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Alexandre Brongniart, 1770-1847, a French army engineer (1794-1800), director of the Sèvres porcelain factory (1800-47) and professor of mineralogy (from 1822), coauthored with Cuvier a ground-breaking geological Essay on the Mineral Geography of the Paris Region, 1808, and map, 1811.

was a method that worked remarkably well, although Smith did not know or perhaps even care why it worked so well. We now know that thanks to evolution organisms change through time. And, once we have worked out how exactly they change and what organisms are typical of the successive intervals of time, we can use them as crude relative 'chronometers' for Earth Time. To illustrate how the scheme works and how it developed we are going to trace William Smith's line of section from London to Snowdon.

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