tion of a stratum occurs at some distance,
detached from the general mass of the formation to which it belongs,
some practical mineral surveyors call it an _outlier_, and the term
is adopted in geological language.

OVATE. The shape of an egg. _Etym._, _ovum_, egg.

OVIPOSITING. The laying of eggs.

OXIDE. The combination of a metal with oxygen; rust is oxide of
iron.

OXYGEN. One of the constituent parts of the air of the atmosphere;
that part which supports life. For a farther explanation of the
word, consult elementary works on chemistry.

PACHYDERMATA. An order of quadrupeds, including the elephant,
rhinoceros, horse, pig, &c., distinguished by having thick skins.
_Etym._, [Greek: pachus], _pachus_, thick, and [Greek: derma],
_derma_, skin, or hide.

PACHYDERMATOUS. Belonging to Pachydermata.

PALAEOTHERIUM, PALEOTHERE. A fossil extinct quadruped, belonging to
the order Pachydermata, resembling a pig, or tapir, but of great
size. _Etym._, [Greek: palaios], _palaios_, ancient, and [Greek:
therion], _therion_, wild beast.

PALEONTOLOGY. The science which treats of fossil remains, both
animal and vegetable. _Etym._, [Greek: palaios], _palaios_, ancient,
[Greek: onta], _onta_, beings, and [Greek: logos], _logos_, a
discourse.

PELAGIAN, PELAGIC. Belonging to the _deep_ sea. _Etym._, _pelagus_,
sea.

PEPERINO. An Italian name for a particular kind of volcanic rock,
formed like tuff, by the cementing together of volcanic sand,
cinders, or scoriae, &c.

PETROLEUM. A liquid mineral pitch, so called because it is seen to
ooze like oil out of the rock. _Etym._, _petra_, rock, and _oleum_,
oil.

PHAENOGAMOUS or PHANEROGAMIC PLANTS. A name given by Linnaeus to those
plants in which the reproductive organs are apparent. _Etym._,
[Greek: phaneros], _phaneros_, evident, or [Greek: phaino],
_phaino_, to show, and [Greek: gamos], _gamos_, marriage.

PHLEGRAEAN FIELDS. Campi Phlegraei, or "the Burnt Fields." The country
around Naples, so named by the Greeks, from the traces of igneous
action everywhere visible.

PHONOLITE. See "Clinkstone."

PHRYGANEA. A genus of four-winged insects, the larvae of which,
called caddis-worms, are used by anglers as a bait.

PHYSICS. The department of science which treats of the properties of
natural bodies, laws of motion, &c.; sometimes ca