DISTINCT GLACIAL EPOCHS. 65 



epoch from another. Changes of this sort have often furnished 

 the basis for the major and minor divisions of time in other 

 parts of geological history, so that there can be no question as 

 to their adequacy, if they were of sufficient magnitude. We 

 hold that the intervention of orographic or other important geo- 

 logic changes might reduce to a minimum the amount of reces- 

 sion, the duration of the recession, and the warmth of the inter- 

 vening climate necessary to constitute the separate ice advances 

 separate ice epochs. The absence of great orographic or other 

 changes in glaciated regions between successive advances of 

 the ice would be no proof that such advances should not be 

 regarded as separate epochs. Divisions of equal importance 

 have often been made without evidence of such changes. 



From the foregoing discussion, brief as it is, it will be seen 

 that within certain narrow limits the definition of a glacial 

 epoch, as distinct from other glacial epochs, must be more or 

 less arbitrary. It is less important that an arbitrarv definition 

 should be accepted, than that the same meaning should be 

 attached to technical terms in common use among geologists. 

 In the interest of harmony and of a common understanding, 

 and without the violation of any truth of science, we believe it 

 would be well if the conception of a glacial epoch, as framed by 

 those who are our leaders in position and in fact, were made the 

 basis for our usage of the term. 



III. The Criteria of Distinct Glacial Epochs. 



If there have been differences of opinion concerning the 

 nature of ice epochs, as distinct from each other and from ice 

 periods, there has been a failure to adequately apprehend the 

 nature, the extent, and the meaning of the real criteria on which 

 the final recognition of separate ice epochs, if such there were, 

 must be based. 



Such criteria are several in number. They are of unequal 

 value. In some instances a single one of them might be quite 

 sufficient to establish the fact of two ice epochs. In other 

 cases, single criteria which might not be in themselves demonstra- 



