74 THE JOURNAL OF GEOLOGY. 



It has been urged against the criterion of differential weath- 

 ering that superglacial material is or may be thoroughly oxidized 

 before, its deposition, and that a layer of oxidized drift between 

 layers of till may be no more than superglacial debris deposited 

 during a minor recession of the ice/ We believe that this 

 attempt to eliminate the value of this criterion rests partly on an 

 exaggerated idea concerning the amount of superglacial material, 

 but more especially on a failure to apprehend the real meaning 

 of the argument for the validity of the criterion, and upon a 

 failure to note the limitations imposed upon it by its advocates. 

 It is not affirmed that a layer of oxidized drift between beds of 

 unoxidized drift is per se proof of two glacial epochs ; but it is 

 affirmed that if such layer of weathered drift can be shown to 

 extend far below any possible superglacial till, into the sub- 

 glacial till below, in such wise as to indicate that it is the result 

 of subaerial exposure in a warm climate subsequent to its 

 deposition and prior to the deposition of the overlying till, it 

 constitutes the best possible evidence of an interglacial epoch, 

 especially when accompanied by the corroborative testimony of 

 other criteria. It is further affirmed that if the second sheet of 

 drift failed to reach the limit of the first, and if the drift which 

 was deposited by the first and never covered by the second ice- 

 sheet, is more thoroughly and more deeply weathered than that 

 deposited by the second, and especially if the two types of drift 

 surface meet along a definite and readily traceable line, the argu- 

 ment becomes, in our judgment, irrefragable. In its application, 

 this criterion would be infallible only in the hands of one who 

 could distinguish between superglacial and superglacially oxidized 

 material on the one hand, and material subaerially weathered 

 after its deposition, on the other. 



In circumstances and relations where the weathering of the 

 drift is not in itself conclusive, it might still have corrobora- 

 tive value in association with other lines of evidence. 



The absence of an oxidized and disintegrated zone of drift 



■ This point was urged at the reading of the paper at Ottawa, by Prof. C. H. 

 Hitchcock, Mr. Upham, and others. 



