246 



GEOLOGY. 



it does not depart much from the summer isotherm of 32°, though ii 

 may be weh above this isotherm where the snowfall is light. That 

 the snow-Hne is not a function of temperature only is shown by its posi- 

 tion in various places. In the equatorial portion of the Andes, for 

 example, 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 3000 or 4000 feet 

 lower on the south side than on 

 the north. 



While in equatorial regions 

 the snow-line has an altitude of 

 15,000 to 18,000 feet, it ap- 

 proaches or even reaches sea- 

 level in the latitude of Antarctica 

 and North Greenland. In inter- 

 mecUate latitudes it has an inter- 

 mediate position. 



While temperature and snow- 

 fall are the most important 

 factors controlHng 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 



The borders ^^^^^ ^^d ice. 



The passage of snow into 

 nev^ and ice. — The snow does not He on the surface long before it 

 undergoes obvious change. The hght flakes soon begin to be trans- 

 formed into granules, and the snow becomes ^'coarse-grained.'' The 

 granular character, so pronounced in the snow of the last banks which 

 remain in the spring in temperate latitudes, is even more distinct in 

 perennial snow-fields, either at the surface or just beneath it. This 

 granular snow is called neve, or firn. Still deeper beneath the surface, 

 where the thickness of the snow is great, the neve becomes more 



Fig. 222.— Map of Greenland. 



only are free from ice. (Stieler.) 



