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Skarns

Limestone or marble permeated by hot magma can sometimes create deposits: 'skarns'. Silicon and iron in the magma combine with calcium and magnesium in the limestone to form silicate minerals such as wollastonite. Also, hydrothermal solutions can leave ore minerals such as tungten, iron and zinc. 

Slickenside

A polished or smooth fault surface. The direction of the last fault movement can be determined by feeling which direction is smoothest with your hand.

Sole Structures

Found in sedimentary rocks, a sole structure or sole mark is a cast found on the base of the rock unit. They are often well developed when a coarse grained rock overlies a fine grained rock. Sole structures can be formed by sticks or stones being carried by a current or by pebbles allowing a vortex to erode the underlying rock in places, forming a depression.

Sparite

Crystalline carbonate. Also known as sparry calcite.

Species

Different definitions are available dependent upon the characters being used to define a species. See also biological species concept, morphospecies concept and phylogenetic species concept.

Stabilising selection

Individuals with intermediate trait values have greatest fitness. E.g. selection on body mass at birth in humans. Mortality rate is lowest at intermediate body mass and the optimum mass has the lowest mortality rate, which is close to the population average.

Stratigraphy

The study of layered sedimentary or metamorphic rocks and how they related to each other, particularly their ages.

Strike-Slip Fault

A fault caused by horizontal shear.

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Stylolite

A pressure-dissolution structure commonly found in chalks and other carbonates. The stylolite appears as a jagged line on the rock surface. Increasing pressure causes the carbonate ions to dossolve and migrate. This does not occur along a straigh line, bit along crystal boundaries, which in turn gives rise to the jagged appearence of the stylolite. See http://www.geo.lsa.umich.edu/~vdpluijm/animations/stylolite.mov for a video of formation.

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Supersaturation

Supersaturation is the dissolving of a larger quantity of solute in a solution than would normally be possible. For example sea water is supersaturated with respect to carbonate by a factor of 3 or 4.

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