Rock Types

Basalt  is a good example of an extrusive igneous rock that formed as lavas surfaced and spread. It is black and has very small crystals (reflecting fast cooling).

Granite is an example of an intrusive igneous rock. It is white or pink-coloured and, reflecting a much slower cooling, may have large crystals. In the early nineteenth century, granite taken from Dalkey hill in south Dublin was used to build the great 'asylum harbour' at Dun Laoghaire .

Also, sedimentary rocks can be classified in more detail. Examples are shales, limestones, and sandstones. Limestones, as already noted, formed in shallow, quiet, warm seas, whereas sandstones formed in desert conditions. Ripple marks  are sometimes evident on sedimentary rocks formed in shallow water.

Metamorphic rocks can likewise be subdivided, some of the major types being quartzites, slates, schists, and gneisses. The Valentia Island slate quarry flouished during the nineteenth century. Valentia slate was exported to Britain and used to roof the House of Commons in London.