206 BULLETIN OF THE 
which all glacial material is subjected, the débris from Iron Hill affords 
the best indication of such variation which I have ever found. Itisa 
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 havé 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 body of the ice. 
Before passing from this part of our inquiry, it is worth 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. I.; there are several outcrops of a crystalline limestone 
which are sufficiently limited in area to afford distinct boulder trains. 
Although this highly metamorphosed limestone has the hardness of 
ordinary marble, and by its structure affords 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 favors 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 waste, 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 
