82 THE JOURNAL OF GEOLOGY. 



to the other was effected. On the other hand, the farther north 

 of the limit of the later ice advance the sluggish drainage accom- 

 panying the earlier ice-sheet may be traced, the farther must 

 the ice have receded before the changes resulting in vigorous 

 drainage occurred. Under certain relations, the retreat of the 

 ice might be shown to have been great enough, before the 

 orographic movements which altered the nature of the drainage, 

 to constitute in our judgment, a re-advance a distinct ice epoch. 

 If for example throughout the course of a long river whose basin 

 was largely covered with ice, there be evidence that sluggish 

 drainage obtained during the maximum ice advance, and during 

 all stages of the ice retreat until the basin was free from ice, and 

 if there be evidence of a vigorous glacial drainage in the same 

 valley at a later time, with no gradations between the two types, 

 we have proof positive of at least a great recession, and of a 

 considerable elevation of the land after the ice had receded 

 beyond the limits of the drainage basin and before it again 

 reached it in its re-advance. We hold that these phases of 

 glacial drainage deposits may be so related to each other, to the 

 valleys in which they occur, and to more or less distinct bodies 

 of glacier drift, as to prove so great a recession of ice between 

 the diverse phases of drainage deposition, as to constitute the 

 second advance a distinct ice epoch. 



The absence of evidence that the land stood at different 

 elevations during different parts of the period of drift deposi- 

 tion, does not in any way militate against the theory of recur- 

 rent and distinct ice epochs. A constant attitude of the land is 

 the thing to be assumed, until positive evidence to the contrary 

 is adduced. 



(12) Vigor and Sluggishness of Ice Actio?i. If it can be shown 

 that during one epoch of glaciation, we will say the epoch of maxi- 

 mum ice extension, the ice action was relatively sluggish, while 

 during a later and minor advance its action was vigorous, the 

 difference of action might be regarded as presumptive evidence 

 of distinct ice epochs. Evidence of the two phases of ice action 

 here referred to are difficult of definition, but they have been 



