THE WORK OF SNOW AND ICE 247 



with water is potentially, but not sensibly, warmer than ice which 

 has just reached the melting temperature. The warm wave of con- 

 duction dies out below, like the cold wave. The warm wave descend- 

 ing by the flow of water stops where the freezing temperature of 

 water is reached, except where crevasses extend to greater depths. 



The foregoing considerations warrant the generalization that 

 glaciers normally consist of two zones (1) an outer or upper zone 

 of varying temperature, and (2) an under zone of nearly constant 

 temperature. The under zone obviously does not exist where the 

 thickness of the ice is less than the thickness of the zone of fluctu- 

 ating temperature, as may be the case in very thin glaciers and in 

 the thin ends and edges of all glaciers. 



The temperature of the bottom. The internal heat of the earth 

 is slowly conducted to the base of a glacier where it melts the ice 

 at the estimated average rate of about one-fourth of an inch per 

 year. It is probable that in all thick glaciers the temperature of 

 the bottom is constantly at the melting-point. In glaciers or in 

 parts of glaciers so thin as to lie wholly within the zone of fluctu- 

 ating temperature, the temperature of the bottom is obviously not 

 constant. 



Temperature of the interior of the ice. The variation of tem- 

 perature of the surface of a glacier has already been stated to lie be- 

 tween a maximum of 32 Fahr. and the minimum temperature of 

 the region where the glacier occurs. In the zone of varying temper- 

 ature, the variation is less and less with increasing depth. In the 

 zone of constant temperature, the range is from the mean annual 

 temperature of the region at the top of the zone (provided jiiis is 

 not above the melting-point of ice at this depth) to the melting 

 temperature of the ice at the bottom. Within these limits the 

 range may be great or slight. 



Considering only the effects of the external seasonal tempera- 

 tures and the internal heat of the earth, it appears that all the ice 

 in the zone of constant temperature in the lower end of a typical 

 alpine glacier should have a melting temperature constantly, for 

 the average annual temperature of regions where the ends of such 

 glaciers .occur is usually above 32 Fahr., and this determines a 

 temperature of 32 Fahr., approximately, at the top of the zone, 



