Portrush Sill

Image: © Anne Burgess Taken: 1 Jun 2007

These rocks were the site of one of the great controversies in the history of geology. There were two opposing schools of thought: the Neptunists believed that all rocks were formed by sediments accumulating on the sea bed, and the Vulcanists, who believed that some rocks, in particular basalts, were formed by volcanic activity. Here in these rocks at Portrush were found ammonites, apparently in basalt, so the Neptunists cited these rocks as evidence supporting their hypothesis. Later study showed, however, that the ammonites were in fact in the underlying layer of mudstones, which had been baked so hard by the heat of the basalt on top of them that they had taken on a similar appearance and hardness as the basalt. The Neptunists' hypothesis has since been clearly demonstrated to be false. In this view, the flat sloping surface is the baked mudstone, and the isolated blocks are the igneous rocks, now classified as dolerite rather than basalt.

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