Igneous rock the partial melting of rocks within the mantle and crust. Extrusive igneous rocks cool and solidify quicker than intrusive igneous rocks. They are formed by the cooling of molten magma on the earth's surface. The magma, which is brought to the surface through fissures or volcanic eruptions, solidifies at a faster rate. Hence such rocks are smooth, crystalline and fine-grained. Basalt is a common extrusive igneous rock and forms lava flows, lava sheets and lava plateaus. Some kinds of basalt solidify to form long polygonal columns. The Giant's Causeway in Antrim, Northern Ireland is an example. The molten rock, with
The Holy Cave, Hunterston 19th century. The visible remains suggested a souterrain, but the near absence of records of these in south-west Scotland make this unlikely. The passage is lined with corbelled drystone walling, roofed with capstones, and leads into a natural cave section which may have been formed by wave action as it shows a water-scour likely to have been created by wave action. Some areas of undisturbed stratified deposits have been found, together with evidence of occupation, such as a hearth, bones, and a fragment of glass. A similar cave or rock shelter survives on Holy Isle (off the east coast of