It is formed of basalt, trachyte and phonolite, and the soil is very fertile.
- Recent volcanic eruptive rocks (including rhyolite, trachyte, phonolite, andesite and basalt) have been examined at a number of points throughout both the geological regions of the island.
Of the Sauerbrunnen lies the Bofen or Biliner Stein (1763 ft.), a large mass of phonolite or clinkstone, with rare flora and fine view.
West of the volcanic region, nearer to Lake Victoria and the Eastern province, ironstone, granite, gneiss and schistose formations predominate, with phonolite in places.
Staffordshire, the diorites of Warwickshire, the phonolite of the Wolf Rock (to which he first directed attention), the pitchstones of Arran and the altered igneous rocks near the Land's End were investigated and described by him during the years1869-1879in the Quarterly Journal of the Geological Society and in the Geological Magazine.