496 T. C. HOPKINS — CAMBRO-SILURIAN LIMONITE ORES 



Where there is sufficient vegetable growth, all of the iron may be ex- 

 tracted, ;is in some of the underclays of the Coal Measures. 



The organic, acids are an important factor in the transference of the 

 iron, as well as in its solution. In fact, so important and direct is their 

 action that many geologists raise the question whether every great 

 accumulation of iron ore does not necessarily imply the presence of 

 organic matter. The ferrous salts are unstable in the presence of oxygen, 

 and hence would not remain long in solution in meteoric waters unless 

 some reducing agent were present. The organic acids have such a strong 

 affinity for oxygen that as long as they are present they prevent the 

 oxidation of the iron to the insoluble oxide, or, what amounts to the 

 same thing, speedily reduce it to the soluble form again. 



The meteoric waters, charged with organic and other acids, reduce, 

 dissolve, and carry away the iron in solution. The next step is its pre- 

 cipitation, which is in most cases from the bicarbonate solution, since 

 carbonic acid is the final stage of all the organic acids. The precipita- 

 tion may be caused by (1) relief of pressure, (2) oxidation, (3) desiccation 

 or concentration, and (4) neutralization or chemical reaction by another 

 salt in solution, or by replacement of another base in the solid, or it may 

 be a combination of two or more of these causes. The iron may be pre- 

 cipitated as the hydroxide, as the carbonate, or as one of the organic 

 salts, but, in whatever form, it is only a question of time until it is brought 

 to the stable form of the hydroxide. The precipitation from the sulphate 

 solution may be by oxidation alone or, what is more probable, oxidation 

 and neutralization combined. 



The precipitation may take place quite remote from the original posi- 

 tion or it may be quite near or even at that point, as is the case in the 

 pseudomorphs of limonite after siderite and pyrite. In the limestone 

 regions it is thought that the iron is not carried far in solution, because, 

 as the waters become saturated with lime, which is more soluble than 

 the iron, the latter would be precipitated. A large part of the iron is 

 probably deposited in the upper or cavernous portion of the limestones. 



Part of the iron is deposited in cavities and caverns in the limestone, 

 as already indicated in the description of the ore forms. The concentric 

 layers of fibrous ore, lining some of the nodular masses, clearly indicate 

 deposition in a cavity, as do the iron oxide stalactites. The loose masses 

 of pipe ore indicate pretty conclusively that they were formed in caverns, 

 The nodules that were formed by the iron oxide, coating rock fragments. 

 were formed probably in caverns or in the residual material. The brec- 

 ciated ores were formed by the deposition of the iron in a mass of loose 

 rock material ; it may have been in caverns, on the surface, or in the 

 residual material. The sheet ore is, without doubt, formed in seams in 



