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



hydrate would be deposited in the pores of the shell or upon its inner 

 surface. 



In attempting to picture these reactions and their results, it is 

 important to remember the extremely slow rate at which oxidation 

 proceeds under such conditions. Even at the surface where decom- 

 position is comparatively rapid, the oxidation of pyrite appears to 

 progress at a very slow rate, perhaps not exceeding an inch or a 

 few inches in depth in several hundred years. 



If the iron be present as carbonate, a precisely similar series of 

 reactions may be conceived, in which carbonic acid transposes the 

 silicates, freeing silicic acid and removing the bases as soluble 

 carbonates. 



Other observers have noted the occasional presence of a central 

 core of siderite or pyrite in bombshell ore, but have generally attrib- 

 uted the presence of such cores to concretionary action and replace- 

 ment by sulphates (accompanied by reduction to sulphide) or 

 carbonates in solution. 



The foregoing theory, advanced to account for the origin of 

 bombshell ore, is based upon a study of these deposits dating back 

 to 1885 — when the writer was personally engaged in mining brown- 

 hematite ore — and upon examinations of many specimens which 

 show more or less clearly the character of the original material from 

 which such ore is formed. It will form an integral part of a broader 

 statement, extending the application of this theory to the genesis 

 of limonite ores, and including a discussion of the original sources 

 of the iron, methods of mineralization, and subsequent decomposition 

 and precipitation. 



