198 BULLETIN OF THE 



movement of the rocks from the central path of the train is about one 

 tenth of the direct forward movement. 



At the source of the train the peridotite boulders are plentifully in- 

 termingled with, and to a certain extent overlaid by, materials brought 

 from the region which lies to the northward of Iron Hill. It is evident 

 that, these schistose rocks yielded an abundance of erratics, and it is 

 therefore not surprising to find that even at the outset the train has 

 not more than one tenth of its mass made up of the fragments of ore. 

 The remarkable fact is that the boulders of peridotite rather plentifully 

 occur on the surface of the drift in the section immediately to the 

 south of the place whence they were derived. This thorough com- 

 mingling of the debris from different parts of the erosion field clearly 

 indicates that there was some feature of movement in the ice which 

 tended to comingle the detritus which it bore, and to effect the com- 

 bination in a very rapid manner. In other words, the vertical scatter- 

 ing of the fragments in the body of the glacier evidently went on 

 simultaneously with the horizontal dispersion which brought about the 

 widening of the belt over which they were dispersed. At the distance 

 of not more than half a mile from the source of supply the intermixture 

 of the peridotite boulders with those derived from the bed rock appears 

 to be complete. The occasional artificial sections in road cuttings and 

 wells show this feature in a clear manner. 



It should be noted that the average distance between the pebbles of 

 ore rapidly increases as we depart from the source of the ti-ain, and that 

 this increase is accompanied by a somewhat proportionate diminution 

 in the size of the fragments. This is approximately represented in the 

 following table, which assembles the observations made on many thou- 

 sand erratics in the section of the train between Iron Hill and Provi- 

 dence. South of that point, as before remarked, the greater part of 

 the trail is covered by the waters of Narragansett Bay. 



Average Diameter. Largest Fragments obserred. 



3 feet. 5 feet. 



2 " 6 in. 4 " 



1 «' 6 " 3 " 



1 foot. 2 " 



10 in. 1 foot 6 in. 



9 " 1 " 3 " 



8 " 1 " 2 " 

 7 " 12 " 



It should be observed that this table must not be taken as repre- 

 senting accurately determined data at the several points on the train 



