FULLER : NOTES ON A CARBONIFEROUS BOULDER TRAIN. 257 
Character and Size of Fragments. 
Not only do the size and angularity of the fragments decrease 
rapidly, in a general way, as the distance from the parent ledges 
becomes greater, but their character also simultaneously undergoes 
a considerable change. As previously indicated, the sandstone 
possesses, even in the more compact layers, a prominent lamination, 
while in the softer and more slialy portions a perfect cleavage exists. 
Naturally such portions would offer the least resistance to the 
moving ice and would, therefore, furnish a large percentage of the 
boulders. An examination of the till in the vicinity of the ledges 
shows clearly that this is the case; the thinness and angularity of 
many of the fragments, and the lamination and cleavage of others, 
all point to such an origin. A transportation of two or three miles 
is sufficient to destroy most of the boulders having a perceptible 
cleavage, while a transportation of twice this distance is usually 
sufficient to reduce to small fragments all boulders showing any 
considerable evidences of lamination. At a distance of ten miles, 
only those of a practically uniform texture are to be found. 
Meanwhile the angularity is lost, the rounding becoming more 
and more marked as the distance from the source of the boulders 
increases, until finally, in some cases at least, all traces of the origi¬ 
nal angles are lost, the boulders being as truly rounded as those 
shaped by the action of water. It is possible that this rounding 
of the boulders of the till may not be due entir 
ice, but in part also, as suggested by Professor Shaler (’93, p. 205), 
to that of water by which they may have been taken up and assort¬ 
ed again and again before finally being taken up by the ice for the 
last time. In general, of course, it was found that the pebbles and 
boulders of the modified drift were more perfectly rounded than 
those of the till. The slight rounding of many of the former was 
nevertheless quite noticeable, and probably indicates an englacial 
derivation, the shortness of the interval during which it was sub¬ 
jected to the action of water before its final deposition being the 
cause of the angularity. 
In the study of the Iron Hill train a perfectly regular decrease 
in the size of the boulders was found to occur as the source became 
more distant. This of necessity is true in a general way for all 
trains, but there are sometimes, for short distances at least, marked 
ely to the action of 
