THE MECHANICS OF GLACIERS 925 



Professor Chamberlin has suggested that a partial cause of 

 the vertical and overhanging ends is the greater effect of the 

 nearly horizontal rays of the sun on the steep edges than on the 

 sloping backs of the glaciers ; this is undoubtedly a true cause ; 

 he thinks, however, it is not the whole explanation, but that 

 there must be some special shearing at the debris-bearing layers. 



Fig. 3. Diagrammatic section of end of Bryant glacier. 



This seems to me unnecessary ; and Professor Russell^ has lately 

 made clear that the presence of debris in a glacier diminishes, 

 instead of increasing, its power to shear. 



The endings of these glaciers present a close analogy to the 

 endings of tide-water glaciers. The lower layers of the latter 

 have their extent determined by the equality between the rate 

 of motion and the melting due to the salt water, and the upper 

 layers project over them until they break off.^ In these Green- 

 land glaciers the extent of the lower layers depend on their 

 rates of motion and of melting, and the upper layers push over 

 them until they break off from lack of support, as is shown in 

 the illustration referred to by the pieces of ice lying in front of 

 the glacier. Above the vertical cliffs, thus produced, the ordi- 

 nary conditions of melting prevail, and the surface of the glacier 



' This Journal, Vol. Ill, 823-832. The proof depends on the quasi-viscous 

 theory of glacial movement. 



-Studies of Muir glacier, Nat. Geog. Mag., IV, 47. 



