MUSEUM OF COMPARATIVE ZOOLOGY. 199 



designated by the distance from the source. It indicates, however, in a 

 diagrammatic way, the average of the observations, and may be taken 

 as a fair approximation to tlie facts. In this connection it is well to 

 note that one of the three fragments of ilmenite which have been found 

 on the western extremity of Martha's Vineyard had a mean diameter of 

 about five inches. 



The greater part of the fragments of peridotite which constitute the 

 Iron Hill boulder train are contained in the till deposits which occur in 

 its path. All those of large size are found in the drift of this nature. 

 A few smaller bits, however, and these genei-ally much rounded, have 

 been discovered in the washed gravels along the shores of Narragansett 

 Bay. The infrequency of the occurrence of this material in the stratified 

 deposits of glacial waste is doubtless due to the fact, which has been well 

 observed in this part of Xew^ England, that these water-borne sands and 

 gravels have always been conveyed from a much greater average distance 

 than the matter contained in the till or ground moraine. Observations 

 which I have made in this field indicate that the average distance to 

 which the ordinary rocks in the till have been transported is probably 

 not more than three miles, while the mean carriage of the stratified 

 materials is at least four times as great. 



A careful inspection of the fragments observed in the trail shows that 

 the diminution in size of the erratics in their southward journey has 

 been mainly due to attrition. Not one per cent of the fragments 

 indicate by their form that they have been subjected to division along 

 joint planes since they parted from the original bedding. As they de- 

 crease in diameter they become more and moi-e spherical or subovate in 

 form, until they attain a size no greater than an ordinary billiard ball. 

 It is a noteworthy fact that none of these bits have been found hav- 

 ing a diameter of less than an inch, and the number of those which 

 approach this size is remarkably small. As a deliberate and extended 

 search has been made for these smaller fragments, it seems likely thnt 

 their appareiit absence is not due to their inconspicuousness. I am in- 

 clined to explain it by the supposition that the smaller the erratic the 

 more likely it is to be crushed into fragments by the rude strains which 

 have served to round the larger bits. It is evident that the resistance 

 to pressure arising from being squeezed between other boulders, or be- 

 tween erratics and the bed rock, is in a measure proportional to the 

 diameter of the pebble. A strain which would be withstood by a frag- 

 ment six inches in diameter might crush to the state of powder one 

 which contained only two or three cubic inches of material. It may in 



VOL. XVI. — NO 11. 2 



