CHAPTEE IX. 



THE CAUSE OF THE GLACIAL PERIOD. 



In searching for the cause of the Glacial period, it is 

 evident that we must endeavor to find conditions which 

 will secure over the centre of the glaciated area either a 

 great increase of snow-fall or a great decrease in the mean 

 annual temperature, or both of these conditions combined 

 in greater or less degree. As can be seen, both from the 

 nature of the case and from the unglaciated condition of 

 Siberia and northern Alaska, a low degree of temperature 

 is not sufficient to produce permanent ice-fields. If the 

 snow-fall is excessively meagre, even the small amount of 

 heat in an arctic summer will be sufficient to melt it all 

 away. 



From the condition of Greenland, however, it appears 

 that a moderate amount of precipitation where it is chiefly 

 in the form of snow may produce enormous glaciers if at 

 the same time the average temperature is low. In south- 

 eastern Alaska, on the other hand, the glaciers are of enor- 

 mous size, though the mean annual temperature is by no 

 means low, for there the great amount of snow-fall amply 

 compensates for the higher temperature. 



Snow stores the cold and keeps it in a definite place. 

 If the air becomes chilled, circulation at once sets in, and 

 the cold air is transferred to warmer regions ; but if there 

 is moisture in the air, so that snow forms, the cold becomes 

 locked up, as it were, and falls to the earth. 



The amount of cold thus locked up in snow is enor- 



