40 Van Htse — Iron Ores of the Penokee- Gogebic Series. 



composed in place to iron oxide, the rock becoming a hematitic 

 chert. Along the seams, waters bearing iron in solution have 

 passed. These waters have particle by particle dissolved out 

 the chert and replaced it with iron oxide, and where once was 

 lean sideritic chert is rich ore. A part of the iron oxide is due 

 to the oxidation in place of iron carbonate, but the larger part 

 has come from a greater or less distance there to be deposited. 

 The seams of iron oxide at this point are but a few inches in 

 thickness, but it is probable that the series of changes which 

 have here taken place upon a small scale, will upon a large scale 

 explain the concentration of workable ore-deposits. 



Time at which concentration of the main ore-bodies occurred. 

 — It has been stated that the iron belt rocks are much less 

 altered as a whole in the upper horizons, being there composed 

 largely of unaltered cherty carbonate, while the lower hori- 

 zons contain very little carbonate and are mostly composed 

 of ferruginous chert and iron ore. It follows as a deduction 

 from this succession, that the series of changes which have so 

 completely altered the lower horizons of the formation have 

 occurred subsequently to the uplifting of the series. The al- 

 terations can only be explained by the action of percolating 

 waters bearing oxygen, and which therefore came from above. 

 If the 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 would be the 

 case if the alteration was subsequent to the tilting, for the up- 

 per layers of the member would partially escape the action of 

 percolating waters, as will readily be seen by glancing at figs. 

 1 and 9, and taking into consideration the nature of the belt of 

 rock above the ore formation. It is a member composed of 

 black and gray clay-slates, greywacke and greywacke-slates, all 

 of which contain a large amount of clay, rocks particularly im- 

 pervious to water. A rock underlying any great thickness of 

 such a formation in a horizontal position could not be greatly 

 affected by waters from above. However, when the series had 

 been uplifted and eroded, this upper impervious member would 

 have been removed, and the waters would come directly in 

 contact with the lower horizons of the ore formation, while the 

 upper horizons of the belt would still be somewhat protected. 



Before going farther, it is necessary to consider the porosity 

 of the rocks which underlie the ore formation. They have 

 been said to be composed of quartzites and feldspathic quartz- 

 slates. Included in the latter are clay-slates ; consequently the 

 rocks of this member are also almost impenetrable to perco- 

 lating waters. The uppermost layer of the member, the 

 quartzite, is not itself a perfect barrier to the passage of water, 

 on account of the joints which are always found in such a brit- 



