MUSEUM OF COMPARATIVE ZOOLOGY. 205 



sideruble 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 debris. 



When first laid down, after transportation by these currents of water, 

 tlie 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 accunuilations 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 

 debris 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 justiiied 

 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 fixct that the fine debris 

 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 b}' 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 boid- 

 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 debris is by supposing that it 

 was borne away to a considerable distance by the subglacial ciu'rents 

 of free water. 



Although there is considerable difference in the measure of wear to 



