Formation

Formation has by English geologists been generally applied to a group of strata united by some character which they have in common, whether of age, origin, or composition, as the Carboniferous Limestone formation, which, together with the formations of the Millstone Grit and the Coal-measures, constitutes the Carboniferous System. The term is therefore of subordinate importance to system. Foreign geologists seldom use formation in this sense. With them the word is descriptive of the materials composing strata, as chalk formation, meaning thereby not the Cretaceous System, but beds composed of chalk; so carboniferous formation is a group of beds containing coal. To bring our nomenclature into uniformity with that of foreign geologists some purists propose dropping the term formation out of our systems of rock classification. But to do that would require despotic authority, and the term will probably survive in spite of its supposed inconvenience.