130 
ECONOMIC GEOLOGY 
cent. of iron, and ferrous, FeO, with 77:7 per cent.), each 
forming a series of compounds. 
Iron is readily soluble; it is present in most spring waters, 
and gives many their medicinal qualities. Itispresent in them 
as bicarbonate, as sulphate, and as double salts of ammonia 
and humic acid, formed, in the absence of air, by organic 
acids. Iron is readily precipitated from these solutions; 
from the bicarbonates on exposure to air, and from the sul- 
phates by organic matter, when the iron is deposited as 
pyrite in a dust too minute for recognition under the micro- 
scope ; it causes the dark blue colour of clay, which is altered 
to brown or yellow on conversion of the pyrite into iron 
oxide. The humates may be decomposed by oxidation, 
iron-secreting bacteria, or decomposing organic matter. 
Ore Suppries—Iron near the earth's surface is subject 
to alternate solution and redeposition in a concentrated form 
as iron ore. Ninety per cent. of the available iron ores have 
been thus formed. According to the estimates by Eckel 
{Iron Ores, 1914, p. 41) of the world’s iron ores 63 per cent. 
are sedimentary; 5 per cent. are normal replacements; 
10-7 per cent. are contact ores or of doubtful origin; 133 
per cent. are residual; and 7-9 per cent. are secondary con- 
centrations. Of the sedimentary ores, 40 per cent. are 
Jurassic in age, 15 per cent. Carboniferous, and 16 per cent. 
Ordovician. Of the total iron ores of Europe (Roesler, 
U.5.G.S., Bull. 706, 1921, p. 18) 35°2 per cent. occur in France 
owing to the enormous quantity of Lower Jurassic ore in 
Lorraine ; the British Isles have 18-2 per cent., Sweden 12-5 
per cent., Germany III per cent., and Spain § per cent. 
The quantity of known iron ore is colossal. According to 
Roesler’s estimate the sedimentary ores known in 1921 
amounted to over 8,400,000,000 metric tons, of which France 
holds nearly half. The supplies already known will last for 
1000 years at the output of 1913. There are still larger 
quantities of material which contain iron but is not re- 
garded as ore. Whether an iron-bearing material can be 
ased profitably depends upon its composition, the cost 
of fuel, and accessibility to markets. An igneous rock 
with 35 per cent. of iron may be of no commercial value, 
while material with half as much may be worth mining as 
an iron-bearing flux. Some iron ores are now useless owing