Ch. 28] TRANSPORTATION OF IRON 507 



areal extent, shape and thickness of the deposit, the texture, mineral 

 composition, and the presence of undesirable impurities, such as silica, 

 alumina, phosphorus, and sulphur. These physical and chemical prop- 

 erties are largely the result of the environment that existed during or 

 immediately after deposition of the sediment. Because iron deposits 

 form under a variety of environments, these properties may be ex- 

 pected to show considerable range, but fortunately they are rather 

 constant for each specific environment. The sedimentary iron deposits 

 may therefore be grouped according to mineral composition and origin 

 into various types which have physical and chemical attributes in com- 

 mon. This is of practical value, for, if the type of deposit is known, 

 the physical and chemical characteristics may be fixed within fairly 

 well-defined limits. For instance, certain types, such as the bog and 

 placer deposits, tend to be thin, lenticular, and low in grade t whereas 

 others, such as the Clinton and Lake Superior deposits, are remarkable 

 because of their thickness, lateral extent, or lack of impurities. The 

 physical and chemical characteristics of each type of deposit are 

 largely a heritage of the source, mode of transportation, and the en- 

 vironment of deposition of the iron. The geologist must appreciate 

 these genetic factors fully, or he will be without guidance in searching 

 for new ore bodies, carrying out development work, estimating ton- 

 nages, or prospecting intelligently for the continuity of known ore 

 bodies either laterally or in depth. 



SOURCE OF IRON 



Nearly every rock exposed at the surface of the earth contains some 

 iron, but the basic igneous rocks are especially rich in this element. 

 Mechanical and chemical weathering release this iron and make it 

 available as the largest single source for sedimentary deposits. Van 

 Hise and Leith (1911, p. 499) have suggested another potential source 

 of iron. They point out the close association of the pre-Cambrian 

 iron formations with contemporaneous submarine lavas and suggest 

 that the iron was derived directly from magmatic sources, either by 

 magmatic emanations from basic lavas poured out on the ocean floor 

 or by rapid decomposition of these rocks, owing to their contact while 

 hot with sea water. 



TRANSPORTATION OF IRON 



Iron may be transported mechanically in the form of various iron- 

 bearing minerals, as a colloid, or in true solution. 



