OF CENTRAL CANADA PART III. 



161 



FIG. 85. 



mencenaent of the Drift Period, the country was again depressed 

 beneath the ocean, and covered with the clays, sands, and boulders 

 of this latter time. Another period of elevation must then have 

 succeeded, bringing up both the Silurian and the Drift formations to 

 their present levels above the sea. 



(b) Denudation : This term, in its geological employment, signi- 

 fies the removal or partial removal of rock masses by the agency of 

 water. The abrading action of the sea, of rivers, &c., acting under 

 ordinary conditions, has already been alluded to; but the erosive 

 effects of water may be seen in numerous localities in which this action 

 is no longer in force. Sections of the kind shewn in the accompanying 

 figure, for instance, are met with almost everywhere, producing un- 

 dulating or roll- 



ing countries. 

 Here it is evi- 

 dent that the 

 strata were once 

 continuous in 



the space between A and B. Valleys which thus result from the re- 

 moval of strata, are termed " valleys of denudation." Some of these 

 valleys are many miles in breadth. Their excavation, consequently, 

 could not, in the majority of instances, have been effected by atmos- 

 pheric agencies, or by the streams which may now occupy their lower 

 levels ; but must have been caused essentially by the denuding action 

 of the sea during the gradual uprise of the land, or during alternate 

 movements of elevation and depression, in former geological epochs. 

 If the bed of the Atlantic, for example, were now being raised from 

 beneath, at the rate of a few inches in a year or series of years, an 

 enormous valley would probably be scooped out along the course of 

 the Gulf Stream ; and in other places where currents prevail, more 

 or less continuous valleys would also be formed. Isolated patches of 

 strata have been frequently left by denudation at wide distances 

 from the rocks of which they originally formed part. These are 

 termed " outliers." Thus in Western Canada, small isolated areas, 

 occupied by bituminous shales of the Devonian series, occur in the 

 townships of Bosanquet and Warwick, and constitute outliers or 

 outlying portions of the Chemung and Portage group (see Part V.) 

 largely developed in the adjoining peninsula of Michigan. The 

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