ENGLACIAL DRIFT. 249 



a relatively slow movement, it will be evident that the average rate of 

 movement for the border of the great ice-sheet of Greenland can not be 

 high; and the average rate of this border is the nearest available analogue 

 to the border movement of the still more extended perij^hery of the ancient 

 American or Laurentian glacier. 



The present writer also differs with Mr. Uphani in his views respecting 

 the amount and distribution of the englacial drift. That considerable dtibris 

 is borne in the l>asal ])ortion of the ice is not questioned; indeed, the term 

 "englacial drift" was proposed by the present writer in recognition of its 

 importance. Our best evidence of the amount and disti-ibution of this is 

 derived from the continental glacier of Greenland. It is there observed 

 that debris prevails in the lower 50 or 75 feet of the ice-sheet, and occa- 

 sionally reaches up to 100 feet, or perhaps even 150 feet. The amount of 

 this d(ibris, if it were let down directly upon the glacier's bottom by melting 

 in situ, without concentration by the forward motion of the ice, would be 

 measured by a very few feet, or by a fraction of a foot. The forward 

 motion of the ice concentrates this at its edge, so tliat it may there reach, 

 theoretically, any dimension, entirely without regard to its amount in any 

 given vertical section of the ice. The thickness of the deposit formed from 

 the englacial drift is quite as much dependent upon the length of time dur- 

 ing which the edge of the ice remains at one line as upon the amount of 

 drift which the ice may carry in any given vertical section. No safe 

 inferences from the thickness of deposits of englacial drift can therefore be 

 drawn with reference to the amount of englacial material present in any 

 given portion of the glacier. If the ice were absolutely stagnant the 

 deposit of englacial drift A\()uld be precisely that which was held in the ice 

 above the point of deposit If there was any forward motion of the 

 ice Avhile it was being melted away, there would necessarily be a concen- 

 tration. If there be 1 foot of englacial debris in a given section, and the 

 ice moves forward 40 feet while the external heat causes a retreat of 1 foot, 

 the englacial deposit should be 40 feet deep. The thickness of the englacial 

 drift may therefore be (juite as much an expression of prolonged time as of 

 a larg-e content of dcliris within the ice. 



