5o8 REVIEWS 



these that lend the dark color to streams flowing through regions of 

 abundant vegetation. Finally, a smaller amount of iron may be carried 

 as sulphate. 



Iron deposits occur in the form of hydroxides and oxides, carbonates, 

 and silicates. Iron is readily precipitated as the hydroxide, when a solu- 

 tion containing an excess of carbon dioxide is induced, through changes in 

 pressure or temperature, to give up its gas with a concomitant saturation 

 with oxygen; if the solution merely loses its carbon dioxide without 

 undue oxidation, ferrous carbonate tends to be precipitated. These 

 several iron precipitates generally accumulate in bogs, lakes, or lagoons. 

 Elsewhere rapidly flowing waters may (rarely) form iron deposits. 



Most deposits of bedded hematite, bog ore, and brown iron ore were 

 laid down originally as hydroxides, chiefly through biologic, but also in 

 part through chemical, agencies. Iron sulphide, too, may be formed 

 in either way, but ferrous carbonates and silicates are not definitely 

 traceable to bacterial action. Iron phosphates and basic ferric sulphates 

 are chemical precipitates. Ferrous siUcates tend to be deposited where 

 alkaline silicates are abundant in regions of precipitation of ferrous 

 carbonates — formed as indicated above. Probably 90 per cent of all 

 the iron ores being worked today are of the sedimentary type. A list 

 and description of those thought to be originally laid down as ferric 

 hydroxide includes the Clinton ores, the Wabana ores, the Lake Superior 

 hematite-chamosites, the itabirites of Minas Garaes, (Brazil) the 

 hematite-magnetites of the Dharwar terrain of India, and the ores of 

 the Norwegian Lias. Bog iron ores, too, were probably deposited in the 

 same form and are now widely distributed, being especially abundant in 

 the eastern part of Canada and the United States, in Sweden, and in the 

 glaciated sections of Europe and Asia; ferric hydroxide is also present 

 in large amounts in the red mud off the coast of Brazil. 



Another type of sedimentary iron ore is that originally deposited 

 as the carbonate; this is represented by the ''black band" ores in Ohio, 

 Pennsylvania, and West Virginia, the oolitic siderites of eastern England, 

 and the siderites of the Lake Superior region. 



The silicates present a third type of sedimentary iron ore. The 

 forms in which the iron occurs include glauconite, bavalite, thueringite, 

 bertherine, and chamosite; of these glauconite is the most widespread. 

 Greenalite was probably the original constituent of the Mesabi ores, 

 while the chamosite ores are predominant in the Bohemian Brdagebirge. 



Ferric and ferrous sulphides represent the fourth type of iron deposits. 

 Pyrite is important in the Huelva region of southern Spain, in the 



