898 GEOLOGY 



also on variations in the obliquity of the ecliptic; but none of them 

 has gained much acceptance. They encounter most of the diffi- 

 culties of the Crollian hypothesis, in somewhat different forms. 

 There have been speculations upon the possible passage of the 

 earth through cold regions of space, but there is no astronomical 

 basis for them. 



The hypothesis of a wandering pole. It was early suggested 

 that the axis of the earth may have been shifting its geographic 

 position, and that the Pleistocene glaciations were but polar glaci- 

 ations of the existing type, at a time when the north pole was 15 

 or 20 south of its present position. So long as the theory of a thin 

 crust resting on a liquid nucleus, and capable of sliding over it, was 

 accepted, the mechanical difficulties of this hypothesis did not seem 

 insuperable; but if the earth is essentially rigid, as seems almost 

 certain, the dynamic objections to the hypothesis seem fatal to it. 

 The distribution of the ice-sheets of the earlier glacial epochs also 

 raises objections to this as a general theory. 



Atmospheric Hypotheses 



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 con- 

 ditions prevailing in Pleistocene times. Furthermore they have 

 not been worked out into detail so as to fit the facts- of periodicity 

 and localization, facts which all hypotheses must meet before they 

 can have serious claims to acceptance. Winds and clouds were 

 no doubt factors in glaciation. 1 



The leading hypothesis of the atmospheric class is based chiefly 

 on a postulated variation in the constituents of the atmospln 

 especially in its amount of carbon dioxide and water. Both 1: 

 elements have high capacities for absorbing heat, and both arc be- 

 ing constantly supplied and constantly consumed. Periods of gi 

 land elevation, extension, and erosion are periods of great con- 

 sumption of carbon dioxide, for, on account of the great contact 

 which the exposed surface presents, the carbon dioxide takes part 

 in the decomposition of rock in a large way. So also, at times of 



'Manson. Am. Geol., Vols. XIV, XXIII, and XXIV. 



