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 the 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 generally 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 New 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 more 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 hay- 
ing « 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 that 
their apparent 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 
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