CHAPTEE XXV. 



GLACIERS. 



Glaciers are rivers of ice, and, like other 

 rivers, some of them are small and some very 

 large. They flow down the gorges from high 

 mountains, whose peaks are always covered 

 with a blanket of eternal snow. Summer and 

 winter the snow is precipitated upon these 

 mountains, and from time to time the heat of 

 the sun's rays softens the snow, when by its 

 great weight it packs more closely together 

 until it is, in many cases, formed into solid 

 ice-cakes. If we take a quantity of snow or a 

 quantity of granulated ice and put it under a 

 sufficient pressure we can produce clear solid 

 ice, and it is by this process that ice is formed 

 out of the snow and hail that falls continually 

 upon the tops of these glacial mountains. We 

 have seen that ice possesses certain viscous or 

 semi-fluidic properties and that it will yield to 

 pressure, but if we put it under sufficient ten- 

 sional strain it snaps like glass or any other 

 brittle substance. As the snows upon these 

 mountains pile up higher and higher the pres- 

 sure becomes greater and greater until it 

 reaches a point where the mass begins to move 



198 



