476 ELEMENTARY GEOLOGY 
throughout the seasons. This variation occurs at long 
intervals of time, and the suggestion is, that the differ- 
ences were at this time sufficient to cause the Glacial 
period. There are various objections to this explanation, 
perhaps the most potent of which is, that the Glacial 
period affected the North Atlantic basin, rather than 
Fig. 204. 
Ideal landscape in the Ice Age. (After Haushofer.) 
the entire northern hemisphere. Had the cause been 
mainly or purely astronomical, it should have been 
more widespread.' 
While it is possible that those astronomical causes 
may have aided in the change of climate, it seems 
1 This theory is known as Croll’s hypothesis, and the astronomical changes 
in question are those resulting from the precession of the equinoxes, and the 
variation in the eccentricity of the earth’s orbit, which will be found explained 
in some astronomies, and in Croll’s Climate and Time. 
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