THE WORK OF SNOW AND ICE. 



319 



result that the thick ice, bathed with water and having a high gradient, 

 develops unusual velocity during the warm season. 



Fig. 295. — Lateral margin of a North Greenland glacier, Inglefield Gulf region 

 The overhanging edges of the successive layers are not altogether the result of 

 shear. They are due in part at least to differential melting along the lines where 

 debris comes to the surface. The debris planes may be shear planes. 



Applications. — By a studious consideration of the cooperation of the 

 auxiliary agencies with the fundamental ones, the peculiarities of glacial 

 movement may apparently be explained. In regions of intense cold, 

 where a dry state and low temperature prevail, as in the heart of Green- 

 land, the snow-ice mass may accumulate to extraordinary thicknesses, 

 for the burden of movement seems to be thrown almost wholly upon 

 compression, with the slight aid of molecular changes clue to internal 

 evaporation and allied inefficient processes. Since the temperature 

 in the upper part of the ice is very adverse (see p. 277), the compression 

 must be great before it becomes effective in melting the ice, and hence 

 the great thickness of the mass antecedent to much motion. Similar 

 conditions more or less affect the heads of alpine glaciers, though here 

 the high gradients favor motion with lesser thicknesses of ice; but in 



