14 



The Scottish Naturalist. 



When the oxygen and iron unite in unequal proportions, say 2 of 

 iron and 3 of oxygen, the compound is called the peroxide of iron, 

 or the sesqui-oxide (perhaps the more significant term, as it tells 

 us the proportions of iron and oxygen, expressed by sesqui (Latin) 

 meaning one and a half) denoted as Fe O . 



Iron, I have already said, is found in all the different rock 

 systems, and is calculated to form 2 per cent of the crust of the 

 earth. When found in beds or deposits it is the result of the 

 decomposition of rocks or substances that previously contained 

 the iron in some form or another. 



Limestone itself is known to contain iron as a carbonate, or as 

 a protoxide or peroxide. But the question naturally rises, In 

 what form was this iron before its deposition in the limestone as 

 Haematite, and how was it deposited ? I have no doubt it has 

 been by infiltration. The iron in the present specimen had 

 existed in another form. It may have been as a carbonate of iron 

 in the limestone itself, or in some superincumbent or underlying 

 stratum ; or it may have been produced by the reduction of iron 

 pyrites to the state of carbonate of iron. A stream of watei 

 passing over or through the carbonate of iron would carry this 

 with it, and if exposed to the atmosphere, the oxygen in the 

 latter combining with the iron held in solution, the resulting 

 deposit would be an oxide of iron. It is a peculiarity of iron that 

 its hydrated peroxide can be reduced into the protoxide by the 

 presence of organic substances. And this protoxide, again, by the 

 presence of carbonic acid gas, generated from decomposing 

 organic remains, can produce the carbonate of iron ; and again, 

 the carbonate of iron, held in solution, is reduced to the peroxide 

 by the oxygen of the atmosphere, as I have just mentioned. 



This has evidently been the process by which this peroxide of 

 iron was first deposited. That the deposit was gradual is evident, 

 as it could take place only by the union of the oxygen of the atmo- 

 sphere with the carbonate of iron, which was being carried along 

 and deposited in the bed of limestone. By and by, however, a 

 change took place, either in the source of supply, or in the mode 

 of its solution and subsequent deposition j as, in place of this 

 less pure and red-ochrey substance, we have a layer of compact 

 haematite, dark, fibrous, and sometimes nearly specular in texture. 

 This layer deposited, a further change in the mode of infiltration 

 or in the supply took place, and it is a remarkable one. The 



