[46 ECONOMIC GEOLOGY 
beds as rhombohedra of siderite), and of iron oxide and ferro- 
silicates. The precipitated iron would have been mixed 
with oolitic grains and shell fragments and have converted 
them into iron carbonate, which has been in some places 
altered into hematite, and in others further reduced to mag- 
netite. The origin of the iron as a direct precipitate has been 
adopted by many authors, including A. F. Hallimond for 
the British Jurassic ironstones (G.S. Gt. Brit, Spec. Rep. 
Min. Res., 29, 1925, pp. 11-14) and by G. Linck (N. Fahrb. 
Min., Beil. Bd., xvi, 1903, p. 497), and Eckel (Iron Ores, 
1014, pp. 58-69). It was adopted by Cayeux (Etude Petrog. 
Roches Sed., Paris, 1916) for the Minette of Lorraine, but in 
his later monograph (Les Minerais de Fer Oolithique de 
France, Fasc. II, Paris, 1922) he attributes the precipitation 
to bacteria, and this view has been accepted by Dr. Rastall 
(Geol. Mag., 1925, p. 91). There is no direct evidence for the 
bacteria, and the main argument for their action is that the 
abundant fossils in the ore indicate that the sea-water cannot 
have contained sufficient iron for precipitation except by 
organic agency. The inorganic precipitation of the iron salt 
need have been no more rapid nor required a greater concen- 
tration of iron in the sea-water than in the formation of 
glauconite, which takes place on sea floors rich in organisms. 
The ore was formed apparently on shoals or in shallow basins 
whence most of the clay and silt were swept away ; accumu- 
lations of oolite grains and shell fragments were buried in 
the wave-concentrated iron precipitate. The calcareous 
constituents were altered into hematite by the material in 
which they were imbedded and not bv iron solutions that 
infiltered from outside. 
BrackBanp OrEs—Some sedimentary iron ores are of 
indirect organic origin, such as the blackband ironstones 
found in the coal-fields of the S.W. of Scotland, which con- 
tained so much carbonaceous matter that they were smelted 
without additional fuel. They usually consist of a breccia 
of ironstone fragments in clay; they were formed on the 
floor of a swamp or lagoon by the alternate deposition of 
carbonate of iron and coaly carbonaceous mud. As the beds 
shrank owing to the loss of water and compression, the 
ironstone crusts were broken and the fragments surrounded 
by the mud.