Andesite is an extrusive rock intermediate in composition between rhyolite and basalt. It is the volcanic equivalent of Diorite with a silica comtent of 52-63% plus pyroxene ( Augite) & feldspars (Plagioclase) which are someimes large enough to occur as phenocrysts. Andesite lava is of moderate viscosity and forms thick lava flows and domes. The name Andesite is derived from the Andes Mountains in South America, where it can be found in abundance. These particular samples are from Borrowdale in the Lake District, Uk & are of Late Ordovician age. Specimens are supplied in a labelled card tray.