CHAPTER XX 

 THE GLACIERS OF MOUNTAIN AND CONTINENT 



Conditions essential to glaciation. Wherever for a suffi- 

 ciently protracted period the annual snowfall of a district is in 

 excess of the snow that is melted, a residue must remain from 

 each season to be added to that of succeeding ones. Eventually 

 so much snow will have accumulated that under its own weight 

 and in obedience to its peculiar properties, a movement will begin 

 within the mass tending to spread it and so to reduce the slope 

 of its upper surface (Frontispiece plate) . The conditions favorable 

 to glaciation are, therefore, heavy precipitation and low annual 

 temperature. If the precipitation is scanty, the small snowfall 

 is soon melted ; and if the temperature be too high, the moisture 

 is precipitated not in the form of snow but as rain. It is impor- 

 tant here to keep in mind that snow is a poor heat conductor 

 and itself protects its deeper layers from melting. 



The snow-line. Because of the low temperatures glaciers 

 should be most abundant or most extensive in high latitudes and 

 in high altitudes. The largest are found in polar and sub-polar 

 regions, and they are elsewhere encountered only at considerable 

 elevations. The largest glaciers are the vast sheets of ice which 

 inwrap the continents of Greenland and Antarctica, but glaciers 

 of large size are to be found upon other large land masses of the 

 Arctic, as well as in Alaska, in the southern Andes, and in New 

 Zealand. Much smaller glaciers are characteristic of certain 

 highlands within temperate and tropical regions, but because 

 of specially favorable conditions both of altitude and precipi- 

 tation the Himalayas, although in relatively low latitudes, nourish 

 glaciers of large proportions. In general, it may be said that 

 the nourishing grounds of glaciers are largely restricted to those 

 areas where snow covers the ground throughout the year. The 

 lower margin of such areas is designated the snow line, and varies 

 but little from the line on which the average summer tempera- 

 ture is at the freezing point of water the so-called summer 



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