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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 he 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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