RELATIVE RICHNESS OF GALENA AND CERUSSITE. 555 



ther from the surface. In the case of the Iron mine the statement to this 

 effect by those in charge of the mine was supported by actual figures ; in 

 other cases it was an opinion founded on general observation, which was 

 still worthy of credence. Whether it will continue when the unoxidized 

 deposits are reached remains to be proved. 



A ready explanation for this condition of things may also be found in 

 the relative solubility of the products of alteration and in the fact that the 

 deposits near the present surface may be considered to be simply the relics 

 of larger deposits, gradually removed by erosion as the alteration by sur- 

 face waters went on. The original deposits may be assumed to have been 

 a mixture of galena, pyrite, and blende in proportions which, while subject 

 to a wide local variation, would bear a certain average relation throughout 

 the region. Given a ton of this mixture, by the alterations which have been 

 above noted it would probably decrease in weight, though its volume might 

 remain sensibly the same ; zinc and iron would be removed as soluble sul- 

 phates, the former in greater part, the latter in ver}^ appreciable amount, 

 and their volume would be replaced in part by the increase in volume of 

 the lead during its transformation into carbonate, in part also by silica and 

 earthy bases brought in mechanically by the waters. Although silver salts 

 have been assumed above to be, in this case, more soluble than those of 

 lead, they are much less so tlian those of iron and zinc, and a relatively 

 small proportion of the silver would have been removed from the given 

 ton of ore. The space formerly occupied by this ton of ore would, therefore, 

 be occupied by oxidized material weighing much less than the original 

 ton, but which would contain, in proportion to that weight, more lead and 

 silver and less zinc and iron. In accordance with this explanation, the pro- 

 portion of lead to the ton of ore should also decrease in depth as a general 

 average of the district. 



Again, as by gradual erosion the deposits — say, for instance, those in 

 the easterly-dipping beds of Carbonate and Iron Hills — approached the sur- 

 face, actual surface waters running- down along the dip would meet bodies 

 composed mainly of carbonate of lead and chloride of silver. If carbonate 

 of lead is soluble in water containing free carbonic acid, the portion thus 

 dissolved would be carried away entirely, for in their continued passage 

 through limestone it may be assumed that the waters would preserve their 



