ORIGIN A]ND DEVELOPMENT OF ORE-REPOSITS. 355 



subject to decomposition, with the production of sulphuretted 

 hydrogen. It may indeed happen in rare cases that the 

 sulphuretted hydrogen thus produced will subsequently by a 

 reverse process give rise to sulphides in the upper parts of 

 fissures, but this can hardly account for the immense deposits of 

 sulphides met with in some lodes ; as for instance the great 

 copper deposits of Clifford Amalgamated and Devon Great 

 Consols, and the large pyrites lodes at Wheal Jane. 



The well-known mutual relations of ore-deposits and 

 country rocks referred to in a previous section, certain rocks 

 being "congenial" and others "uncongenial," lend great 

 support to the hypothesis of lateral secretion, which is so closely 

 connected with the descension hypothesis as to be hardly 

 distinguishable from it. 



If however the percolating solutions were alkaline, as from 

 the kaolinization of felspathic rocks, and especially if these 

 waters had been circulating through deep-seated rocks where 

 the temperature was high and the pressure great, sulphides 

 might no doubt be dissolved, transferred, and re-deposited 

 without change, thus forming an example of ascension deposits. 

 And in fact, deep-seated sources do seem to be required for the 

 vein substances of group 4 in very many cases, and for groups 

 5 and 6 in all cases, either to give sufficient dissolving power to 

 the solutions, or to supply the characteristic non-metallic 

 components, or in some cases probably to supply the metallic 

 components. 



Let us consider these three cases separately. Suppose the 

 surface waters to reach far down into the interior of the earth 

 before making their way into a fissure and upward current. 

 They will become hot, they will be subject to great pressure, 

 and we may fairly suppose that they will in most cases become 

 alkaline from the decomposition of silicates. If as Sandberger 

 supposes, many of the silicates thus decomposed contain such 

 metals as tin and copper, the solution will be charged with these 

 metals. If sulphides are present in the rock, these will probably 

 be dissolved unchanged and without decomposition ; if chlorine or 

 fluorine is present, and also oxide of tin (as a rock-component) 

 fluoride or chloride of tin will be formed in the solution, and 

 when it makes its way into the fissures or cavities, metallic 



