MANNER OF DEPOSITION. 101 



Were the ore composed entirely of pyrite it would shrink on oxidizing. 

 Although the hydrated oxide of iron occupies molecule for molecule more 

 space than the pyrite, yet the quantity of iron left in place after the oxida- 

 tion of a mass of pyrite is not equal to the amount of that metal that the 

 pyrite originally contained, owing to the fact that in the process of oxidation 

 soluble salts are formed which are carried off. To the action of such soluble 

 salts is no doubt due the staining of the limestone with ferric oxide in the 

 neighborhood of ore bodies. On the other hand, if the ore were composed 

 entirely of galena it would increase in bulk when changed into carbonate, 

 though the carbonate of lead is to some extent soluble in waters carrying 

 carbonic acid. 



observations made in a mass of pyrite. — From what has been observed of a mass of 

 pyrite containing but a little blende and galena on the eleventh level of the 

 Eureka mine, it would appear that oxidation is followed by a slight shrink- 

 age of the ore body. This body of mineral is a compact mass which seems, 

 as far as explorations have developed it, to touch the limestone everywhere 

 throughout its surface. The pyrite, however, is separated from the lime- 

 stone by a coating of ferric oxide nearly a foot thick. The ferric oxide 

 contains grains and rounded fragments of limestone, and by its structure 

 shows that the pyrite from which it was derived was brought into place by 

 substitution. Pseudomorphs of pyrite after calcite are well known, although 

 no specimens have been observed in Eureka; pseudomorphs of pyrite after 

 fragments of limestone, however, are often found on Ruby Hill. The ferric 

 oxide is also much more porous and less compact than the pyrite. A shrink- 

 age of the mass took place upon the leaching which followed oxidation. 



Evidences of the contraction of ore bodies since oxidation. As OVritO Originally COmDOSed 



more than half the volume of the ore bodies, it is highly probable that a 

 considerable contraction has taken place in their mass since oxidation began. 

 Supposing such a decrease of volume to have been brought about by oxid- 

 ation, an opening would be produced above the ore body proportioned to its 

 size, as the ore becoming porous would naturally settle by its own weight. 

 Add to this the action of surface waters carrying carbonic acid, which 

 would enlarge these cavities and to some extent redistribute the ore, and a 

 condition of things is brought about precisely similar to that which at 



