896 GEOLOGY 



subject to considerable variation. This does not alter the total 

 amount of heat received by the earth, or by either hemisphere, from 

 the sun; but it affects the distribution of heat within the year, 

 shortening or lengthening the cooler and warmer seasons, according 

 as they fall in the perihelion or the aphelion part of the earth's orbit. 

 Thus the hemisphere which has summer in perihelion has a short 

 summer with much heat per hour; the other hemisphere has a long 

 summer with less heat per hour. The precession of the equin< 

 reverses the seasonal relations of the hemispheres every 10,500 

 years. At present the earth is nearest the sun in winter in the 

 northern hemisphere (summer in the southern hemisphere). In 

 10,500 years the earth will be nearest the sun in the summer of the 

 northern hemisphere (winter of the southern hemisphere). \Ye 

 shall then have a shorter summer with more solar heat per hour than 

 now, and a longer winter with less heat per hour. Croll's hypothesis 

 is built upon the belief that snow-accumulation would be favored 

 by long winters, and snow-melting reduced by short summers. 

 The hypothesis is that the glacial epochs were the times of aphelion 

 winters during periods of great eccentricity. 



It is admitted that these astronomical relations are insufficient 

 in themselves to produce the observed glaciation, and so certain 

 terrestrial conditions are made important elements in the working 

 force of the hypothesis. Thus it is held that the zone of the trade - 

 winds and the thermal equator would be shifted from the glaciated 

 hemisphere toward the warmer one, and that this shifting would 

 turn a large part of the warm equatorial waters away from the cooler 

 hemisphere. Croll held that if the trade- wind belts were shift ed 

 southward a few degrees, a large part of the equatorial run-out 

 would be south of Cape St. Roque, and so turned into the South 

 Atlantic, greatly lowering the temperature of the northern hemi- 

 sphere. When the southern hemisphere was passing through 

 cold period, nearly all the equatorial current would be north ol 

 Roque, and this would give the northern hemisphere a moist inter- 

 glacial epoch. 



If the hypothesis were correct, (1) glacial epochs should alte 

 between the northern and the southern hemispheres, and i'J) their 

 duration should be limited to an appropriate fraction of the j 



