Two dikes observed near Sruhanavarnis stream, in Donegal, northwest Ireland, cut across flow banding in the main Donegal granite, and at 120 degrees seem to be along one of the joint directions. As the dikes, the granite, and xenoliths appear to have crystallized together, it is suggested that the joint system may have been partly formed before crystallization was completed and that the dikes represent metasomatized xenolith material rather than differentiation products of the granitic magma. The petrography of the dikes is described and the origin compared with that of the hypersthene gabbro of Ardnamurchan.

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