ISLANDS OF THE SOUTH ATLANTIC OCEAN

Gough Island.—Lat. 40° S. Long. 10° W.

Gough Island lies roughly 200 miles south of the Tristan da Cunha group. It is 8 miles long by 3 miles wide.

Topography

The island forms a monoclinal block with dip slopes to the west and escarpments to the east. The highest point on the long ridge which runs down the longer axis of the island is about 2,915 feet above sea level.

The west side of the ridge goes down in a long slope to the cliffs bordering the sea.

The escarpments on the east side are cut by three or four glens. The largest one, about half-way down the coast, gives access to the interior.

The most striking feature, looking up the glen, is the great stock of an acid intrusive rock, which rises to 2,270 feet. It can best be described in the words of Scott:

“Shooting abruptly from the dell

Its thunder splintered pinnacle.”

Geology

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.

Photo: Wilkins

GENTOO PENGUIN WITH TWO CHICKS

Photo: Wilkins.

NESTING GROUND OF THE MOLLYMAUK

Photo: Wilkins

GIANT PETREL AT NEST

Tristan da Cunha.—Lat. 37° S. Long. 12° W.

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:

(1) Rocks with felspar almost nil. Probably consist mainly hornblende and pyroxene, with perhaps some olivine, apatite and magnetite.
(2) Rocks with a little felspar and characterized by large poikilitic plates of hornblende. These contain abundant pyroxene, and some olivine, apatite and magnetite.
(3) Rocks with long, thin blades of hornblende in a fine-grained matrix of labradorite, and with some patches of black “glass” and abundant minute prisms of apatite. In hand specimens these look like dyke-rocks, but I think the texture and the patches of magnetite show that they are segregations.
(4) Coarse-grained rocks with perhaps more felspar than hornblende. Hornblende in large crystals in a matrix of labradorite. The texture is coarser than in the preceding type. Felspars reach 2 or 3 mm. in diameter. The hornblende includes some small crystals of yellow pyroxene. Apatite and magnetite are given abundant.

All four types appear to be closely inter-related. They contain the same minerals in varying proportions and probably grade one into the other.

The obsidian and the pieces of red glass are basalt glass, and are probably similar to the specimen described by Renard in the Report on the Challenger Collection, p. 82. He states that the inhabitants use the rock for striking fire.

OBSERVED SECTION FROM THE PEAK TO HERALD POINT

Elevations.	Rock provisionally named.	Remarks.
FEET
6,400	Scoriæ and vesicular basalt.	Forming summit.
6,200-5,700	Loose scoriæ and bombs.	Crater cone.
5,700-5,500	Basalt.
5,000-4,550	Trachy-basalt.
4,400	Vesicular basalt.
4,300	Trachytic agglomerate.
4,250	Compact basalt.
4,150	Red scoriæ.
3,950	Basalt.
3,750	Scoriæ.
3,600	Scoriæ and basalt.	A contact.
3,520	Basalt and scoriæ.	A contact.
3,420	Scoriæ and basalt.	A contact.
3,220	Grey basalt.
(Break in the observations).
1,200	Basalt and scoriæ. A contact.
(Break in the observations).
225	Basalt.	This rock is used for building the dwellings by the inhabitants.
223	Cinder.
222	Scoriæ.
220	Tuff.
216	Agglomerate.
To sea level.	Compact basalt.	Rough columnar structure.

A number of vapour vents were observed at different points.

It is apparent that the small craters mentioned above as existing on the settlement plain sprang up after the main period of eruption when the island was built.

To the west and about 22 miles from Tristan there are the four islands—Nightingale, Middle, Stoltenhoff and Inaccessible.

Nightingale

Topography

This island, which is the most southerly of the group, is rectangular in plan, one mile by three-quarters. High cliffs bound the south, east and west sides. The northern slopes descend gradually to the sea, where they terminate in cliffs about 30 feet high.

The highest point is on the east side of the island, and is about 1,000 feet above sea level. It is connected by a low featured col to the high land to the south-west. To the west, that is, towards the interior of the island, there is a depressed area which now has a small pond in it. It is probable that this was once the crater from which the lavas issued.

NIGHTINGALE IS

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Geology

One day only was available for work on this island, and orders were that the supposed guano deposits which were reported at the north side were to be examined. These deposits are of no economic value, and an analysis is here appended.

Certificate of Analysis.
Ogston and Moore,
Analytical Chemists,
89 Aldgate, London.
July 28, 1922.

Guano from Nightingale Island.

Moisture	72.12
Organic matter and ammonia salts	24.70
Phosphoric acid	nil
Lime	nil
Magnesia, alkalies, etc.	1.60
Silicious matter	1.58
	100.00

Guano from Cave on Middle Island.

Moisture	17.00
Organic matter and ammonia salts	15.15
Phosphoric acid	3.85
Lime	5.10
Magnesia, alkalies, etc.	10.20
Silicious matter	48.70
	100.00

The rocks, however, appeared to be in general of a trachytic nature.

Middle Island

Topography

Middle Island lies less than half a mile to the north of Nightingale. It is in plan about a quarter of a mile square, and rises to a height of about 200 feet. It is flat-topped, with minor depressions.

Geology

There have been questions asked as to the origin of Middle Island, and to the writer, who had this in mind when visiting the island, the following were the reasons for its existence.

The trachytic flows from Nightingale probably extended at one time about a mile farther to the north than the present northern shore of Nightingale. This is evidenced by the trachytic agglomerate and trachyte seen on Middle. Following this there was an effusion of a hard, compact lava from a neck which exists on the latter island. The border of the neck is marked by a breccia. The dykes emanating from this lava are not seen on Nightingale, but some of the rocks which infest the channel between the islands are probably their eroded remains. The action of the sea on the mass of altered trachyte between Middle and Nightingale Islands has in the course of time cut a channel through.

Stoltenhoff

It is not possible to land on this island, as it rises sheer from the sea to about 200 feet. It is flat-topped, and in area about 500 yards by 150 yards. The rock of which it is composed appears to be of a trachytic nature, and may be the northern limit of the flow’s from Nightingale, which have already been mentioned; it may, however, be a centre of activity, such as is described as existing on Middle Island.

Inaccessible Island

Topography

Eleven miles to the N.N.W. from Stoltenhoff is this island, which is the most northerly one of the group. In plan it is pear-shaped, being about 3 miles by 2½.

In its general features it is a basin, being a great caldera, the south-east side of which has been blown out. A cone rises to about 1,500 feet towards the north-east of the depressed central area. The interior is broken country clothed in verdure, and on account of the high rim, which affords protection from the winds, would be suitable for human habitation. A stream winds through the interior, finally falling in a beautiful cascade to the beach at the north-east shore, where a landing is easily made if the wind is not from the north.

Inaccessible Island

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Geology

The central cone is a mass of scoriæ, and the section from here to the sea near the waterfall shows that there have been successive flows of basalt and trachyte. The high cliffs to the west of the landing are cut by a series of parallel dykes, which are an outstanding feature.