NIAGARA FALLS AND VICINITY 53 



The valley of Georgian bay is continued northwestward in the 

 channel known as North passage, a narrow body of water lying 

 between the Manitoulin islands and the Canadian old-land. The 

 southward continuation of the lowland is blocked by drift; but a 

 number of borings, between the southern end of Georgian bay and 

 Lake Ontario, east of Toronto, have developed the existence of a 

 buried channel, which connects these two valleys. This channel is 

 considered by Spencer to mark the pathway of his former Lauren- 

 tian river. It is clear however that this valley is merely the buried 

 connecting part of the inner lowland which extends along the base 

 of the entire Niagara escarpment. This portion of the lowland 

 was originally occupied by two streams flowing, the one northwest- 

 erly into the ancient Saginaw, the other southeasterly into the Dun- 

 das. The divide between the two may have been in the neighbor- 

 hood of Lake Simcoe. It is however not at all improbable that 

 the tributary of the Dundas may have, owing to favorable condi- 

 tions, gained an advantage over that of the Saginaw, and pushed 

 the divide northward. Such a migration of the divide might have 

 resulted in the diversion of the upper waters of the Saginaw by cap- 

 ture, so that they eventually became tributary to the Dundas. This 

 would account for the greater depth of the Georgian bay lowland, 

 which, after the capture of the upper Saginaw waters, could be 

 deepened independently of the notch in the cuesta through which 

 its waters were formerly carried out. This of course is merely sup- 

 positional, and the truth can be established only by more detailed 

 study of the ground. It is however what we might expect to hap- 

 pen in the normal adjustment of a coastal plain drainage. This 

 hypothetic relation is illustrated in fig. 6. 



The Huron lowland and the Chippewa and Tonawanda valleys. 

 On the yielding strata of the Salina group a second lowland was 

 carved out by subsequent streams, leaving an escarpment capped 

 by the Devonic limestones on the south. This, as we have seen, 

 becomes prominent eastward in the Helderberg range, where the 

 third upper Devonic escarpment unites with it. In the Niagara 

 region it faces the Tonawanda and Chippewa lowlands, which were 

 probably opened out by a subsequent stream tributary to the an- 



