220 LULLETIX OF THE 



thickened, thereby iucreasing the amount of pressure molten water, a 

 portion of the fluid would be squeezed under the ice of the peripheral 

 zone, thei-eby augmenting the thickness of its section, and at the same 

 time thinning the ice in the central area. In this manner we cani con- 

 ceive the creation of a balance in the impulses and resistances affecting 

 the movement of the ice which was softened by pressure melting, so 

 that the drift of the material toward the margin of the continental 

 glacier would be slow and uniform. 



The conditions of our hypothesis require us to suppose that the effects 

 of pressure melting would first be felt in the deeper parts of the glacier, 

 those portions of its mass which lay in the valleys, and that the soften- 

 ing of the ice might there be completely effected while the frozen water 

 was still in contact witli the earth at the higher levels of the surface. 

 It thus might well happen that the considerable elevations of the coun- 

 try, those hard parts which had survived under tiie conditions of ordi- 

 nary land erosion, would be much more effectively worn down than the 

 rock beneath the river valleys. We can thus account for the destruction 

 of such a prominence as Iron Hill, which w^as probably a sharp peak of 

 considerable altitude when the glacier began its work, while the neigh- 

 boring valleys were but little worn by the action of the glacier. 



So long as a glacier is receiving a considerable annual contribution of 

 snow which is built into its mass at a low temperature, it may well be 

 that the accumulations of heat due to the work done near the base of the 

 ice would not affect any considerable portion of the central section of the 

 mass. If now for a time the annual snowfall diminished to the point 

 where, by the thinning of the glacier, pressure melting ceased to take 

 place, the vrhole section of the ice might gradually acquire a relatively 

 high temperature, so that any sudden increase in pressure might bring 

 about very extensive melting. If in this condition of the deposit the 

 amount of snowfall should, for a number of years, be greatly increased, 

 the result might be a great development of pressure molten water, which 

 would be pushed forward towards the margin of the glacier to the point 

 ■where, owing to the diminution in the thickness of the ice, it could be- 

 come refrozen. In this way we may perhaps account for those sudden 

 and temporary advances in the margin of the glacier which are so clearly 

 indicated at various points in this country. 



No direct verification of the hypothesis above deduced is to be ob- 

 tained by observation or experiment. The only approach to proof which 

 we can hope to secure is by an inspection of the facts exhibited in the 

 records of glacial action with a view to ascertaining how far they may 



