VI.—The Petrology of Arran

Abstract

The Riebeckite-Orthophyre of the Holy Isle.—Four Occuxrences of acid and sub-acid igneous rocks containing the rare soda-amphibole riebeckite have been recognized in Great Britain, of which that of Ailsa Craig is the best known. The occurrences of Ailsa Craig and Mynydd Mawr, Carnarvonshire, are riebeckite-microgranites or paisanites. They contain ragged moss-like areas of riebeckite, together with microphenocrysts of quartz and alkalifelspar, in a microcrystalline groundmass of quartz and felspar. Riebeckite was also found by Dr. Teall in the granophyre of Meall Dearg and the neighbouring area of Druim an Eidhne, Skye. Harker described the riebeckite in these rocks as occurring in two forms, one having the usual ragged, sponge-like appearance, and the other being idiomorphic, the faces in the prism zone being well-defined, but with irregular terminations. The fourth occurrence differs somewhat from the others. This rock occurs as an intrusion into the Upper Old Red Sandstone, or Calciferous Sandstone of Easter Eildon Hill, Melrose, and was described by Barron as riebeckite-trachyte or phonolite. It consists principally of sanidine, occurring both as microphenocrysts and in the groundmass, with interstitial patches of riebeckite, and a little nepheline. Harker describes the rock as an orthophyre.