CHAP. IX GEOLOGICAL CLIMATES 205 



condition of the northern hemisphere is the result of the 

 peculiar distribution of land and water upon the globe ; 

 and the general permanence of the position of the con- 

 tinental and oceanic areas — which we have shown to be 

 proved by so many distinct lines of evidence — is also im- 

 plied by the general stability of climate throughout loug 

 geological periods. The land surface of our earth appears 

 to have always consisted of three great masses in the 

 north temperate zone, narrowing southward, and termi- 

 nating in three comparatively narrow extremities re- 

 presented by Southern America, South Africa, and Aus- 

 tralia. Towards the north these masses have approached 

 each other, and have sometimes become united ; leaving 

 beyond them a considerable area of open polar sea. 

 Towards the soutn they have * never been much further 

 prolonged than at present, but far beyond their extremities 

 an extensive mass of land has occupied the south polar 

 area. 



This arrangement is such as would cause the northern 

 hemisphere to be always (as it is now) warmer than the 

 southern, and this would lead to the preponderance of 

 northward winds and ocean currents, and would bring 

 about the concentration of the latter in three great streams 

 carrying warmth to the north-polar regions. These streams 

 would, as Dr. Croll has so well shown, be greatly increased 

 in power by the glaciation of the south polar land ; and 

 whenever any considerable portion of this land was ele- 

 vated, such a condition of glaciation would certainly be 

 brought about, and would be heightened whenever a high 

 degree of excentricity prevailed. 



It is now the general opinion of geologists that the 

 great continents have undergone a process of development 

 from earlier to later times. Professor Dana appears to 

 have been the first who taught it explicitly in the case of 

 the North American continent, and he has continued the 

 exposition of his views from 1856, when he discussed 

 the subject in the American Journal, to the later editions 

 of his Manual of Geology in which the same views are ex- 

 tended to all the great continents. He says : — 



" The North American continent, which since early 



