216 
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 
