[46 ECONOMIC GEOLOGY

beds as rhombohedra of siderite), and of iron oxide and ferrosilicates.
 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 magnetite.
 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 concentration
 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 ; accumulations
 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 contained
 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.