XII.— Camtonitic Dyke Rocks from Inishowen, County Donegal, Ireland

Abstract

This paper is a petrographic account1 of a collection of camptonitic dyke-rocks from Inishowen, County Donegal, made by Dr. W. J. McCallien. His field notes show that these thin dykes are mostly concentrated in Malin Head and Inishtrahull Island round about Lag, Carrickvady and Dough; in the south, in Inch Isles and Buncrana; and in the west in Fanad. For a brief note on the geology of the region see preceding paper (No. XI). The rocks studied fall into five groups:— 1. Camptonitic rocks including the typical camptonites. These embrace types with panidiomorphic texture, the essential minerals being titanaugite, strongly pleochroic brown barkevikitic hornblende, and acid or basic andesine. Variations are seen first in the texture, viz., by the subhedrism of the mafic minerals which become ophitic or subophitic towards felspar, giving rise to dolerites, and secondly, in the composition, viz., in the relative proportion of titanaugite and brown barkevikitic hornblende. The rocks are microporphyritic, with pseudomorphs of olivine occurring both as euhedral phenocrysts and as irregular xenocrysts. Parallel growth between the augite and the hornblende, described by Flett from dyke rocks in the Orkney Islands (see “The Trap Dykes of the Orkneys” Trans. Roy. Soc. Edin., 1900, vol. xxxix, p. 865), is found in the Inishowen rocks also. The ocelli, typical of camptonites, are mostly composed of fibrous spherulitic chlorite, calcite, felspar, or, in some cases, of analcite. They are surrounded often tangentially by biotite and felspar, the latter showing replacement by chlorite. 2. Camptonitic olivine-dolerite, a more coarse-grained type, with This 250-word extract was created in the absence of an abstract