Other volcanoes
The hills of central Arran, lower and less dramatic than those in the north, mark the site of another volcano. This area never rose so high as the northern volcano, so it has been eroded down only to the lower part of the actual volcano, exposing the caldera into which the surface rocks subsided. Hence, there are blocks of lavas and volcanic ash which formed during the volcanic episode, as well as blocks from the overlying Cretaceous chalk and Jurassic marine clays with fossils. It is because the rocks are variable and fragmented that the hills are not so prominent.
Ailsa Craig is another volcanic centre. Known as Paddy’s Milestone because of its position midway between Glasgow and Ireland, Ailsa Craig, sticking above the sea, is all that is presently visible of this ancient volcano. The granite is distinctive and easily recognisable; it contains a blue mineral, riebeckite. Despite its remote location the quality of the rock was so outstanding, particularly for making curling stones, that the island’s shape has been altered by quarrying.