190 Transactions of the Society. 



tion yields by spontaneous evaporation olive green or yellow six- 

 sided tables of the basic salt (Fea O3 . 2 SO3) 2(K20 . SO3) 6 HgO 

 (Maus). 



It is also well known tbat ferric sulphate is easily formed by 

 oxidation in the air of ferrous sulphate, and that this latter is fre- 

 quently produced naturally from pyrites through the agency of air, 

 light, heat, and moisture. While the oxidation of ferrous sulphate 

 is in progress, ferric oxide is precipitated. 



This ferric oxide is soluble in the simultaneously produced ferric 

 sulphate, giving rise to a series of basic ferric sulphates which are 

 more or less insoluble, and most of which have been but little and 

 very imperfectly studied by chemists. 



From what we have just stated, it is clear that, beginning with 

 a solution of ferrous sulphate and submitting it to oxidation under 

 varying circumstances, almost any of the possible basic ferric salts 

 may be produced. If we begin with a lode of superficial deposit of 

 iron pyrites, exposed to the action of air and moisture (preferably 

 warm), and situated at such a distance from the sea and at such 

 an altitude that the escaping drainage from the lode would have 

 sufiicient fall and sufficient distance to travel ; or if it be collected 

 into a pool or lake so as to allow its ferrous salts to be more or less 

 perfectly converted by long exposure into ferric salts, we have all 

 that is required to explain the formation of such a basic sulphate 

 as that of the mineral cyprusite. It is true that the mineral 

 contains sulphate of alumina, but I am inclined to consider this 

 rather as an accidental admixture. Most of the surrounding rocks 

 are highly aluminous,* and from the fact that on the ground, at a 

 short distance from and lower down than these deposits of cyprusite, 

 there occur great incrustations or efflorescences, as noticed by Dr. 

 Eeinsch and myself, of soluble sulphate of alumina, one can scarcely 

 doubt that this soluble salt is being slowly dissolved out of the 

 cyprusite deposit, leaving behind the insoluble tribasic ferric 

 sulphate, admixed with organic silica. The further fact that 

 various specimens of the cyprusite, analysed by Mr. Fulton, 

 contained varying proportions of alumina and sulphuric acid con- 

 firms this. The sulphate of alumina probably at first came there 

 by spontaneous evaporation of the mother-liquors after the up- 

 heaval of the bed, or the drying up of the stream or deposit in 

 which it had collected. 



We have therefore only to imagine at first a stream of water 

 issuing or oozing from a pyrites lode, and carrying with it in 

 solution the products of decomposition of the lode and its walls, 



* An analysis of the compact dolerite or melaphyre, often altered into wake, 

 of this region, gives 54 '90 per cent, of silica, 26 '19 per cent, of alumina, and 

 14*53 per cent, of peroxide of iron as its composition. The percentage of 

 alumina is higher than in any other rock of this class known to me. 



