202 BULLETIN OF THE 



the ice over the whole trail would not necessarily alter its width. It is 

 necessary to suppose, if we are to account for the widening of the field 

 in this way, that the movements were at once so numerous and so or- 

 ganized in relation to each other that they resulted in a dispersing 

 action on the boulders, which was the greater the ftirther they were 

 away from their source. In other words, the difficulty with this 

 hypothesis is that it will not account for the gradual and essentially 

 uniform widening of the train from its source to the sea. This feature 

 seems to indicate some dispersing agent which has served in a constant 

 manner to increase the distance between the frajrments during their long 

 southward journey, without at the same time deflecting the path from a 

 tolerably direct line. I shall now submit the hypothesis which it seems 

 reasonable to frame to meet these conditions. 



"Where a continental glacier rests on and moves over a surfixce of rock 

 it is clear that its bed is subjected to powerful al^rading forces. The 

 energy of position of the glacier breaks up the rock, grinds the greater 

 part of it into small fragments, and thereby converts a certain amount 

 of energy into heat. The experiments made by Robert Mallet, which 

 are set forth in his paper in the Philosophical Transactions in 1873, 

 show that the energy required to crush certain of the harder kinds of 

 rock is sufficient to elevate the temperature of the material to about 

 200° Fo-hrenheit. This amount of heat appears to be sufficient to melt 

 somewhere near an equal bulk of ice, provided the ice was at the tem- 

 perature of 32° when the melting began. 



The effect of this rock crushing, which is accomplished at or near the 

 base of the glacier, is necessarily to bring about the melting of a certain 

 amount of the ice. Another and doubtless a more important supply of 

 lieat in this deeper part of the ice arises from the shearing movement 

 of the particles of the glacier over each other. There can be no doubt 

 that the principal part of the energy of position of the glacier is ex- 

 pended in the work of impelling the particles of ice by each other in 

 the unceasing movement to which they are subjected in the lower por- 

 tion of the sheet. Some part of its energ}' is doubtless expended in the 

 differential movement which takes place in the upper part of the section, 

 but by far the greater portion of it must be spent in bringing about the 

 tortuous shearing motions which we know from the glacial scratches 

 occurred in the section immediately adjacent to tlie i)ed rock. The 

 value of this heat-giving action cannot be well estimated, but there is 

 reason to believe that in amount it is many times as great as that 

 derived from the disruption of the rock which occurs at the base of the 

 glacial mass. 



