LOUIS AGASSIZ 77 



will, of course, return), or there may 

 have been abrupt changes in the consti- 

 tution of the earth herself and a change 

 in the geographical situation of the 

 pole. Not much is known of the geol- 

 ogy of the arctic regions, and it is not 

 even absolutely sure whether the glacial 

 period was one of simultaneous cold 

 over the whole earth. Geologists are 

 not at one as to whether several distinct 

 periods or only one period with inter- 

 ruptions can be traced. Geologically 

 speaking, the time was extremely re- 

 cent it has been put within the exist- 

 ence of mankind. Such a sheet of ice 

 as that which covered the North Ameri- 

 can continent would distinctly alter the 

 level of the ocean ; and for this an 

 immense accumulation of snow is neces- 

 sary, while meteorology is still so far 

 from being an exact science that her 

 best men shrink from saying what 

 would be the effect of great changes in 

 the distribution of land and water, and 



