I06 GLACIERS 



are in contact with one another, not separated by air. To con- 

 vert snow into ice, therefore, it is only necessary to expel the air 

 and bring the crystals into contact, for which pressure alone is 

 not ordinarily sufficient. 



The first step in the transformation is the partial melting of the 

 upper layers of snow, for which a change of temperature is neces- 

 sary, though the change need not warm the air, but may be due 

 to the direct rays of the sun. Glaciers are rare in the tropics 

 because of the constancy of the temperature, and the small area 

 which extends above the snow-line, which seldom permits the for- 

 mation of extensive snow-fields. Sometimes, however, the condi- 

 tions of glacier formation are fulfilled even in the equatorial zone ; 

 for example, there is a glacier on one of the peaks of Ecuador. 



When the surface layers of snow have been partially melted, the 

 water thus formed trickles down into the snow beneath, expelling 

 much of the air. This underlying snow has still a temperature 

 much below the freezing-point, and the percolating water is soon 

 refrozen into little spherules of ice. This substance, midway be- 

 tween snow and ice, is called neve, and may be seen every winter 

 wherever the snow lies for any length of time. The hardened 

 "crust" which forms by the refreezing of partly melted snow is 

 neve. The air, which is now in the form of discrete bubbles, is 

 largely expelled by the increasing pressure of the overlying snow 

 masses, which are continually added to by renewed falls, and the 

 neve is thus converted into ice. 



It follows from this that glaciers can be formed only where 

 there is a relatively large snow supply, or at least where the snow 

 accumulates to great thicknesses, and cannot be disposed of by 

 either melting or evaporation. Hence, glaciers are rare or absent 

 in dry regions, where the snow does not increase to great depths, 

 as in most of the Rocky Mountains within the limits of the United 

 States. It also follows that the ground upon which the snow lies 

 must be so shaped as to allow great masses of it to gather, with- 

 out rushing downward in avalanches. 



Wherever, then, more snow falls in winter than can be melted 

 in summer, and continues to accumulate, glaciers will be formed. 



