78 THE JOURNAL OF GEOLOGY. 



beneath the drift into the underlying rock, these valleys would be 

 conclusive evidence of a long interval between the deposition of 

 the two bodies of drift. The argument is of especial force when 

 such excavations in the rock beneath the drift can be shown to 

 have taken place at great distances within the margin of the 

 newer drift. For valleys in such situations imply that the ice 

 had receded at least as far to the north as they lie, during the 

 interval between the two drift depositions, and may be so sit- 

 uated as to show that the ice had wholly left the drainage basin 

 where they occur. 



The absence of evidences of deep valley excavations in any 

 given region during a supposed interglacial epoch, is no proof 

 that such interval did not exist. The conditions may not have 

 been everywhere favorable for erosion within the limits of any 

 narrowly circumscribed area, and the absence of interglacial 

 valleys would be only negative evidence against an interglacial 

 epoch. The absence of such evidence everywhere would bear 

 against the existence of an interglacial epoch of much duration 

 in such wise as to be more than negative evidence. 



(9) Different Directions of Movement. If, after its maximum 

 advance, the ice suffered merely a minor recession and then 

 remained stationary, or nearly so, for a time, the general direc- 

 tion of its movement in a subsequent advance would probably 

 be essentially the same as in the earlier. But if, after its maxi- 

 mum advance, the ice receded to a great distance, and especially 

 if it entirely disappeared, a subsequent ice-sheet might have a 

 very different direction of movement, since its center of accu- 

 mulation and dispersion might be very different. It is con- 

 ceivable that this center might shift during the history of a 

 single ice-sheet. In this case there should be a gradual change 

 in the direction of ice movement, not an abrupt one. If, there- 

 fore, there be found one sheet of drift made by an ice move- 

 ment in one direction, overlaid by another sheet of drift depos- 

 ited by ice moving in a very different direction, with an abrupt 

 transition between them, such drift sheets would be presumptive 

 evidence of distinct ice epochs. An exception would need to 



