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Overburden
The unwanted layer(s) of rock above an ore or coal body.
Oxide minerals
A group of minerals in which oxygen is attached to a metal (for example, iron oxide – haematite – and aluminium oxide – bauxite).
Permeable rock
P
Parent material
The rock or other material on which a soil develops. There are many kinds of parent material in addition to solid rock. Alluvium – the material deposited by a river – is a common parent material, as is wind-blown loess and the bouldery clay material called till, left when ice sheets melt away. Most soils north of about 48°N are formed on till rather than on solid rock. (See also: C horizon.)
 Parent material – A soil develops from solid rock, river alluvium, or some other parent material. In this photograph a brown soil is shown developed on a slate parent material. The start of the parent material is marked by the tip of the trowel blade.
Peat
An accumulation of decomposed plant remains. It occurs when water does not drain away, and so the normal processes of decay cannot occur. Peat can be cut and used as a fuel. (See also: Muck.)
Pegmatite
An igneous rock (for example, a dike) of extremely coarse crystals. Pegmatites are a rich source of ores and mineral crystals of great size and value.
Percolation
The downwards movement of water through a soil or rock. A permeable rock is one in which percolation can occur.
Period
A large unit of geological time. It begins and ends with some easily recorded and dramatic change in the rocks, such as a mass extinction
of living things. Geological periods last for tens of millions of years and have names like Jurassic and Permian. The rocks associated with
a geological period make up a geological system. (See also: Epoch.)
Permeable rock
A rock that will allow a fluid (water, crude oil, or natural gas) to pass through it. Permeable rocks that contain water are called aquifers. This is not the same as porous, which simply means a rock or soil containing small cavities or holes. To be permeable, the pores in
a rock must connect to allow a liquid to pass through. (See also: Impermeable and Percolation.)
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