332 POPULAR LECTURES AM) ADDRESSES. 



at all stand upon that number; he is satisfied with 

 a small fraction of it, 3,OOO or 5,000, or 12,000 feet, 

 instead of 65,000 feet. lie is satisfied with an ice- 

 cap of a comparatively moderate thickness as a 

 sufficient cause for some most important fluctuations 

 of sea level which geological history proves to have 

 taken place. It seems to me that ('roll is here 

 meeting my case, and we may find the most prob- 

 able explanation of some of our familiar chan 

 of sea level familiar even to people who are not 

 geologists in Croll's supposition of shiftings of ice 

 cither on the Antarctic or the Arctic hemisphere. 



In the first place I shall ask you to imagine that 

 the Antarctic continent for some unknown cause 

 had at one time a distribution of ice over it thicker 

 by I,OOO feet than at another time. To be more 

 accurate I would say 1,200 >ond 



with the area equivalent to 1,000 feet of water. 

 Mr. Murray has made a very careful cstima1< 

 the area of the Antarctic continent, which shows 

 that the area is about one-fortieth of the area of 

 the whole earth, ('roll makes it more; but Mr. 

 Murray has given us the more recent and more 



