522 THE JOURNAL OF GEOLOGY. 



between the successive layers of ice, as stated by Mr. Upham on 

 pages 38-9 of the article referred to (quoted below). This dif- 

 ferential motion arises from friction at the bottom and extends 

 to the summit. It was natural, therefore, to take it for granted 

 that the supposed rising current extended as far as its postulated 

 cause. It was to be assumed, of course, that the current would 

 rise less rapidly in the upper part if the difference of movement 

 of successive ice layers were less there than below, but it would 

 seem that the rise must be supposed to continue at some rate 

 so long as the differential motion continued, i. e., until the sur- 

 face was reached. The accession of snow-fall within the zone of 

 accumulation would, to be sure, prevent erratics from reaching 

 the new surface thus continually formed, but it would not pre- 

 vent their reaching the surface in the zone of wastage. It is 

 this latter zone with which our problems of deposition and many 

 of our problems of derivation have chiefly to do. The career of 

 some erratics is wholly confined to it. It goes without saying 

 that ablation brings the surface down and is a factor in every ex- 

 posure within the zone of wastage, but this does not prevent the 

 erratics rising (by hypothesis) until they meet it. This conception 

 of rising currents met by a plane of ablation I supposed without 

 question to be that entertained by Mr. Upham and others. To 

 be sure, in a strict and complete statement under this view the ex- 

 posure of englacial erratics at the surface would be attributed to 

 the joint result of the upward movement and the downward melt- 

 ing, but the liberties of brief and convenient statement would 

 permit it to be referred to in terms of either factor, and I have inter- 

 preted the expressions of these writers on this basis. The cor- 

 rection does not, so far as I can see, in any serious way affect the 

 main question under discussion. If there were rising currents 

 bearing erratics to heights of 500 or 1000 feet above the base of 

 the ice the result in ultimate deposition would be essentially the 

 same as if the currents rose to the surface. If the rising currents 

 are a misinterpretation, it is immaterial whether they be sup- 

 posed to bear erratics to varying heights up to 500 or 1000 feet, 

 whence these erratics move forward parallel with the base of the 



