THE CAUSE OF THE GLACIAL PERIOD. 463 



immobility of ice that enables it to produce by its accumula- 

 tion such extraordinary effects in physical geography and in 

 climate as we see in the glaciers of Switzerland, and the ice- 

 capped interior of Greenland.* 



Theories respecting the causes of the glacial period alto- 

 gether number more than half a score, principal of which are 

 the following : 1 . A decrease in the original heat of the planet ; 

 2. The shifting of the polar axis; 3. A former period of 

 greater moisture in the atmosphere; 4. The depletion of 

 carbon dioxide in the atmosphere by chemical union and 

 oceanic absorption; 5. Variations in the temperature of 

 space; 6. Variations in the amount of heat radiated by the 

 sun; 7. The combined effect of the precession of the equinoxes 

 and of the changing eccentricity of the earth's orbit; 8. 

 Changes in the distribution of land and water; 9. Elevation 

 of the lands in northern Europe and America to a higher level 

 than that now occupied. 



Though these causes cannot in all cases, and perhaps not in 

 any case, be supposed to act except in combination with one 

 another, it will be profitable to consider them separately. 



If, according to the first theory, the Glacial period was 

 due to a decrease of the original heat of the planet, the period 

 should not have culminated in the past, but we should still 

 be looking for its culmination in the future ; for both the 

 earth and the sun are cooling off. We may, therefore, drop 

 out the first theory. 



If, according to the second theory, the cause had been the 

 shifting of the earth's axis of rotation, we should not find, as 

 we now do, evidences that the warm climate which preceded 

 the Glacial period approached the poles along the present 

 circles of latitude ; but, as it is, we find that the temperate 

 flora which covered the arctic regions at the close of the 

 Tertiary period approached the pole not only in Greenland 

 and British America, but also in Spitzbergen and Nova 



* Wallace's " Island Life," pp. 127, 128. 



