THE WORK OF S^OW AND ICE. 277 



or wholly from the rock beneath. In glaciers or in parts of glaciers so 

 thin as to lie wholly within the zone of fluctuating temperature, the 

 temperature of the bottom is obviously not constant. 



Temperature of the interior of the ice. — The variation of temperature 

 of the surface of a glacier has already been shown to lie between a 

 maximum of 32° Fahr. and the minimum temperature of the region 

 where the glacier occurs. Lower, in the zone of fluctuating tempera- 

 ture, the variation is less, and where the zone of fluctuating temperature 

 passes into the zone of constant temperature, variation ceases. The 

 thickness of the zone of fluctuating temperature varies with the tem- 

 perature of the region where the glacier occurs, being greatest where 

 the winters are coldest. In the case of all glaciers except thin ones 

 in very cold regions, the temperatures within the zone of constant 

 temperature range from the mean annual temperature of the region 

 at the top of the zone (provided this 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 sHght. 



If we consider only the effects of the external seasonal temperatures 

 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 constant melting temperature, for the average tempera- 

 ture of regions where the ends of such glaciers occur is usually above 

 32° Fahr., and this determines a temperature of 32° Fahr. (or a little less) 

 at the top of the zone, while a melting temperature is maintained at 

 the bottom by the earth's interior heat. In thin glaciers of very cold 

 regions, where the zone of constant temperature has relatively shght 

 thickness, 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 freezing temperature. In all other cases, the ice at the bottom 

 of the under zone has a melting temperature, while that above is prob- 

 ably colder. 



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, on the average, be below the melting-point, unless heat enough 

 is generated in the interior of the ice to offset the effect of the tempera- 

 ture above. For example, where the mean annual temperature is 20° 



