The Island of Arran has long been celebrated among geologists for the variety and extensive development of its igneous rocks. The basaltic group is well represented in its various forms of structure and modes of occurrence; the more highly silicated series by felsites and porphyrites, forming dykes and huge amorphous or columnar masses; while the glassy varieties are illustrated by dykes and veins of pitchstone. All these rocks may be readily studied in the small area comprised within the southern half of the island, the greater part of which consists of eruptive rocks, and sandstones of the Carboniferous period. It is among these sandstones that most of the igneous rocks have been intruded. Some, however, are evidently interbedded and contemporaneous, forming great sheets of melaphyre between the Carboniferous strata.