MUSEUM OF COMPARATIVE ZOOLOGY. 195 
The reason for this relatively large wear of the rock surfaces by the 
process of scratching and grooving is easily seen: every part of the 
base of the ice was armed with bits of stone, which, held in the moving 
glacier, were dragged over the subjacent surface. When one fragment 
was worn out in the rude work, another was pressed down in its place. 
Each bit of rock worn from these grooves in turn became a cutting 
point : even the finest served to polish, and in so doing to remove, a 
portion of the rocky matter over which the ice moved. Thus the scor- 
ing and grooving work continuously went on, but the plucking out of 
fragments took place intermittingly. Im order to have a mass thus 
plucked out from its bed it was necessary to have the strain which the 
moving ice applied to it so disposed as to lift it from its bed place, and 
in most cases this movement required that the detached rock should be 
overturned in a manncr which required it to rise up into the lower part 
of the glacier. It is evident that the disruption of blocks of rock from 
their bed places would be more easily accomplished on a slope which 
inclined away from the course of the ice than in other conditions of 
exposure to the glacial flow, for in the first named position the blocks 
would have less support, and would need to rise to a less distance into 
the ice, than where the slope faced a current. To these conditions we 
may fairly attribute the ruder character of the surface observable on the 
southern slopes of many rocky hills as compared with the northern 
aspects of the same elevations. Unfortunately, the southern end of 
Iron Hill is so much covered with glacial waste that it is not possible 
accurately to determine the relative amount of plucking which went 
on there. The exposed surfaces, though limited in area, appear to indi- 
cate, however, that the amount of degradation due to this cause was not 
materially greater than upon the northern and more visible part of 
the hill. . 
Although in its present condition the surface of Iron Hill exhibits 
but few boulders of its own material, the history of the place makes it 
clear that in its natural state it was plentifully scattered over with 
these erratics, the greater portion of which have been removed to be 
used in the manufacture of iron. Within the period of my own memory 
of the locality a large part of the erratic material has thus disappeared, 
’ and at the same time a large number of boulders from the southern end 
of the train have likewise been taken away. There still remain upon 
the surface of the elevation a number of considerable erratics, which 
being from parts of the deposit where the ore was of low grade have not 
been deemed worthy of export. The evidence goes to show that the pro- 
