The little land-locked harbour of Pwllheli in Caernarvonshire owes its existence to a boss of igneous rock, which forms a natural breakwater, and which is known in Wales as the " Gimlet Rock."

The large quarries afford excellent exposures of the rock, which is worked into " setts " and used largely for paving purposes in the large towns of Lancashire, etc. It is a coarse ophitie dolerite, in which the augite is a very conspicuous mineral. The rock is traversed by many segregation veins, and is much jointed. The faces of the larger jointplanes are covered with crystals of quartz, calcite, a little natrolite, brookite and prehnite. But the three last-named minerals are of rare occurrence, and are always associated with quartz.