CHAP. VIII THE CAUSES OF GLACIAL EPOCHS 161 



while on the other hand the surface markings produced 

 by the ice have been extensively preserved ; and taking 

 all these facts into consideration, the period of about 

 200,000 years since it reached its maximum, and about 

 80,000 years since it passed away, is generally considered 

 by geologists to be ample. There seems, therefore, to be 

 little doubt that in increased excentricity we have found 

 one of the chief exciting causes of the glacial epoch, and 

 that we are therefore able to fix its date with a consider- 

 able probability of being correct. The enormous duration 

 of the glacial epoch itself (including its interglacial mild 

 or warm phases), as compared with the lapse of time since 

 it finally passed away, is a consideration of the greatest 

 importance, and has not yet been taken fully into account 

 in the interpretation given by geologists of the physical 

 and biological changes that were coincident with,'' and 

 probably dependent on, it. 



Changes of the Sea-level Dependent on Glaeiation. — It has 

 been pointed out by Dr. Croll, that many of the changes 

 of level of sea and land which occurred about the time of 

 the glacial epoch may be due to an alteration of the sea- 

 level caused by a shifting of the earth's centre of gravity ; 

 and physicists have generally admitted that the cause is a 

 real one, and must have produced some effect of the kind 

 indicated. It is evident that if ice-sheets several miles in 

 thickness were removed from one polar area and placed on 

 the other, the centre of gravity of the earth would shift 

 towards the heavier pole, and the sea would necessarily 

 follow it, and w^ould rise accordingly. Extreme giacialists 

 have maintained that durinof the heioiit of the glacial 

 epoch, an ice-cap extended from about 50° N. Lat. in 

 Europe, and 40° N. Lat. in America, continually increasing 

 in thickness, till it reached at least six miles thick at the 

 pole ; but this view is now generally given up. A similar 

 ice-cap is however believed to exist on the Antarctic pole 

 at the present day, and its transference to the northern 

 hemisphere would, it is calculated, produce a rise of the 

 ocean to the extent of 800 or 1,000 feet. We have, how- 

 ever, shown that the production of any such ice-cap is 

 improbable if not impossible, because snow and ice can 



M 



