Ten dense black glassy basalts. All appear free of olivine. Some are crowded with minute laths of plagioclase; others contain fewer minute laths but show a few small phenocrysts of plagioclase, or of augite, or both.
Four dense dark-brown glassy olivine-basalts, some containing many crystals of plagioclase, and a few crystals of olivine and augite. The glass is crowded densely with magnetite and sometimes with other undetermined microliths.
Four rather paler basalts with holocrystalline-porphyritic texture. These contain very small phenocrysts of plagioclase and sometimes of augite, in a ground mass of very minute laths of felspar and grains of augite and magnetite. The texture of the ground mass is intergranular. One of the specimens contained no augite phenocrysts, but rather numerous microphenocrysts of magnetite.
Two small fragments of pale basalt-glass, deep olive-buff in colour. Microliths are absent in one specimen, but they are abundant in the other and consist of small laths of plagioclase, and minute prisms of augite and a few crystals of what is probably olivine. The felspar laths gave extinction angles of 15°, but only a very few measurements could be made. This material resembles the pale patches of glass in the palagonite tuffs of Sicily and of Kerguelen Land,[17] and a somewhat similar though darker coloured rock has been described from Schwartzenfels Hesse as vitrophyric basalt, and has been elegantly figured by Berwerth.[18]
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.