Darwin wrote "Regular pale purplish sandstone".
This fine-grained rock consists of many tiny fragments of volcanic glass which exhibit 'shard' shapes disgnostic of explosive fragmetnation during eruption and occasionally completely enclosed bubbles. They are now crystallized so are not isotropic between crossed polars. Crystallisation probably occurred appear shortly after eruption as the glass was deposited while still hot and deformable. Crystallisation of the fragments probably occurred during the welding process that led to the fine-lamination which is a bedding/compaction feature. Collected in July 1835.
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.