MUSEUM OF COMPARATIVE ZOOLOGY. 205 
siderable disruption of the glacier, by thus favoring the movements of 
the imprisoned waters, would be likely to bring about the transverse 
scattering of the rock débris. 
When first laid down, after transportation by these currents of water, 
the detrital materials would naturally have the washed and bedded 
character proper to deposits such as occur in kames, but we know by 
observation that it often happens that such accumulations were soon dis- 
rupted by the motion of the glacier, the fragments taken into the mass 
of the ice to be redeposited with the aspect of ordinary till. Much of 
the drift material in Southeastern New England evidently consists of 
débris which has recently been in the form of washed and stratified 
gravels. A careful study of the drift in this section of the country has 
convinced me that by far the greater part of its mass has been at least 
once, and probably again and again, assorted by water before it was 
finally taken into the ice for the last time, to be laid down in the shape 
in which we now find it. It therefore seems to me that we are justified 
in supposing the horizontal dispersion of the materials contained in the 
boulder train from Iron Hill to have been mainly brought about by the 
violent movements of subglacial water. 
Attention has already been called to the fact that the fine débris 
derived from the scoring and polishing of Iron Hill,-and from the com- 
minution of the boulders which are plucked from it, is not distinctly 
recognizable in the path of the boulder train. The evidence of wearing 
afforded by the hill itself clearly shows that at least three fourths of 
the erosion which took place upon its surface delivered the iron ore 
to the glacier in the form of fine sand, such as is. ground out from gla- 
cial striations or worn from the polished surfaces between the grooves. 
Moreover, by far the greater part of the mass of the erratics which were 
plucked from the rock was reduced to a similar state of division by the 
attrition to which the fragments were subjected. If this iron sand 
had been transported in substantially the same manner as the larger 
boulders, we should be entitled to expect evidence of the material in, 
- the path of the trail ; but, as before noted, this comminuted magnetite 
is scarcely, if at all, more abundant in the field occupied by the boul- 
ders of the substance than in the other parts of the country to the 
‘north, east, and west of the train. The only way in which I can ac- 
count for the disappearance of the fine débris is by supposing that it 
was borne away to a considerable distance by the subglacial currents 
of free water. 
Although there is considerable difference in the measure of wear to 
