ORES OF IRON 145

and are estimated to amount to 5000 million tons of ore with
from 50 to 55 per cent. of iron, and from -2 to +7 per cent.
of phosphorus; and the Wabana ores of Bell Island, Newfoundland,
 which are also Ordovician, and their five beds,
varying from § to 20 feet in thickness, along a three-mile
outcrop, contain, according to A. O. Hayes (G.S. Canada,
Mem. 78, 1915), between 2000 and 3000 million tons of ore
with about 53 per cent. of iron and ‘85 per cent. of phosphorus
and 7000 million tons of lower grade ore.
These bedded ores have proved one of the puzzles of sedimentary
 petrology. The banded ironstones, which consist
of thin layers of quartz and hematite or magnetite, in the
pre-Palzozoic rocks of India, Western Australia, and Africa,
and the Itabirites of Brazil, are probably altered representatives
 of this type.
The bedded ores are generally associated with iron silicates
 including glauconite, and also chamosite and thuringite
which are chloritic hydrosilicates of alumina and iron oxide
(FeO). Some of the oolitic grains contain a skeleton of silica
that indicates their formation from silicates, and conversion
‘nto ore during the formation of the deposit. These bedded
ironstones have been generally formed under shallow water
marine conditions. The Clinton and Wabana ores contain
marine shells and Bryozoa replaced by Hematite, and the
mud-cracks and ripple-marks show that the deposits were
Occasionally above water; the deposition may have been
under lagoon conditions. Three chief explanations have been
offered of these ores. According to the first, the metasomatic
 theory (Sorby, Hudleston, and later Hatch and Rastall,
Petrol. Sed. Rocks, 1913, pp. 210-12), they are limestones
altered to carbonate of iron, and perhaps later to oxide.
The second explanation of some of the ore was by saturation
and partial replacement of sandstone by carbonate of iron
(Judd). These conclusions rested mainly on the oolitic
grains and marine shells which were formed as carbonate of
lime. These explanations are inadequate as many of the
oolitic grains had been altered into hematite before they had
reached their present position, and their matrix shows no
sign of alteration by infiltration.
The third theory is that the ore was formed by precipitation
 on to the sea floor of iron carbonate (which occurs in the
Ie