_Metamorphism._--Any change in rocks effected in the earth by heat, pressure, solutions, or gases. A common cause of the metamorphism of rocks is the intrusion into them of igneous rocks. Rocks that have been so changed are termed metamorphic. Marble, for example, is metamorphosed limestone. _Mineral._--An inorganic substance of definite chemical composition found ready made in nature, e.g. calcite, quartz. _Mississippian._--A period of the Paleozoic era of geologic time--in order of age, the third from the last of the era. _Moraine._--Glacial drift carried on, within, or under a glacier and deposited at the end, along the sides, or under the glacier. _Oil-pool._--An accumulation or body of oil in sedimentary rock that yields petroleum on drilling. The oil occurs in the pores of the rock and is not a pool or pond in the ordinary sense of these words. _Ordovician._--Next to the earliest period of the Paleozoic era of geologic time. _Ore._--A metal-bearing mineral or rock of sufficient value to be mined. _Outcrop._--That part of a rock formation which appears at the surface. The appearance of a rock at the surface or its projection above the soil. Often called an exposure. _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.