The deposits of the useful minerals & rocks; their origin, form, and content . though owing to the abundanceof FeCOg and FeSiOg, that is, of iron originally chiefly in the ferrous andnot in the ferric condition, these ferruginous schists may not be com-pared with lake ore. The siderite- and greenalite-schists are invariably in the hanging-wallof particularly extensive and thick sheets of principally basic eruptives—basalts—which nuist be regarded as submarine outpourings. From thesesubmarine sheets the iron content of the schists has probably in greaterpart been derived. In addition to iron, a
The deposits of the useful minerals & rocks; their origin, form, and content . though owing to the abundanceof FeCOg and FeSiOg, that is, of iron originally chiefly in the ferrous andnot in the ferric condition, these ferruginous schists may not be com-pared with lake ore. The siderite- and greenalite-schists are invariably in the hanging-wallof particularly extensive and thick sheets of principally basic eruptives—basalts—which nuist be regarded as submarine outpourings. From thesesubmarine sheets the iron content of the schists has probably in greaterpart been derived. In addition to iron, a little manganese, and silica. 1066 OEE-DEPOSITS some alkali was also conveyed to the water from the basic eruptive. Theformation of the greenalite is then explained as follows : FeCla + = (oreenalite) + + NaaO. 3S1O2. nH20 = FeO. SSiO,. -r NagSO^. The precipitate resulting from the latter reaction consists actually ofgreenalite, , and free silica. The siderite-schist, which consists principally of FeCOg and Si02, may. Fig. 432.—Thin section showing greenalite, the ilark spheres, in fine-giained quartz,from the Mesabi district ; niagnifieil 40 times. Leith. in part have been directly precipitated, and in part indirectly formed fromthe greenalite, thus : FeSiOg + CO2 = FeCOg + SiOa. Though in exceptional cases these deposits may represent primary,relatively rich deposits in the formation of which secondary enrichmentplayed but a subordinate part, as a rule they have resulted from far-reaching secondary processes by which (1) the silica of the originalferruginous schists was removed, or (2) this removal of silica took placesimultaneously with an entry of iron. These processes depended upon theaction of descending meteoric waters containing oxygen, carbon-dioxide. THE lEON ORE-BEDS IN CRYSTALLINE SCHISTS 1067 etc., and in addition alkali salts, probably derived from the basiceruptive sheets. Such aqueous solutions collecte
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectminesandmineralresou
