CHAPTER IV. 



Chronological relations of mineral masses the first object in geological classifica- 

 tion Superposition, proof of more recent origin Exceptions in regard to 

 volcanic rocks Relative age proved by included fragments of older rocks 

 Proofs of contemporaneous origin derived from mineral characters Variations 

 to which these characters are liable Recurrence of distinct rocks at successive 

 periods Proofs of contemporaneous origin derived from organic remains 

 Zoological provinces are of limited extent, yet spread over wider areas than 

 homogeneous mineral deposits Different modes whereby dissimilar mineral 

 masses and distinct groups of species may be proved to have been contempo- 

 raneous. 



DETERMINATION OF THE RELATIVE AGES OF ROCKS. 



IN attempting to classify the mineral masses which compose 

 the crust of the earth, the principal object which the geologist 

 must keep in view, is to determine with accuracy their chrono- 

 logical relations, for it is abundantly clear, that different rocks 

 have been formed in succession ; and in order thoroughly to 

 comprehend the manner in which they enter into the structure 

 of our continents, we should study them with reference to the 

 time and mode of their formation. 



We shall now, therefore, consider by what characters the 

 relative ages of different rocks may be established, whereby we 

 may be supplied at once with sound information of the greatest 

 practical utility,, and which may throw, at the same time, the 

 fullest light on the ancient history of the globe. 



Proofs of relative age by superposition. 



It is evident that where we find a series of horizontal strata, 

 of sedimentary origin, the uppermost bed must be older than 

 those which it overlies, and that when we observe one distinct 

 set of strata reposing upon another, the inferior is the older 

 of the two. In countries where the original position of mineral 



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