GEOLOGICAL CHANGES. 417 



the ice, it must still be twelve miles thick at the South 

 Pole. 



Now if the climatal conditions altered, so that the northern 

 hemisphere became colder, and the southern on the contrary 

 warmer, and if one-sixth of this ice, or two miles, was removed 

 from the latter, and a similar amount deposited on the former, 

 the result would be such a displacement of the earth's centre 

 of gravity as to cause, according to Mr. Croll, a rise of the 

 ocean at the North Pole of 380 feet; and in the latitude of 

 Edinburgh 312 feet, while according to other calculations it 

 would be much greater. 



Again, it is very probable that ice would be melted at the 

 one Pole more rapidly than it would be formed at the other. 

 At the lowest estimate, the ice forming the present Antarctic 

 ice cap would be sufficient to raise the general level of the 

 ocean several hundred feet. The removal of two miles of ice 

 from the Antarctic continent, and the deposition of one mile 

 over a corresponding area round the North Pole, would alter 

 the sea-level no less than 485 feet at the Pole, and 435 feet 

 in the latitude of Edinburgh, as follows : the removal of the 

 two miles would affect the centre of gravity of the earth 190 

 feet, the deposition of the ice round the North Pole would 

 carry it 95 feet further, while the additional water resulting 

 from the melting of the one mile of ice would raise the general 

 level of the ocean 200 feet, making, as before, a total change 

 at the North Pole of 485 feet. 



Sir C. Lyell attempted* to form an estimate of the dura- 

 tion of the glacial epoch, on the assumption that the dif- 

 ferent movements of elevation and depression proceeded at 

 an average rate of 2| feet in a century. As the simplest 

 " series of changes in physical geography which can possibly 

 account for the phenomena of the glacial period," he gave the 

 following : 



* Antiquity of Man, pp. 2S2 ; 285. 

 2 E 



