926 HARRY FIELDING RE ID 



follows the ordinary alpine form. The debris is, however, 

 an important factor in increasing the melting of the lower 

 layers. It acts in two ways : it absorbs the sun's rays more 

 abundantly than the ice does, and its presence reduces the 

 effective density of the ice, so far as melting is concerned, just 

 as air bubbles would. Its existence seems almost necessary for 

 the development of overhanging ends, for there are glaciers in 

 the same region which end like ordinary alpine glaciers ; ^ and 

 Professor Salisbury tells me that, so far as his observation went, 

 all those, and only those, containing much debris in their lower 

 layers, have vertical or overhanging ends well developed. Pro- 

 fessor Chamberlin does not think this is universal. 



Both of these endings are forms of stable equilibrium ; for 

 an increase of velocity, accompanied, as it must be, with an 

 increase of differential velocity, would cause the upper layers to 

 project more, resulting in more breakage ; and the surface slope 

 , of the lower layers would overhang more, thus presenting a 

 larger surface to the salt water or to the air, as the case may be, 

 and increasing the melting; for we must remember that even in 

 northern Greenland some of the melting comes from contact 

 with the air, which, during the summer, is well above the freez- 

 ing point. 



Variations of glaciers. — Although the sloping surface of 

 alpine glaciers is a surface of equilibrium, it is unstable, and any 

 cause, such as a few years of greater melting or accumulation, 

 which would alter this slope, would destroy the equilibrium, and 

 the surface would tend to depart more and more from its equi- 

 librium form. When the equilibrium of the surface is destroyed 

 by diminished melting or increased flow, the upper layers every- 

 where advance over the under ones ; of course, the higher and more 

 rapidly moving layers advance at a greater rate, but as they are 

 near the neve-line, where the direction of motion makes but a 

 small angle with the glacier's surface, they cause but a very 

 small swelling ; nearer the end, however, the direction of motion, 



^ Chamberlin, Bull. Geol. Soc. Amer., 1895, VI, 202. Salisbury, this Journal, 

 189s, III, 887-890. 



