432 THE CANADIAN NATURALIST. [Vol. ix. 



may be deposited either as hydrous peroxide (limonite, etc.,) as 

 carbonate (siderite), as silicate, or as sulphide (pyrite, etc.), 

 in all of which forms iron is found in sedimentary deposits. As 

 regards the formation of siderite, he described experiments which 

 show that solutions holding five grammes of ferrous carbonate 

 dissolved as di-carbonate in a litre of water, are spontaneously de- 

 composed in close vessels at the ordinary temperature, and deposit 

 two-thirds of their iron as a white crystalline (hydrated) mono- 

 carbonate, with liberation of carbonic-dioxide. This serves to 

 render more intelli";ible the reduction and se^res-ation of iron as 

 siderite in earthy sediments, as long since pointed out by W. B. 

 Kogers, for the ores of the coal-measures. 



The intervention of the soluble sulphates, and their reduction 

 through organic agency to sulphides, determines the formation of 

 sulphide of iron in sediments. The generation of a bi-sulphide 

 (pyrite or marcasite) was then discussed, and it was shown that 

 the ferrous mono-sulphide, which naturally is first generated, 

 may fix a further portion of sulphur, and thus form a more stable 

 compound. One example of this is seen when recently preci- 

 pitated hydrous ferrous sulphide is brought in contact with a 

 solution of a ferric salt, which takes up a portion of the iron, 

 leaving sulphur free to unite with the undecomposed sulphide, 

 and form therewith a very stable higher sulphide of iron. Ex- 

 periments now in progress lead the writer to believe that sulphur 

 liberated from soluble sulphides may, in a similar manner, unite 

 with ferrous sulphide, and thus help us to explain the generation 

 of pyrites in nature, in the presence of water, at ordinary tem- 

 peratures. 



The changes of siderite and pyrite under atmospheric influences 

 were next considered. The latter by oxidation yields, as is well 

 known, ferrous sulphate. Its frequent conversion by sub-aerial 

 decay into limonite was conceived to be due to the intervention 

 of water holding carbonates, which conjointly with oxygen, changes 

 it into hydrous peroxide (limonite), which latter often retains the 

 form of the pyrites. The transformation of carbonate of iron 

 into hydrous peroxide is a familiar fact. 



Limonite ores may thus be produced in three ways. They are 

 sometimes formed by the peroxidation and precipitation of dis- 

 solved salts, as in the so-called bog-ores ; but more frequently 

 from the alteration in situ of deposits of pyrite or siderite. Such as 

 these are the limonites which mark the outcrops of beds or veins 



