2l6 ALASKA GLACIERS 



very small, the pressure of a tidal glacier on its bed is 

 diminished by the sea. 



It thus appears that there is no important difference, as 

 respects pressure on the rock bed, between a glacier rest- 

 ing on the land and one which is partly bathed by the 

 water of a fiord; and, so far as glacial erosion is con- 

 ditioned by pressure, the presence of the sea does not 

 diminish the efficiency of the glacier. It is possible, how- 

 ever, that the rate of erosion is affected by changes in the 

 thickness of the capillary film of water. In the familiar 

 case of the grindstone, the application of water modifies 

 abrasion in two ways: It acts as a lubricant to diminish 

 friction and reduce the efficiency of the stone; and it acts 

 as a carrier to remove the product of abrasion, prevent 

 clogging, and thus enhance the efficiency. The sub- 

 glacial film doubtless has both these functions, and both 

 should be affected by its thickness; but as the two are 

 antagonistic in relation to abrasion, it is not at once 

 evident whether a thickening of the film should increase 

 or diminish the erosive work of the glacier. 



It seems furthermore possible that the influence of film 

 thickness on abrasion may not be the same for waste par- 

 ticles of all sizes. It may be important with reference to 

 the finest rock flour, where the diameters are of the same 

 order of magnitude as the depth of the film, and quite un- 

 important with reference to the work of sand and still 

 coarser waste. And if we turn from abrasion to plucking, 

 we seem to pass altogether out of the field of influence of 

 the subglacial film. 



While it is not altogether easy to picture the combina- 

 tion of molar and molecular forces associated with the sub- 

 glacial film, and while it is still more difficult to analyze 

 the effect of variation in that film on so complicated a proc- 

 ess as glacial erosion, I am nevertheless confident that 

 the influence of the sea in diminishing the pressure of a 



