Perpendicular olivine and plagioclase crystals form the selvage of a dolerite vein that has a smooth contact cutting across individual crystals of the wall rock of the dolerite plug that it intruded. The plug itself is a part of Slemish mountain (northeast Ireland) and is of Tertiary age. The occurrence of such columnar crystals in nature indicates unusual conditions of crystallization, and various factors are mentioned as possible explanations. The temperature gradient across the vein contact may have been the determining factor.

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