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
Cave S sea level and below the Great Gibraltar Sand Dune that was once used to capture rainwater which served as Gibraltar's main water supply. The cave is about from the north end of the Rock and opens on the east face of the Rock just above Holy Boy's Cave. The cave provides evidence of marine life indicating that despite its current altitude it was once a sea cave. This is not the only evidence of rising and falling sea levels in Gibraltar which have historically varied greatly. Wynfrid Duckworth described the cave's floor as falling in height towards the entrance and