Dolomite, CaMg(CO3)2. Tan y Foel quarry, Cefn Coch, Nr Welshpool, central Wales. The quarry provides a section through the Wenlock, mid Silurian, sedimentary deposits. The rocks belong to the Penstrowed Grits Fm. The specimen is an example of material from the occasional fault within the quarry, and shows veins filled with dolomite and quartz. A small vugh has allowed the growth of clear quartz crystals and numerous tiny dolomite crystals which display the familiar curved faces.A very similar specimen was obtained from Penstrowed quarry, Newtown.