Van Hise — Iron Ores of the Penokee-Qogebic Series. 4& 



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 in solution, and consequently also more iron carbonate, 

 it would be less able to carry silica, and would deposit that 

 material as chert in the lower horizons. The water thus travel- 

 ing on with an increasing amount of iron carbonate, would 

 finally reach a dyke and be deflected toward the foot-wall 

 quartzite. It would follow this dyke until the apex of the 

 trough was reached; here it would mingle with a larger 

 amount of water more directly from the surface bearing oxy- 

 gen, and therefore capable of oxidizing the iron carbonate. 

 The iron would then be precipitated in the apex of the trough 

 as more or less hydrated sesquioxide of iron. 



Upon the other hand, the silica would here be dissolved ; 

 for the carbon dioxide solution containing iron carbonate would 

 be greatly diluted by the large amount of water which bore 

 the precipitating agent for the iron, and the resultant abundant 

 dilute solution of carbon dioxide bearing perhaps alkalies with 

 it, would be capable of taking up silica which was either origi- 

 nally present or had been subsequently deposited in the apex 

 of the trough. Such solutions may have furnished the silica 

 which has enlarged the particles of quartz in the foot-wall and 

 thus indurated it. The result of this leaching would be to 

 steadily add iron oxide to and remove the silica from the 

 apices of the trough formed by the quartzite and dyke, and 

 thus to form ore-bodies. At the same time the other parts of 

 the formation would be steadily impoverished in iron content. 

 Much of that which remained disseminated through the forma- 

 tion would have been changed from carbonate to oxide. In 

 its lower part, the silica which was taken into solution in the 

 upper part of the water's course would be precipitated.* 



The processes thus outlined would penetrate to deeper parts 

 of the formation as erosion steadily advanced until the present 

 surface of the country is reached, and the ore-bodies thus 

 formed at depth are now found at surface. It follows that a 

 large amount of iron found in the ore-bodies was originally 

 in rock which has been removed by erosion. So far as the 

 deposits are at the surface, all of the iron oxide, except that 



* The chemistry of the processes thus outlined assumes the following : that the 

 oxygen of percolating waters is sufficient to oxidize iron carbonate not in solution 

 and set carbon dioxide free; that the resultant carbonated waters are sufficient to- 

 take iron carbonate in solution; that if such waters bearing dissolved carbonates 

 are mingled with other waters bearing oxygen, the iron carbonate or a por- 

 tion of it will be precipitated ; that silica may be carried in percolating waters; 

 that carbon dioxide is sufficient to precipitate silica from such solutions ; and that 

 a carbon dioxide solution strong enough to precipitate silica, by dilution, may be 

 made so weak in carbon dioxide that it would be capable of taking silica into 

 solution. All of these facts and principles of chemistry are so well known that 

 no discussion of them or reference to authorities is needed. 



