CHANGES OF CLIMATE. 79 



this cause tliat we must mainly look for the changes which have 



OCCIIlTCcl. 



Such changes of level must, as has been long since shoAvn by Sir 

 Charles Lyell, modify and change climate. Every diminution of the 

 land in Arctic America must tend to render its climate less severe. 

 Every diminution of land in the temperate regions must tend to reduce 

 the mean temperature. Every diminution of land anywhere must 

 tend to diminish the extremes of annual temperature ; and the condition 

 of tlic southern hemisphere at present shows that the disappearance 

 of the great continental masses under the water would lower the mean 

 temperature, but render the climate much less extreme. Glaciers 

 might then exist in latitudes Avhere now the summer heat would 

 suffice to melt them — as Darwin has shown that in South America 

 glaciers extend to the sea level in latitude 46° 50', — and at the same 

 time the ice would melt more slowly and be drifted farther to the 

 southward. Any change that tended to divert the Arctic currents 

 from our coasts would raise the temperature of their waters. Any 

 change that would allow tlie equatorial current to pursue its course 

 through to the Pacific, or along the great inland valley of North 

 America, would reduce the British seas to a boreal condition. 



The boulder formation and its overlying fossiliferous beds prove, 

 as I have in a previous paper endeavoui'cd to explain with regard to 

 Canada, and as has been shown by other geologists in the case of 

 other parts of America and of Europe, that the land of the northern 

 hemisphere underwent in the Post-tertiary period a great and gradual 

 depression and then an equally gradual elevation. Every step of this 

 process would bring its modifications of climate, and when the de- 

 pression had attained its maximum there probably was as little land 

 in the temperate regions of the northern hemisphere as in the southern 

 now. This would give a low mean temperature and an extension to 

 the south of glaciers, more especially if, at the same time, a consider- 

 able Arctic continent remained above the waters, as seems to be 

 indicated by the effects of extreme marine glacial action on the rocks 

 under the boulder clay. These conditions, actually indicated by the 

 phenomena themselves, appear quite sufficient to account for the 

 coldness of the seas of the period; and the wide diffusion of the Gulf 

 Stream caused by the subsidence of American land, or its entire 

 diversion into the Pacific basin,* would give that assimilation of the 



* This is often excluded from consideration, owing to the fact that tlie marine 

 fauna of the Gulf of Mexico differs almost entirely from tliat of the Pacific coast; but 

 the question still remains, whether this difterence existed in the Later Tertiary period, 

 or has been established in the Modern epoch, as a consequence of changed physical 

 conditions. 



