THE WORK OF SNOW AND ICE. 



245 



In addition to these limited fields of snow in mountain regions, there 

 are fields of much greater extent covering wide expanses of plain and 

 plateau in the polar regions. The greater part of the island of Green- 



FiG. 221. — An alpine snow-field. 



land is covered with a single field of ice and snow, the size of which 

 is variously estimated at 300,000 to 400,000 square miles (Fig. 222)— 

 an area 400 to 600 times as large as the snow-and-ice-covered area 

 of Switzerland. Numerous islands to the west of North Greenland 

 are also partly covered with snow, the areas of the snow-fields far ex- 

 ceeding those of most mountain regions. In Antarctica there is 

 believed to be a still larger field, the largest of the earth. Its area 

 is not even approximately known, but such data as are at hand in- 

 dicate that it may have an extent of 3,000,000 or 4,000,000 square 

 miles. 



The only condition necessary for a snow-field is an excess of 

 snowfall over snow waste. The lower edge of a snow-field, the snow- 

 line, is dependent chiefiy on temperature and snowfall. In general 



