FULLER : NOTES ON A CARBONIFEROUS BOULDER TRAIN. 259 
Of course the assumption that the erosion in traversing a given 
distance is the same in the case of the boulders of different sizes 
mentioned above, is not to be maintained. It is manifest, however, 
that the amount of surface exposed to erosion in proportion to bulk 
is less in large fragments than in small. This, in connection with the 
fact that erosion of fragments depends almost wholly on factors 
foreign to themselves and is therefore independent of size, indicates 
that the principle of decrease in size, as shown in the above table, 
holds good, though not to the full extent indicated by the figures. 
That the same relation holds good in the deposits of modified 
drift was well brought out in an examination made by Professor 
Crosby and myself, with several others, of the composition of a 
prominent esker on the northeast shore of Weymouth. Quoting from 
Professor Crosby’s description (’96, p. 142), we find that, “North of 
this point in the line of glacial movement are three broad belts of 
rocks : First, slates and conglomerates of the Boston Basin (Carbonif¬ 
erous) about thirteen miles; second, hornblendic granites, diorite, and 
felsite, with some Cambrian slate and quartzite, eight to ten miles; 
third, mica schists, muscovite granites and gneiss, pegmatite, etc., 
extending into New Hampshire. We found, on looking over some 
tons of material, that of all which was coarse enough for easy identifi¬ 
cation, about 50 per cent is from the first belt, 40 per cent from the 
second, and 10 per cent from the third.” The average size of the 
pebbles and boulders, however, varied, as in the case previously 
described, inversely with the distance from which they were 
derived. 
The greater regularity in the decrease of size in the case of the 
Rhode Island train is undoubtedly to be explained by the greater 
homogeneity of the iron ore, and the consequent slighter differences 
in the size of the fragments furnished. A regular jointing would also 
be favorable to a supply of fragments of similar size. 
The results which I find to hold good as to the absolute size of the 
pebbles are also at considerable variance with the results obtained 
from the study of the Cumberland train, in which, according to 
Professor Slialer, no pebbles were found, in the till at least, having a 
diameter of less than an inch. This is certainly not the case in the 
train under discussion. In the many sections of till which I examined 
within the area of the sandstone train, T found not only an abundance 
of pebbles under an inch in diameter, but many that are less than 
half that size, while not infrequently they are found with diameters 
