Darwin wrote: “Mica, quartz and feldspar (porphyritic with large crystals of do.) arranged in plane; containing garnets and joining to 467. (It is a true gneiss, a most beautiful rock).”
The rock is a garnet-biotite augen gneiss. Some individual augen have cores of plagioclase feldspar mantled by quartz. Other augen are entirely quartz.
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.