30 Proceedings of the Ohio State Academy of Science 



or the other of a mean, rather than an evolutionary progress in 

 one direction. This fact must be considered in any theory of the 

 cause of glaciation. 



TIjc Cause of Glacial Periods. 



In the middle eighties, various theories of the cause of the 

 glacial age were held, though the only one which had anything 

 like common acceptance was Croll's Hypothesis. Croll's hypoth- 

 esis is primarily astronomic, and secondarily geographic. At 

 present, winter in the northern hemisphere comes when that 

 hemisphere is nearest the sun (perihelion). 10,500 years hence, 

 due to the precession of the equinoxes, northern winter will occur 

 when the eartti is at aphelion. But the eccentricity of the el- 

 liptical orbit of the earth slowly changes, under the attraction of 

 other planets on the earth. Today the eccentricity is slight. At 

 other times it has been and will be greater. Croll's hvDOthesis 

 assumes that the glacial period will .occur in the northern 

 hemiphere wdien that hemisphere is turned from the sun in 

 aphelion at a time of great eccentricity. Aphelion winter recurs 

 every 21.000 years; periods of maximum eccentricity are much 

 longer and more irregular. Certain other changes, such as 'the 

 shifting of the heat equator, and of the equatorial current, and a 

 consequent variation of the proportion of that current turned 

 into northern and southern latitudes, were believed by Croll to 

 work with the astronomical factors. 



If Croll's hypothesis is the true explanation of the glacial 

 period, we should expect ( i ) the recurrence of glaciation many 

 times in the history of the earth; (2) that in any one period 

 there would be several alternations of glacial and non-glacial con- 

 ditions, with intervals of 10,500 years between the culminations 

 of successive glacial periods, the development of each ice-sheet 

 occupying perhaps not more than five to six thousand years; (3) 

 that glacial conditions in the northern and southern hemispheres 

 would alternate. Glacial field studies have shown, on the contrary, 

 that the ice advances were vastly longer than Croll's hypoth- 

 esis admits of, and that the intervals between them do not cor- 

 respond with the theory. Chamberlin summed up the situation 



