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. In 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 manner 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- 



