ANCIENT GLACIERS. 117 



1. So far as we can estimate, a temporary retreat of 

 the front, lasting a few centuries, would be sufficient to 

 account for the vegetable accumulations that are found 

 buried beneath the glacial deposits in southern Ohio, In- 

 diana, central Illinois, and Iowa, while a temporary re- 

 advance of the ice would be sufficient to bury the vegeta- 

 ble remains beneath a freshly accumulated mass of till. 

 Thus, as Dr. Bell suggested, the interglacial vegetal de- 

 posits do not necessarily indicate anything more than a 

 temporary oscillation of the ice-front, and do not carry 

 with them the necessity of supposing a disappearance of 

 the ice from the whole glaciated area. Thus the introduc- 

 tion of a whole Glacial period to account for such limited 

 phenomena is a violation of the well-known law of parsi- 

 mony, which requires us in our explanations of phenomena 

 to be content with the least cause which is sufficient to 

 produce them. In the present instance a series of com- 

 paratively slight oscillations of the ice-front during a 

 single glacial period would seem to be sufficient to ac- 

 count for all the buried forests and masses of vegetal 

 debris that occur either in the United States or in the 

 Dominion of Canada. 



2. Another argument for the existence of two abso- 

 lutely distinct glacial periods in North America has been 

 drawn from the greater oxidation of the clays and the 

 more extensive disintegration of certain classes of the bould- 

 ers found over the southern part of the glaciated area of 

 the Mississippi Valley, than has taken place in the more 

 northerly regions. Without questioning this statement 

 of fact (which, however, I believe to be somewhat exag- 

 gerated), it is not difficult to see that the effects probably 

 are just what would result from a single long glacial pe- 

 riod brought about by such causes as we have seen to be 

 probably in operation in America. For if one reflects 

 upon the conditions existing when the Glacial period be- 

 gan, he will see that, during the long ages of warm cli- 



