DISTINCT GLACIAL EPOCHS. 6/ 



the northward that its re-advance might, in our judgment, appro- 

 priately be regarded as a separate ice, epoch. 



It has been suggested in opposition that temperate condi- 

 tions may obtain even up to the edge of the ice, and that inter- 

 bedded vegetal remains indicating temperate" climate do not 

 prove any considerable recession of the ice. The phenomena 

 about existing glaciers have been appealed to in support of this 

 demurrer. But the objection is not well taken. The climatic 

 conditions which obtain about the borders of small, local glaciers, 

 are not a safe guide as to climatic conditions which obtained 

 about the margin of a continental ice -sheet, any more than the 

 climatic conditions which obtain about a small inland lake are 

 a safe criterion as to the climatic conditions about a sea -coast. 

 The general principles of climatology, as well as specific facts con- 

 cerning plant distribution, seem to us to indicate that the climate 

 about the border of a continental ice-sheet must have been arctic. 



It is evident that the greater the distance north of the over- 

 lying drift remains of temperate plants are found, the more con- 

 clusive becomes the evidence. Plant remains indicating tem- 

 perate climate at the very margin of the drift sheet which over- 

 lies them, would be less conclusive than similar evidences one 

 hundred miles to the northward. It might be difficult to prove 

 in any given instance that the ice which deposited the drift over- 

 lying plant remains advanced one hundred miles, or any other 

 specific distance, south of any particular underlying forest bed. 

 If the forest bed were continuous for the whole distance, the 

 case would be clear. It would also be conclusive if the con- 

 tinuity of the drift overlying a forest bed at any point with that 

 of a remote point to the south, could be demonstrated. In spite 

 of these difficulties in its application, the vegetal beds constitute 

 a valuable criterion in making the discriminations under consid- 

 eration, when they are properly applied. Under proper circum- 

 stances the criterion may be conclusive when taken alone, and it 

 may have corroborative significance when not itself conclusive. 



The absence of forest beds and of all traces of vegetal deposits 

 whatsoever between beds of drift, is no proof of the absence of 



