1881.] ^-L • [Spencer, 



five fathoms deep, but with a channel of about fifty fathoms depth running 

 through it, towards the direction of the north angle of the Au Sable river, 

 near Brewster's mills. Saginaw bay, belonging to this section, is like 

 Green bay, shallow even at its mouth, where it is less than 100 feet deep. 



Lake St. Clair is a flat plane, with its bed varying from 18 to 21 feet 

 below its surface, and is altogether modern. 



At Detroit the drift is 130 feet deep. The three south-western coun- 

 ties of Ontario are low and flat, and covered with drift varying gen- 

 erally from 50 to 100 feet in thickness below the level of Lake Erie. In. 

 places it is known to be absent to a depth of 200 feet below the same lake 

 in portions of a buried channel to be noticed below. In fact, all the evi- 

 dence appears to show that the southern end of Lake Huron and the west- 

 ern end of Lake Erie, with the intervening region, constituted one plane 

 underlaid by a considerable depth of Erian shales, reposing on the thick 

 development of Coniferous limestone, and traversed by deep channels 

 running through it. 



The section of Lake Huron under consideration is mostly excavated out 

 of Upper Erian shales in a direction at right angles to the trend of the 

 formations. The denuding action was lessened when the waters in the 

 deeper northern part of the lake subsided to a level having a southern mar- 

 gin bounded by hard Coniferous limestone, covered to no very great depth 

 with Upper Erian shales subjected to only sub-aerial action — the whole 

 traversed with water courses in deep channels. 



The second division into which, for convenience, I have made of Lake 

 Huron, is that portion between the line drawn from Presqu' lie to Kincar- 

 dine, and 'the Manitoulin islands to the northward. This is the deepest 

 portion of the lake and extends in a direction running from north-west to 

 south-east. It consists of a broad plane traversed by several deep channels. 

 The average depth of this plane below the surface of the lake does not ex- 

 ceed 75 fathoms, although there are channels much deeper, one of which is 

 represented by a depth of 117 fathoms. There is also one isolated sounding, 

 which reaches 125 fathoms or 750 feet, this being the deepest spot known. 



The deeper channels appear to lead from the northern portions of the 

 lake, and unite as they proceed southward, being separated by elevations 

 indicating peninsulas or islands. Two of the principal channels appear to 

 proceed from Mississagua strait (between Manitoulin and Cockburn 

 islands), and from south of Manitoulin island, eastward of the Duck 

 islands. However, the channels in the marginal portions of the lake are 

 generally more obscured by drift or silt than towards the central waters. 

 The channel, if such you can call it, proceeding from the Mackinaw straits 

 is of inferior depth to those leading from the more northern end of the lake. 



This portion of the lake is excavated out of the rocks of the various for- 

 mations from the Niagara to the Coniferous limestones. But most largely 

 out of the more or less soft rocks of the Onondaga group, along the strike 

 of these formations, thus giving the eroding agencies the power of remov- 

 ing the softer basal rocks, and of producing au escarpment of the Conifer- 



