THE QUATERNARY PERIOD 363 



glaciation during geologic history and still more may be discovered. 1 

 Also it has been proved that there were five, and probably six, 

 ice advances and retreats corresponding to colder and warmer 

 stages during the Quaternary Glacial epoch. As regards the dura- 

 tion of this Ice age and the time since its close, it seems impossible 

 to imagine seven or eight advances and retreats of the ice within 

 160,000 years unless we postulate rates of advance and retreat 

 much greater than studies of existing glaciers show. Also the best 

 geological evidence does not place the close of the Ice age so far 

 away as 60,000 to 80,000 years. One of the most serious objections 

 to CrolPs hypothesis is the fact that, during the Permian period, 

 there was widespread glaciation in comparatively low latitudes 

 (20° to 35°) either side of the equator. 



Chamberlin's Atmospheric Hypothesis. 2 — Among the atmos- 

 pheric hypotheses, the one which Chamberlin has put into its 

 best form "is based chiefly on a postulated variation in the con- 

 stituents of the atmosphere, especially in the amount of carbon 

 dioxide and water. Both these elements have high capacities for 

 absorbing heat, and both are being constantly supplied and con- 

 stantly consumed. . . . The great elevation of the land at the close 

 of the Tertiary seems to afford conditions favorable both for the 

 consumption of carbon dioxide in large quantities, 3 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 (elevation) hypothesis form a 

 part of this hypothesis. . . . By variations in the consumption of 

 carbon dioxide, especially in its absorption and escape from the 

 ocean, the hypothesis attempts to explain the periodicity 4 of 



1 It should of course be remembered that the proper temperature con- 

 ditions for glaciation may have recurred many times when other factors 

 such as requisite precipitation of snow may not have obtained, and hence 

 great ice sheets may not actually have formed. 



2 For a fuller treatment of this hypothesis see Chamberlin and Salisbury's 

 Geology, Vol. 3, pp. 432-446. 



3 Much carbon dioxide is used up in the decomposition or carbonation of 

 the rocks. 4 That is, the successive advances and retreats of the ice sheets. 



