278 GEOLOGY. 



Fahr. or lower, as in middle Greenland, Spitzbergen, and Franz Josef 

 Land, and at certain high altitudes in more southerly latitudes, the 

 mean temperature in the zone of constant temperature should range 

 from 20° Fahr, at the top to 32° Fahr. (or a little less) below; i.e., it 

 should average about 6° below the melting-point. Under these con- 

 ditions, all the ice in the zone of constant temperature, except that 

 at its bottom, must be permanently below the melting-point, but it is 

 perhaps worthy of especial note that much of it is but little below. 

 In alpine glaciers the part of the ice affected by this constant low 

 temperature (below freezing) is presumed to be chiefly that which lies 

 beneath the snow-fields. In polar glaciers the low temperature 

 probably prevails beneath the surface, not only throughout the 

 great ice-caps, but also in the marginal glaciers which descend from 

 them. 



From these theoretical considerations we may deduce the gener- 

 alization that in the zone of constant temperature within the area oj 

 glacial growth, the temperature of the ice is generally below the melting- 

 point, while within the area of wastage, the temperature of the corre- 

 sponding zone is generally at the melting-point. 



Compression and friction as causes of heat. — The foregoing conclu- 

 sions are somewhat modified by dynamic sources of heat. The 

 compression arising from gravity, and the friction developed where 

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

 motion takes place, the heat which it generates is secondary and may, 

 for present purposes, be neglected. Compression not only lowers the 

 melting-point slightly, but it produces heat at the point of compression. 

 Where the ice is granular, the compression, due to weight, takes place 

 at the contacts of the grains. At intermediate points the pressure 

 tends to cause them to bulge, and this has the effect of lowering the 

 temperature of the bulging points. If therefore the compression be 

 considerable, the granules may be warmed to the melting-point where 

 they press each other, while at other points their temperature may 

 be lower. In this case melting will take place at the points of com- 

 pression, and the moisture so produced will be transferred to the adja- 

 cent parts of the granule and immediately refrozen. 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. Thus the melting, the refreezing, 



