224 THE CANADIAN NATURALIST. [Vol. ix. 



of the glacier. Accordingly we find the greater part of the drift 

 in any county consists of material brought from the counties to 

 the northward, mixed with a smaller quantity from a greater 

 distance, and some metamorphic boulders or pebbles from Ca- 

 nada. It becomes thinner as we go southward, probably because 

 the propelling power of the ice became less with decreasing thick- 

 ness. By noting the depth of the glacial drift in the northern 

 part of the State, therefore we take it at its maximum, and we 

 deal with material brought for the most part from the region of 

 the great lakes. We can thus obtain approximately the amount 

 of erosion which that region suffered during the glacial era. The 

 following figures from the Geological Survey of Ohio show us 

 the thickness of the drift through the three northern tiers of 



♦This thickness is measured in the deeply excavated and buried channel of 

 the Cuayhoga, and is therefore far above the average. 



These figures shew plainly that we have no grounds for making 

 any enormous estimates of the erosion of our State and the lake 



