before the change is 

 shown in their lower portions. Professor Tarr's explanation seems 

 entirely satisfactory and is supported by the observation of Mr. 

 A. H. Brooks, who in September, 1899, was on the eastern side of 

 the St. Ehas chain and reports that he heard unusually large avalanches 

 falHng from the mountains. Examples are known of glaciers which 

 have advanced when neighboring glaciers were retreating, due to the 

 protection of their surfaces by avalanches of snow or by land slides; 

 but the present case seems to be far more remarkable than any hereto- 

 fore reported and it is greatly to be hoped that observations will be 

 continued and the future changes recorded.^ At present we only 

 have sketch maps of these regions, but at some future time, when 

 better maps are made we may be able to show more clearly the causes 

 of the different behavior of the various glaciers. 



1 Ralph S. Tarr and Lawrence Martin, "Recent Changes of Level in the Yakutat 

 Bay Region, Alaska," Bulletin of the Geological Society of America, 1906, Vol. XVII, 

 pp. 29-64. 



2 Professor W. H. Sherzer has described some moraines in the Canadian Rockies, 

 made up entirely of large blocks of rock. He thinks that this material may have been 

 shaken down upon the glaciers by earthquakes. See " Glacial Studies in the Canadian 

 Rockies and Selkirks," Smithsonian Miscellaneous Collections, 1905, Vol. XLVII, 

 Part. 4, pp. 494-96. 



