CHAP. XVIII. TO AFFECT CLIMATE. 365 



graphical change in post-pliocene times. But a difficulty of 

 another kind presents itself. We have seen that, during the 

 glacial period, the cold in Europe extended much farther 

 south than it does at present, and in this chapter we have 

 demonstrated that in North America the cold also extended 

 no less than 10° of latitude still farther southwards than in 

 Europe; so that if a great body of heated water, instead of 

 flowing northeastward, were made to pass through what is 

 now the centre of the American continent towards the Arctic 

 circle, it could not fail to mitigate the severity of the winter's 

 cold in precisely those latitudes where the cold was greatest, 

 and where it has left monuments of ice-action surpassing in 

 extent any exhibited on the European side of the ocean. 



In the actual state of the globe, the isothermal lines, or 

 rather the lines of equal winter temperature, when traced 

 eastward from Europe to North America, bend 10° south, 

 thei'e being a marked excess of winter cold in corresponding 

 latitudes west of the Atlantic. During the glacial i)eriod, 

 viewing it as a whole, we behold signs of a precisely similar 

 deflection of these same isochimenal lines when followed 

 from east to west; so that if, in the hope of accounting for 

 the former severity of glacial action in Europe, we suppose 

 the absence of the Gulf Stream and imagine a current of 

 equivalent magnitude to have flowed due north from the 

 Gulf of Mexico, we introduce, as we have just hinted, a 

 source of heat into ])rccisely that part of the continent where 

 the extreme conditions of refrigeration are most manifest. 

 Viewed in this light, the hypothesis in question would render 

 the glacial phenomena described in the present chapter more 

 perjjlcxing and anomalous than ever. But here another 

 question arises, whether the eras at Avhich the maximum of 

 cold was attained on the opposite sides of the Atlantic were 

 really contemporaneous? We have now discovered not only 

 that the glacial period was of vast duration, but that it passed 



