288 TBE PENOKEE IRON-BEARING SERIES. 



contained alkalies — which might readily be obtained from the alteration of 

 the basic dikes — of taking up a small amount of silica. Other water falling 

 upon higher layers of *the formation would make its way slowly and with 

 difficulty through these less altered parts, and would oxidize iron carbonate 

 until all oxygen had been extracted from it. This oxidation of a part of 

 the iron carbonate would liberate carbon dioxide, which would be taken 

 into solution and added to tlie carbon dioxide which the water already con- 

 tained.' Such water would take into solution unaltered iron carbonate. It 

 would also, in its upper course, take up what silica it was able to carry. As 

 it penetrated farther and took more carbon dioxide into solution, and con- 

 sequently also more iron carbonate, it Avould be less able to carry silica, 

 and would deposit that material as chert in the lower horizons. This water, 

 thus traveling on with an increasing amount of iron carbonate, would 

 finally reach a dike and be detlected toward the foot-wall quartzite. This 

 dike it would follow until the apex of the trough was reached. Here it 

 would mingle witli a larger amount of water more directly from the surface, 

 bearing oxygen, and therefore capable of oxidizing the iron carbonate. 

 The iron would then l)e precipitated in the apex of the trough as more or 

 less hydi'ated sesquioxide of iron. 



Upon the other hand, the silica woukl here be dissolved, for the I'arbon 

 dioxide solution coTitaining iron carbonate would be greatly diluted by the 

 large amount of wiiter which l>ore the precipitating agent for the inni, and 

 the resultant abundant dilute solution of carbon, dioxide, bearing j^erhaps 

 alkalies witli it, would l)e capable of taking up silica, which was either 

 originally present or had been subsequently deposited in the apex of the 

 trough. Such solutions may have furnished the silica Avhich has enlarged 

 the particles of quartz in tlie foot wall and thus indurated it." The result 

 of this leaching would be to steadily add iron oxide to and to remove the 



' 111 tliis (liscussiou carboiiateil water Is taken :i8 the agent of solution. It is likely enoiijjli that 

 oiganii' acids have helped to take the iron earboiiate in solution and bear it to the jioint.s ol' ]irecipi- 

 tation. 



•"I'be iheiiiistry ol' the process thus oulliued assumes the Ibllowiiif; : That the ox.vj;eii of (lerco- 

 hiiiug waters is sufficient to oxidize iron carbonate not in solution and set carbon dioxide free; that 

 the resultant carbonar(^(l waters are sufticicnt to take iron carbonate in solution; that if such waters 

 bearing diss(dved carbonates are ininf;lcil with other waters lieariny oxysjeu, the iron carbonate 

 or a )iortlciii of it will lie iiriM-ipitated ; that, silica may be carried in percolating waters; that carbou 



