246 



GEOLOGY 



descent of the surface cold may be called the winter wave of low 

 temperature. The degree of cold becomes rapidly less below the; 

 surface, and at no great depth the temperature is not much affected 



by the seasons. In middle latitudes, 

 the zone of no seasonal variation of 

 temperature on land is only 50 or 60 

 feet below the soil. Its depth in ice 

 is not known, but it is not probable 

 that the cold of winter has much 

 effect below a similar depth. Con- 

 duction alone considered, the tempe- 

 rature of the ice at the depth where 

 the cold wave dies out, should corre- 

 spond, approximately, to the me t an 

 annual temperature of the region, 

 provided that temperature is below 

 the melting-point of ice. At this 

 depth, the temperature of the ice in 

 middle Greenland should be about 

 20 Fahr. the mean annual tem- 

 perature of the region. 



With the coming of summer, ihe 

 temperature of the surface of the ice 

 may be raised to 32, and this rela- 

 tively high temperature makes itself 

 felt below the surface as the warm 

 wave, if the temperature of ice can 

 ever be said to be warm. The 

 relatively high temperature of the 



surface makes itself felt below both by conduction and by the 

 descent of water. The former affects the ice at all temperatures, 

 the latter only after the melting-point of the ice at the surface 

 is reached. The first conforms measurably to the warm wave 

 affecting other solid earth-matter, while the second is governed 

 by other laws. After the surface portion of the ice is brought to the 

 melting temperature, the additional heat which it receives melts ice. 

 and is transformed from sensible into potential heat. Ice charged 



Fig. 202. A part of the verti- 

 cal side of a North Green- 

 land glacier. The vertical 

 even overhanging faces 

 ai~ sometimes more than 

 100 feet high. 



