Spencer,] '^^^ [March 18, 



The streams ran from the north and west of Lake Huron, south-eastward 

 towards Goderich ; thence southward, and entered the Au Sable near its 

 northern angle ; it turned eastward, near the southern angle of the same 

 river, and with a gentle sweep, having Vienna on the left and Port Stanley 

 on the right bank (receiving the upper-waters of the Au Sable and Thames 

 as tributaries), it passed Long point, flowing near the present Canadian 

 shore, and entering the Grand river (reversed) south of Cayuga ; after- 

 wards it passed down the Dundas valley towards north-east, into the 

 basin of Ontario, and then along the foot of the buried Hudson River es- 

 carpment to near Oswego. * 



Along its course, it received, probably, a small stream from near Detroit, 

 the Maumee, Sandusky. Chagrin, Cuyahoga, Grand (of Ohio, and its tribu- 

 tary, the Mahoning), Conneaut, Allegheny, and other rivers from the 

 American States, and afterwards the Genesee and various other streams in 

 its course through the basin of Ontario. 



The rate at which this basin was excavated, of course depended on the 

 wear of the limestone rocks after entering the Grand river. But as this 

 was very slow, the softer shaly rocks would gradually be worn down, and 

 the basin of these lakes, in the shaly rocks, across their beds, would be 

 greatly widened, as we see it. 



, No great pitch in the rivers would be required to occasion a flow of the 

 waters, a very few inches in the mile Avould suffice. If we observe the 

 deepest portions of Lakes Huron and Ontario, we have a difference in 

 altitude of 360 feet (both being below sea-level) in 400 miles along the 

 route indicated, whilst probably there were lake-expansions along the 

 course, thus causing the fall to be confined to a few places, especially 

 through the Dundas Valley, in the form of a series of rapids after the cap- 

 ping limestones had been removed. 



Excavation of Lake Basins. Having seen the course of the Preglacial 

 drainage, let us ask how the broad lake troughs could be excavated. 

 First let us look at Lake Ontario. 



The river coming down the Dundas valley flowed originally at near the 

 out-crop of the Niagara limestones, elevated by geological causes long ago. 

 The direction of the stream was parallel to its trend. On the one side, 

 were the soft Cambro-Silurian shales, geographically higher, geologically 

 lower ; on the other (southern) side, the Niagara limestones, beneath which 

 were the soft Medina shales, until these were worn away in part. As the 

 shaly rocks were removed the limestones would be undermined, and therefore 

 we had the Niagara escarpment produced. How far these limestones 

 have receded towards the present face and summit of the slope, is a question 

 yet to be decided. As the waters sunk to a lower level a second escarpment 

 was produced (the one noticed at Port Dalhousie, at the present lake 

 level). Afterwards, the Hudson River shales (with some hard rocks) were 

 pierced, whilst yet there were capping Medina shales, forming the surface 

 of the country between the river and the limestone escarpment. 



All this presupposes the continent at a higher level (at least between five 



