led rock in which detached crystals or felspar or some
other mineral are diffused through a base of other mineral
composition. _Etym._, [Greek: porphyra], _porphyra_, purple.

PORTLAND LIMESTONE, PORTLAND BEDS. A series of limestone strata,
belonging to the upper part of the Oolite Group, found chiefly in
England in the Island of Portland on the coast of Dorsetshire. The
great supply of the building-stone used in London is from these
quarries.

POZZUOLANA. Volcanic ashes, largely used as mortar for buildings,
similar in nature to what is called in this country Roman cement. It
gets its name from Puzzuoli, a town in the Bay of Naples, from which
it is shipped in large quantities to all parts of the Mediterranean.

PRECIPITATE. Substances which, having been dissolved in a fluid, are
separated from it by combining chemically and forming a solid, which
falls to the bottom of the fluid. This process is the opposite to
that of chemical solution.

PRODUCTA. An extinct genus of fossil bivalve shells occurring only
in the older secondary rocks. It is closely allied to the living
genus Terebratula.

PTERODACTYL. A flying reptile: species of this genus have been found
in the Oolite and Muschelkalk. Some of the finger-joints are
lengthened, so as to serve as the expansors of a membranous wing.
Hence the name _wing-fingered_. _Etym._, [Greek: pteron], _pteron_,
a wing, and [Greek: daktylos], _dactylos_, a finger.

PUBESCENCE. The soft hairy down on insects. _Etym._, _pubesco_, the
first growth of the beard.

PUDDINGSTONE. See "Conglomerate."

PUMICE. A light spongy lava, chiefly felspathic, of a white color,
produced by gases or watery vapor getting access to the particular
kind of glassy lava called obsidian, when in a state of fusion; it
may be called the froth of melted volcanic glass. The word comes
from the Latin name of the stone, _pumex_.

PURBECK LIMESTONE, PURBECK BEDS. Limestone strata, belonging to the
Wealden Group, which intervenes between the Greensand and the
Oolite.

PYRITES. (Iron.) A compound of sulphur and iron, found usually in
yellow shining-crystals like brass, and in almost every rock,
stratified and uustratifled. The shining metallic bodies so often
seen in common roofing slate are a familiar example of the mineral.
The word is Greek, and comes from [Greek: pyr],