E. H. L. Sehwarz — Gold in Cape Colony. 375 



the necessary solvent for the gold would be produced. But against 

 this we have two undoubted facts in the area we are describing : the 

 sandstones remain blue and have not the sulphides removed, even 

 though we have very palpable evidence that Posepuy's vadose water 

 is circulating through them ; and, secondly, the Malmesbury or 

 pre-Cape rocks in the neighbourhood, through which the deep-seated 

 water would have had to pass to get to the mineral veins in the 

 Table Mountain Sandstone, show no signs of having clefts or fissures 

 which are filled in with ore and veinstone. The mineralizing water 

 seems, therefore, to be undoubtedly that derived from the surface, 

 and at the same time is not an oxidizing medium ; if we are 

 searching for a solvent, I think we should look to the ferric sulphide 

 and not the sulphate as probably the active printsiple. The forma- 

 tion of ironstone gravel beneath the soil in so many places in the 

 neighbourhood shows that the surface water does contain a very 

 appreciable quantity of iron in solution. 



Doelter has shown that the sulphide is soluble to a certain extent 

 in pure water, and sodium sulphide is a solvent for gold ; there are 

 no sources which I can see whence a sodium salt could be derived, 

 but it seems reasonable to suppose that the ferric salt might take its 

 place. Until experiments have been carried out in this direction, it 

 is naturally impossible to make the assertion that any one salt, if 

 present in the water, would have dissolved the gold, but an enquiry 

 like this may be useful to show chemists in what direction geologists 

 want their help. I think, however, I have shown sufficiently clearly 

 that gold will go into solution in water containing salts which 

 naturally occur in the rocks, and that it is not necessary for the salt 

 to be a compound of oxygen. 



Whatever may have been the solvent, temperature and pressure 

 would increase its action. Certain salts like sodium sulphate are 

 more readily soluble with increasing temperatures and pressure up 

 to a certain point, when there is a sudden decrease, while if there is 

 a contraction in volume when the salt goes into solution, pressure 

 and heat decrease the solubility.^ These cases, however, are rare 

 exceptions to the general rule that the rate of increase of solubility 

 is a function of temperature and pressm*e, and it is a fairly safe 

 statement that in the case of gold the rate of solubility is far greater 

 10,000 feet below the surface than it is at 1,000 feet. On this fact 

 depends the question whether the reefs become richer with depth, 

 and unless the gold behaves exceptionally with its solvent we may 

 say that they do. The banket naturally, being a stratified deposit, 

 would be unaltered with depth, but serving as a source of gold for 

 the quartz-veins, and being near the cracks down to great depths, it 

 is almost certain that there is increased solubility and consequent 

 deposition of gold. 



1 therefore define the Millwood gold-reefs as true fissure veins, 

 limited, however, in depth by the extent of the disturbance caused 

 by the arm of granite, and filled in with veinstone and ore by 



1 Ostwald : Algemeine Chemie, pp. 1046, 1048. 



