228 



GEOLOGY 



small snow-fields in the western mountains, from Mexico on the 

 south to Alaska on the north, their number and size increasing in 

 the latter direction. In the United States there are few snow- 

 fields south of the parallel of 36 30', and most of the many hun- 

 dreds north of that latitude (excluding Alaska) are small. Farther 

 north, the snow-fields of the western mountains attain greater size. 

 Snow-fields comparable to those of the northwestern part of the 

 United States and British Columbia occur in the higher mountains 

 of Europe and Asia, while in South America there are snow-fields 



of small size even in equatorial lati- 

 tudes, and in the Chilean Andes 

 there are some of considerable size. 

 Small snow-fields occur on the high- 

 est peaks of tropical Africa, and in 

 the mountains of New Zealand. For 

 reasons which will appear later, 

 much of every large snow-field is 

 really ice. 



Besides these fields of snow in 

 mountain regions, there are fields 

 of much greater extent in polar 

 regions. The greater part of Green- 

 land is covered with a single field 

 of ice and snow, the size of which 

 is estimated at 300,000 to 400,000 

 square miles (Fig. 186), an area 

 400 to 600 times as large as the 

 snow-and-ice-covered area of Swit- 

 zerland. Numerous islands to the 

 west of North Greenland are also 

 partly covered with snow. In Ant- 

 arctica there is a still larger field, the largest of the earth. Its uiva 

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

 indicate that it may have an extent ten times as great as that of 

 Greenland. 



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

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



Fig. 186. Map showing the ice- 

 cap of Greenland. Only the 

 borders (shaded parts) of 

 the island are free from ice. 



