THE IRON SERIES (IN PART). r /3 



Limestones invariably have at least small amounts, and at times 

 very considerable percentages. Sandstones are often low, but not 

 seldom are stained through and through. The metamorphic rocks 

 offer close analogies to the igneous. In general distribution and in 

 quantity, iron leads the list of the distinctively metallic elements. 

 Its peculiar property of possessing two oxides, of different chemical 

 quantivalence, assists greatly in the formation of ores and the 

 general circulation of the metal. This is set forth under the 

 following examples. 



LIMONITE. 



2.01.05. Example 1. Bog Ore. Beds of limonite, superficially 

 formed in marshes, swamps, and pools, of standing water. The 

 general circulation of water through the rocks enables it very 

 frequently to take up iron in solution. Ferruginous i&inerals are 

 among the first and easiest that fall a prey to alteration. Car- 

 bonic acid in the water aids in dissolving the iron, which thus, in 

 waters containing an excess of CO 2 , passes into solution as the 

 protocarbonate FeCO 3 . Organic acids may also play a part. The 

 alteration of pyrite affords sulphuric acid and ferrous sulphate, 

 and the latter enters readily into solution. On meeting calcium 

 carbonate, both ferric and ferrous sulphate are decomposed, yield- 

 ing in the first case calcium sulphate, ferric hydrate, and carbonic 

 acid; in the second, if air is absent, ferrous carbonate and calcium 

 sulphate, but on the admission of air ferric hydrate soon forms. 

 (See F. P. Dunnington, Amer. Jour. Sci., iii., XXXVI. 176. Ex- 

 periments 10 and 11. See also Addenda.) 



2.01.06. Bodies of limonite that become exposed to a reduc- 

 ing action from the favorable presence of decaying organic mat- 

 ter likewise furnish the protocarbonate. In general it may be 

 stated that free oxygen must be absent or only in small quantity 

 where solution takes place. Sooner or later the ferruginous (or 

 chalybeate) waters come to rest, especially in swamps. The proto- 

 salt is exposed to the evaporation of the excess of CO 2 , that held 

 it in solution, and also to the action of oxygen. Two molecules 

 of carbonate, together with one atom of oxygen and some water, 

 break up into CO 2 and Fe 2 O 3 , x H 2 O. The latter forms as a scum 

 and then sinks to the bottom and accumulates in cellular masses. 

 The sesquioxide is insoluble, and as against ordinary waters free 

 from reducing agents it remains intact. Deposits of mud and 

 peat forming above may cover the beds with a protecting layer. 



