VIII.— The Islay — Jura Dyke Swarm

Abstract

1. Introduction Portions of the Islay—Jura dyke swarm have been dealt with petrographically in the Geological Survey Memoirs covering sheets 19, 27 and 28. In the Knapdale Jura and Kintyre memoir, Flett (1911, p. 117) coined the term “crinanite” for the ophitic analcite-olivine-dolerites so abundantly represented in the area, but it is perhaps unfortunate that the name indicates their occurrence round Crinan — that is in the Mull swarm — whereas the analysed type rock was taken from Jura and is a member of the Islay—Jura swarm. A more general treatment of the swarm was given by Tyrrell (1928, pp. 238-239) who considered that the Islay—Jura swarm was connected with a plutonic focus in Arran in spite of the gradual diminution in the number of dykes in the direction of that island. McCallien (1932, pp. 52-55) suggested that on the other hand the Arran swarm should be divided into (1) the Islay—Jura swarm consisting almost entirely of crinanites and olivine dolerites and (2) the Arran swarm, which is compound and associated with more than one centre. He suggested further that the focus of the Islay—Jura swarm may be N.W. of Islay since the dykes become more numerous in that direction, but that the complex Arran swarms comprising crinanitic and tholeiitic injections of different ages, has two centres, one being North Arran and the other (dominantly tholeiitic) to the south of the island. Richey (1939, p. 425) in his comprehensive survey of the dykes of Scotland, accepts McCallien’s suggestion as to This 250-word extract was created in the absence of an abstract