THE WORK OF SNOW AND ICE. 279 



and the attendant movement might go on until the hmits of the power 

 of gravity in this direction were reached. From considerations already 

 adduced, it appears that the temperature in some parts of every con- 

 siderable body of ice must be such as to permit these changes. The 

 heat due to depression and friction may modify the theoretical deduc- 

 tions drawn above from atmospheric and internal influences. 



Summary. — If the foregoing generalizations be correct, (1) the sur- 

 face of a glacier is likely to be melted during the summer, (2) its imme- 

 diate bottom is slowly melting all the time (unless the thickness of the 

 ice be less than the thickness of the zone of annual variation or of per- 

 manent freezing temperature); (3) its subsurface portion in the zone 

 of waste is generally melting, owing to descending water, compression, 

 and friction ; while (4) its subsurface portion in the zone of growth is 

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

 temperature by compression, friction, and descending water, and at 

 the bottom by conduction from the rock beneath. 



Movement under low temperature. — Glacier motion will not be dis- 

 cussed at this point, but one of the bearings of the preceding conclu- 

 sions on glacier motion may be pointed out. Since there must be motion 

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

 mental cause of motion must be operative in bodies of ice the mean 

 temperature of which is below the melting-point, unless the dynamic 

 sources of heat are considerable. This fundamental cause does not exclude 

 the cooperation of causes that work only (1) at the melting temperature, 

 or (2) where the ice is bathed with water, or (3) in the plane of con- 

 tact between wet ice above and dry ice below. These may be auxiliary 

 causes which abet the fundamental one in producing the more rapid 

 movement of warm seasons, or in bringing about the especially rapid 

 movement in situations where there is abundant water, or in inducing 

 the shearing which is such a remarkable feature of arctic glaciers. 



Evaporation. — The ice wastes by evaporation as well as by melting, 

 and while the former process is far less important than the latter, its 

 results are probably larger than is commonly apprehended. One of 

 the most remarkable features of some of the deposits of ancient glaciers 

 is the shght evidence they afford of escaping waters. The most plausi- 

 ble explanation seems to He in the supposition that the ice was largely 

 wasted by evaporation. This conclusion finds support in many places 

 in the presence of a mantle of fine silt over the drift, the silt being appar- 



