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 sight 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 
im the successive advances and recessions of the ice during the closing 
stages of the Glacial Period. We 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 Railway abundant proof that there 
were 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 ftom 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 away 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 angle to those which were antecedently formed. 
It will be easily understood that a uniform shifting in the course of 
