Normal Groups. Groups of certain rocks taken as a rule or standard. Etym., norma, rule or pattern.
Nucleus. A solid central piece, around which other matter is collected. The word is Latin for kernel.
Nummulites. An extinct genus of the order of molluscous animals, called Cephalopoda, of a thin lenticular shape, internally divided into small chambers. Etym., nummus, Latin for money, and λιθος, lithos, stone, from its resemblance to a coin.
Obsidian. A volcanic product, or species of lava, very like common green bottle glass, which is almost black in large masses, but semi-transparent in thin fragments. Pumice-stone is obsidian in a frothy state; produced, most probably, by water that was contained in or had access to the melted stone, and converted into steam. There are very often portions in masses of solid obsidian, which are partially converted into pumice.
Ochre. A yellow powder, a combination of some earth with oxide of iron.
Ogygian Deluge. A great inundation mentioned in fabulous history, supposed to have taken place in the reign of Ogyges in Attica, whose death is fixed in Blair's Chronological Tables in the year 1764 before Christ. See p. [341].
Old Red Sandstone. A formation immediately below the Carboniferous Group. The term Devonian has been recently proposed for strata of this age, because in Devonshire they are largely developed, and contain many organic remains.
Oligoclase. A mineral of the felspar family.
Olivine. An olive-colored, semi-transparent, simple mineral, very often occurring in the form of grains and of crystals in basalt and lava.
Oolite, Oolitic. A limestone; so named because it is composed of rounded particles like the roe or eggs of a fish. The name is also applied to a large group of strata, characterized by peculiar fossils, in which limestone of this texture occurs. Etym., ωον, oon, egg, and λιθος, lithos, stone.