GLACIERS 



105 



snow is disposed of by direct evaporation, and on high mountains 

 avalanches carry the snow down to lower levels, where it melts. 

 Where the snow-line is at sea-level, avalanches are obviously of 

 no avail. In places where the excess of snow cannot be disposed 

 of in either of these ways, glaciers are formed and thus keep up 

 the circulation of the waters, by carrying the surplus snow down 

 to lower levels at which it can melt, or by entering the sea and in 

 the shape of icebergs (which are fragments of glaciers) being 

 floated to warmer latitudes. 



FIG. 33. — The Dalton Glacier, Alaska. (U. S. G. S.) 



Though even at the present time there are in various parts of 

 the world great tracts of glacier-ice, they cannot be called com- 

 mon and are found only where certain conditions concur. The 

 nature of these conditions will be best understood by examining 

 the process of glacier formation. 



Snow is made up of minute, hexagonal crystals of ice, which 

 are intimately mixed with air and thus separated from one another. 

 Though the individual crystals are transparent, snow is white and 

 opaque, as always results when a transparent body is intimately 

 mixed with a gas, as in the foam on water, or in powdered glass. 

 Ice is composed of the same kind of crystals as is snow, but they 



