THE WORK OF SNOW AND ICE 229 



snow-line, is dependent chiefly on temperature and snow-fall. In 

 general it does not depart much from the summer isotherm of 

 32, though it may be above this isotherm where the snow-fall is 

 light. That the snow-line is not a function of temperature only, 

 is shown by its position in various places. Thus in the equatorial 

 portion of the Andes, the snow-line has an altitude of about 16,000 

 feet on the east side of the mountains, where the precipitation is 

 heavier, and of about 18,500 feet on the west side, where it is 

 lighter. For the same reason the snow-line in the Himalayas is 

 lower on the south side than on the north. Though temperature 

 and snow-fall are the most important factors controlling the position 

 of the snow-line, both humidity and the movements of the air are 

 of some importance, since both affect the rate of evaporation of 

 snow and ice. 



The passage of snow into neve and ice. The snow does not lie 

 on the surface long before it undergoes obvious change. The light 

 flakes are transformed into granules, and the snow becomes " coarse- 

 grained." The granular character, so pronounced in the last banks 

 of snow in the spring, is even more distinct in perennial snow-fields. 

 This granular snow is called neve. Where the thickness of the snow 

 is great, the neve becomes more compact below, and finally grades 

 into porous ice. Ice is found in some snow-fields at no great depth 

 from the surface. 



Structure of the ice. Ice formed beneath a snow-field is in 

 some sense stratified. It is made up of successive falls of snow which 

 tend to retain their individuality to some extent. Thus the snow 

 of one season, or of one period of precipitation, may have been 

 considerably changed before the succeeding fall of snow. Again, 

 the surface of the snow-field at the end of the melting season is 

 often covered with a visible amount of earthy matter, some of 

 which was blown up on the surface during the melting season, and 

 some of which was concentrated at the surface by the melting of 

 the snow in which it was originally imbedded. The amount of 

 earthy matter is often sufficient to define snows of successive years, 

 and makes distinct the stratification which would otherwise remain 

 obscure. 



In addition to its stratification, the ice of the deeper portions 



