8 SURFACE GEOLOGY. 
thickness for every two hundred miles that we advance towards the head 
of its outflow at the north. Over the highlands between the St. Law- 
rence valley and Hudson bay it would have been three or four miles 
in depth, and at the same time probably much deeper over Greenland. 
Even with this vast accumulation of ice we have so gentle a slope to 
produce its motion that we can scarcely suppose this progress, at least 
in its lower portion, which passed over the very uneven surface of the 
land, to have exceeded one twelfth that of the glaciers in the Alps. This 
would give us an advance of twenty-five feet yearly, requiring 21,000 
years to move one hundred miles. If these conclusions are any ap- 
proximation to the truth, the highest rate of motion which could be 
attained by the ice-sheet at its greatest depth, continuing through half 
of this time, would seem quite inadequate to plough up and remove 
the extensive and thick deposits of stratified gravel, sand, and clay 
which we now find in New Hampshire, so that scarcely any traces of 
them would remain. Similar deposits of modified drift would have been 
formed at each melting away of the ice; and their almost complete 
removal in the epochs during which this theory supposes the ice-sheet to 
prevail seems improbable, when we consider the slowness of its motion. 
The accumulation of the vast thickness of ice which must have existed 
at the north, probably amounting to twenty thousand feet, seems also to 
require a longer time than Mr. Croll’s theory allows. The average rain- 
fall of New England is about three and a half feet, three fifths of which 
are evaporated from the surface, while two fifths flow to the sea. This 
rain-fall exceeds that of the continent northward and westward. Proba- 
bly it was from a precipitation of snow and rain of no greater amount 
that the ice-sheet increased in thickness from year to year. Melting and 
evaporation must have removed a large portion of this; and an annual 
addition of two feet of ice seems to be too high an estimate. The forma- 
tion of the ice-sheet would thus occupy all the time through which it is 
supposed to act in any single glacial epoch. 
Another consideration which adds to the probability that the ice-sheet 
continued through the whole period of great eccentricity, being princi- 
pally formed in the successive epochs when the winters occurred near 
aphelion, but not disappearing when winters fell at perihelion, is found 
in the great elevation of these ice-fields which over the White Moun- 
