268 EARTH FEATURES AND THEIR MEANING 



mountain glaciers rock may project at any level but always above 

 the highest snow surface. Ice caps may be regarded as interme- 

 diate between the two main classes of mountain and continental 

 glaciers (Fig. 297). Because of the large role which continental 



FIG. 297. View of the Eyriks-Jokull, an ice-cap of Iceland (after Grossman). 



glaciers have played in geological history, it is thought best to con- 

 sider them first, leaving for later discussion the no less interest- 

 ing but less important mountain glaciers. 



The nourishment of glaciers. The life of a glacier is depend- 

 ent upon the continued deposition of snow in aggregate amount 

 in excess of that which is lost by melting or by other depleting 

 processes. Whenever, on the other hand, the waste exceeds the 

 precipitation, the glacier is in a receding condition and must 

 eventually disappear, if such conditions are sufficiently long con- 

 tinued. The source of the snow is the water of the ocean evapo- 

 rated into the atmosphere and transported over the land in the 

 form of clouds. We are to learn that the changes which this 

 moisture undergoes before its delivery to the glacier are notably 

 different for the classes of continental and mountain glacier. 



The upper and lower cloud zones of the atmosphere. Be- 

 fore we can comprehend the nature of the processes by which gla- 

 ciers are nourished, it will be necessary to review the results of 

 recent studies made upon the earth's atmospheric envelope. It 

 must be kept in mind that the sun's rays are chiefly effective in 

 warming the atmosphere through being first absorbed by some 

 solid body such as rock or water and their heat then communicated 

 by contact to the immediately adjacent air layers. The layers thus 

 warmed being now lighter than before, they rise and are replaced 

 by colder air, which in its turn is warmed and likewise set in up- 

 ward motion. Such currents developed in the air by contact 

 with warmer solid bodies constitute the process known as con- 

 vection. 



