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Volcanoes - Dyke


View at Porta Leacach, south-east coast of Arran, Buteshire

Tertiary basalt dyke (0.6 m. broad) cutting Triassic sandstones.

Pale, subhorizontally-bedded sandstone dips gently seaward. Joints are spaced c. 2 metres apart. The dyke (dark) is relatively fine-grained, because it is narrow, and therefore cooled quickly. Here its hardness is compensated by its relatively close (0.15 m.) jointing, and it does not stand proud of the sandstone. Most of the large loose blocks are dolerite, either from dykes or from the Dippin sill.

SCRAN: 000-000-146-226-C; British Geological Survey


University of Glasgow