THE CAUSE OF THE GLACIAL PERIOD. 309 



hundred thousand miles farther from the sun in winter 

 than in summer; at which times, also, the winter was 

 nearly twenty-eight days longer than the summer. Such 

 an extreme elongation of the earth's orbit occurred about 

 two hundred and fifty thousand years ago. 



It is easy to assume that such a change in astronomical 

 conditions would produce great effects upon the earth's 

 climate ; and equally easy to connect with those effects the 

 vast extension of ice during the Glacial period. Since, 

 also, this period of extreme eccentricity terminated only 

 eighty thousand years ago, the close of the Glacial period 

 would, perhaps, upon Mr. Croll's theory, be comparatively 

 a recent event ; for if the secular summer of the earth's 

 eccentricity lags relatively as far behind the secular move- 

 ments as the annual summer does behind the vernal 

 equinox, we should, as Professor Charles H. Hitchcock 

 suggests, have to place the complete breaking up of the 

 Ice period as late as forty thousand years ago.* 



We have no space to indicate, as it deserves, the com- 

 parative merits and demerits of this ingenious theory. It 

 would, however, be a great calamity to have geologists 

 accept it without scrutiny. It is, indeed, a part of the 

 business of geologists to doubt such theories until they 

 are verified by a thorough examination of all accessible 

 terrestrial evidence bearing upon the subject. There is no 

 reason to question the reality of the variations in the rela- 

 tive positions of the earth and the sun assumed by Mr. 

 Croll ; though there may be serious doubt whether the 

 effects of those changes upon climate would be all that is 

 surmised, since equal amounts of heat would fall upon 

 the earth during summer, whether made longer or shorter 

 by the cause referred to. During the short summers 

 the earth is so much nearer the sun that it receives 

 each season absolutely as much heat as it does during the 



