OF CENTRAL CANADA PART III. 



199 



VI. CLASSIFICATION OF ROCK MASSES IN ACCORDANCE WITH 

 THEIR RELATIVE PERIODS OF FORMATION. 



Viewed in reference to their modes of derivation or general form- 

 ative processes, rocks admit, as we have seen, of a distribution into 

 three leading groups : comprising Sedimentary, Metamorphic, and 

 Eruptive rocks ; but they admit also of another and far more interest- 

 ing classification, based on their relative ages or periods of formation. 



It is now universally conceded, on proofs the most unanswerable, 

 that the various sedimentary and other rocks which make up the solid 

 portion of our globe, were not formed during one brief or transitory 

 period, but were gradually elaborated or built up during a long succes- 

 sion of ages. In areas of very limited extent, for example, even on 

 the same cliff-face, or in excavations of moderate depth, we often find 

 alternations of sandstones, limestones, clays, &c., lying one above 

 another, and thus revealing the fact that the physical conditions pre- 

 vailing around the spot in question must have been subjected to 

 repeated changes. The same thing is also proved by alternations of 

 marine and fresh-water strata in particular localities : and of deep- 

 sea and shallow-sea deposits, in others. Again, the sedimentary rocks 

 are frequently found in unconformable stratification, as explained 

 above : horizontal beds resting upon the sloping surface or upturned 

 edges of inclined strata. Here it is evident that the inclined beds 

 must have been consolidated and thrown into their inclined positions 

 before the deposition of the horizontal beds which rest upon them. 

 In the absence of particular sets of strata in special localities, proving 

 extensive denudation or long-continued periods of upheaval and de- 

 pression in the vast metamorphic changes effected throughout many 

 districts in the upward limitation of 

 faults (Fig. 109), as sometimes seen 

 and, briefly, in the worn and denuded 

 surface which a lower formation often 

 .presents in connexion with strata rest- 

 ing conformably upon it, we have 

 additional evidence of the lapse of long 

 intervals of time during the elaboration 

 of these rocks generally. FIG. 109. 



But a still more conclusive proof of this fact is to be found in the 

 limited vertical distribution of fossil species of plants and animals, the 



