CHAP. VIII THE CAUSES OF GLACIAL EPOCHS 155 



Spain. The consequence of such a state of things would 

 probably be that the southward flowing Arctic currents 

 would be much reduced in velocity ; and the enormous 

 quantity of icebergs continually produced by the ice-sheets 

 of all the lands bordering the North Atlantic would hang 

 about their shores and the adjacent seas, filling them with 

 a dense ice-pack, equalling that of the Antarctic regions, 

 and chilling the atmosphere so as to produce constant 

 clouds and fog with almost perpetual snowstorms, even at 

 midsummer, such as now prevail in the worst portions of 

 the Southern Ocean. 



But when such was the state of the North Atlantic (and, 

 however caused, such must have been its state during the 

 height of the glacial epoch), can we suppose that the mere 

 change from the distant sun in winter and near sun in 

 summer, to the reverse, could bring about any important 

 alteration — the physical aoid geographical causes of glaciation 

 remaining unchanged ? For, certainly, the less powerful 

 sun of summer, even though lasting somewhat longer, 

 could not do more than the much more powerful sun did 

 during the phase of summer in perihelion, while during the 

 less severe winters the sun would have far less power than 

 when it was equally near and at a very much greater 

 altitude in summer. It seems to me, therefore, quite 

 certain that whenever extreme glaciation has been brought 

 about by high excentricity combined with favourable 

 geographical and physical causes (and without this combina- 

 tion it is doubtful whether extreme glaciation would ever 

 occur), then the ice-sheet will not be removed during the 

 alternate phases of precession, so long as these geographical 

 and physical causes remain unaltered. It is true that the 

 warm and cold oceanic currents, which are the most 

 important agents in increasing or diminishing glaciation, 

 depend for their strength and efficiency upon the compara- 

 tive extents of the northern and southern ice-sheets ; but 

 these ice-sheets cannot, I believe, increase or diminish to 

 any important extent unless some geographical or physical 

 change first occurs.^ 



^ Dr. Croll says that I here assume an impossible state of things. He 

 maintains " that the change from the distant sun in winter, and near sun 



