In the 1790s, William Smith hypothesized that if two layers of rock at widely differing locations contained similar fossils, then it was very plausible that the layers were the same age.
I 1790'erne fremsatte den britiske geolog William Smith den hypotese, at hvis to klippelag på vidt forskellige steder indeholdt fossiler af samme type, var det meget sandsynligt, at lagene havde samme alder.