138 CHANCE— THE ORIGIN OF BOMBSHELL ORE. [May 15, 



If such sandstone or slate, is broken and fissured by faulting 

 and crushing, and by the development of cleavage planes, oxidation 

 by percolating waters will proceed along the joints or planes which 

 form the channels through which these waters circulate, and in each 

 fragment of the mineralized rock oxidation will commence upon the 

 outside and progress towards the center. 



In this way on T)uter skin or shell of limonite first forms on the 

 outside of the fragment, for if the iron be present as pyrite or mar- 

 casite while some of it may be removed as ferrous sulphate, this salt, 

 if formed, may immediately be oxidized and precipitated in situ as 

 ferric hydrate. The sulphuric acid formed by the oxidation of the 

 remaining molecule of sulphur will attack and decompose the clay 

 of the gangue, removing the bases as sulphates in solution ; the 

 silicic acid also escaping in solution, or combining with iron oxides 

 to form iron silicates, remains as an integral part of the ore. 



If clay be present in large quantity a portion will remain unde- 

 composed in the center of the bomb, together with all of the sand 

 originally present in the gangue. 



Hence, if the original pyritic material has a clayey (slate) gangue, 

 bombs may form containing no residual matter, or containing more 

 or less clay; if the gangue be sand and clay (arenaceous slate), the 

 sand only, or sand and clay may remain ; if the gangue be sand only, 

 some of this will remain as an impurity in the limonite forming the 

 body of the shell, and some as a partial filling of the interior of the 

 bomb. 



It is now well known that pyrite (or marcasite) oxidizing under- 

 ground, whether by waters carrying free oxygen, or by waters con- 

 taining no uncombined oxygen, or by reactions involving hydrolysis, 

 does not behave in the same way as when oxidized by exposure to the 

 air above-ground. One of the most common reactions above-ground 

 is that in which sulphur is set free, often written : 



FeSo-i-40 = FeS04 + S, 



but this rarely occurs beneath the surface, for the gossans of pyritic 

 veins seldom carry free sulphur, although there are a few noted 

 examples in which large deposits of sulphur are found between the 

 surface and the unoxidized portions of such veins. 



