Unglacial Drift. — Crosby. 
231 
feet of ice has escaped from the ice-sheet through subglacial 
channels; and during its entire subglacial course the main 
body of the englacial drift has been within its reach and 
undergoing modification. This consideration, and the con¬ 
templation of the deposits being made at the present time by 
the Fountain stream, the Yahtse, and other rivers emerging 
from the base of the Malaspina glacier, not to multiply exam¬ 
ples, satisfy me that not only is the modified drift, so far as 
it is being formed at the present time, the product chiefly of 
subglacial streams acting on englacial drift, but that it pro¬ 
bably was so in Pleistocene times. When it is generally recog¬ 
nized that the modified drift requires not one but several 
theories, criteria will, doubtless, be established by which we 
may determine for any normal example whether it has been 
derived chiefly from subglacial, englacial, or superglacial 
drift. 
The Transportation Argument. 
Notwithstanding the abundant and indubitable evidence 
that a small part of the drift of the Pleistocene ice-sheet is 
far-travelled, it is generally conceded that the great bulk of 
the drift is of relatively local origin; and good authorities 
hold that this is substantially true for the modified drift as 
well as till. My own studies in the Boston basin have satis¬ 
fied me, however, that the modified drift and till of this region 
are somewhat contrasted in this respect, though perhaps not 
more than we should expect, considering that the modified 
drift was transported by water as well as by ice. For exam¬ 
ple, with the aid of several students in the Massachusetts In¬ 
stitute of Technology, I examined the composition of a prom¬ 
inent esker on the northwest shore of Weymouth. North of 
this point in the line of glacial movement are three broad 
belts of rocks: first, slates and conglomerates of the Boston 
basin (Carboniferous), about thirteen miles; second, horn- 
blendic granites, diorite and felsite, with some Cambrian slate 
and quartzite, eight to ten miles; third, mica schists, musco¬ 
vite granites and gneiss, pegmatite, etc., extending into New 
Hampshire. We found, on looking over some tons of mate¬ 
rial, that of all which was coarse enough for easy identifica¬ 
tion about 50 per cent, is from the first belt, 40 per cent, from 
the second, and 10 per cent, from the third. Subsequently at 
