206 BULLETIN OF THE 



which all glacial material is subjected, the debris from Iron Hill affords 

 the best indication of such variation which I have ever found. It is a 

 noticeable fact, that within half a mile of the source of the material we 

 occasionally find boulders which have been completely rounded com- 

 mingled with others which have been subjected to such slight attrition 

 that their original form and size has hardly been altered at all. Pro- 

 ceeding down the train, we note the fact that gradually all the fragments 

 become more and more rounded, but even at thirty-five miles away some 

 of the bits appear to have retained some share of their original outline. 

 It is true that the angularity of these fragments may in part be at- 

 tributed to the successive fracturing to which they have been sub- 

 jected, but for some miles from the source it is evident that many of 

 the erratics have been in a manner preserved from attrition with the 

 bed rock, or against other moving fragments. This has probably been 

 brought about by a process which uplifted the well preserved erratics 

 into the bodv of the ice. 



Before passing from this part of our inquiry, it is woi-th while to note 

 the striking contrast exhibited by this train of peridotite boulders as 

 compared with certain other trains of softer material which are traceable 

 in this section. In the region to the southwest of Iron Hill in the town 

 of Smithfield, R. 1., there are several outcrops of a crystalline limestone 

 which are sufficiently limited in area to aff'ord distinct boulder trains. 

 Although this highly metamorphosed limestone has the hardness of 

 ordinary marble, and by its structure afl'ords erratics which are on the 

 average larger than those plucked out from the ice at Iron Hill, the 

 trains which are formed of it cannot be traced for more than five or six 

 miles to the southward of the outcrops. We thus perceive the measure 

 in which the singular hardness of the rock from Iron Hill fixvors the 

 preservation of the boulders derived from that locality, which has been 

 able to journey more than ten times as far as the hard marbles of the 

 Smithfield district. 



I may allude, in passing, to the fact that the relative hardness of the 

 bed rock of any district, as compared with that of the fragments borne 

 over it by glaciers from other fields, is of much importance when we 

 seek to explain the distribution of glacial drift. Where the bed rock of 

 any locality is hard, and the rocks lying just above it in the path of the 

 glacial flow are soft, we generally find the surface of these hard ma- 

 terials occupied by little coarse detrital Avaste, and this for the reason 

 that the fragments are readily ground out against the nether millstone. 

 If the conditions are reversed, and the rocks from which the glacier came 



