The Mourne Dyke Swarm

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Abstract
I. Introduction J. F. Berger (1816) in his paper read before the Geological Society noted the uniform north-west and south-east direction and the parallelism of practically all the dykes of the north of Ireland now known to be of Tertiary age. The exhaustive researches of the Geological Survey upon the Tertiary province of Scotland confirmed this observation. The Tertiary dykes of North Ireland may be roughly divided into two major sets:— (1) The Antrim set—more or less coinciding with the area of plateau-lavas. (2) The Donegal-Down set, stretching diagonally across North Ireland and probably continuing across the floor of the Irish Sea to Anglesey, North-West Wales, and the English Midlands. The dykes of this set are frequently exposed in elevated regions inland, but are best seen along the seashore. The coast from the entrance of Strangford Lough southwards to Dundalk Bay affords an excellent section for the study of the Donegal-Down set, and here three concentrations of dykes can be discerned. Provisionally, we propose to regard these as three separate swarms: namely, (a) the Ardglass swarm, (b) the Mourne swarm, and (c) the Carlingford swarm. As rock-exposure largely fails along the coast between these three concentrations, it is not known whether they are separated by areas in which intrusions become sparse. The fact, however, that the Carlingford and Mourne dykes coincide geographically with plutonic massifs suggests that they are indeed swarms in the accepted sense of the term. In the case of the Ardglass concentration, however, there is no visible

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