MYNYDD PRESELI DOLERITE
The rocks that make up the famous prehistoric structure of Stonehenge in Britain are often referred to as “Stonehenge Bluestone”. They are Ordovician-aged porphyritic dolerites that have been subjected to low-grade metamorphism during the Paleozoic assembly of Pangaea.
Here's a sample of spotted dolerite from the Preseli Hills of Wales It consists of (mostly) albitized plagioclase feldspar (whitish & whitish-gray), pyroxene (blackish), and the low-grade metamorphic minerals chlorite, epidote, and actinolite (all greenish).
Unit & Age: Mynydd Preseli Dolerite Suite, Llanvirnian Stage, early Middle Ordovician
Locality: Preseli Hills, North Pembrokeshire, southwestern Wales, southwestern Britain.
Mynydd Preseli Dolerite (6.7 cm across at its widest), the rock type that makes up the prehistoric structure at Stonehenge.