1872.] 119 [Perry. 
We accordingly find in a continental ice-sheet an instrument fully 
able to plane, smooth and striate the rocky floor of the country as it 
now appears, and thus to account for the debris in its manifold forms, 
as almost everywhere encountered in greater or less abundance. ‘The 
various different phases of the subject, which have been just brought 
into view—the manifold facts connected with the underlying rocks, 
with the overlying material, and the incidental phenomena referred 
to, point by point—while they receive little confirmation from the ice- 
berg hypothesis, while some of them utterly fail of an intelligible ex- 
planation in all the light we have been able to get from it, seem per- 
fectly natural on the supposition that they were brought about in 
connection with glacier-agency. In fact they all, so far as appears, 
are in every essential respect just what one may presume they would 
have been, in case the region were at the time covered by an immense 
mass of ice. And the same is substantially true in regard to the rel- 
ative duration of the Glacial period. Evidence is already found, and 
it is constantly aceumulating, of a kind and in a way, to show that 
the time was very protracted ; that it was much longer than has been 
usually supposed ; indeed, that it was far more extended than even 
geologists have been generally inclined to believe. 
§16. The Dissolution of the Ice-Sheet. 
The time finally came, as we have seen, when the vast mass of ice 
began to melt, especially on its southern limits, and thus steadily to 
recede, so as to occupy only the more northern portions of the coun- 
try. On the whole, however, this recession must have been very 
gradual, and therefore the field of ice a long time in taking its de- 
parture even from New England, to say nothing of the whole north- 
ern portion of the continent. The face of the region suggests that 
there was a comparatively rapid retreat from Long Island to Middle- 
town, Connecticut ; that there a high wall of ice was for a long while 
nearly stationary; that the next retreat was to Mount Holyoke; that 
thence, after a protracted halt in that neighborhood, the front of the 
glacial mass receded to Turner’s Falls; that thus by recessions, suc- 
ceeded by delays, it retreated to Brattleboro, to Bellows Falls, to 
Charleston, to Windsor, and so on until it reached the neighborhood 
of the White Mountains, which occasioned a long-protracted rest. 
Thus there were probably stages of progress, and places of repose; 
In given ‘nstasces retreats may have been made by gigantic strides, 
follo~ delevs of great duration ; for in some neighborhoods, no 
