PORPHYRY DYKE IN GRANITE (CUT AND POLISHED) - Cornwall

Hingston Down Quarry, Gunnislake, Cornwall

A gorgeous slice of beautifully polished granite with a conspicuous dyke. Polished on both sides. It is one of the most interesting British rocks we have had for sale. It is presented in a vintage, foam-lined, lidded box.

A dyke is a sheet of formerly molten rock that was forced up through a vertical crack in a pre-existing rock. The dykes cutting through the fine grained Hingston Down granite consist of dark (boron-rich) igneous rock containing large crystals of feldspar. Rocks with large crystals in a fine-grained groundmass are described as having a ‘porphyritic’ texture.

This example also shows the ‘baked margins’ due to the heat of the intrusion metamorphosing the granite either side of the intrusion – proof that the dyke is younger than the granite.

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