- (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.