844 A TREATISE ON METAMORPHISM. 



as iron carbonate mainly, but subordinately iron sulphate and other com- 

 pounds, are brought by underground solutions to shallow bodies of water, 

 snch as lagoons. By oxidation and hydration the material is thrown down as 

 hydrated hematite or limonite, with a greater or less amount of basic ferric 

 sulphate. But the larger masses of limonite and hematite are usually mainly 

 produced by segregation subsequent to the formation of the original 

 sediments. These, like the limonites, are largely replacement deposits of 

 limestone or of some other carbonate rock. The Clinton hematite ore 

 deposits are partly replacements of limestone which may have contained a 

 small amount of iron carbonate, but according to various authors the hema- 

 tites, as such, in large measure were deposited with the associated lime- 

 stones. But the most extensive deposits of hematite known, those of the 

 Lake Superior region, are segregations in formations which were originally 

 in large measure iron-bearing carbonate or hydrous ferrous silicate. For 

 these deposits the iron as carbonate has been transported to the places of 

 deposition; there it meets other solutions bearing oxygen, and in conse- 

 quence of the mingling of such solutions the iron is thrown down as a 

 partially hydrated hematite, the reaction being: 

 2FeC0 3 +0+nH 2 0=Fe 2 3 +nH a O+2C0 2 . 



It may be said, therefore, that limonitic hematite is either a direct 

 chemical precipitate in the zone of katamorphism or is due to aqueous 

 concentration in that zone. 



Ordinarily, so long as limonite and limonitic hematite remain in the 

 horizon in which they form, they are not further altered, since they are end 

 products of metamorphism for the zone of katamorphism. But at or near 

 the surface in arid regions, or in times of drought in humid regions, the 

 more hydrous forms of the oxide may be dehydrated and thus pass into 

 hematite. 



The more important alterations of limonite and hydrated hematite take 

 place when these ores pass into the zone of anamorphism. In that zone 

 dehydration occurs. The limonites and more hydrated hematites are grad- 

 ually decreased in amount, and where dehydration is complete hematite is 

 formed. Where the change is from pure limonite to pure hematite a 

 decrease in volume of 38 per cent is involved. 



Where the rock is subjected to strong orogenic movements, not only 

 does dehydration occur, but the iron becomes specular, and a laminated 



