The island is the result of a series of fissure flows of a basaltic and trachytic nature. These flows have been intruded by the stock just mentioned above, and many fissures were opened by it. These have subsequently been filled by dykes. The rock forming the dykes is very hard, with the result that they are now a very prominent feature, and stand up in some cases about 50 feet above the surrounding country. This is due, of course, to differential weathering. It is probable that the east coast represents a fault plane, but as the erosion has been great, direct evidence is wanting. Apart from this fault no faulting nor folding was observed. [Illustration: GENTOO PENGUIN WITH TWO CHICKS _Photo: Wilkins_] [Illustration: NESTING GROUND OF THE MOLLYMAUK _Photo: Wilkins._] [Illustration: GIANT PETREL AT NEST _Photo: Wilkins_] TRISTAN DA CUNHA.—_Lat. 37° S. Long. 12° W._ [Illustration: TRISTAN da CUNHA] _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: (1) Rocks with felspar almost _nil_. Probably consist mainly hornblende and pyroxene, with perhaps some olivine, apatite and magnetite.