MUSEUM OF COMPARATIVE ZOOLOGY. 201 



the rate of about a thousand feet for the mile of distance. It is evi- 

 dently a difficult matter to determine the cause of this remarkable 

 feature. At first siglit it seemed to me possible that the result was due 

 to the action of floating ice, operating during the retreat of the glacier, 

 and more or less aided by the action of the ocean waves during the 

 submergence which apparently continued in this field throughout the 

 closing stages of the Ice Period ; but the fact that the fragments with 

 rare exceptions are found in the till renders it difficult, if not impos- 

 sible to accept this explanation. Moreover, the gradual widening of 

 the trail at the uniform rate above stated seems of itself to make this 

 hypothesis quite untenable. 



There is another apparently possible cause of dispersion to be found 

 in the successive advances and recessions of the ice during the closing 

 stages of the Glacial Period. "VVe might conceive that the successive 

 forward movements varied somewhat in direction, and that the waste 

 from Iron Hill might thus have been shoved about so as to widen the 

 field which it occupied. In the region in which this boulder train lies 

 there is no distinct evidence of such successive movements of advance 

 and retreat of the ice, but in a region about twenty miles to the east of 

 this line, in the valley of the Taunton River, we find from the sections 

 along the line of the Old Colony Piailway abundant proof that there 

 Avere many successive, though slight movements of this nature. In a 

 portion of the valley of the above named river corresponding in length 

 and position to that extending from Iron Hill to Providence, there is 

 good evidence of at least six of these successive movements of advance 

 and retreat. These oscillations were slight and temporary, as is shown 

 by the fact that the ice in each southward going did not clear awaj' the 

 previously formed incoherent deposits, nor were there any distinct 

 frontal moraines formed at the margin of the ice field. The facts indi- 

 cate that these variations in the position of the ice front amounted to 

 only a few hundred feet of distance in the axis of the motion. In these 

 successive advances and recessions of the glacial margin there may pos- 

 sibly have been some alterations in the direction of the ice flow. It is 

 a well observed fact that the margin of a glacier, if the ice sheet have 

 a considerable front, is apt from time to time to put forth lobes which 

 push forward in directions somewhat independent of the general course 

 of the ice field. Moreover, we often find, in a district where the glacial 

 scratches are well preserved, that the last scorings inflicted on the rock 

 lie at a considerable ansjle to those which were antecedentlv formed. 



It will he easily understood that a uniform shifting in the course of 



