200 BULLETIN OF THE 



sreucral 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 wliich 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 

 debris 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, howevei', 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 b}' 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 f;ir 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 Vinej^ard. 



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 di.stance of about thirty-five miles, at 



