270 EDWARD STEIDTMANN 



Their conclusions on the origin of the sedimentary iron formations 

 from which the ores were derived by secondary concentration will 

 be considered here. The unaltered iron formations consist essen- 

 tially of alternating bands of chert, iron carbonate, oolitic greenalite, 

 and iron oxides, varying in thickness from a few feet to more than 

 a thousand feet. It is inferred that they are chemical sediments 

 from their composition, lack of fragmental texture, and bedding. 

 They show a great variety of lithologic associations, subaqueous 

 ellipsoidal greenstones, tuffs, acid extrusives, and normal sediments, 

 viz., quartzites, Umestones, and slates. 



From the fact that the iron content of the thick formations and 

 the sediments associated with them is greater than the iron content 

 of the rocks which antedate them, and since normal weathering 

 tends to fix iron in place rather than to cause its transportation and 

 deposition elsewhere on a large scale, it is concluded that the iron 

 in these thick iron formations is not the result of a normal cycle 

 of erosion, but was contributed from some unusual source. Further 

 evidence for this conclusion consists in the fact that their thickness, 

 composition, and structural characteristics do not resemble either 

 the bog or the glauconite deposits of the present time. Further- 

 more, their calcium-magnesium ratio is the reverse of that prevailing 

 in normal sediments. On the other hand, the small bodies of iron 

 formation, particularly those interbedded with slates, are similar in 

 their composition, thickness, and association to deposits formed by 

 the solution, transportation, and deposition of iron by processes of 

 normal weathering. The unusual source of iron as well as of the 

 silica of the thick deposits Van Hise and Leith believe may have 

 been from the submarine extrusions of basic lavas, with which the 

 iron formations are more or less associated in time and in place. 

 They postulate that the lavas may have contributed the iron 

 solutions directly, or that iron solutions may have been formed 

 from the interaction of the hot lava and sea water, and that the 

 lavas may have furnished iron solutions by weathering. 



They show that salt water acting on hot basalt forms sodium 

 silicate, and that sodium silicate in the presence of iron salts will form 

 ooHtic iron silicate and silica which are thrown down in alternating 

 bands, and that when carbonic acid gas is present, iron carbonate 



