USEFUL MET 201 



Three important theories have been advanced to account for 

 the origin of these ores. 



(1) Sedimentary origin. James Hall, C. H. Smyth, Jr., and 8. 

 W. McCallie have advocated that the ores are of contemporaneous 

 origin with t he inclosing rocks and that they have been dep< 

 upon the Silurian sea floor as chemical precipitates. Hall be- 

 lieved that the essential iron of the present ore was leached out 

 of the older crystalline rocks and deposited as a chemical pre- 

 cipitate. Smyth considers that this leached iron was trans- 

 ported from the associated crystalline areas into shallow basins 

 in the Silurian sea and deposited around sand grains or any nuclei 

 that were available. McCallie believes that the original iron 

 mineral was either glauconite or greenalite. The continuation 

 of the ores with depth, as in Alabama where the ores are en- 

 countered 800 ft. below the surface, and in Missouri where the 

 ores have been encountered by drillings, is an additional proof of 

 the sedimentary origin of these ores. 



(2) Residual Enrichment. I. C. Russell advocated the theory 

 that the Clinton iron ores were derived from ferruginous lime- 

 stones by weathering agencies. The iron representing the less 

 soluble portions of the formations would remain in a more con- 

 centrated form due to the loss of the limestone through solution 

 and transportation. Russell cites a 57 per cent, iron content at 

 the surface of the Clinton limestone near Attalla, Alabama, and 

 an iron content of only 7.75 per cent, at a depth of 250 ft. E. C. 

 Eckel has pointed out that at a depth of 250 ft. the iron content 

 sometimes rises as high as 42 per cent., therefore the validity of 

 the theory is called into question. 



3. Metasomatic Replacement Deposits. According to J. J. 

 Rutledge the ores are of much later origin than the associated 

 terranes. He believes that the iron content of the overlying 

 shales was leached out by weathering processes and deposited 

 as a metasomatic replacement of calcium carbonate by the 

 anhydrous iron oxide. He cites in substantiation of his theory 

 the following arguments: (1) The invariable association of the 

 soft rich ore with leached decolorized shales together with the 

 association of the hard lean ores with bright unweathered shales. 

 (2) The relation of the ores with the shattered sandstones and 

 to the topographic situation of the ores. (3) The fact that analo- 

 gous replacements are in the process of formation at the present 

 time in the Medina sandstone. (4) The progressive downward 



