History of the geologic time scale
British geologists were most active in the 19th century. The "Cambrian," (the Roman name for Wales) and the "Ordovician," and "Silurian", named after ancient Welsh tribes, were periods defined by Welsh rocks. The "Devonian" was named after the English county of Devon, and the name "Carboniferous" was simply an adaptation of "the Coal Measures," the old British geologists' term for the same set of strata. The "Permian" was named after Perm, Russia, because it was defined using strata in that region by a Scottish geologist Roderick Murchison. British geologists were also responsible for the grouping of periods into eras and the subdivision of the Tertiary and Quaternary periods into epochs. In general, periods were named after places where the rocks were easy to see.