204 THE CANADIAN NATURALIST. [Vol. viii. 



continental glacier produced so small an effect ? In short we 

 have as yet no evidence that either local glaciers or a continental 

 ice-sheet could excavate the basins of the great American lakes. 

 The effect seems more justly attributable to the slow action of a 

 river during part or the whole of the Secondary and Tertiary 

 eras. 



The three lakes, Huron, Erie and Ontario, are therefore on 

 the view here maintained, only the broad portions of the valley 

 of an ancient river, the narrow parts of whose channel were 

 filled up with drift during the glacial era. A glance at the 

 geology of the region confirms this view. The western end of 

 Lake Erie lies on the hard Corniferous limestone, but the greater 

 part of the lake basin is excavated out of the Hamilton and Erie 

 shales, which are comparatively soft. At the eastern end of the 

 lake the Upper Silurian ridge of Niagara limestone crosses the 

 course of the river. The Ontarian basin also is cut out chiefly 

 in the Hudson River shales, while hard rock again ensues be- 

 tween it and the sea. Accordingly we have the gorge at Detroit 

 in the hard limestone, the broad open valleys of Erie and On- 

 tario in the softer shales, and the channel at Niagara between 

 them, worn in the Upper Silurian limestone. It is easy to see 

 that the rate of erosion in the softer rocks must have been 

 limited by the rate at which the hard limestone barriers could 

 be cut down. The river meandered, as is usual with rivers, 

 hither and thither over the wide plain, gradually excavating the 

 valley by cutting down and carrying away the material as fast 

 as the rocky bars were lowered. And when the length of time 

 during which the work may have been in progress is considered, 

 no one familiar with the phenomena of subaerial erosion will 

 deem the cause insufficient. Let any one who doubts reflect on 

 the examples to be found in other parts of the world. Let him 

 turn to south-western Ohio, and see how the hard Niagara lime- 

 stone has been swept away over a wide district, where existing 

 outliers prove that it was formerly present, and by rivers of com- 

 paratively insignificant size — the two Miamis and their tribu- 

 taries. Let him also realize how large a lake would be formed 

 by damming back the Ohio at Cincinnati with a mole one or two 

 hundred feet in height, and we think all difficulty will vanish in 

 admitting the erosion which we here imply, vast as it is, during 

 the time that elapsed from the Carboniferous to the Quaternary 

 era — that is during the whole Secondary and Tertiary ages. 



