Paleozoic.—An old era of geologic time—third back from the present.
Peneplain.—A region reduced almost to a plain by the long-continued normal erosion of a land surface. It should be distinguished from a plain produced by the attack of waves along a coast or the built-up flood plain of a river.
Pennsylvanian.—Next to the last period of the Paleozoic era of geologic time.
Period.—A name applied to one of the subdivisions of an era of geologic time, e.g. Cambrian period.
Permian.—The last period of the Paleozoic era of geologic time.
Petrology.—The study of rocks, including igneous, sedimentary, and metamorphic rocks.
Physiography.—The study of the relief features of the earth and how they were produced.
Placer Deposit.—A mass of gravel, sand, or similar material resulting from the crumbling and erosion of solid rocks and containing particles or nuggets of gold, platinum, tin, or other valuable minerals, which have been derived from rocks or veins.
Plutonic Rock.—An igneous rock solidified from a molten condition well within the earth. (See [Igneous Rocks].)
Proterozoic.—Next to the earliest known era of geologic time.