8i8 THE POPULAR SCIENCE MONTHLY. 



Iceland as part of tlie general rise on the American and Euro- 

 pean sides this would account for greater precipitation on the 

 borders of the ocean, and especially over its western border, the 

 American. 



But leaving this source of increased precipitation out of con- 

 sideration, it is plain that in the Glacial period the difference in 

 amount of precipitation over the high eastern border made into a 

 lofty ice plateau by the accumulation of snow and ice, and over 

 the broad medial belt from Wisconsin and Minnesota northwest- 

 ward, should have been much greater than it is now. Moreover, 

 this central valley of North America would have had something 

 of the existing disadvantage of a relatively warm summer tem- 

 perature. At the present time, in July, a mean temperature of 70 

 Fahr. extends beyond the latitude of Lake Winnipeg even to 56 

 north, and this is 10 in latitude, or nearly seven hundred miles, 

 farther north than the position of the same heat line over New 

 England. 



The advantages for ice-making of eastern over central North 

 America were, therefore, very great, both as regards temperature 

 and precipitation. When the conditions over the interior were 

 sufficient to jjroduce a small annual gain of ice, those over New 

 England would have been making a very large annual gain. A 

 small gain continued for many scores of centuries would make 

 finally a great thickness of ice. But with the conditions over the 

 interior near the critical point, a small unfavorable meteorological 

 change, if long continued, might carry off the ice for scores or 

 hundreds of miles from a southern limit, with proportionate floods 

 from the melting, while the eastern border was all the time gain- 

 ing in ice, or was making only a short retreat. 



The actual facts correspond with these views. The distance 

 in the upper Mississippi basin between the farthest southern limit 

 of the ice and the line of the great moraine, or that of the so- 

 called "second Glacial epoch," is over five hundred miles; but to 

 the eastward it narrows through Indiana, Ohio, Pennsylvania, 

 and New Jersey ; and in New England a corresponding moraine 

 interval can not be certainly made out, and nothing exists that 

 could not be better explained by reference to short retreats in a 

 single glacier. 



I leave the subjects here for the consideration of geologists of 

 the East and West. The cause appealed to explains at least why 

 the geologists of the East and West are divided on the subject ; 

 and also why the grand display of terminal and retreat mo- 

 raines characterizing the West produces there the stronger opin- 

 ions and the stronger expressions of opinions; and why also a 

 complete survey of the facts will probably lead to a general 

 agreement in favor of a single Glacial epoch only. 



