structure - the very root of all our architecture.   Preseli Bluestone Geology   Preseli Dolerite (Bluestone), which is over 500 million years old,is a Metamorphic and Igneous rock made up of Plagioclase Feldspar and Augite and is a Pyroxene mineral (calcium magnesium aluminosilicate. Ca, Mg, FE, AI, (Al,Si,206). It is