_ 1872.] | 105 (Perry. 
amount of disengaged matter. They were also likely to take not a 
little from the sides of the mountains that chanced to skirt the val- 
leys which they traversed. 
During the Middle Glacial times, the wok must have gone on 
with even greater vigor. The vast accumulations of ice, spreading 
far and wide, of great thickness, would abrade the surface at 
large, and in some respects tend to plough deeper furrows than in 
the earlier portion of the cold period. Nearly every height being 
covered by the broad mantle of ice, the material must have been 
mainly furnished from the surface of the underlying solid rocks. 
This matter, as disengaged from the subjacent rocky floor, might be 
borne along, and to a considerable extent broken and ground to 
powder, by the immense rock-crusher that slowly and with irresisti- 
ble force moved over the country. There would thus be a large 
amount of detrital material especially beneath, and to some extent 
in front of the great ice-sheet.? 
On the final wasting of the stupendous mountain of ice, one might 
expect all this matter to be left in a confused jumble, overlying the 
surface of almost the entire region. Such a mass of material now 
covers nearly all New England. It must have been likewise spread, 
very largely, over what is to-day the submarine border of this por- 
tion of the continent, the level of the sea perhaps being at that time 
still somewhat depressed. Accordingly, the submarine valleys, ex- 
tending from the mouths of the Hudson and of the Connecticut, 
would be, as they are now found, overspread. by typical drift. 
But this leads me to notice what seems to be a misconception, on 
the part of Professor Dana, as to actual facts in the production and 
distribution of drift. He evidently has the impression, that the 
boulders and comminuted material which were abraded from the 
underlying rocks, were taken up into the body of the ice, and only 
dispersed over the surface of the country, in connection with its final 
melting. Referring to the abrasion of the subjacent surface, he says 
1The statements in the above paragraph suggest the prevailing marks of the 
yast blanket of matter underlying the glacial mass in a comparatively level coun- 
try like ours, and wherein it is likely to differ from the drift cf a region traversed 
by lofty mountains. In confirmation of this, I take pleasure in quoting from one 
of the lectures of Professor Agassiz:—‘‘ The characteristic difference of the drift in 
North America, as compared to that of Europe, is that here, owing to the absence 
of high mountain chains, the drift is almost entirely sub-glacial, 7. e., rounded, 
polished, and scratched, and the supra-glacial angular boulders are comparatively 
few, and mainly found in the vicinity of our higher hills (the White Mountains, 
for instance).”’ 
