340 University of California Publications in Geology [Vol. ll 



which was leached out was redeposited in the form of bodies of iron 

 ore. At places, he believed, the iron carbonate reacted with mag- 

 nesium carbonate during the progress of silicification and there 

 resulted the magnesian iron silicates that characterize the magnetitic 

 and amphibolitic schists. 



N. H. and H. V. Winchell, 96 in 1889, suggested that the Keewatin 

 iron ores and jaspers might be chemical precipitates. They showed 

 that these jaspers often contained considerable amounts of chalcedonic 

 silica, a form which is produced only as a chemical precipitate. The 

 banding of chalcedonic silica and hematite was regarded as indicat- 

 ing that the iron formations were deposited in water in regular strata. 

 They showed that the Keewatin contained large amounts of volcanic 

 material which they believed was largely erupted beneath the sea. 

 They believed that the waters of the ocean, on coming in contact 

 with molten material during these eruptions, dissolved out large 

 amounts of magnesia, iron, potassium, and sodium, together with 

 silica. The precipitation of the iron and silica from these waters 

 resulted in the production of the iron formations. This hypothesis 

 appears to be an early formulation of the one now adopted by Van 

 Hise and Leith. 



J. E. Spurr, 97 in 1894, presented a somewhat different theory of 

 origin of the iron ores in the Mesabi Range. Spurr found certain 

 specimens in the iron formations that consisted of a ground mass of 

 finely crystalline silica in which was embedded rounded or subangular 

 bodies of a green mineral. This mineral was a hydrous silicate of iron 

 which he believed to be glauconite, though he was aware of the fact 

 that the potash content was low. He believed that the original rock 

 of the iron formations was an altered greensand, and thought that 

 the alteration of the iron silicate gave rise to silica, iron oxides and 

 iron carbonates. He regarded the banding of the iron formations as 

 due to the deposition of the products of oxidation along zones of 

 weakness in the original rock. 



Van Hise and Leith 98 state that since the green granules contain 

 no potash they cannot be regarded as glauconite. They suggest the 

 name greenalite for the green granules of ferrous magnesian silicate. 



Van Hise and Leith are convinced that the iron-bearing sediments 

 were, for the most part, originally in the form of greenalite rock with 



96 On a Possible Chemical Origin of the Iron Ores of the Keewatin in Minne- 

 sota, Am. Geol., vol. 4, p. 291, 1889. 



97 The Iron Ores of the Mesabi Eange, Am. Geol., vol. 13, p. 335, 1894. 



98 Monograph 52, pp. 168-69. 



