MOTION OF GLACIERS. 



Fig. 15. THE MER DE GLACE, SWITZERLAND. 



cannot retreat up the slope against the enormous weight 

 always pressing it downwards and onwards. 



Then, too, it seems probable that the freezing and con- 

 sequent expansion of the molecules of water, which must 

 drain into it whenever the surface is ever so slightly melted, 

 also help to urge the glacier onward. 



