_Paleontology._--The study of the world's (geologically) ancient life, either plant or animal, by means of fossils.
_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.
_Quartzite._--A metamorphic rock composed of sand grains cemented by silica into an extremely hard mass.
_Quaternary._--The later of the two periods of the Cenozoic era of geologic time.
_Rejuvenated._--Any region which has been subjected to erosion for a greater or less length of time and then reelevated so that the streams are renewed in activity.
_Rock._--Any extensive constituent of the crust of the earth, usually consisting of a mechanical mixture of two or more minerals, e.g. granite, shale. Less commonly a rock consists of a single mineral (e.g. pure marble), or of organic matter (e.g. coal).
_Sandstone._--A sedimentary rock consisting of consolidated or cemented sand. Often shaly or limy.
_Schist._--A rock that by subjection to heat and pressure and usually moisture within the earth has undergone a change in the character of the particles or minerals that compose it and has these minerals arranged in such a way that the rock splits more easily in certain directions than in others. It is a metamorphic rock derived from either sedimentary or igneous rock, more commonly the former.