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 2i,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 ej^ochs 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- 



