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 aav theonv of the 

 cause of glaciation. 



The Cause of Ghwial 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 geograpliic. 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 earth is at aphelion. But the eccentricity of the el- 

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

 other i)lanets on the earth. Today the eccentricitv 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 when that hemisi)here is turned from the sun in 

 aphelion at a time of great eccentricity. Aphelion winter recurs 

 every 21,000 \ears ; periods oi 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 ]M-o]M)rti(Mi of that cm-rent turned 

 into northern and southern latitutles. were believed bv CroU to 

 work witli the ast'/onomical factors. 



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

 period, we should expect ( i ) the recurrence of irlaciati:)n many 

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

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

 ditions, with inte'/vals of 10.500 years between the cidminations 

 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 ac!mits of, and that the intervals letween them do not cor- 

 respond with tlie theory. Chamberlin smumed up the situation 



