284 THE PENOKEE IRON-BEARING SERIES. 



dioxide dissolved tlie iron earboiiate. This carbon dioxide ma)- liave beeil 

 furnislied by the oxidation of other iron carbonate, which would relieve the 

 percolating water of oxygen at the same time that it became capable of 

 takingf iron carbonate into solution. Such water holdinj>' iiYtn carbonate 

 would retain it until some more or less open cleft or passage was reached, 

 by lueans of Avhich the carbon dioxide could escape, and perhaps oxygen 

 reach these solutions dissolved in other waters, and thus precipitate the 

 iron oxide. The only other chemical change necessary to explain this con- 

 centration of ore is that of the sokition of the silica, which in ])art j^robably 

 occupied the place now taken by the ore. It is now well established 

 that great quantities of silica are carried in solution and deposited in rocks. 

 The large amount of water which has traversed these jiassages has been able 

 to take the silica into solution at the same time that it oxidized the iron 

 carbonate coming in Ijy lateral secretion, and thus simultaneously precipi- 

 tated iron oxide and removed silica. There is every reason to lielieve that 

 these are the chemical changes which have formed the narrow rich seams 

 of ore described. It is most pi-oliable that these chemical changes, taking 

 place upon a large scale, have produced the large workable deposits. 



Time at which roncrntration of the main ore bodies occurred. — It has been 

 stated that the iron belt rocks are much less altered as a whole in the 

 higher horizons. In ])assing downwards, it will be remembered that an 

 increasing proportion of the rock varies from its original condition, and at 

 the lowest horizons there is nowhere known any unaltered carbonate. It 

 follows as a deduction from this succession that the series of changes which 

 have so completely altered the lower ])art of the formation have taken 

 place subsequent to the uplifting of the series. The alterations can only 

 be explained by the action of percolating waters bearing oxygen, and 

 which therefore came from abo\ t». If tlie layers were' horizontal when the 

 changes occurred, the waters passing downward would have altered most 

 that part of the formation nearest the surface. The reverse woidd be the 

 case if the alteration was snbsecpient to the tilting, i'or the upper layers oi 

 the iron member would jjartially escape the action of percolating waters, 

 as will be readily seen b}' glancing at any of the sections across the series 

 in that part of the range under discussion (e. g., Pis. vi, viii andx;) and 



