CiiAr. Yiii.] THE CAUSES OF GLACIAL EPOCHS. 



143 



important differences are : the open southern ocean, the longer 

 and colder winter, and the general low temperature caused by 

 the south polar ice. But the great accumulation of south polar 

 ice is itself due to the great extent of high land within the 

 Antarctic circle acted upon by the long cold winter and furnished 

 with moisture by the surrounding wide ocean. These conditions 

 of high land and open ocean we know did not prevail to so 

 great an extent in the northern hemisphere during the glacial 

 epoch, as they do in the southern hemisphere at the present 

 time ; but the other acting cause — the long cold winter — existed 

 in a far higher degree, owing to the excentricity being about 

 three times as much as it is now. It is, so far as we know or 

 are justified in believing, the only efficient cause of glaciation 

 which was undoubtedly much more powerful at that time ; and 

 We are therefore compelled to accept it as the most probable 

 cause of the much greater glaciation which then prevailed. 



Geogm'pJiical changes, how far a Cause of Glaciation. — Messrs. 

 Croll and Geikie have both objected to the views of Sir Charles 

 Lyell as to the preponderating influence of the distribution of 

 land and sea on climate ; and they maintain that if the land 

 were accumulated almost wholly in the equatorial regions, the 

 temperature of the earth's surface as a whole would be lowered, 

 not raised, as Sir Charles Lyell maintained. The reason given 

 is, that the land being heated heats the air, which rises and thus 

 gives off much of the heat to space, while the same area covered 

 with water would retain more of the heat, and by means of 

 currents carry it to other parts of the earth's surface. But 

 although the mean temperature of the whole earth might be 

 somewhat lowered by such a disposition of the land, there can 

 be little doubt that it would render all extremes of temperature 

 impossible, and that even during a period of high excentricity 

 there would be no glacial epochs, and perhaps no such thing as 

 ice anywhere produced. This would result from there being no 

 land near the poles to retain snow, while the constant inter- 

 change 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 



