228 ORIGIN OF METALLIFEROUS DEPOSITS. [XII. 



ochre, or, under somewhat different conditions, becomes aggre- 

 gated as a massive iron-ore. A process identical in kind with 

 this has been at work at the earth's surface ever since there 

 were decaying organic matters, dissolving the iron from the 

 porous rocks, clays, and sands, and gathering it together in 

 beds of iron-ore or iron-ochre. It is not necessary that these 

 rocks and soils should contain the iron in the state of pro- 

 toxide, since these organic products (which are themselves 

 dissolved in the water) are able to remove a portion of the 

 oxygen from the insoluble peroxide, and convert it into the 

 soluble protoxide of iron, being themselves in part oxidized 

 and converted into carbonic acid in the process. 



"We find in rock-formations of very different ages beds of 

 sediments which have been deprived of iron by organic agen- 

 cies, and near them will generally be found the accumulated 

 iron. Go into any coal region, and you' will see evidences that 

 this process was at work when the coal-beds were forming. 

 The soil in which the coal-plants grew has been deprived of its 

 iron, and when burned turns white, as do most of the slaty 

 beds from the coal-rocks. It is this ancient soil which con- 

 stitutes the so-called fire-clays, prized for making bricks which, 

 from the absence of both iron and alkalies, are very infusible. 

 Interstratified with these we often find, in the form of iron- 

 stone, the separated metal ; and thus from the same series of 

 rocks may be obtained the fuel, the ore, and the fire-clay. 



From what I have said it will be understood that great 

 deposits of iron-ore generally occur in the shape of beds ; al- 

 though waters holding the compounds of iron in solution have, 

 in some cases, deposited them in fissures or openings in the 

 rocks, thus forming true veins of ore, of which we shall speak 

 further on. I wish now to insist upon the property which 

 dead and decaying organic matters possess of reducing to 

 protoxide, and rendering soluble, the insoluble peroxide of iron 

 diffused through the rocks ; and reciprocally the power which 

 this peroxide has of oxidizing and consuming these same 

 organic matters, which are thereby finally converted into car- 

 bonic acid and water. This last action, let me say in passing, 



