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.
000-000-146-226-C; British Geological Survey