592 TRANSACTIONS OF SECTION C. 



the 'Canadian Shield,' and through Dana's account of the 'V Formation,' about 

 which the North American Continent was built up. 



It must be remembered, however, that, though most of the territory has been 

 roughly traversed by Bell, Tyrrell, Low, and other explorers, only a few dis- 

 tricts in the south have had their geology worked out in detail, because of their 

 valuable deposits of silver, nickel, and iron ores. It is only in these districts and 

 comparatively recently that the succession of Pre-Cambrian formations has been 

 determined with certainty. In the wide spaces of the north only the most general 

 relationships are known. 



It is intended to bring together here our knowledge of the most ancient 

 chapters in the history of North America as disclosed by recent field-work. 



Physiographic Features. 



In its physiography the Canadian Shield shows the features that might be 

 expected from one of the oldest and most stable land areas of the world. It was 

 reduced in very early times to a peneplain, but later was elevated, permitting the 

 rivers to begin a process of dissection. This process had a recent interruption 

 by the Pleistocene Ice Age, which blocked many of the valleys with moraines and 

 gave rise to the most extensive tangle of lakes in the world. Physiographically as 

 well as geologically, the region shows a dramatic mingling of extreme youth 

 with extreme old age. 



The best account of this rejuvenated peneplain has been given by Dr. A. W. G. 

 Wilson, 1 who shows that the gradients are very gentle, and suggests that two or 

 more facets can be distinguished as having slightly different inclinations and as 

 having been carved at different times. Here it will be unnecessary to take the 

 matter up except in a general way. 



The peneplain has been unequally elevated, parts standing 3.000 or 4,000 feet 

 above the sea, and other parts sinking beneath its surface. Only at two marginal 

 points can the Archaean surface be said to rise as mountains — in the Adirondacks, 

 projecting south-east into the State of New York ; and in the Nachvak peninsula, 

 just east of Ungava Bay. 



To the south-west and eouth the shield sinks, almost inperceptibly in many 

 places, beneath the older Palaeozoic rocks, and the same is true around the 

 central depression of Hudson Bay. Towards the south-east the shield breaks off 

 suddenly along the great fault of the Lower St. Lawrence, and apparently the 

 precipitous north-east shore of Labrador indicates faulting on even a larger scale. 

 It has been suggested that Greenland, the Highlands of Scotland, Scandinavia, 

 and Finland may have been parts of a single great shield, now separated through 

 the settling down of the sea-bottoms. 



In detail the region is full of variety of hill and valley, waterfall, river, and 

 lake; but, on the whole, it is monotonous to the ordinary traveller from the con- 

 stant repetition of similar forms, since there are no real mountain ranges and 

 few outstanding ' monadnock ' hills to break the sky-line. The sweep of horizon 

 from every hilltop seems horizontal, the summits around seldom rising more 

 than 200 or 300 feet above the valleys, and all reaching nearly the same elevation. 



The geologist finds, however, that this impression of general flatness is 

 deceptive. In reality the rock structures are usually more nearly vertical than 

 horizontal, as in most Archaean regions. The schistose rocks, which form so 

 much of the surface, commonly show dips of more than 60°, so that it is clearly 

 a mountain region planed down to its foundations. The arrangement of valleys, 

 ridges, and hills generally follows more or less closely these ancient rock forms. 



Geological Structure. 



Until recently most of the geological work done in this northern territory has 

 been track surveys following Indian canoe routes. Here and there moraines or 

 old lake deposits hide the rocks for a SDace, but usually the geology is admirably 

 displayed as one's canoe threads the intricate waterways of sprawling lakes 



1 'The Laurentian Peneplain': Jour. Geol., vol. si., No. 7, pp. 615-659. 



