Dolerite

Dolerite is a medium-grained, mafic, intrusive, igneous rock with a chemical composition similar to that of basalt and gabbro. The rock usually forms as dykes and sills in basaltic provinces. It may also occur as dyke or sill swarms.

This piece of Olivine Dolerite was kindly sent to me by a fellow S260 student.

"It came from Titterstone Clee Hill Quarry Shropshire and is known locally as "dhustone" (which is probably a variation of ddustone, ddu being Welsh for black). It occurs as a sill intruded into horizontal coal measures, although, in places, it lies unconformably on top of the upper Devonian. The sill is thick - up to 90 m and shows poorly developed columnar jointing. In the 19th century, it was used as stone sets for roads and pavements and hardly wears at all. There is still one quarry working which produces material for roadstone".