The principle, first recognized at the beginning of the nineteenth century by William Smith
(1769–1839), that different strata each contain particular assemblages of fossils
by which the rocks may be identified and correlated over long distances; and that these fossil forms succeed one another in a definite and habitual order. This law, together with the law of superposition of strata
(i.e. that sedimentary strata are deposited sequentially, so that in an undisturbed sequence each stratum is younger than the one beneath it), enables the relative age
of a rock to be deduced from its content of fossil faunas and floras.
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