282 
The American Geologist , 
April, 1896 
points only two to live miles from the northern edge of the 
Boston basin, I found the proportion of material from the 
first belt in the modified drift very small, 10 per cent, or less. 
Hence it is probably safe to assume that more than half of 
the coarser material of the modified drift of the Boston basin 
is five to ten miles from its source, and a good fraction as 
much as twenty miles. 
But conceding that the readily identifiable constituents of 
the drift, whether modified or unmodified, are chiefly of dis¬ 
tinctly local origin, it may still be doubted whether much 
weight should be attached to this fact as an argument against 
the view that practically the entire volume of the drift was 
englacial in the earlier and maximum stages of the ice-sheet. 
For the purpose of this discussion, the drift may be divided 
into three parts: first, the preglacial detritus, which must 
have been chiefly of a residuary and clayey character and 
highly oxidized (red and yellow), like the residuary soils of 
the South; second, the finer products of glacial erosion, rock 
flour, etc., formed chiefly on stoss slopes and for the most part 
unoxidized; third, the coarser part of the drift, the identifi¬ 
able rock fragments, which must be almost wholly of glacial 
origin and derived chiefly from the lee slopes. 
The preglacial residuary and sedentary soil was probably 
partly swept away by aqueous erosion during the elevation of 
the continent and before the formation of the ice-sheet. What 
was left of it probably became incorporated with the ice-sheet 
in its earliest stage; and we may well suppose that during 
the various vicissitudes of the ice-sheet, and through the 
cooperation or alternation of glacial, lacustrine and fluvial 
transportation, it has been carried in large part beyond the 
limits of the glaciated area. Certainly there is little indica¬ 
tion of its presence in the composition of the drift; and ex¬ 
periment shows that an admixture of a very small proportion 
of highly oxidized residuary clay, like that of the South, with 
a typical till is readily detected in the change of color. It is 
a natural suggestion, therefore, that the Lafayette and Co¬ 
lumbia formations of the South have been derived, along 
their northern borders, in part from the preglacial residuary 
soils of the North. The finely comminuted and unoxidized 
glacial detritus constitutes now the basis or matrix of the till 
