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._, [Greek:
oon], _oon_, egg, and [Greek: lithos], _lithos_, stone.

OPALIZED WOOD. Wood petrified by siliceous earth, and acquiring a
structure similar to the simple mineral called opal.

OPHIDIOUS REPTILES. Vertebrated animals, such as snakes and
serpents. _Etym._, [Greek: ophis], _ophis_, a serpent.

ORGANIC REMAINS. The remains of animals and plants (_organized_
bodies) found in a fossil state.

ORTHOCERATA OR OHTHOCERAE. An extinct genus of the order of
molluscous animals, called Cephalopoda, that inhabited a
long-chambered conical shell, like a straight horn. _Etym._, [Greek:
orthos], _orthos_, straight, and [Greek: keras], _ceras_, horn.

OSSEOUS BRECCIA. The cemented mass of fragments of bones of extinct
animals found in caverns and fissures. _Osseous_ is a Latin
adjective, signifying bony.

OSTEOLOGY. That division of anatomy which treats of the bones; from
[Greek: osteon], _osteon_, bone, and [Greek: logos], _logos_, a
discourse.

OUTLIERS. When a por