They are at a depth of about 12 ft., in slaty shale containing Llandeilo fossils and contemporaneous felspathic ash and scoriae.
Many of them have been originally micaceous or felspathic sandstones.
These are separable into a lower group (Kundelungu) of red felspathic grits and into an upper group (Lubilasch) of white friable sandstones.
The basin in which these sandstones were laid down is limited on the east by ancient gneisses and schists overlain by the highly inclined red felspathic grits.
Hornblende schists also occur and a compact felspathic rock in the Suris defile.
The rocks of Tristan da Cunha are felspathic basalt, dolerite, augite-andesite, sideromelane and palagonite; some specimens of the basalt have porphyritic augite.