650 THE PLEISTOCENE PERIOD 



to an appropriate fraction of the precessional period. This appropriate fraction is 

 probably about that which effective winter bears to the whole year. In the middle 

 latitudes, the effective period of cold would perhaps be 5,000 or 6,000 years. These 

 features of the hypothesis afford a means of testing it. If it be true, the glacial 

 epochs should be of equal length; all of them should be short, and all of those in the 

 same period of eccentricity, equally distant from each other in time. If the com- 

 puted periods of eccentricity are correct, there could be only a few alternations of 

 glaciation between the hemispheres within a given period of high eccentricity, 

 and none of them could be more recent than 60,000 years. Croll placed the close 

 of the glacial period 80,000 years ago. 



The glacial studies of recent years seem to show that the intervals between the 

 different invasions are of very unequal duration, and that the most recent is 

 relatively young. It has also been found that glaciation was extended notably 

 beyond its present limits on the lofty mountains of the equatorial regions, though 

 climate there should not have been much affected. The Labradorean and Kee- 

 watin ice-sheets pushed out from what appear to have been their centers about 1,600 

 and 1,500 miles respectively. If one foot per day be allowed for the advance of 

 the margin an estimate much beyond the probabilities it would take more 

 than 20,000 years for the ice-edge to reach the extension observed. This is almost 

 the whole of a precessional period. Nor is the difficulty escaped by assuming 

 that the snow-field grew up simultaneously over the whole area, or some large part 

 of it, for bowlders are found 600 to 1,000 miles from their probable sources. To 

 allow time for the residue of winter snow above summer melting to build itself up 

 to a height capable of giving effective motion, and then to allow time to carry 

 drift this great distance at any probable rate of motion, taxes the hypothesis very 

 severely, to say the least. 



Other astronomical hypotheses. Attempts have been made to base other theories 

 on the eccentricity of the earth's orbit, and also on variations in the obliquity of 

 the ecliptic; but none of them has gained much acceptance. They encounter most 

 of the difficulties of the Crollian hypothesis, in somewhat different forms. There 

 have been speculations upon the possible passage of the earth through cold regions 

 of space, tut there is no astronomical basis for them. 



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 glacia- 

 tions 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 now 

 seems certain, the dynamic objections to this hypothesis are fatal. 



Atmospheric hypotheses. The leading hypothesis of the atmos- 

 pheric class is based chiefly on a postulated variation in the constitu- 

 tion of the atmosphere, especially in its amount of carbon dioxide 

 and water. Both these elements have high capacities for absorbing 

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

 sumed. Periods of great land elevation and extension are periods 

 of great erosion and of great consumption of carbon dioxide, for 

 under these conditions weathering is at a maximum, and carbon 



