THE GLACIERS OF MOUNTAIN AND CONTINENT 263 



this coast, the clouds may travel over vast stretches of country 

 before being arrested by mountains and robbed of their moisture. 

 Thus in tropical Brazil the glaciers are found in the Andes upon 

 the Pacific coast though nourished by clouds from the Atlantic. 



FIG. 291. Map showing the distribution of existing glaciers, and the two im- 

 portant wind poles of the earth. 



Sensitiveness of glaciers to temperature changes. How 



sensitive is the adjustment between snow precipitation and tem- 

 perature may be strikingly illustrated by the statement on ex- 

 cellent authority that if the average annual temperature of the air 

 within the Scottish Highlands should be lowered by only three 

 degrees Fahrenheit, small glaciers would be the result; and a 

 moderate temperature fall within the region surrounding the 

 Laurentian lakes of North America would bring on glaciation, 

 otherwise expressed as a depression of the snow line of the region. 

 The cycle of glaciation. Though to-day buried beneath its 

 ice mantle, it is known that Greenland had more than once in earlier 

 geological ages a notably mild climate, and in some future age 

 it may revert to this condition. In other regions, also, we have 

 evidence that such a rotation of climatic changes has been suc- 

 cessively accomplished, the climate having steadily increased in 

 severity towards a culminating point, and been followed by a 

 reverse series of changes. Such a complete period may be called 

 a cycle of glaciation. While the climate is steadily becoming 

 more rigorous, we have to do with an advancing hemicycle of 



