EDITORIALS. 523 



ice, or whether they be supposed to continue to rise (more and 

 more slowly) till they meet the descending plane of ablation. 



If currents rise by reason of differential movements to cer- 

 tain heights, but not beyond them, notwithstanding the extension 

 of the differential movements all the way up to the surface, a very 

 distinct statement of this limitation and of the dynamics invol- 

 ved, qualitative and quantitative, would be appropriate. Perhaps 

 such an explanation is intended in the following quotation from 

 Mr. Upham, which I introduce to give ampler expression to his 

 views, though I dissent from his interpretations of the crevasses 

 of the alpine glaciers and of the esker, Bird's Hill, as well as 

 from his fundamental proposition. 



"The conditions of the flowing ice which seem to me to have 

 been efficacious to carry drift upward into it from tracts of plane 

 or only moderately undulating contour, were the more rapid 

 onflow of the ice-sheet in its upper and central parts and even in 

 the portion near the ground but not in contact with it, than upon 

 the bed of the ice-sheet where its movement was much retarded 

 by friction. A very good analogy with the slowly rising cur- 

 rents which I believe to have existed in many portions of the 

 base of the ice-sheet is afforded by the edges of alpine glaciers, 

 where the crevasses extending diagonally up stream into the 

 glacier testify that the movement of its friction-hindered border 

 is from the side of the valley into the ice mass. But the arched 

 surface of the glacier and the great supply of its central current 

 prevent the drift so worn off and borne away from being carried 

 into the axial portion of the ice stream. Similarly the steady 

 accession to the mass of the ice-sheet over any place by onflow 

 from its thicker central part and by the accumulating snowfall 

 forbade the drift of the upwardly moving basal current from 

 being carried far into the ice in comparison with its total thick- 

 ness. The evidence of the esker called Bird's Hill, near Winni- 

 peg, Manitoba, shows that much englacial drift had there been 

 uplifted from a nearly level country to a height of more than 500 



