A schist is a metamorphic Roche of sedimentary origin (often a Argile) which, under the action of the pressure and of the temperature, a regular flow in parallel plans acquired that one calls plans of Schistosité. The plan of schistosity is perpendicular to the direction of applatissement. Two episodes of deformation of different directions leads to the formation of two different directions of schistosity, and to the creation of " frites" (pieces of rock lengthened, of rather small section, typically 1 cm or less).

Only some species of trees adapted to these conditions can evolve/move there. It is the case, for example, of the pubescent birch, which can push in a ground of this type.

Among the notable schists, the Slate, very plane and of schistosity marked, is output in fine flagstones being used for the cover of the Toiture S. One can also use the Lauze schist, thicker. One in the case of speaks about Micaschiste a Métamorphisme of higher rank, which leads to the presence of Mica S white (Muscovite) or blacks (Biotite) in the plan of schistosity.

The schists drawn from the Spoil heap S Coal rs (of the Nord-Pas-de-Calais in particular), are used with the realization of sitted as Chaussée S. It exists two types about it:

• red resulting from one cooking inside spoil heaps entered spontaneously in combustion, which gave them a better mechanical resistance,
• black , at the natural state, more friable and more fat.

There also exists of corneal, another type of schist, obtained by metamorphism starting from sedimentary rocks. It is often in coastal regions, in particular in Normandy.

the schist (black stone), drawing tool :

In XVe century in Italy, in particular, the argillaceous schist with tightened grain, was used with the manner of a pencil which gives a feature whose color varies black with the gray for returned modelled, volumes and consequently for the study of the human figure. Two other techniques came to supplant it, the charcoal and the black lead.

See too

• Schists of Burgess

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