Darwin wrote "Blackish grey sonorous, even semi-conchoidal porphyry, with many acicular crystals of glassy feldspar & common do. & hornblende; forming part of contemporaneous vein (it has character of phonolite)"
Euhedral microphenocrysts of plagioclase feldspar and brown hornblende are the major constituents of this rock. They are accompanied by large xenolithic fragments of green amphibole (actinolite) and quartz. The groundmass contains all the previously mentioned species and is peppered with tiny (black) iron oxide crystals. Collected June-July 1834.
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.