1893.] evidence for the Recurrence of Ice Ages. 235 



it can be shown that throughout the ages earth movements have 

 been repeated of such a kind and degree as would fully account 

 for the most severe cold of which we have any record, which with 

 such precipitation as can be easily accounted for by the geo- 

 graphical distribution of the regions affected, would furnish the 

 lai'gest ice-sheets of which we have any suggestion, it follows that 

 we have another cause in operation which also must produce 

 recurrent glacial conditions and we have to ask whether the 

 astronomical or the geographical cause is the most potent. 



The amount of the effect which could be produced by the 

 astronomical combinations has been calculated and various intensi- 

 fying circumstances have been taken account of. 



So also, in the case of the geographical causes^ on the hypothesis 

 of recurring elevation and depression over any area, the rate of 

 increase of cold as we ascend into higher regions of the atmo- 

 sphere is a matter of observation, and, given sufficient precipitation, 

 the effect is cumulative as the ice creeps down to lower regions. 

 We can thus approximately estimate the height at which in any 

 latitude glaciers must be formed. 



In the case of the astronomical theory the most favourable 

 conditions for the production of glacial conditions would be entirely 

 neutralized by a distribution of land and water having an opposite 

 tendency. 



Whereas the geographical theory is not dependent on astro- 

 nomical conditions, but the most unfavourable combination of them 

 may be overcome by a slight increase in the amount of upheaval, 

 while keeping well within the limit of observed earth movement. 



The astronomical effects are small and contingent, the geo- 

 graphical large and independent. 



The astronomical theory that the cause of ice ages must be 

 the coincidence of winter Aphelion with great excentricity of the 

 orbit, with concomitant aggravations, requires the secular recurrence 

 of circumpolar glaciation which is not confirmed by observation. 



The geographical theory allows the probability of the recurrence, 

 and perhaps the secular recurrence, of glacial conditions but in 

 connection with earth movements, the existence of which is a 

 matter of observation. But the glacial conditions need not be 

 and are not circumpolar except so far as the temperature gene- 

 rally falls as we approach the poles. 



VOL. VIII. PT. III. 18 



