248 GEOLOGY 



while a melting temperature is maintained at the bottom by the 

 earth's interior heat. In thin glaciers of very cold regions, the low 

 temperature descending from the surface may so far overcome 

 the effect of internal heat as to keep the bottom of the ice at a freez- 

 ing temperature during the winter season; but in all other cases 

 the ice at the bottom of the under zone probably has a melting 

 temperature, while that above is probably colder, except where the 

 ice is warmed by the descent of surface water. 



In the higher altitudes and in the polar latitudes, where glaciers 

 are chiefly generated, the mean annual temperature of the surface 

 is usually below the melting-point of ice. Here the temperature 

 of the ice between the top and bottom of the zone of constant 

 temperature must be below the melting-point, on the average, unless 

 heat enough is generated in the interior of the ice to offset the effect 

 of the temperature above. For example, where the mean annual 

 temperature is 20 Fahr. or lower, as in middle Greenland, 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 some 6 below the melting-point, disregarding the 

 effect of water descending from the surface. Under these condi- 

 tions, all the ice in the zone of constant temperature, except thai 

 at its bottom, must be permanently below the melting-point ; 

 but it is 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 temporal u re 

 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 ucn- 

 eralization that in the zone of constant temperature within the 

 of glacier growth the temperature of the ice is generally below the 

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

 the corresponding zone is generally at the melting-point. 



Compression and friction as causes of heat. The fore<roinii con- 

 clusions are somewhat modified by dynamic sources of heat. Both 

 the compression arising from gravity, and the friction developed 



