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• Igneous rocks
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Marble and Granite

Igneous rocks

Igneous rocks are those which cooled and consolidated from a fluid melt (magma) of rock material. The magma may have cooled quickly to give fine-grained rocks or slowly, to produce coarse-grained crystalline rocks. The nature will depend also on their chemical composition.

Many have been found in Great Britain, but only one - granite - has been used on any scale. South-West England and Scotland are the great granite producing areas for building.

Sedimentary rocks were originally deposits of sediment (the eroded material from pre-existing rocks) laid down, mostly, on old sea-floors. When compacted and cemented, sedimentary rocks result. Their composition depends on the nature of the original sediment, but only sandstones (composed of quartz grains) and limestones (chiefly of calcium carbonate) have been used on any scale. In general terms, limestone predominates in southern Britain and sandstone in the north, but some limestones such as Portland stone have been used ubiquitously; and some sandstones, for example York stone are widely used especially for paving.

Metamorphic rocks result from the modification of pre-existing rocks by heat, pressure or both. The one which has been most widely used in Britain is slate. Other indigenous metamorphic rocks have been little used. Slate is found chiefly in Scotland, the Lake District, North Wales and Cornwall.

There are constraints inherent in stone which demand that the material is properly used in accordance with its unique characteristics. Igneous rocks may contain minerals which on exposure to the atmosphere may break down with consequent damage to the stone. Rising salts, also, may cause spalling. Of prime importance with sedimentary rocks is the placing of the bedding plane so that it is at right angles to the thrust imposed upon it. Metamorphic rocks may have deleterious (harmful) minerals present. The greatest restraint in the use of stone is that of the jointing. All rocks are jointed and the size of a block that can be wrought from a quarry is controlled by joints.

Such constraints are minor compared with the benefits: Stone from all these groups, whether used in classical idiom or in contemporary form, will have a durability other materials cannot match, provided it is properly chosen and properly used.