68 sosman: peoblems of the oxides of iron 



is the explanation offered for many of the deposits of titaniferous 

 magnetite, such as the Ramsoy deposits of Norway. 24 It has 

 also been suggested that the magnetite may have separated by 

 gravity as a liquid or partly liquid layer while the entire magma 

 was still molten. 25 These explanations based on magmatic 

 differentiation have met with various difficulties, geological as 

 well as chemical and physical. The separation of a liquid layer 

 of pure magnetite, for instance, is hardly believable, since the 

 melting point of magnetite is 1580°, whereas the intruding rock 

 bodies from which it separated could hardly have been at a 

 temperature of more than 1000? Whether small quantities of 

 other substances might greatly lower the melting temperature of 

 magnetite is one of the problems yet to be solved experimentally. 



The other mode of origin assumes that the iron was carried 

 in the hot solution or vapor given off by an intruding igneous 

 mass. There are many deposits for which this mode of origin 

 seems well established. Examples of these are the magnetite 

 deposits of Cornwall, Pennsylvania, 26 and the hematite-magnetite 

 ores of the Island of Elba. 27 



An interesting feature of the latter, which we discovered in 

 the course of some magnetic measurements, is the zonal de- 

 velopment of certain crystals. 28 The hematite crystals from 

 Elba are well known for their large size and excellent devel- 

 opment. They are not pure Fe 2 3 , however, but contain a 

 considerable percentage of ferrous iron. By chemical analysis 

 and by magnetic measurements we have shown that the ferrous 

 iron is zonally distributed, being higher in the core and base 

 of the crystal and lower in its outer surface and free-growing tip. 

 Evidently, the temperature or oxygen concentration, or both, 

 were changing continuously in one direction while these crystals 

 were being deposited. 



The reactions that will lead to the deposition of hematite, 

 hematite-magnetite, or pure magnetite from a vapor or a liquid 



24 Foslie, S. Norges Geol. Unders., vol. 4, 1914. 



25 Daly, R. A. Origin of the iron ores at Kiruna. Vetensk. prakt. Unders. 

 Lappland. Geology, No. 5. 1915. 



26 Spencer, A. C. U. S. Geol. Survey, Bull. 359. 1908. 



27 Lotti, B. Mem. descr. carta geol. Italia, II. 1S86. 



28 Jour. Wash. Acad. Sci., 6: 309. 1916. 



