THE PLEISTOCENE OR GLACIAL PERIOD 897 



cessional period (21,000 years). This appropriate fraction is prob- 

 ably 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; in the high latitudes, one-half or more of the 

 precessional period. These peculiarities of the hypothesis afford 

 a means of testing it. If it be true, the glacial episodes should bear 

 evidences of equal length; they should all be short, and all of those 

 in the same period of eccentricity, equally distant from each other 

 in time. If the computed periods of eccentricity are correct (which 

 has been questioned), there could only be a few alternations of gla- 

 ciation between the hemispheres within a given period of high 

 eccentricity, while none of them could be more recent than 60,000 

 years; indeed, Croll placed the close of the glacial period 80,000 

 years ago. 



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

 betweeii 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 this climate 

 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 nearly 

 the whole of the 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 numerous 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. On 

 the whole, the result of prolonged study of the hypothesis has been 

 to weaken, rather than strengthen it. 



Other astronomical hypotheses. Attempts have been made 

 to base other theories on the eccentricity of the earth's orbit, and 



