Mississippi basin in the Glacial period. 329 



northward, as well as on the Pacific side, the continent stood 

 ranch above its present level, and that the elevation was the 

 culmination of that which was in progress during the closing 

 part of the Tertiary era, — as urged by Prof. Upham. How- 

 ever much the surface of the great medial valley of the conti- 

 nent was raised, it cannot be reasonably questioned that the 

 border mountain regions experienced the greater amount of 

 elevation. Hence, with the mountain condensers on the east 

 so much increased in altitude and extent, the differences be- 

 tween the eastern and interior regions as to precipitation would 

 have been greatly augmented, to the advantage of the eastern 

 region. 



Further, the Glacial period was probably a time of greater 

 precipitation than now, as well as of greater cold. Some have 

 said, of greater precipitation, and not of greater cold ; but the 

 former of these two statements has general acceptance. If the 

 surface waters of the Atlantic basin were warmer than now — 

 owing to a rise of land along a belt from southeast to north- 

 west through Iceland as part of the general rise on the Ameri- 

 can and European sides — this would account for greater pre- 

 cipitation 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 

 northwestward, 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 temperature. At the present time, in July, a 

 mean temperature of 70° F. extends beyond the latitude of 

 Lake Winnipeg even to 56° N., and this is 10° in latitude, or 

 nearly 700 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 tempera- 

 ture and precipitation. When the conditions over the interior 

 were sufficient to produce a small annual gain of ice, those 

 over Xew England would have been making a very large 

 annual gain. A small gain continued for many scores of cen- 

 turies 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, 



