232 The Size of the Ice-grain in Glaciers 



In lake-ice of moderate thickness the crystalline axis 

 is perpendicular to the surface of the lake. Consequently, 

 Tyndall's planes of fusion are parallel to this surface. When 

 exposed to a powerful sun, and with an air temperature even 

 much below o C., the ice weathers into horizontal laminae 

 separated by Tyndall's planes of fusion, and into vertical 

 columns. The column in lake-ice and the grain in glacier- 

 ice are homologous features. They express the form which 

 the individual crystal takes in these different varieties of 

 natural ice. 



Were it not for the fact that a glacier is made up of dis- 

 tinct grains of ice, and that this substance has the property of 

 melting and freezing at different temperatures, according to 

 the composition of the water with which it comes in contact 

 and to the pressure to which it is subjected, there is little 

 doubt that a glacier would be as motionless as any other 

 mass of crystalline rock. 



