All the volcanic rocks of these islands are submarine stratified tuffs which are penetrated here and there by andesite or diabase dikes.
The lavas and ashes ejected by these volcanoes consist of liparite, dacite, andesite and basalt.
Howson andesite 250 ft.
Keechelus andesite series 4000 ft.
The rolled crystals of sapphire occur, with garnet and other minerals, in glacial deposits, and have probably been derived from dykes of igneous rocks, like andesite and lamprophyre.
At the close of the Eocene period great eruptions of augite-andesite took place from two fissures which ran along the west coast.
Augite-andesite predominates, but basalt and rhyolite also occur.
Certain flat oval nodules from a decomposed lava (augite-andesite) in Uruguay present a cavity lined with quartz crystals and enclosing liquid (a weak saline solution), with a movable air-bubble, whence they are called "enhydros" or water-stones.
The bulk of the mountain consists of andesite, but porphyry, obsidian, trachyte, basalt, and.
Finally the basic dykes of dolerite, basalt and augite-andesite are abundant and traverse the various sedimentary formations and the granite.
The recent lavas of the still active volcanoes of the south are olivine-bearing hypersthene-andesite and basalt.'
Very extensive layers of melaphyre and andesite, as also of conglomerates and volcanic tuffs, cover the middle portions of the peninsula.
- 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.
The rocks of Tristan da Cunha are felspathic basalt, dolerite, augite-andesite, sideromelane and palagonite; some specimens of the basalt have porphyritic augite.