William Smith · A Delineation of the Strata of England and Wales, with part of Scotland · London, 1815 · and Strata Identified by Organized Fossils (1816–1819)
Smith's claim is built on one observation, made over years of surveying canals, quarries and coal pits: the strata of England succeed one another in a fixed order, and each stratum holds its own characteristic fossils — so a layer can be recognised, anywhere, by the organic remains within it. What follows is a structural map of that work; the central principle is given verbatim in Smith's own words.
The principle of faunal succession — in Smith's words
Smith traced how he came to see that visually similar rocks could be told apart by what they contained:
it was the nice distinction which those similar rocks required, which led me to the discovery of organic remains peculiar to each Stratum.
“Organic remains peculiar to each Stratum” is the whole idea: the fossils of one layer belong to that layer and no other. Find them, and you have found the layer.
The column of England (young above, old below)
Reading downward from the youngest, Smith's southern-English sequence runs through the Chalk, the Greensand, the great Jurassic Oolites, the Lias clays, the Coal Measures, and the older slates beneath — each with its own fossils. By the law of superposition the lower bed is the older; faunal succession then lets the same beds be recognised even where they dip, thin, or are partly worn away.
Correlation and prediction
Once the order and the fossil signatures were fixed for one district, the same fossils identified the same strata elsewhere — letting Smith match an outcrop here to one fifty miles off, reconstruct the hidden structure of a region, and predict from surface clues the rock that lay below. This is what made geology useful: where to sink for coal or water, where building stone would be found.
The map, and the man
He set it all on a single hand-coloured sheet some 2.6 m tall — “A Delineation of the Strata of England and Wales” (1815) — shading each formation darker along the base of its outcrop to show the order of dipping beds. A self-made surveyor without standing, Smith was slighted by the Geological Society; a cheaper rival map drawing on his results undercut his sales. Bankrupt, he passed through a debtors' prison in 1819 and lost his home and fossil collection. Vindication came in 1831, when the Geological Society gave him its first Wollaston Medal and Adam Sedgwick named him “the Father of English Geology.”
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London · 1815