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 



*CroU's Climate and Time, Amer. ed., pp. 76-78, etc. 



