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Dear John, Northern Ireland's coast is of course replete with dykes and s ills. Less well known is the Blind Rock Dyke on the north coast of Donegal Bay. I bring this to your attention because its width, about 15 metres, appears to be exceptional in the context of Ireland. A quick check of accessible literature suggests that dykes thicker than 4 metres are a rarity. This dyke is intruded into Lower Carboniferous Bundoran Shale and extends for over a kilometre. Photos attached. Best wishes Malcolm.