226 THE CANADIAN NATURALIST. [Vol. ix. 



And with yet stronger reason we may ask, " Is it possible that 

 similar ice-tongues — a mere fringe of the great Canadian ice-sheet 

 — could cut out the valleys of Michigan and Huron 800 and 900 

 feet below their present water surface and more than 1500 feet 

 below much of the surrounding land, and thus perform from 

 sixteen to thirty times as much work as the massive grinding 

 continental ice-sheets have performed ? " Surely this is varying 

 the effect inversely with its cause. 



Summing up the results thus obtained we find : — . 



1st, That the objection founded on the beaded nature of the 

 old Mohawk valley is overruled by high authority in Physical 

 Geography and by the phenomena of existing rivers. 



2nd. That the objection drawn from the great and unequal 

 depth of the lake-beds is answered by appealing to the elevation 

 of the northern part of the continent at the time referred to. 



3rd. The same reply meets the objection drawn from the want 

 of sufficient fall for the old river in question. 



4th. That Dr. Newberry's own theory of the origin of these 

 lake-beds is open to the following very serious objections, if not 

 indeed altogether untenable. 



a. The evidence afforded by glacial striae in the bed of Lake 

 Erie is not sufficient to prove the excavation ot the whole of that 

 valley by the action of ice. 



b. The same is true of similar markings in the bed of Lake 

 Ontario. 



c. Instead of attributing to the action of local glaciers the 

 excavation of the deep vales of Michigan, Erie, Huron and 

 Ontario, we are compelled to assume the pre- existence of these 

 valleys as the only possible cause of the local glaciers ; for the ex- 

 istence of such local glaciers without the lake-beds, and perhaj^s 

 with thenij is incompatible with the laws of the motion of ice. 



d. We cannot assume the erosion of the great continental ice- 

 sheet at more than 50 feet over the lake district. 



e. Granting for the sake of argument the existence of these 

 local glaciers it is utterly impossible to suppose them capable of 

 eroding broad valleys 500 to 1000 feet deep, for this would be to 

 suppose them capable of accomplistiing many times as much 

 work as their more massive continental predecessor had per- 

 formed. 



