274 



GEOLOGY. 



The variation of temperature at the surface is due primarily to the 

 varying temperature of the air. During the cold season, a wave of low 

 temperature (the winter wave), starting at the surface, penetrates the 

 ice, and during the warm season a wave of higher temperature (the 



Fig. 249. — Side view of a North Greenland glacier (East glacier), showing position 

 of debris and structure of the ice. 



summer wave) takes the same course. The day and night waves and 

 other minor variables are, for present purposes, negligible. 



The winter wave. — There are but few observations on the internal 

 temperatures of glaciers during the winter season, but it seems certain 

 that the winter wave diminishes rapidly downward and dies out below, 

 much as does the winter wave which affects land surfaces not covered 

 with ice. Conduction alone considered, the temperature of the ice where 

 the cold wave dies out, should correspond, approximately, to the mean 

 annual temperature of the region, provided that temperature is below 

 the melting point of ice. 



