7 
200 BULLETIN OF THE 
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general be assumed that the resistance which rocks of this intensely 
hard nature oppose to crushing strains is inversely as their size, and in 
this way we may account for the rapid manner in which most of the 
observed boulder trains disappear, so that they are not traceable for more 
than a few miles from their origin. 
After observing the deficiency of small pebbles of material, derived 
from Iron Hill, I endeavored to trace the comminuted material within 
and near the evident path of the trail by determining the quantity of 
magnetic sand which the glacial deposits afforded. Examinations for 
this purpose were made at several hundred points by carefully separating 
_ the grains of magnetite attracted by a horseshoe magnet from the other 
débris with which it was mingled. At first it seemed likely that this 
method of inquiry would give important results, for it was at once made 
evident that the till material of the district contained from two to six 
per cent by weight of magnetic sand. It soon, however, became certain 
that this material could not be accepted as an indication of the trail, for 
an extended search showed that something like this amount of magnetic 
sand commonly exists in the glacial waste derived from the metamor- 
phosed schistose rocks of Southern New England. ‘The fact is that these 
ancient strata throughout contain a notable percentage of crystalline 
magnetite. An effort to discriminate the fine-grained material from Iron 
Hill by the amount of titanium it contained also proved fruitless, for the 
reason that this substance very commonly occurs as an ingredient in the 
ferruginous sands of the drift. Thus the result of about a hundred 
assays made by my assistant, Mr. Robert Robertson, was purely nega- 
tive, so far as the study of the boulder train was concerned, though it 
served to throw a good deal of light on the mineralogical constitution of 
our glacial deposits. It proved that this heavy and little oxidizable iron 
ore is in a measure concentrated by the actions which have brought 
about the formation of our glacial deposits. 
I have now set forth the most important features concerning this train 
which are not made sufficiently evident by the delineation of its path on 
the map. I next propose to make these facts the basis for some consid- 
erations as to the nature of the actions which distributed the material 
over the surface between Iron Hill and Martha’s Vineyard. 
CAUSE OF THE FANNING OUT OF THE TRAIN. 
I have already noted the fact that the Iron Hill boulder train widens 
from its source to the sea, or for a distance of about thirty-five miles, at 
