Darwin wrote: “Purple porphyry, abounding with crystals of quartz and feldspar; Britannia Rock.”
The sample is an altered welded tuff containing phenocrysts of embayed quartz and euhedral feldspar crystals. The groundmass of the sample is partly crystalline and partly glassy and in places classic fiamme (cuspate glass shards) can be seen.
This collection was a collaboration between The Open University and the Sedgwick Museum, created in 2009 to celebrate the bicentennial of the birth of Charles Darwin.
The Sedgwick Museum opened a new gallery, 'Darwin the Geologist', and created a museum-based virtual microscope to showcase rocks he collected during the Voyage of the Beagle. We enjoy this collection because Darwin did not always pick up a representative sample from the islands he visited; it's often the unusual rocks that caught his eye. Just explore the collection and you'll see what we mean. For those who have the opportunity, a visit to the museum is strongly recommended.