64 . THE JOURNAL OF GEOLOGY. 



Analogous subdivisions in pre-Pleistocene formations have been 

 frequently recognized. 



(2) The application of the time element is hardly sus- 

 ceptible of quantitative statement. We are inclined to think that 

 it would be generally agreed that, with a given amount of reces- 

 sion of the ice, its re-advance would be more properly regarded 

 as a distinct glacial epoch if the interval which had elapsed 

 since the first advance were long. Whether a longer time 

 between the separate advances might reduce the amount of 

 recession necessary in order to constitute the second advance a 

 second epoch, we are not prepared to assert; but we are inclined 

 to think it might. 



(3) The third element is perhaps somewhat more tangible 

 than the second. If, during the retreat of the ice, the climate of a 

 region which was twice glaciated became as temperate as that of 

 the present day in the same locality, we should be inclined to 

 regard the preceding and succeeding glaciations as distinct ice 

 epochs, especially if the intervening recession were great and its 

 duration long. 



Unfortunately for simplicity and ease of determination, there 

 are difficulties in determining with precision how far the ice 

 retreated between successive maxima of advance, how long the 

 interval during which it remained in retreat, and the extent to 

 which the climate was ameliorated, as compared with that which 

 went before and that which followed. 



(4) If changes of any sort which interrupt the continuity 

 of geological processes intervened between successive maxima 

 of advance of the ice, the separation of the later advance 

 from the earlier, as a distinct ice epoch, would be favored. 

 How great the intervening changes should be in order to 

 constitute the re-advance a distinct ice epoch, is a point 

 concerning which there might be difference of opinion. But it 

 is altogether possible that such changes might intervene as alone 

 to give sufficient basis for the separation. Orographic movements, 

 resulting either in continental changes of altitude or attitude are 

 among the events which might come in to separate one ice 



