TRISTAN da CUNHA

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Topography

Tristan is an island octagonal in plan, about 8 miles across. It rises as a prism for about 2,000 feet, and then tapers off as a cone to about 6,400 feet above sea level. The crater is now filled with water, and at that level is about 200 feet across. The rainfall on the upper slopes is very great, and they are deeply eroded. At the foot of the cliff, on the northern shore, there is a gently sloping lava plain, upon which the settlement is situated. In extent it is about 3½ miles long by half a mile wide. About midway between the extremities there are a few small craters rising above the plain. The plain is grass clothed, and the upper slopes are covered in moss, bracken and scrub trees. This vegetation continues up to about 4,000 feet, above which point the rocks are bare.

Geology

The island consists of a great series of lava flows which have poured from the volcano, and are of the nature of scoriæ, cinder, trachyte and basalt in succeeding and alternating layers. As is so common on these volcanic islands, the lower lava is generally a hard, compact basalt showing rough columnar structure.

Only one section was observed, which is placed below, but there is good reason to believe that to the west, in the neighbourhood of Swain Bay, more complex conditions exist, as many samples of bombs of a rock carrying large crystals of felspar and hornblende and other coarse grained rocks were given to the writer by some of the islanders, who stated that they came from this locality.

Preliminary note by W. Campbell Smith, M.C., M.A., on the samples given by islanders at Tristan da Cunha and reported to have come from the neighbourhood of Swain Bay. The specimens can be grouped in four types: