ORES OF IRON 147 
Boe Iron Ores—The famous microscopist, Ehrenberg, 
described as Gaillionella ferruginea, a fresh-water alga with 
stems charged with hydrous iron oxide, which has been 
regarded as a normal secretion of the living organism like 
the carbonate of lime in shells and coral. A. Gages has 
described a mould, Penicillium, that grew in the tanks of the 
College of Science, Dublin, and extracted so much iron 
that if burnt it left a skeleton of hydrous iron oxide. Ac- 
cording to some authorities the deposition of the iron oxide 
in these plants is a post-mortem process due to the reduction 
of iron salts by the decomposing tissues. According to 
D. Ellis (Iron Bacteria, 1919, pp. 191-2, p. xvii, etc.) the living 
iron bacteria secrete iron directly and thus help the formation 
of bog iron ore; but according to H. Mélisch (Pflanze in 
Bezichungen zum Eisen, 1892, p. 80) their contribution is 
insignificant, and usually nothing. 
Bog iron ore though of good quality occurs in comparatively 
small quantities. The most important deposits were those in 
the Swedish lakes which are renewed and re-dredged at in- 
tervals. Larger masses occur in many parts of the world, 
such as the Hill of the Pines, or Mesa de los Pinos, at Rio 
Tinto ; it contains fossil leaves and was deposited in a swamp 
from iron derived from the adjacent masses of pyrites. 
SURFACE ORrES—EFFLORESCENT, RESIDUAL, AND ALLUVIAL 
Ores—Superficial sheets of iron ore are widespread. They 
occur as residual deposits left by the solution of limestone 
as in Franconia; as a sheet of spherosiderite or crust of 
hematite which breaks up into ironstone or “crowfoot 
gravel, over weathered basalt; as crusts or concretions of 
ironstone formed as efflorescent deposits over sandstones or 
sands; as laterites due to evaporation during the dry season 
of water which has soaked into iron-bearing aluminous beds ; 
as the sheets of nodular hezmatites in some of the rich ores 
in Bengal. These ores are high in phosphorus, usually 
ranging from 2 to 2 per cent., and are low in titanium, 
Alluvial iron ores are concentrates of iron oxides on river 
beds and sea beaches, such as the black iron sands due to 
the disintegration of igneous rocks containing magnetite, 
These black sands are collected in alluvial mining, but as 
the magnetite is titaniferous they have hitherto been of no 
commercial value, though they may be useful now that titan- 
lum oxide is used as a paint.