Note on the Spongy Ironstone of Angola

Extract

IN the interior of Africa, in the high region of the Portuguese colony of Angola, in localities lying at an altitude of from 3,000–4,000 feet, where the soil consists of a thick layer of sand, clearly derived from the disintegration of the underlying sandstones, one constantly meets with peculiar and interesting out-crops of iron ore in the form of brown ironstone (limonite) of a spongy texture. This brown ironstone has in some places a greyish metallic colour, in places a reddish-brown, brown, brownish-yellow, or other tint characteristic of this hydrated iron oxide, but in structure it is different from all other varieties of this kind of iron ore. The entire mass is full of irregular cavities exactly like those in an ordinary toilet sponge or in pumices, and gives the impression of a ferruginous mass inflated with gases. It also recalls the unforged iron produced in the old-fashioned methods of “burning out” metallic iron direct from the ore used by ancient metallurgists.