THE DATE OF THE GLACIAL PERIOD. 575 



beliefs that long chronology carries with it. If any one 

 chooses to believe that kettle-holes can stand one hundred 

 thousand 3 r ears, and fill up only twentj^-four feet from the 

 apex of the inverted cone, he must run the risk of being 

 considered credulous. 



In rejecting the theory of Mr. Croll concerning an indefinite 

 succession of glacial periods, we did not mean to foreclose 

 the discussion connecting the question whether there have 

 not been several Pleistocene glacial epochs. This question 

 must, therefore, now be considered with more particular 

 reference to its bearing upon matters of chronology. As 

 the reader doubtless observed in the remarks upon Croll's 

 theory, quoted from Mr. Gilbert and President Chamber- 

 lin, in the preceding chapter, each of them spoke of an "In- 

 terglacial Period" as clearly indicated in North American 

 geology. The calculations j ust made relate to the chronology 

 of what President Chamberlin called the "second glacial 

 epoch." Niagara Falls, the Falls of St. Anthony, the kettle- 

 holes of Massachusetts, and the valley of Plum Creek, 

 are none of them upon the extreme border of the glaciated 

 region. Raccoon Creek is nearer the margin. Calculations 

 respecting those interior points, therefore, do not give the 

 date of the extreme marginal deposits. Hence it becomes 

 a matter of prime importance to consider to what extent 

 the ice retreated during the various climatic episodes 

 which characterized the epoch. Many, perhaps most, of 

 the authorities on glacial subjects at the present time hold 

 that during two or three of these episodes the ice retreated 

 as far as the Laurentian Highlands and then re-advanced to 

 the limits of what are called respectively, the Iowan, the 

 Illinoisan and the Wisconsin boundaries of glacial drift. 

 It is necessary, therefore, to discuss these questions in con- 

 siderable detail. 



The most obvious evidence adduced in favor of inter- 

 glacial epochs in America consists of the so-called " inter- 



