196 BULLETIN OF THE 



cess of plucking out boulders from this exceedingly firm set rock contin- 

 ued down to the very time when the ice ceased to act upon the surface. 

 I have found these facts very difficult to reconcile with tlie common 

 view as to the circumstances under which the glacier passed away from 

 this part of the country. The opinion is, in effect, that the ice 

 gradually lost its energy of movement, and slowly came to an inactive 

 or stagnant state. 



The evidence as to the suddenness with which glacial action ceased 

 at this point is paralleled at many other places in New England which 

 lie at elevations within a few hundred feet of the shore line. It seems 

 to me that this feature may be accounted for on the supposition that 

 this region was considerably depressed beneath the sea at the time 

 when the ice lay over it, and that the glacial mass was not removed by 

 gradual melting, but floated away in the form of icebergs. The transi- 

 tion from the conditions of the ice sheet between the time when it 

 rested on and moved over the surface to that in which it disappeared 

 in the form of icebergs, may well have been brought about suddenly 

 by the progressive melting of the mass. I cannot discuss this hypoth- 

 esis as to the conditions under which the glaciers of this district ceased 

 to act, but I may be permitted to remark that this view seems more 

 consonant with the evidence than that which holds the glaciers in this 

 part of the country to have gradually stagnated and passed away by 

 sub-aerial decay. 



Detailed Description of the Iron Hill Boulder Train. 



The general course of this boulder train, and its relations to the sur- 

 face on which it lies, are fairly well shown by the map which accom- 

 panies this paper. The most striking feature of the train is the pro- 

 gressive widening of the belt of country which it occupies from its 

 source to the sea. At the point of origin the train is not over 900 feet 

 in width, and saving a few small scattered blocks which may owe their 

 dispersion to the action of sea waves during a post-glacial submergence 

 or to the interference of man, the width of the trail at its source does 

 not exceed 700 feet ; yet near Providence, R. I., a point about 75,000 

 feet south of Iron Hill, it has widened so that the distance between its 

 borders slightly exceeds 20,000 feet, and at the sea the belt occupied 

 by the Frratics is probably more than 40,000 feet in width. 



Throughout its extent this boulder train is in its central part most 

 distinctly marked by the erratics. From this middle line the boulders 



