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.
Na década de 1790, William Smith hipotetizou que se dúas capas de rochas en localizacións moi afastadas contiñan fósiles similares, entón era moi plausible que as capas fosen da mesma idade.