OF CENTRAL CANADA PART III. 



201 



geological periods or " ages," is conventionally adopted, as in the 

 annexed tabular view. 







Notes on the above Table. 



As the rock- formations enumerated in this table comprise a known 

 thickness of many thousands of feet, it is evident that they can never 

 exhibit a complete series at any one locality. But they are known 

 to occur in this order, by a comparison of their relative positions at 

 different places. Thus, in one district, we find (in ascending order) 

 the Silurian and Devonian series ; in another, the Devonian and 

 Carboniferous, and so on. 



(2) One or more of several consecutive formations are often want- 

 ing or absent at a given spot. The Carboniferous rocks may thus, in 

 certain districts, be found resting on the Silurian, without the inter- 

 vention of the Devonian series. But the relative positions of these 

 groups are never reversed. The Devonian beds are never found 

 under the Silurian, for example, nor the Cretaceous under the Jur. 

 assic. The absence of particular strata, at a given locality, is 

 accounted for by the elevation of the spot above the sea-level during 

 the period to which the strata in question belong ; or otherwise it is 

 explained by denudation; or by the district having been situated 



