38 The American Geologist. J"^^' ^^^-■ 
many millions of cubic yards of sand, gravel and bowlders 
have been delivered through a single narrow channel, not to 
mention the still greater volumes of clay and quartz flour which 
we know must once have been incorporated with the coarser 
detritus. If the subglacial hypothesis be true, the region to the 
north of some of our extensive sand-plains ought to have been, 
in large part, swept bare of till or ground moraine ; but we 
do not find it so ; and it could not be so, as long as the ice-sheet 
rested upon and protected it. 
Davis concluded that the Newtonville and Auburndale 
eskers must be products of subglacial streams acting on sub- 
glacial drift, because they contain fragments of slate and con- 
glomerate which must have been derived from the ledges with- 
in two to four miles to the northward ; and he thinks it improb- 
able that englacial drift could, in so short distance, rise to the 
level of superglacial streams. The proportion of material 
from near-by sources is, however, very small, certainly not 
more than ten and possibly not more than five per cent. ; and 
the elevation of this small fraction of the drift a hundred feet 
or so in, say, three miles does not impress me as offering any 
special difficulty. Still farther within the Boston basin, as in the 
vicinity of Newton Upper Falls and West Roxbury, at dis- 
tances of from four to eight miles from the granitic northern 
border of the basin, the proportion of drift of local origin 
is still very small, the granite rocks from the northern high- 
lands constituting from 90 to 99 per cent, of the whole ; 
while neighboring sections of till show, as usual, that it is 
mainlv of strictly local origin. As I have previously stated, 
this contrast is what we should expect in any case, since the 
modified drift has been transported by water as well as by ice; 
but it is also evidence, if not complete proof, that its glacial 
transportation was in part englacial. In the eskers to which 
Davis particularly refers, the detritus of relatively local origin 
is found near the top as well as near the bottom of the sec- 
tion, though perhaps less abundantly ; and that detritus is sure- 
ly even more of a difficulty for the subglacial than for the 
superglacial hypothesis, since it presupposes that the sub- 
glacial stream could erode the bed-rock or till above which it 
had already aggraded its channel fifty to one hundred feet. 
XoTE. — Since this paper was written, I have learned that to 
Prof. N. H. Winchell properly belongs the credit of first sug- 
