CHAPTER VIII. 



MANNER OF THE DEPOSITION OF THE ORE. 



Derivation and circulation of the metalliferous solutions. In the foregoing chapter it liaS 



been stated that the ore was probably derived from some massive rock by 

 solfataric action. The solutions containing the ore penetrated the limestone, 

 passing through fissures and interstices in the broken rock, and deposited 

 the ore where conditions of temperature, pressure, and chemical activity 

 were favorable to its precipitation. The irregularity of the deposits and 

 their connection with fissures and other phenomena have already been 

 described and accounted for, but as yet no attempt has been made to 

 explain the causes which led to the release of the minerals from the solu- 

 tions which contained them and their aggregation in immense chambers. 

 It is impossible to determine what may have been the chemical composition 

 of these solutions, but it is not improbable that they consisted in great part 

 of sulphides of the heavy metals dissolved in alkaline sulphides. These 

 solutions were necessarily formed under the influence of heat and pressure. 

 Rising into the shattered limestone at a diminishing pressure and tempera- 

 ture, the liquids lost much of their solvent power and many of the metals 

 that they contained were precipitated. 



Manner in which the ore was deposited. As tO the Uiannei* of this precipitation, tWO 



theories only are admissible, either that the ore was precipitated from the 

 solutions in pre-existing large openings, or that it was substituted directly 

 for the limestone, that rock being dissolved and metallic minerals being left 

 in its place. In other words, the ore was either deposited in caves and 

 other openings, or the caves found above the ore bodies were caused by a 

 shrinkage of the ore and the action of dissolving waters. 



Importance of the manner of deposition. At fil'St Sight this question does llOt Seem 



of great practical importance, for if the mineral-bearing solutions came 



(93) 



