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Granite
An acidic, igneous rock formed deep below the surface of the Earth, mainly in the magma chambers that once fed volcanoes. Because this material was insulated from
the surface, it cooled slowly, giving time for crystals to form.
The biggest crystals in a granite are usually feldspar. They make the grey, white, or pink opaque shapes in a granite. Black micas form much smaller crystals (see: Biotite). A mass of crystalline quartz (looking like greyish glass) fills in the spaces between the feldspar and mica crystals.
Granites are easily recognised and colourful rocks. Large masses of granite are found in batholiths. (See also: Acid soil;
Arkose;
Rhyolite.)
Gold Gold
A soft, golden yellow metal that is usually uncombined with other metals. It is one of the heaviest metals that is common. Gold is often found in veins that were once close to magma chambers, alongside silver and copper.
Grain
A particle of a rock or mineral. (See also: Clast.)
Grey–brown soil
Soils of the cool humid regions, often found under coniferous forest or heath. They are mid-way between brown soils and podzols. They have poorly decomposing acidic humus surface layers. Rainwater passing through the humus carries acids into the soil. The acids react with iron from
the topsoil, making it soluble and allowing it to be washed into the subsoil. This leaves the topsoil greyer in colour. (See also: Acid soil; Brown soil; Podzol.)
Greywacke
A type of sedimentary rock formed by an underwater landslide. All of the particles in a greywacke are arranged chaotically. Greywackes are important sedimentary rocks in mountain chains. Before being lifted up
into mountains, they formed in geosynclines.
Grit
Grains larger than sand (2mm across) but smaller than stones. Grit stones look like coarse sandstones. Grit stones were once valued as grinding stones and were often used in flour mills. They are still used today for producing ‘stone-ground’ flour.
Gulleying
A form of accelerated soil transport that mainly occurs in places where people have stripped off the natural vegetation and left it exposed to heavy rainstorms. As the rainwater runs off the surface, it gathers speed until it can begin to carry soil particles along. First, small channels called rills form. Rills are shallow enough that they can later be ploughed out. If rills combine to make deeper channels that cannot be ploughed out, they are called gullies. (See also: Erode, erosion and Soil erosion.)
Gypsum
A mineral made of calcium sulphate. Gypsum is soft (hardness 2 on the Mohs’ scale of hardness) and translucent white. It is an evaporite mineral.
H
Haematite
Iron oxide, one of the main ores of iron. It is found
in igneous, metamorphic and sedimentary rocks. The word
haematite comes from the Greek haimatites, meaning
colour of blood. Haematite
Black biotite
 Granite – Granite is typified by a mixture of large pink or grey feldspar crystals and small black biotite crystals set in a ground of glassy quartz.
Pink (plagioclase) feldspar
Quartz
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