174 ALLEN: ORE DEPOSITION 



conclusions relating to the formation of these minerals in nature. 

 (1) We know that the mineral marcasite can be formed by the 

 action of hydrogen sulphide on ferric sulphate solutions, or, 

 what comes to the same thing, a solution containing ferrous sul- 

 phate 2 and hydrogen sulphide to which the air finds some access. 

 If the solution contains as much as 1 per cent of free sulphuric 

 acid and the temperature is not above 100°, the product is all 

 marcasite. It is also not improbable that pure marcasite can 

 form at a higher temperature if the solution is still more acid. 

 Now, these conditions agree strikingly with what we find about 

 the genesis of marcasite from field study. It is always a surface 

 mineral, presumably formed from surface solutions at low tem- 

 peratures. It is also a matter of common knowledge that such 

 solutions when they contain iron usually carry it in the form of 

 sulphate, together with sulphuric acid, since they result from the 

 oxidation of an older body of pyrite or marcasite. (2) We know 

 that pyrite forms from solutions which are neutral or alkaline 

 by the action of alkaline polysulphides on ferrous salts and by 

 the action of hydrogen sulphide on ferric hydroxide. Both reac- 

 tions are essentially the union of ferrous sulphide and sulphur 

 in a neutral or alkaline medium. Coming again to the deduc- 

 tions from field observation, it is found that pyrite is always 

 deposited by hot waters instead of marcasite, and that deep veins 

 which formed under comparatively high temperatures also con- 

 tain pyrite, never marcasite. Now, hot terrestrial waters are 

 practically always alkaline, not only because silicates are hydro- 

 lized by hot waters with the necessary formation of an alkaline 

 solution, 3 but because if a strong acid were allowed to act on rocks 

 at the high temperatures which prevail in the depths of the 

 earth's crust, chemical action would soon neutralize it by the 

 decomposition of carbonates, silicates, etc. (3) Pyrite and mar- 

 casite form together when hydrogen sulphide acts on ferric sul- 

 phate unless the percentage of free acid and the temperature are 



2 It is not essential to use the sulphate; any other ferrous salt will suffice. 



3 This, of course, postulates the presence of some alkali or alkaline earth metal 

 in the rocks, but some of these metals are practically always present. 



