ORES OF MANGANESE AND CHROMIUM 149 
Eocene sandstones of the Southern Caucasus. Manganese 
ores, probably also of sedimentary origin, occur in pre- 
Cambrian metamorphic rocks, such as spessartite beds in 
the Gondite Series in India, and the extensive -deposit at 
Wigg, in Minas Geraes in Brazil. 
Manganese deposits are still being formed in the sea; 
nodules of pyrolusite due to the decomposition of volcanic 
fragments litter the ocean floor and grains and nodules are 
formed in shallow seas, as in Loch Fyne, by precipitation 
from river water. 
The residual ores are economically the most important. 
They occur where manganese disseminated through a rock 
has been concentrated by removal of the rest. Thus the 
Silurian Batesville Limestone of Arkansas contains manganese 
which has been left as nodules where the rock has been dis- 
solved (R. A. F. Penrose, Ann. Rep. G.S., Arkansas, for 1890, 
i, 1801, p. 177). The botryoidal manganese ore of the La- 
fayette district of Brazil is regarded by Miller and Singewald 
(Min. Dep. S. Amer., 1919, pp. 182-3) as residual from a 
siliceous pre-Cambrian limestone. The manganese ores of 
Bahia are attributed by the same authors (tbid., p. 189) to 
the superficial decomposition of rocks containing manganese. 
This process has also formed the manganese ores with the 
laterites of tropical countries, such as India, East Africa, 
and the Gold Coast. The high-grade Gold Coast ore is 
residual and due to the weathering of phyllites, schists, and 
a quartzite containing spessartite (Kitson, Gold Coast G.S,, 
Bull. i, 1925, pp. 12-16). Lateritic ore often contains so 
much iron that it is sold as manganiferous iron ore. Iron 
and manganese ores present a gradual passage from iron 
ore containing less than 5 per cent. of manganese, through 
manganiferous iron ores containing between 5 and 30 per 
cent. of manganese and over 30 per cent. of iron, and ferru- 
8!10Us manganese ores containing from 25 to 50 per cent. of 
manganese and from 10 to 30 per cent. of iron, to manganese 
ore which contains over 40 per cent. of manganese and less 
that 10 per cent of iron. 
The price of manganese ore is fixed by the unit or per- 
centage of manganese. Usually the ore must contain 50 
per cent. of manganese, but 45 per cent. ore is used ; before 
the War the price varied from od. to Is. for each unit in the