THE WORK OF SNOW AND ICE 249 



where there is motion, are causes of heat. Since friction occurs 

 only when motion takes place, the heat which it generates is second- 

 ary and may, for present purposes, be neglected. But compression 

 produces heat at the point of compression, and also lowers the melting- 

 point slightly. If the compression is considerable in ice which is 

 already near the melting temperature, the granules may be warmed 

 to the melting-point where they press each other. In this case 

 melting will take place at the points of compression, and the water 

 so produced will flow to points of less pressure, and be re-frozen im- 

 mediately. Melting at the points of compression would result in 

 some yielding of the mass, and in some shifting of the pressure to 

 new points, where compression and melting would again take 

 place. From considerations already adduced, it appears that the 

 temperature in some parts of every considerable body of ice must 

 be such as to permit these changes. The heat due to compression 

 and friction may modify the theoretical conclusions deduced above. 



Summary. If the foregoing generalizations are correct, (1) 

 the surface of a glacier is likely to be melted during the summer; 

 (2) its immediate bottom is slowly melting all the time (unless the 

 thickness of the ice is less than the thickness of the zone of annual 

 variation) ; (3) its sub-surface portion in the zone of waste is gener- 

 ally melting, owing to descending water, compression, and friction; 

 while (4) its sub-surface portion in the zone of growth is probably 

 below the melting-point except as locally brought to that temper- 

 ature by compression, friction, descending water, and, at the bottom, 

 by conduction from the rock beneath. 



Glacier motion is not under discussion at this point, but it may 

 be pointed out that since there must be motion in the area of 

 growth in order to supply the loss in the area of waste, the funda- 

 mental cause of motion must be operative in bodies of ice whose 

 mean temperature is below the melting-point, unless the dynamic 

 sources of heat are considerable. 



Drainage. Some of the water produced by surface melting 

 forms little streams on the ice. Sooner or later they plunge into 

 crevasses or over the sides and ends of the glacier. In the former 

 case, they may melt or wear out well-like passages (moulins) in the 

 ice, and even in the rock beneath. Much of the surface water sinks 



