528 THE ICE AGE IN NORTH AMERICA. 



every subcenter where the snow-fall was larger than the 

 average would, to some extent, make its influence felt upon 

 the shape of the margin. 



Here is a field for the mathematician. When the prop- 

 erties of ice are more fully understood from experimental 

 investigations, and the laws of its fluidity brought under 

 mathematical formulae, it will doubtless be possible, from a 

 study of the contour of the glacial boundary, to calculate the 

 position of all the principal areas of largest precipitation 

 during the Glacial period. 



Those remarkable lobe-like projections in southern Ohio 

 and Indiana, for example, indicate subcenters of accumu- 

 lated ice not far back from the margin. The still more 

 remarkable prolongation of the loops in the kettle-moraine 

 in Wisconsin, and its extension through the States farther 

 west, point, as President Chamberlin sagaciously and cor- 

 rectly supposes, not merely to greater snow-fall over the 

 regions from which the ice-movement radiated, but to the 

 conservative influence of the deeper valleys and depressions 

 to the north, which were filled with ice. These loops of the 

 kettle-moraine sustain a remarkable relation to the valley of 

 Green Bay, and to the northeast and southwest axis of Lake 

 Superior, while the ice-lobe which occupied the valley of the 

 Minnesota and extended to the center of Iowa is evidently 

 related to the great valley of the Ked Kiver of the North. It 

 is not improbable that the depth of ice in such a depression 

 as Lake Superior would, by its very thickness, tend for a long 

 time to increase the snow-fall over its own area, and in other 

 ways to resist the antagonistic agencies which were gradu- 

 ally driving the ice-front back to the north. The driftless 

 area of southwestern Wisconsin is situated just where it es- 

 capes these several ice-movements dominated by the depths 

 of Lake Michigan and Lake Superior, and it is to this day — 

 as Professor Dana has pointed out — a region of light precipi- 

 tation. 



If this discussion of the cause of the Glacial period 

 seems unsatisfactory, the justification is that the present 



