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 farther 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 fragments 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 surface of rock 
it is clear that its bed is subjected to powerful abrading 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° Fahrenheit. 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 
heat 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 energy 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 the bed 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. 
