GEOLOGY. 317 



Second. That this sesquioxide, in the presence of the changing vegetable 

 matter with which certain of the strata abounded, was converted into proto- 

 carbonate, Avhich remained in part diffused through these beds, or by process 

 of filtration and segregation was accumulated in particular layers. 



It is well known that during the slow chemical changes by which vege- 

 table matter, enclosed in moist earth, is converted into lignite or coal, both 

 light carburetted hydrogen and carbonic acid are evolved, and that these 

 gases are even eliminated from coal seams and their adjoining carbonaceous 

 strata. The reducing agency of the carbon and hydrogen, as they separate 

 in their nascent state from the organic matter, is capable, as we know, of 

 converting certain sulphates into sulphurets, and even more readily of trans- 

 forming the sesquioxide of iron into protoxide. The latter change would 

 doubtless be favored by the affinity of the carbonic acid present in the mass 

 for the protoxide, as formed, and in this way the sesquioxide would be entirely 

 converted into proto-carbonate of iron. 



Conceiving a like process to have operated on a large scale in the coal- 

 measures or other strata, containing, when deposited, a mixture of sesquioxide 

 of iron and organic matter, we have a simple explanation of the general con- 

 version of this oxide into carbonate, and of the loss of the reddish coloring in 

 which these materials more or less participated. As these actions must be 

 supposed to have commenced in each stratum as soon as the organic matter 

 contained in it began to suffer chemical change, we may conclude that the 

 formation of the proto-carbonate was already far advanced in the earlier strata 

 when only beginning in those deposited at a later period. Each layer of 

 vegetable matter, as it was transformed into coal, would not fail to impregnate 

 the adjoining beds of shale and sandstone with the proto-carbonate, and thus 

 the development of this compound was, as it were, coeval with that of the 

 coal. 



The gathering of the diffused proto-carbonate into bands and courses of ore 

 began, no doubt, as soon as the production of this compound had made some 

 progress, but it probably continued until long after the completion of the 

 chemical changes above described, and, indeed, it is possible that in some 

 strata it is not yet entirely finished. In this process, which finds a simple 

 explanation in the combined action of infiltration and the segregating force, it 

 can hardly be questioned that tJie carbonic acid, pervading the mass of sedi- 

 ment, acted a very important part. The large amount of this gas evolved 

 from the beds of vegetable matter, undergoing change, would impart to the 

 water of the adjoining strata the power of dissolving the diffused proto-carbo- 

 nate, which, being then carried by infiltration through the more porous beds, 

 would accumulate above and within the close argillaceous or shaly layers, 

 forming, in some cases, bands of rock ore, in others, courses of nodular and 

 plate ores. Of these, the former would seem to have resulted from the accu- 

 mulation by gravity of the dissolved carbonate in the substance of sandy 

 shales, near the upper limit of the more impervious beds, while we may regard 

 the latter as having been collected in all directions from the general charge of 

 proto-carbonate accumulated in the argillaceous mass, its mobility in the 

 dissolved condition greatly aiding the gathering process of the segregating force. 



