XI.— The Dyke Rocks of County Donegal and the adjoining part of County Tyrone, Ireland

Abstract

The material on which this paper is based was collected by Dr. W. J. McCallien, formerly of the Geological Department of Glasgow University, and has been made available to the writer by Dr. G. W. Tyrrell, under whose direction the present investigation has been carried out.1 The counties of Donegal and Tyrone in northern Ireland are built up for the most part of rocks belonging to a metamorphic series which represents an extension of the metamorphic series of the south-west and central Grampians of Scotland. They consist of crystalline schists, quartzites, limestones and dolomites of Dalradian age. Intrusions into this series comprise: ( a ) an Epidiorite group (designated on the geological maps ‘hornblendic igneous rocks’) forming part of the Dalradian; ( b ) a Spilite-Keratophyre group of Ordovician age; ( c ) a Lamprophyre-Porphyrite group (the felstones and felstone porphyries of the geological maps) of Caledonian age; ( d ) a Camptonite-Monchiquite group of Permian age; and ( e ) a group of Crinanites, Olivine-basalts and Tholeiites (referred to on the geological maps as ‘basalts and dolerites of Tertiary age’). Much of the region, especially in the west, is occupied by granite which is later than the epidiorites but older than the other dykes. Previous research on this region is very scanty. The present paper deals only with the Caledonian and Tertiary dykes; the former have a NE-SW trend while the latter strike NW-SE or NNW-SSE veering at times to E-W. They occur mostly as minor stringers of only a few feet in width (2 to 12 feet), with This 250-word extract was created in the absence of an abstract