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Show 34 CAUSE OF VIOtATibNS OF CONTINUITY. [Ch.III. one being Roman, while, as in the former example, the lowest was Greek, and the uppermost Italian, he would then perceive the fallacy of his formeL' opinion, and would begin to suspect that the catastrophes, whereby th<: cities. were inhumed, might have no relation whatever to the fluctuations in the language of the inhabitants; and that, as the Roman tongue had evidently intervened between the Greek and Italian, so many other dialects may have been spoken in succession, and the passage from the Greek to the Italian may have been very gradual, some terms growing obsoleteJ while others were introduced from time to time. If this antiquary could have shown that the volcanic paroxysms of Vesuvius were so governed as that cities should be buried one above the other, just as often as any variation occurred in the language of the inhabitants) then, indeed, the abrupt passage from a Greek to a RomanJ and from a Roman to an Italian city, would afford proof of fluctuations no less sudden in the language of the people. So in Geology, if we could assume that it is part of the plan of nature to preserve, in every region of the globeJ an unbroken series of monuments to commemorate the vicissitudes of the organic creation, we might infer the sudden extirpation of species, and the simultaneous introduction of othersJ as often as two formations in contact include dissimilar organic fossils. But we must shut our eyes to the whole economy of the existing causes, aqueousJ igneous, and organic, if we fail to perceive that such is not the plan of Nature. ( 35 ) CHAPTER IV. Chronological relations of mineral masses the first object in geological classification- Super}JOsition, proof of more recent origin-Exceptions in regard to volcanic 1·ocks-Relative age proved hy inclucled fragments of older rocksProofs of contemporaneous origin derived f1·om mineral characters-Variations to which these characters are liable-Recurrence of distinct rocks at successive periods-Proofs of contemporaneous origin derived from organic remainsZoological provinces are of limitecl 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 contemporaneous. DETERMINATION OF THE RELATIVE AGES OF ROCKS. IN attempting to classify the mineral masses which compose the crust of the earthJ the principal object which the geologist must keep in view, is to determine with accuracy their chronological 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 utilityJ and which may throwJ 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 tha~ 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 D2 |