THE PROBABLE AGE OF THE WORLD. 6 73 



sun than she is now, the climate of the northern hemisphere would be 

 very different from what it is. 



One such period of great eccentricity occurred about two million 

 five hundred thousand years ago. Fifty thousand years later there 

 was another. Again, eight hundred and fifty thousand years ago 

 there was a third, and two hundred thousand years ago a fourth. 

 Those periods were characterized by cold such as we have no concep- 

 tion of. More than arctic winter lingered far on into the spring, and 

 unmelted ice of one year accumulated through the next, till from the 

 pole to the south of Scotland the earth was covered with a vast ice- 

 cap, probably several miles in thickness. 



Now, in Europe and America, wherever in fact any records are 

 left of the glacial epoch, it is remarked that a general subsidence of 

 the land followed closely on the appearance of the ice. This fact led 

 certain geologists to conclude that there was some physical connection 

 between the two phenomena, and Mr. Jamieson suggested to the Geo- 

 logical Society that the crust of the earth might have yielded under 

 the enormous weight of the ice. Mr. Croll, however, gives a different 

 explanation ; and the more it is understood the more it appears to 

 gain ground with those capable of forming an opinion. He says that 

 the surface of the ocean always adjusts itself in relation to the earth's 

 centre of gravity, no matter what the form of the earth happens to be. 

 If a large portion of the water of the ocean were formed into solid ice, 

 and placed round the north pole, its weight would naturally change 

 the centre of gravity of the earth. The centre would be changed a 

 little to the north of its former position. The water of the ocean 

 would then forsake its old centre, and adjust itself with reference to 

 the new. The surface of the ocean will therefore rise toward the 

 north pole, and fall toward the south. The land will not sink under 

 the sea, but, what amounts to the same thing, the sea will rise upon 

 the land. The extent of submergence will be in proportion to the 

 weight of the ice. 



It is easy to see that glaciation would not be contemporaneous on 

 both hemispheres. One hemisphere would be covered with ice and 

 snow, while the other would be enjoying a perpetual spring. A gla- 

 cial epoch resulting from the eccentricity of the earth's orbit would 

 extend over a period of a hundred thousand years. But, for the reason 

 o-iven above, the glaciation would be transferred from one hemisphere 

 to another every ten thousand years. A glacial epoch extending over 

 a hundred thousand years would therefore be broken up into several 

 warm periods. The warm period in one hemisphere would coincide 

 with the cold one in the other, and there would be elevation of the 

 land during the warm period and subsidence during the cold. 



This cause would be quite sufficient to effect the alternate upheaval 

 and depression. During the successive ages that each pole alternately 

 was subjected to glaciation, the winter ice, unmelted by the brief sum- 



TOL. IX. 13 



