igoS] . 



CHANCE— THE ORIGIN OF BOMBSHELL ORE. 139 



In the absence of oxygen, carbonates in solution may, as shown 

 by Dr. N. H. Stokes, completely oxidize the iron of pyrite or mar- 

 casite thus : 

 8FeS, -f i5NaoC03 = 4Fe,03 + nXa.S + Na.S^Og + 15CO0, 



and under proper conditions of temperature and pressure the ferric 

 oxide thus formed may be deposited as hydrate ; but these reactions 

 do not satisfy the observed conditions and it seems more probable 

 that oxidation near the surface has proceeded as indicated by some 

 of the following reactions : 



FeS, + HoO + 70 = FeSO, + HoSO^ ; 



aFeSs + 8H0O + 22O = Fe.Og, sH^O + FeSO, + sH.SO, ; 



2FeSO, + 5H2O + O = Fe.Og, 3H0O + 2H2SO, ; 



2FeS2 + 7H0O + 15O = Fe.Os, SH.O + 4H0SO, ; 



2FeS2 + 4H2O + 15O = FcoOs + 4H,SO, ; 



2FeS04 + 2H2O + O = FcoOa + 2H0SO, ; 



6FeSO, + 30 = FcoOg + 2Fe,(SOj3 ; 



6FeSO, + 3H,0 + 30 = Fe^Og, 3H,0 -f 2Fe, ( SOJ 3 ; 



2Fe,(SO,)3 + 2FeS. + 4H,0 + 12O = 6FeSO, + 4H,SO,. 



The sulphuric acid having been formed in direct contact with the 

 gangue, it is reasonable to suppose that it must at once attack any 

 clay or other decomposable material, and the removal of the soluble 

 silicates and silicic acid by transfusion through the walls of the bomb 

 is readily pictured. It is, however, possible that the colloidal silicic 

 acid may be retained, and further that it may perhaps often 

 be set free in a gelatinous condition. This latter hypothesis may 

 account for the frequent presence in such ores of a skeleton of 

 amorphous silica which appears to completely ramify some parts of 

 the limonite. 



If the oxidation proceed according to these equations, the succes- 

 sive additions of layers of limonite to the interior of the shells is 

 doubtless due to the further oxidation of the ferrous sulphate as 

 above shown, the oxidation of the solution occurring at or in the wall 

 of the shell where the solutions, in escaping by transfusion through 

 the walls of the shell, are met by oxidizing waters transfusing 

 towards the center of the shell. Under such conditions the ferric 



