CAUSES OF CLIMATAL CHANGE 387 



ing must in all geological times have greatly exceeded, as it 

 certainly does at present, the denudation caused by atmospheric 

 action at the equator, and must have tended to increase the 

 disposition to equatorial collapse occasioned by retardation of 

 rotation.! 



While such considerations as those above referred to tend to 

 reduce the practical importance of Mr. CroU's theory, on the 

 other hand they tend to remove one of the greatest objections 

 against it — namely, that founded on the necessity of supposing 

 that glacial periods recur with astronomical regularity in geolo- 

 gical time. They cannot do so if dependent on other causes 

 inherent in the earth itself, and producing important move- 

 ments of its crust. 



Sir Robert Ball has in a recent work very ingeniously im- 

 proved this theory by showing that CroU was mistaken in 

 assigning equal amounts of heat to the earth, as a whole, in 

 the periods of greater and less eccentricity. This would tend 

 to augment the effect of astronomical revolutions as causes of 

 difference of temperature; but has no bearing on the more 

 serious geological objections to the theory in question. 



A fatal objection, however, to Croll's theory, the force of 

 which has been greatly increased by recent discoveries, is that 

 the astronomical causes which he adduces would place the 

 close of the last Glacial period at least 80,000 years ago, where- 

 as it is now certainly known from geological facts that the close 

 of the last Glacial period cannot be older than about an eighth 

 or a tenth of that time. This difficulty seems to have caused 

 the greater number of geologists, specially acquainted with the 

 later geological periods, to regard this theory as quite inapplic- 

 able to the facts. 



^ Croll, in "Climate and Time," and in a note read before the British 

 Association in 1 876, takes an opposite view ; but this is clearly contrary to 

 the facts of sedimentation, which show a steady movement of debris toward 

 the south and south-west. 



