THE PLEISTOCENE OR GLACIAL PERIOD 899 



great land elevation and extension, the sum total of evaporation 

 of water was reduced, and the average amount of water vapor in 

 the air was correspondingly lowered. The great elevation of land 

 at the close of the Tertiary seems to afford conditions favorable 

 both for the consumption of carbon dioxide in large quantities, and 

 for the reduction of the water content of the air. Depletion of 

 these heat absorbing elements was equivalent to the thinning of 

 the thermal blanket which they constitute. If it was thinned, the 

 temperature was reduced, and this would further decrease the 

 amount of water vapor held in the air. The effect would thus be 

 cumulative. The elevation and extension of the land would also 

 produce its own effects on the prevailing winds and in other 

 ways, so that some of the features of the hypsometric hypothesis 

 form a part of this hypothesis. This hypothesis also takes into 

 account the action of the ocean in absorbing and giving forth 

 carbon dioxide under the varying conditions that prevailed. It is 

 thus a highly complex hypothesis and cannot be fully set forth 

 here. 1 



By variations in the consumption of carbon dioxide, especially 

 in its absorption and escape from the ocean, the hypothesis at- 

 tempts to explain the periodicity of glaciation. Localization is 

 attributed to the two great areas of permanent low pressure hi 

 proximity to which the ice-sheets developed. 



While this hypothesis is still new and on trial, it is almost if not 

 quite the only one which has been worked out into details so far 

 as to fit the leading facts as now developed by studies of the glacial 

 formations. It should be understood, however, that its truth 

 remains to be established, and that modifications and additions 

 may yet be required, 



FORMATIONS OUTSIDE THE ICE-SHEETS 



While the glaciation of middle and high latitudes was the most 

 striking event of the Quaternary period, by far the larger part of 

 the earth's surface was not affected directly by the ice, and outside 



1 For a fuller exposition of this hypothesis see Chamberlin and Salisbury's 

 Earth History, Vol. Ill, pp. 432-446. 



