A TREATISE ON METAMORPHISM. 



metric tons of metallic iron. The question naturally arises, What has 

 become of this enormous amount of iron? The processes of metamorphism 

 have segregated from 30 to 70 per cent of iron in the various iron-bearing 

 formations and in the iron-ore deposits. (See pp. 842-846, 1193-1198.) 



While the iron-ore deposits themselves contain much the larger per- 

 centage of- iron, the main mass of the segregated iron is in the iron-bearing 

 formations rather than in the iron ores. Indeed, the amount in the iron 

 ores is probably insignificant as compared with the amount in the iron- 

 bearing formation. For instance, in the Mesabi district of Minnesota, 

 where the iron-ore deposits are larger than in any other region, Doctor 

 Leith has calculated that in the part of the iron-bearing formation which is 

 exposed at the surface, including no part which passes below the overlying 

 slate, the amount of disseminated iron is probably one hundred times as 

 great as that contained in the ore deposits. And the amount of this 

 formation below the slates, in which there are no known ore deposits, is 

 certainly many times, probably hundreds of times, that exposed. This 

 calculation in reference to the Mesabi range shows how trivial is the 

 amount of iron in the ore deposits as compared with the more widely 

 distributed lower-grade products of the iron-bearing formations Such 

 formations are illustrated by the great pre-Cambrian iron-bearing forma- 

 tions of various parts of the world, such as those occurring in the Lake 

 Superior region ; by the very extensive iron-bearing member and the ores 

 of the Clinton horizon of the Silurian; by the great iron-bearing horizons 

 of the Carboniferous, and by the bog deposits of the Pleistocene. It is 

 believed that if the iron-bearing formations and the iron ores associated 

 with them were represented in the above equation in proportion to their 

 mass and percentage of iron, the excess of iron in them beyond that of the 

 original rocks would be nearly or quite sufficient to account for the great 

 deficit of iron shown by the ordinary sediments. The segregation of the 

 iron in the iron-bearing formations and the ores is treated on pages 

 102-130, 823-846, 1193-1198. 



In the zone of katamorphism many minerals are produced by the 

 alteration of the original iron-bearing minerals. Of these the hydrated 

 oxides of iron, especially limonite, and the silicates, such as the chlorites and 

 the epidotes, are important. Where the rocks pass into the deep-seated zone 

 any of the minerals in which the iron originally occurred may be reproduced. 



