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Stalactites and stalagmites
A stalactite is a rock-like column that hangs from the roof of a cave. A stalagmite is a similar feature that grows up from the floor of a cave.
Both are the result of carbonate rich water flowing through limestone. When the carbonate rich water reaches a cave, some of the carbonate comes out of solution and becomes rock once more.
This happens especially at places where water drips from cave roofs, or where it splashes onto cave floors. Over thousands of years tiny amounts of precipitated calcium carbonate (calcite) build up into spectacular features, sometimes tens of metres long.
 Stalactites and stalagmites – These are needle-like deposits of calcite that mostly form within caves.
Sphalerite
Sphalerite
A colourless mineral also called zinc sulphide. Any colours in it
are caused by iron impurities;
with increasing iron the colours change to green, brown, red and black. Red to yellow sphalerite does not look like a metal, being transparent or translucent and looking something like dull amber. Black sphalerite looks more metallic. Sphalerite containing cadmium is bright orange.
Zinc sulphide is the most abundant and important zinc mineral. It is also called zinc blende. Sphalerite comes from the Greek sphaleros, meaning treacherous.
 Sphalerite – The crystals of sphalerite are the darker ones. Intermixed with them are pyrite (yellow)
and calcite (transparent).
Stock
A vertical protrusion of a batholith that pushes up closer to the surface.
Stratigraphy
The study of the Earth’s rocks to try to find out their history and the conditions under which they formed.
Stratovolcano
A volcano of the common cone type. The cone is formed by layers of ash and lava built up over time and as the result of many eruptions.
Stratum, strata
A layer or layers of sedimentary rock.
Streak
The coloured line produced by rubbing a mineral against a piece of unglazed white porcelain. This is used as a test when identifying minerals.
Structure, soil
The way in which soil particles are held together. Soil particles are held together in clumps by humus and by calcium in the soil. If the soil has enough of both of them,
it will usually form into small ball-shaped clumps. This is called a crumb structure. It is most commonly found in the topsoil.
In the subsoil there is little humus and so a crumb structure does not form. Subsoils in sandy materials do not form any kind
of structure. Subsoils in clay materials are affected by shrinking and swelling as they dry out
and become wet from season to season. This tends to break the soil into blocks and produce a blocky structure.
Structure is important because plant roots need good drainage and aeration. Crumb and blocky structures help provide this.
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