ORES DEPOSITED AS SULPHIDES. 563 



It remains, then, to considei* the possible reactions which may have 

 brought about the deposition of ores under the circumstances and in the 

 manner assumed above; and in considering these it must be borne in mind 

 that it is not possible to reproduce in the laboratory all the conditions that 

 may have prevailed at the depths within the earth's crust at which these 

 deposits were formed, and that therefore reactions may have taken place 

 and combinations may have been formed under these conditions which, 

 from laboratory experience alone, might not be deemed possible. 



The sulphides of the heavy metals may be precipitated, according to 

 Koth,^ from various solutions: first, where they exist as sulphides, by sul- 

 phides of the alkalies and alkaline earths; second, where they exist as carbon- 

 ates and sulphates, when they come in contact with solutions containing 

 the alkalies and alkaline earths or sulphureted hydrogen; third, where they 

 exist as sulphates, which in contact with organic matter are reduced to 

 sulphides. The metallic sulphides are soluble in waters containing alka- 

 line sidphides or sulphureted hydrogen, and silica and the earthy bases in 

 water containing alkaline carbonates. Solfataric waters (that is, hot waters 

 charged with mineral matter arising from some unknown source below) are 

 known to contain sulphureted hydrogen and the alkaline sulphides and 

 carbonates. On the supposition that the metals of these deposits came up 

 from the unknown source below or were derived from pyrite and galena 

 in neighboring rocks, it might be assumed that the iron and lead at least 

 were actually brought in as sulphides ; in this case, however, it is somewhat 

 difficult to conceive the reaction by which the sulphides should replace the 

 cabonates of lime and magnesia, and, so far as laboratory experience 

 teaches, it would seem necessary that the carbonates should have already 

 been dissolved out and carried away before the sulphides were deposited. 

 This apparently involves the pre-existing cavity theory. It is, however, 

 conceivable that the dissolving out of the former so immediately preceded 

 the deposition of the latter that the process was practically an interchange 

 of substance for substance, or the commencement of a change from sul- 

 phide to sulphate may have taken place in presence of the carbonate, and 



'Allgemeiue Geologie, p. 563. 



