In the mid 19th century, two prominent geologists, Roderick Murchison and Adam Sedgwick, used their studies of the geology of Wales to establish certain principles of stratigraphy and palaeontology. From the Latin name for Wales, Cambria (derived from "Cymru"), was derived the name of the earliest geological period of the Paleozoic era, the Cambrian. After much dispute, the next two periods of the Paleozoic era, the Ordovician and Silurian, were named after pre-Roman Celtic tribes of Wales, the Ordovices and Silures.