824 A TREATISE ON METAMORPHISM. 



The bedded iron-bearing carbonates vary from nearly pure carbonate 

 material through siliceous and clayey iron-bearing carbonates to carbon- 

 aceous, siliceous, and argillaceous rocks. The most widespread of the iron- 

 bearing carbonates are of Carboniferous age. Other extensive deposits of 

 iron-bearing carbonates are those of Algonkian ag-e in the Lake Superior 

 region. A subordinate amount of the iron-bearing carbonates occurs as 

 veins, but so far as this is true the material will not be here considered. 



ORIGIN OF SIDERITE, ANKERITE, AND PARANKERITE. 



So far as siderite, ankerite, and parankerite are composed of calcium, 

 magnesium, and carbonic acid, the sources of the materials are the same as 

 for the same materials in the calcium-magnesium carbonate family just 

 considered. But we must account for the iron. As the iron-bearing car- 

 bonates are aqueous sedimentary rocks, the iron must have been deposited 

 from Avater. The formations containing iron carbonate plentifully are gen- 

 erally local. Often they are associated with carbonaceous rocks. Many of 

 them are closely associated with the coals which are known to have formed 

 in lagoons and estuaries, some of which possibly were only imperfectly 

 connected with the sea. Deposits from which coal is derived are formed at 

 times of very luxuriant vegetation near the level of the sea, where the soil 

 is very moist. It is believed that the physiographic conditions for the 

 deposition of iron-carbonate deposits are similar to those which obtain in 

 flat-lying areas near water level, where vegetation is very luxuriant. It 

 may be recalled that under such conditions carbonation of the silicates 

 goes on very rapidly. (See Chapter VI, pp. 476-477.) The iron silicates 

 are decomposed precisely as are the others. It has been fully explained on 

 pages 470-471 that where vegetation is very abundant, in very humid 

 areas, the oxygen of the soil is not sufficient for the oxidation of the 

 vegetation. It was seen that under such conditions ferric oxide oxidizes 

 some of the residual vegetation, and is thereby reduced to the ferrous oxide. 

 The oxidation of the organic compounds produced abundant carbon 

 dioxide. The ferrous oxide originally present and that formed by reduction 

 unites with the carbon dioxide and produces iron carbonate. 



Where soluble ferrous compounds are formed abundantly, as above 

 described, it is probable that the rocks which are being destroyed in the 



