CAUSE OF GLACIAL CLIMATE 651 



dioxide from the air takes part in the decomposition of rock in a 

 lari^e way (p. 264). So also, at times of great land elevation and 

 extension, the sum total of evaporation of water is reduced, and the 

 average amount of water vapor in the air is 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 con- 

 tent of the air. Depletion of these heat absorbing elements was 

 equivalent to the thinning of the thermal blanket which they con- 

 stitute. 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 hypso- 

 metric hypothesis form a part of the atmospheric hypothesis. This 

 hypothesis also takes into account the action of the ocean in ab- 

 sorbing and giving forth carbon dioxide under the varying condi- 

 tions that prevailed. It is thus a highly complex hypothesis and 

 cannot be set forth in detail here. 1 By variations in the consump- 

 tion of carbon dioxide, especially in its absorption and escape from 

 the ocean, the hypothesis attempts to explain the periodicity of 

 glaciation. 



While this hypothesis is still new and on trial, it is the only one 

 which has been worked out into such detail as to fit the leading facts 

 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. 



Hypotheses have been based on the direction of the prevailing winds and also 

 upon the degree of cloudiness; but these have not been satisfactorily connected 

 with known causes and with the conditions prevailing in Pleistocene times. Fur- 

 thermore, they have not been shown to fit the facts of periodicity and localization, 

 facts which all hypotheses must meet before they can have serious claims to accept- 

 ance. 



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 

 the area of the continental ice-sheet, the commoner phases of erosion 



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

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



