1881.] OtJi [Spencer. 



and six hundred feet). Prof. Dana points out that the continent, during 

 at least the Mesozoic, if not the Tertiary times, stood at an altitude equal 

 to this measurement, as shown by the soundings at the mouth of the Hud- 

 son river, which extends 80 miles seaward ; and Prof. Hillgard has shown 

 that the Mississippi also had nearly an equal elevation above that of the 

 present day. 



In the sketch of the topography of Lakes Erie and Huron, we have seen 

 that the whole of the latter lake and the south-western half of the former 

 are excavated mostly out of softer rock ; and the north-eastern half of Lake 

 Huron is excavated along the junction of harder and softer rocks similar 

 to Lake Ontario. 



The rate at which these upper lakes were excavated would depend on 

 the rate of the excavation of the Dundas valley and its extensions through 

 the limestone, at first by a slow abrasion, and the solution of the carbonate 

 of lime by the carbonic acid held in the water, and afterwards by the un- 

 dermining of the hard rocks on the removal of the Medina shales. 



As to Georgian bay and the North channel, these formed independent 

 valleys. That the North channel is excavated out of the Cambro- Silurian 

 shales, along the junction of Niagara limestones on one hand, and the meta- 

 morphic rocks on the other, is apparent at a glance ; as we see that the 

 Spanish, Missifisagua, Thessalon, and other rivers all point in that direc- 

 tion. It has been noticed that the North channel has the same depth as 

 the deepest outlet ; and also that the deeper portions of the nortliern part 

 of Lake Huron are in that direction. An appropriate coincidence is that 

 the strait between Manitoulin and Cockburn islands should be called Mis- 

 sissagua, which was doubtless the ancient outlet of that river ; and False 

 De Tour channel that of the Thessalon river. 



Again, Georgian bay is scooped out of the soft rocks between the crystal- 

 line rocks on the east and the Niagara limestones on the west along the 

 line of junction, similar to the North channel, or to Lake Ontario. 



The Indian peninsula is a perfect counterpart to the Niagara escarpment, 

 and the escarpment submerged beneath Lake Ontario. For here the Ni- 

 agara limestones tower more than 300 feet above Georgian bay, whilst at 

 the foot, but submerged, there is a precipitous descent of 500 feet below 

 the surface of the lake. The deepest outlet into Lake Huron is only about 

 800 feet. "Whether this is filled with drift deposit or not we cannot say. 

 One thing is certain, that a broad depression in the topography of the 

 country extends all the way from the southern end of Georgian bay, in- 

 cluding Simcoe, Balsam, Rice, and a multitude of smaller lakes emptying 

 into the bay of Quinte by the Trent river, to Lake Ontario. A great sys- 

 tem of drainage did exist along this line. According to Sir William Logan, 

 this trough is deeply filled with drift. Lake Simcoe is 130 feet above Geor- 

 gian bay, however, and the height of land in the trough to the east of Lake 

 Simcoe is more than 100 feet higher. It may be said that this trough is 

 bounded by a ridge (known as Oak ridge) which is, according to the levels 

 of the Toronto and Nippissing railway, 893 feet above Lake Ontario, and 



PROC. AMER. PHILOS. SOC. XIX. 108. 2p. PRINTED APRIL . 1881. 



