224 BULLETIN OF THE 



account the minor propositions, we may make the following condensed 

 statement. 



In the growth of a glacial mass, the snow is built into it at a tem- 

 perature below the freezing point, and each annual contribution is ever 

 brought nearer to the surface of the earth, and tends to become molten 

 by pressure. Effective melting near the base of the ice is probably 

 secured by the conditions which make for the development of heat at 

 that level. It is highly probable that, when the ice has attained a depth 

 of a mile or two, its lower part is either converted into water or so far 

 softened that it ceases to be an eroding agent, and may be forced to 

 move in essentially the manner of a fluid towards the zone of less re- 

 sistance. Arriving at a point where, owing to the thinning of the ice, 

 the pressure is sufficiently diminished, this water gradually refreezes and 

 is rebuilt into the firm glacier, and as such pursues the remainder of its 

 journey. We have thus to conceive a deep glacial envelope, such as that 

 which now covers Greenland, to be divided into two realms ; a central, 

 in which the ice does not come in contact with the surface of the earth, 

 and a peripheral, in which it exercises the familiar erosive action on the 

 bed rock. 



During the development of a continental glacier, until the sheet had 

 attained a thickness at which the pressure melting action would begin, 

 the whole of the mass would rest upon the surface of the earth. As 

 the inner parts of the field attained the depth which would cause the 

 ice next the ground to become softened or melted, the erosive work 

 would be limited to the peripheral zone. With the further increase in 

 the profundity of the glacier there would be a tendency, rapidly to push 

 outward the peripheral parts of the accumulation where the glacier 

 rested on the bed rock. When, in the closing stages of the period, the 

 ice sheet thinned, this zone of erosion would gradually be withdrawn to- 

 wards the centre of the field, or towards the point where the glacial 

 conditions lingered longest. In this way we can account for a long con- 

 tinued sojourn of the ice in the fields which we know it occupied, with- 

 out being required to suppose that the aggregate erosion was very large. 

 If the width of the peripheral zone were, say one hundred miles, and 

 the distance from the centre to the farthest point to which the ice ex- 

 tended one thousand miles, the time during which the eroding zone 

 occupied any part of the surface may have been but a small portion of 

 the duration of the Glacial Period. 



The hypothesis of pressure melting enables us to account for various 

 peculiarities of glacial movement which cannot otherwise be readily 



