This specimen of dolerite ( diabase ) was collected on a field trip to a local quarry in June 2001. The Criggion quarry, on the outskirts of Welshpool in Powys is well known for supplying very hard stone, mainly used for road surfacing. The rock type is described as an albitised olivine diorite. The mineralogy and appearance of the rock indicates a degree of chemical alteration due to reactions between original minerals and migrating fluids. The usual appearance of lath shaped plagioclase crystals is absent. Instead the dolerite has a spotted appearance. This assumption is due to my observation, and therefore a better description might be obtained by a laboratory investigation. Note the difference between weathered surface and exposed surface.