CRITERIA REQUISITE TO A GLACIAL AGE 67 



I say "theoretically derivable," because no fully authenticated 

 case of this kind has been presented as yet, so far as I know. 



2. Evidence from assorted drift included i7i till and morai?ies. — 

 Relics found in gravels or sands interbedded with true bowlder 

 clays, when they are clearly of immediate glacial derivation, 

 furnish nearly as good evidence as relics in the bowlder clays 

 themselves, but their identification requires more circumspec- 

 tion. 



3. Evide?ice from kames and eskers. — So also relics found in 

 true kames or eskers may furnish good evidence if the sources 

 of error are sufficiently avoided, but still more caution and 

 alertness are required here. 



Confirmatory evideiice ifi these cases. — Glacial striation, besides 

 being a source of evidence in itself, may afford supplementary' 

 evidence in these cases and may serve asa check on the sources of 

 error previously noted. Unmistakable glacial striation may even 

 make good what would otherwise be worthless evidence. For 

 example, a well-striated bone found in secondary till might 

 be accepted as good evidence of glacial antiquit}', whereas if 

 unstriated it would have to be excluded because the rearrange- 

 ment or disturbance of the till would vitiate its trustworthiness. 

 Striated stone implements must, however, be received with great 

 caution, for, in the first place, stones that had long before been 

 striated may be used in the construction of implements without 

 obliterating all the striation, and, in the second place, use may 

 give rise to grooves and scratches ver}' like true glacial work. 

 Erroneous identification of striae are by no means uncommon 

 in the history of glacial geology. 



Combi?ied evidences. — When well-authenticated striation is 

 combined with imbedment in original glacial deposits, the evi- 

 dence reaches its highest value. We have, however, so far as I 

 know, no such cases as yet in America. 



TJie cidmi?iative value of repetition. —The evidential value of 

 ^ood cases rises rapidly with their multiplication, for the doubts 

 that might legitimately arise in a single case notwithstanding all 

 precautions against error are effectually reduced by additional 

 good cases. But, true as this is, no amount of multiplication of 



