REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 75 



rocks that crop out on the coasts. The difficulties of exploration prevented the naturalists 

 from rambling out of sight of the ship. On considering the nature of the rocks 

 collected, everything leads to the belief that similar conditions would have been 

 observed in the central part of the island. 



The group of Tristan da Cunha comprises the Islands of Tristan, Nightingale, and 

 Inaccessible. On the strength of the relations of the flora, there ought to be added to 

 the same group the small Island of Gough, lying 200 miles to the south. These 

 islands form the summits of a great submarine chain, which traverses the middle of the 

 Atlantic from north to south, and on which, in the southern part of that ocean, rest 

 the St. Paul's Rocks and the Islands of Ascension and St. Helena. 1 



A. Rocks of Tristan Island. 



Tristan, the most important of these islands, lies in the north of the group ;■ it is 

 situated in lat. 37° 2' 45" S., long. 12° 18' 20" W. (Herald Point); it is 1550 miles 

 distant from the Cape of Good Hope, 2000 miles from Cape Horn, and nearly 1320 

 south of St. Helena. The area is about 1 6 square miles. The Island of Tristan is 

 almost circular, an elevated peak occupying the centre. If a circle of 3^ miles radius 

 be described with this mountain as centre, it will touch all the salient points of the 

 coast, except those in the eastern quarter, where the shore projects about half a mile 

 beyond the circumference. This island rises almost vertically from the bottom of 

 the sea, the 100 fathom line occurring close to the coast ; it is bordered by craggy 

 cliffs, which render landing very difficult. The perpendicular rocks that encircle the 

 island attain a height of 1000 to 2000 feet, and form a terrace or plateau, on which 

 stands a conical peak, reminding one of the peak of Tenerife ; its summit, covered 

 with snow for nearly the whole year, attains a height of 7640 feet. According to the 

 inhabitants of Tristan, the peak is a cone of black and red scoriae, with a crater-lake on 

 the top ; the diameter of the crater is about a quarter of a mile. From the coast other 

 eminences of less height are visible on the plateau that forms the centre of the 

 island. These hills are very probably also secondary cones of eruption ; several of 

 them, like the central peak, have crater-lakes. 



The cliffs are formed of nearly horizontal beds of basalt, alternately compact and 

 scoriaceous, with intercalated layers of reddish volcanic tufa. The whole system of beds 

 slopes slightly towards the shore, as can be seen to the east and west of the harbour. 

 These beds are traversed by dykes, generally vertical and of no great thickness. 



1 Starting from the meridian of 35° W., and a little to the south of the parallel of 35° S., the bottom of the sea 

 begins to rise gradually, till it reaches the culminating point of the submarine chain of the South Atlantic. The 

 ground rises to the height of the Islands of Gough and Tristan da Cunha, around which soundings of 1100 fathoms 

 and upwards have been made. To the east of the islands the bottom sinks to 2200 fathoms, between long. 10° W and 

 15° E., and from lat. 30° to 50° S. 



