MODIFIED DRIFT IN NEW HAMPSHIRE. 7 
with the distant Eozoic and Paleozoic past, in which the rocky strata 
of New Hampshire were deposited beneath the sea and upheaved in 
crumpled folds to form our hills and mountains. 
The theory of Mr. Croll,* which supposes that during the long period 
of great eccentricity glacial and warm inter-glacial epochs succeeded 
each other in cycles of 21,000 years, does not seem to be sustained so 
fully as we should expect by evidence of such warm intervals, which he 
thinks even in arctic latitudes would be nearly free from ice and snow. 
A consideration of what we have to explain by the agency of ice, and of 
the mode in which these results are likely to have been produced, seems 
to point to a very long, continuous period of glacial action, with times of 
retreat and advance, but not apparently of complete departure and return 
of a continental ice-sheet. By other writers the glacial climate is be- 
lieved to have been principally caused by a different distribution and 
elevation of the land, attended with changes in the direction of oceanic 
currents. Even if a supposed combination of such conditions could be 
shown to be adequate to produce the ice-sheet, it seems more reasonable 
to attribute its origin to an astronomical cause, which we know to have 
existed, with a tendency to bring about these results. As very intense 
cold is not required for the accumulation and preservation of snow and 
ice, may not the continually cool climate, when winter occurred in peri- 
helion during the period of great eccentricity, have kept the ice-sheet 
which was already formed from being melted? The rare testimony of 
any retreat and subsequent advance of the ice during the glacial period 
in America, with the vast results which were accomplished in this time, 
favor this view. 
The motion of the ice, being produced by the pressure of its own 
weight, and extending immense distances over a comparatively level but 
very irregular surface, must have been exceedingly slow. The average 
yearly progress of the glaciers of the Alps is about three hundred feet. 
The continental glacier, which striated the northern United States and 
Canada, must have had a much less slope. If its upper surface de- 
scended only one foot in two hundred, which in this state is consid- 
ered a very moderate railroad grade, the ice would increase one mile in 
*Croll’s Climate and Time, Amer. ed., pp. 76-78, etc. 
