XXI. A Tertiary Volcanic Vent in the Island of Arran

Abstract

The red sandstone rocks of Arran have long afforded food for reflection and discussion to geologists, and many and various have been the opinions expressed regarding them. It is now generally understood, however, that the great bulk of them south of Brodick are of post-Carboniferous age, and that north of that village they lie unconformably on greatly older, though lithologically not very dissimilar strata; the line of junction passing across the island, nearly along the line of the String Road, to the mouth of the Machrie Water. So slight is the unconformity that it is hardly recognisable at any one locality, yet the newer rocks rest on lower and lower horizons of the Carboniferous series towards the west, until at Machrie Waterfoot they overlap them altogether, and lie directly on the Old Red Sandstone. As most of our members are aware, a very remarkable discovery was made in 1899 by the late Mr. William Gunn, F.G.S. [see Mr. Gunn's own account, antea , page 192], which has furnished conclusive evidence as to the age, not only of the red sandstones, but also of the intrusive igneous rocks of the south of Arran. A very intricate complex has long been known to exist south of the String Road, covering an area of 8 to 10 square miles, and including a group of hills ranging up to 1700 feet over sea-level. The recent detailed survey of this complex has proved it to be a plug or core which fills an ancient volcanic vent, This 250-word extract was created in the absence of an abstract