CHAP, viri THE CAUSES OF GLACIAL EPOCHS 149 



would result from there being no land near the poles to 



retain snow, while the constant interchange of water by 

 means of currents between the polar and tropical regions 

 would most likely prevent ice from ever forming in the 

 sea. On the other hand, were all the land accumulated in 

 the polar and temperate regions there can be little doubt 

 that a state of almost perjoetual glaciation of much of the 

 land would result, notAvithstanding that the whole earth 

 should thoretically be at a somewhat higher temperature. 

 Two main causes would bring about this glaciation. A 

 very large area of elevated land in high latitudes would act 

 as a powerful condenser of the enormous quantity of vapour 

 produced by the whole of the equatorial and much of the 

 temperate regions being areas of evaporation, and thus a 

 greater accumulation of snow and ice would take place 

 around both poles than would be possible under any other 

 conditions. In the second place there would be little or no 

 check to this accumulation of ice, because, owing to the 

 quantity of land around the j^olar areas, warm oceanic cur- 

 rents could not reach them, while the warm winds would 

 necessarily bring so much moisture that they would help 

 on instead of checking the process of ice-accumulation. 

 If we suppose the continents to be of the same total area 

 and to have the same extent and altitude of mountain 

 ranges as the present ones, these mountains must neces- 

 sarily offer an almost continuous barrier to the vapour- 

 bearing winds from the south, and the result would probably 

 be that three-fourths of the land would be in the ice-clad 

 condition of Greenland, while a comparatively narrow belt 

 of the more southern lowlands would alone aftord habitable 

 surfaces or produce any woody vegetation. 



Notwithstanding, therefore, the criticism above referred 

 to, I believe that Sir Charles Lyell was substantially right, 

 and that the two ideal maps given in the Principles of 

 Geology (11th ed.Vol. i. p. 270), if somewhat modified so as 

 to allow a freer passage of currents in the tropics, do really 

 exhibit a condition of the earth which, by geographical 

 changes alone, would bring about a perpetual summer or an 

 almost universal winter. But we have seen in our sixth 

 chapter that there is the strongest cumulative evidence, 



