A Composite Dyke from Eastern Iceland

Abstract

I. Introduction. A Copious literature exists relating to the phenomena of composite dyke-and-sill intrusions, mainly based on a study of the Tertiary rocks of Ireland and Scotland. Composite sills have not yet been noted in Iceland, but composite dykes occur quite commonly in the Tertiary plateau-basalt series of the eastern part of the country. Most of them illustrate characteristics which have been reported from elsewhere, and no detailed account of them is desirable. The dyke here described exhibits, however, some unique features, which have seemed to us to warrant investigation. The field work was carried out by the second-named author, during summer visits to Iceland in 1914, 1920, 1922, and 1924. The only reference in literature to this occurrence is a note by Th. Thoroddsen of a ‘liparite-dyke,’ full of basalt-fragments, bordered on each side by basaltic dykes. The outcrop gives rise to a deep cleft, in the cliffs on the north side of Breithdalsvik, known as Hökulvikurgil (Pl. XIX, fig. 1), about 300 yards east of Snaehvammur farm. The exposure of rock in the gill is continuous from sea-level to the summit of the ridge between Breithdal and Stothvarfjord—about 2400 feet high, with the exception of a break between 15 and 300 feet which is covered by talus and raised-beach material, and another at 400 feet due to gill-deposits. The country-rocks are plateau-basalts, with partings of red lateritic rock, and a massive dolerite-sill comes in between 1450 and 1850 feet. The outcrop trends 7° east of north. It does