92 



DISCOVERY 



Garnet albite schist. 

 Garnet albite schist with mica and 

 banded with a sandy limestone. 



hornblende 



Tristan da Cunha Group 



This group, consisting of the five islands, Tristan, 

 Nightijigale, Middle, Stoltcnhoff, and Inaccessible, lie 

 in lat. 37° S., long. 12° \V., which is about 1,500 miles 

 west of Cape Towii. 



It was discovered by Tristan da Cunha about 1508, 

 but only became inhabited about the beginning of the 

 nineteenth century \\hen Napoleon went to St. Helena. 



Point and westwards to just bej-ond the Hardy Rocks. 

 That a more complex state exists in the neighbourhood 

 of Swain Bay is evidenced by the specimens given by the 

 islanders to the writer. These might almost be said 

 to be plutonic, but it is thought ^ that they are of the 

 nature of bombs. 



To the west and about twent\"-two mUes from 

 Tristan there are the four islands mentioned above. 



Nightingale, the southernmost of the four, is 

 rectangular in shape, and in size one mile by three- 

 quarters. High cliffs bound the south, east, and west 

 sides. The northern slopes descend gradually to the 



-TYPICAL SEDIinSNTARY ROCK SCENERY, PRINCE OI,.\V HARBOUR, SOUTH GE0RGI.\. 



The island of Tristan is a volcanic cone rising to 

 6,400 ft. In shape it is an octagon about eight miles 

 across. The first 2,000 ft. are very steep, but after 

 this the slope is. 17° or less. 



At 3,200 ft. the slopes become deeply indented with 

 drainage ravines. These continue for about 500 ft., 

 when the slopes become verj- rugged with minor rocky 

 knobs. This rough surface continues up to 5,700 ft., 

 where the final cinder cone' begins. The old crater at 

 6,300 ft. now forms a snow-water lake. The vegeta- 

 tion line is about 3,800 ft. above sea level. 



The mountain is built of successive flows of lava 

 mainly basaltic in character. Many vapour vents 

 occur, and the rock varies from compact to very 

 vesicular in texture. The lowest lava forms a lava 

 plain at the north side of the island, and this plain 

 has a number of small cones which represent the second 

 and last stage of vulcanism. These observations were 

 made on the line of section from the Peak to Herald 



sea, where they terminate in cliffs about 50 ft. high. 

 The highest point is on the east of the island and is 

 about 1,000 ft. above sea level. It is connected by a 

 low-featured col to the high land on the south-west. 

 To the west, that is towards the interior of the island, 

 there is a depressed area which has now a small pond 

 in it. It is probable that this was once the crater from 

 which the lavas issued. They are mainly composed of 

 trachyte — a volcanic rock which is common in Scotland. 



Middle Island lies less than half a mUe to the north 

 of Nightingale. It is about a quarter of a mile square 

 and rises to a height of about 200 ft. It is flat- 

 topped with minor depressions. 



There have been questions asked as to the origin of 

 Middle Island, and to the writer, who had this in mind 



1 W. Campbell Smith. M.C., M.A., Brit Mus. (Nat. Hist.). 

 Plutonic rocks are those which have been thrust through over- 

 lying rocks from below in a liquid condition. Bombs have 

 been cast forth as round masses from volcanoes. 



