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Series
Series
The rock layers that correspond to an epoch of time.
Shale
A fine-grained sedimentary rock made of clay minerals with particle sizes smaller than 2 millionths of a metre across. It is formed from mud that built up in calm conditions on an ocean or lake bed.
(See also: Argillaceous.)
Silica, silicate
The mineral silicon dioxide. It is very common, occurring as, for example, quartz. A silicate is any mineral that contains silica.
(For examples of silicates see: Augite; Bauxite; Feldspar; Hornblende; Micas; Olivine.)
 Shale – Bands of rain-eroded shale in Petrified Forest National Park, Arizona.
 Sill – A sill is an igneous rock that forms between other beds of rock.
Sill
A sheet-like body of intrusive igneous rock that has been injected between layers of sedimentary
or metamorphic rock. Sills are often composed of basalt. (See also: Contact metamorphism.)
Silt
Particles of rock between 0.06mm and 0.0002mm across. Less common than sand or clay. Silt particles give a soil a soapy feel (see: Silty soil). (See also: Grain and Grit.)
Silty soil
A soil in which there is enough silt for it to feel soapy to the touch. Silty soils are common in places where wind-blown material called loess occurs. It is found in large areas of China, in parts of the prairies and in northern Europe
and Australia. Silty soils are easily eroded by the wind and rain if exposed, such as when the land is ploughed for growing crops. Parts of the Dust Bowl region of the mid- western United States have silty soils. Huge problems of erosion face the people of China.
Silver
A silvery white, relatively soft, shiny metal. It has been sought after since the earliest times and is regarded as a precious
metal, just as gems are precious stones. Silver is often found in veins that were once close to magma chambers, alongside gold and copper.
 Silver – This is native silver.
Skarn
A mineral deposit formed by the chemical reaction of hot, acidic fluids and carbonate rocks.
Slag
The rock-like material that becomes separated from metal during the smelting of an ore.
Slate
A metamorphic rock produced only by pressure. The clay minerals become arranged parallel to one another. This is the reason why slate is so easy to split into sheets and use for roofing. (See also: Slaty cleavage.)
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