Action of the Atmosphere upon newly -deepened Soil. 471 



3. Bisulphuret of Iron, or Pyrites. — Composed of — 



Iron 46*7 



Sulphur 53-3 



100-0 

 It is a frequent body in all kinds of rocks, being produced both in 

 igneous and sedimentary formations. Less soluble in acids than 

 the preceding. It appears frequently to arise from the decompo- 

 sition of organic matter in solutions of iron. jNIr. Pepys found an 

 interesting instance of this take place by the putrefaction of some 

 dead mice in a solution of sulphate of iron, and Sir H. De la 

 Beche gives another of its formation where a dog had fallen into a 

 solution of iron, its body being found surrounded by bisulphuret 

 of iron or iron pyrites,' {Geological Observer, p. 600.) Its mode 

 of decomposition has been already referred to in showing its 

 decay to be a frequent cause of the disintegration of minerals. 



4. Magnetic Iron is a combination of protoxide and peroxide 

 of iron : — 



Protoxideof iron .. .. 31-23 Iron 72-40 



Peroxide of iron .. .. 68-77 Oxygen ., .. 27-60 



100-00 100-00 



Soluble in hydrochloric acid. Occurs mostly in igneous or meta- 

 morphic rocks, but also in beds and masses, chiefly in the northern 

 parts of the globe, being the most important ore of iron in 

 Sweden and Russia. It changes into oxyhydrate of iron and 

 also into peroxide. 



5. Hoimatite, or specular iron. — It is mostly peroxide of iron, 

 and is slowly soluble in acids : — 



Iron 70-03 



Oxygen 29-97 



100-00 

 It is a very plentiful substance, and is occasionally produced by 

 the gradual change of the carbonate and sulphuret of iron. 



6. Brown IIa:matite, or Limonite, is a peroxide of iron with 

 water : — 



Peroxide of iron So'O 



Water 14-4 



100-00 

 It occurs in rocks of all kinds, being produced often by the de- 

 composition of other ores. Water containing carbonic acid also 

 dissolves out the carbonate of the protoxide, which, on exposure 

 to the atmospliere, is converted into the hydrated peroxide and 

 precipitated, (arl)onic acid being given off. In this manner it 

 mav l)e observed how rain-water, percolating through soils con- 

 taining much iron, issues out of the drains full of t:arbonate of the 



