432 GEOLOGY. 



ally taking place so rapidly as to be detectible in the course of a few 

 months, and when it was found in the progress of field studies that 

 the Alaskan-Asiatic side of the northern hemisphere was not gener- 

 ally glaciated, as the Atlantic side was, there seemed some little hope 

 that a wandering pole might offer the solution of the glacial puzzle. 

 The polar movement, however, proved to be limited to a returning 

 curve of very small radius, without evidence of secular wandering. 

 Geological research also failed to show that there was the northward 

 shift of the warm zones on the unglaciated side of the globe which 

 the hypothesis required. 



Atmospheric Hypotheses. 



In the discussion of the origin and nature of the early atmosphere 

 and its dependence on feeding and depletion (Vol. II, p. 93), we have 

 endeavored to develop a conception of the general atmospheric con- 

 ditions of all the ages that would at least not be inconsistent with 

 glaciation in the early Cambrian, or the Permian, or at any other stage 

 in the earth's history at which a suitable combination of conditions 

 might be presented. 



I. Variations in depletion the working factor. — In the discussion 

 of the problems of the Permian, we have endeavored to connect atmos- 

 pheric conditions with causes springing fundamentally from defor- 

 mation of the earth, and entering into the complex outworkings of 

 the periods following such deformations. 



The deformations of the Pliocene may be presumed to have pro- 

 duced effects on the atmosphere similar to those produced by the post- 

 Carboniferous deformations. The general discussion there given (Vol. 

 II, p. 658) may therefore be regarded as applicable to the Pleistocene 

 glaciations, so far as the general atmospheric conditions are concerned, 

 merely recalling (1) that the oceanic circulation was interrupted by 

 the extension of the land; (2) that vertical circulation of the atmos- 

 phere was accelerated by continental and other influences; (3) that 

 the thermal blanketing of the earth was reduced by a depletion of 

 the moisture and carbon dioxide in the atmosphere, and that hence 

 the average temperature of the surface of the earth and of the body 

 of the ocean was reduced, and diversity in the distribution of heat 

 and moisture introduced. The general conditions for glaciation are 



