DISTINCT GLACIAL EPOCHS. 83 



independently noted by more than one glacialist. It is true that 

 a forward oscillation of the ice edge might be more forceful than 

 an earlier forward movement which might have reached a greater 

 extension. In itself, therefore, this line of evidence can not be 

 regarded as possessing great value. 



It has been indicated that under certain circumstances, and 

 in certain relations, some of the foregoing criteria, taken singly, 

 may be conclusive of glaciations so distinct from each other, as 

 to make their reference to separate epochs proper. But where 

 the facts and relations which constitute one of the criteria are 

 found, the facts and relations constituting one or more of the 

 others are likely to be found as well. Where two of the fore- 

 going criteria are found to be coexistent, their joint force is 

 greater than that of either one. If neither one be absolutely 

 conclusive, the two may still be, since the one may exactly meet 

 the deficiency of the other. If three or more concurrent lines 

 of evidence exist in any locality, the case is still further strength- 

 ened. We maintain that several of the foregoing criteria may 

 be so related to each other and to the formations concerned, as 

 not only to make the recognition of separate ice epochs proper, 

 but to make the failure of such recognition altogether unscien- 

 tific. Even when a single line of evidence, or when double, or 

 triple, or quadruple lines of evidence are not absolutely conclu- 

 sive in ruling out every conceivable technical escape from the 

 conclusion that there were separate ice epochs, their cumulative 

 and corroborative force may still be such as to carry conviction 

 scarcely less positive than that which mathematical demonstra- 

 tion would afford. In the nature of the case not all of these 

 various lines of evidence could be expected to be found in 

 any one locality, or perhaps in any one limited geographic area, 

 but where one occurs, some or all of the others are liable to 

 be found under favoring circumstance. The number of criteria, 

 and the great extent of area where they may hope for applica- 

 tion, afford great possibilities. 



From the foregoing discussion, it will be readily seen that the 

 nature of the criteria and the limitations imposed upon their 



