Plastic Clay. One of the beds of the Eocene Tertiary Period; so called, because it is used for making pottery. The formation to which this name is applied is a series of beds chiefly sands, with which the clay is associated. Etym., πλασσω, plasso, to form or fashion.
Plesiosaurus. A fossil extinct amphibious animal, resembling the saurian, or lizard and crocodile tribe. Etym., πλησιον, plesion, near to, and σαυρα, saura, a lizard.
Pliocene, Older and Newer. Two divisions of the Tertiary Period which are the most modern, and of which the largest part of the fossil shells are of recent species. Etym., πλειων, pleion, more, and καινος, kainos, recent.
Plutonic Action. The influence of volcanic heat and other subterranean causes under pressure.
Plutonic Rocks. Granite, porphyry, and other igneous rocks supposed to have consolidated from a melted state at a great depth from the surface.
Poliparia. Corals. A numerous class of invertehrated animals, belonging to the great division called Radiata.
Porphyry. An unstratified or igneous rock. The term is as old as the time of Pliny, and was applied to a red rock with small, angular, white bodies diffused through it, which are crystallized felspar, brought from Egypt. The term is hence applied to every species of unstratifled rock in which detached crystals or felspar or some other mineral are diffused through a base of other mineral composition. Etym., πορφυρα, 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.