BOULDER BELTS AND BOULDEB TRAINS. 20 



Professor A. Winchell : Some of the phenomena to which President Chamberlin 

 alludes are well known within those regions that have become familiar to my own 

 observation, and particularly within the lower peninsula of Michigan. I have 

 attempted to explain the absence of fragments of Corniferous and Niagara limestones 

 between their northern out-crops and the southern boulder areas by the fact that they 

 are of a calcareous character. We have, for instance, about five hundred definable 

 species and varieties of Archaean boulders, and these boulders have been transported 

 from the regions about Lake Superior, let us say, to the north, and to the south probably. 

 But we have very few boulders derived from the limestones which out-crop in the 

 vicinity of Mackinaw and Drummond Island, and the reason seems to me obvious. 

 The limestones resist the destruction which has been incident to the movement of 

 these boulders far less completely than the Archaean fragments do ; the limestones 

 have been worn out or dissolved, and have disappeared ; but the Archaean boulders 

 have endured the transportation, and hence they are with us. I should think per- 

 haps a consideration of such facts should enter into President Chamberlin's conclusion 

 in reference to currents of boulders that originated from remote points, and those 

 others from the immediate vicinity in which the boulders are discovered. It might 

 be said that there are indeed trains of calcareous fragments, large and small, but 

 particularly small, of a local character that have been derived from the formations 

 over which the glacier has passed within a distance of five or ten miles ; but speaking 

 of boulders of remote transportation, the limestone boulders are few and the Archaean 

 boulders are many. 



Professor G. P. Wright : This paper is of special interest to me because it brings 

 to view familiar phenomena in portions of the country which I have not visited. My 

 own observations have been, to a very considerable extent, on the extreme margin of 

 the glaciated area, and certain phenomena which occur there seem to be analogous, if 

 not altogether identical, with those described by President Chamberlin. What Pro- 

 fessor Lewis and myself denominated the '-fringe " seems to correspond very closely 

 to these bands of boulders in front of the larger deposits. For a time this " fringe " 

 was neglected by us, but as our examination progressed we came to see that there was 

 never, or at least very rarely, a piling up of material at the very margin, but that the 

 piling up occurred somewhat back of the extreme margin. We concluded, both from 

 the nature of the case and from the facts under observation, that the rapidity of 

 motion in the ice, which is well known to be greatest near the middle portion of the 

 current, continually decreased up to the very margin, where of course there was a 

 complete cessation. This would result in what we uniformly found, namely, that 

 there were very generally boulders thrown over to a considerable distance beyond 

 other marks of direct glacial action. The appearance was as if they had been carried 

 over on something corresponding to breakers upon the seashore successively advanc- 

 ing on each other. Probably the advance of the ice-front was interrupted by periods 

 of rest, allowing moraine material to accumulate at various stages of its progress. 

 With every further advance the ice would rise and flow over this moraine and rework 

 the material and drag it along underneath. Finally, at the extreme margin, we have 

 this fringe of boulders from which the ice retreated permanently. If there were 

 periods of cessation in the retreat, wherever a line of equilibrium was established 

 this accumulation of moraine material, with a fringe in front of it, would take place 

 in reverse order and be left for permanent inspection. Thus the bands of boulders 

 of which President Chamberlin has given such an interesting account would seem to 

 be a series of fringes to what I should call the " moraines of retrocession." 



