DRUM LIN, OSAR AND KAME FORMATION. 257 



it was not absolutely at the bottom of the ice, I think, in a gen- 

 etic view, it is to be regarded as basal unless it was lifted so 

 high into the body of the glacier as to be borne onward thence- 

 forth entirely within the body of the ice and free from basal 

 influences so that it was at length carried out to the surface as 

 it approached the terminal edge, and was deposited as super- 

 glacial material. If the material remained approximately at the 

 bottom of the glacier and again descended to the absolute base 

 of the ice, it seems to me best to regard it as basal, even though 

 it may be, for a time, completely enveloped within the ice. 

 This seems best, because it represents the significant factor in 

 the operation. In origin, it was basal, and, in the end, it became 

 basal. It was only englacial by accident, temporarily. 



Opposed to the agencies that tended to carry material from 

 the absolute bottom of the ice into its basal portion to limited 

 heights, there were several agencies that tended to bring it back 

 to the absolute bottom, (i) The conduction of internal heat 

 contributed slightly to this by melting away the base of the ice. 

 The annual amount was undoubtedl}' very small, but the cumu- 

 lative effects upon the bottom of any particular column of ice 

 during the last five hundred miles of its journey (and this much 

 is involved in certain aspects of the problem) was probably very 

 appreciable and was manifestly greater in proportion to the slow- 

 ness of the ice movement. (2) Basal friction undoubtedly gave 

 rise to a much larger wastage and so lowered the embedded 

 debris. (3) The introduction of warm waters from the surface, 

 through the agency of crevasses, also caused wastage of the bot- 

 tom, but this was obviously limited to such portions as were 

 accessible to these waters and the effects were unequally dis- 

 tributed, although the positions of the streams undoubtedly 

 changed from time to time, and this tended to spread the effects 

 more generally over the bottom. (4) It is probable that there 

 was a certain amount of penetration of solar rays through the 

 ice. As the surface of a glacier is usually granular, only a minor 

 portion of the sun's rays probably succeeded in penetrating to 

 the transparent ice below. But such portions as reached this 



