THE IRON BEARING MEMBER. 283 



red banded .slates and (•arl)(>nates in the middle lidrizuns; and for tlie rela- 

 tively mueh more abundant unaltered carbonate in tlic upper horizons. 



A p<iythal(ir occurrence of iron ore. — Before attem})tino- to give a general 

 explanation of these facts it will first be well to again refer to an occur- 

 rence of narrow belts of iron ore U})on the bank of Sunday lake outlet, in 

 Sec. 13, T. 47 N , K. 4G VV., Michigan. Here the actual tnuisfonnation from 

 lean cherty iron carbonate to ore is seen in all its ))hases. In clefts and 

 joints and in partings along the bedding of the exposure are narrow seams 

 of hematite. In {lassing from the seams the hematite liecomes mingled 

 with some chert. This chci-t increases in quantity imtil the chert}' ground- 

 mass contains many rhombohedra of iron oxide. Proceeding farther away 

 from the ore, the iron oxide graduallv changes to siderite, the transition 

 from one to the otlier being quite gradual. The siderite is in perfect rhom- 

 bohedra, and it is evident in thin section that the iron oxide adjacent is 

 pseudomorphous after it, and now the rock is a sideritic chert; it is of a 

 light gray color, aphanitic texture, and breaks Avitli conchoidal fracture. 

 This rock is manifesth' in its original condition at this place. The processes 

 1)V which the seams of iron oxide now occupy the space once taken bv this 

 sideritic chert are jjlain. The iron carbonate has decomposed in place to 

 iron oxide, the rock becoming a hematitic chett. Along the seams waters 

 bearing iron in solution have passed. These waters have particle by parti- 

 cle dissolved out the chert and replaced it with iron oxide, and where once 

 was lean sideritic rock is rich ore. A poilion of the iron oxide is due to 

 the oxidation in place of the iron carbonate, but the larger portion lias come 

 from a greater or less distance there to be deposited. These seams of iron 

 oxide at this place are but a few inches in thickness, Imt it is jiossible that 

 the process of alteration Avhich lias here taken place upon a small scale may 

 upon a large scale explain the concentration of workalile ore deposits. 



Chemistry of the process of concentrdfion. — So far as the iron oxide has 

 formed from iron carljonate in place the process is simjily one of oxidation, 

 and it is only necessary to sujijiose that percolating waters Imve carried 

 suthcient oxygen in st)lution to accomplish this work. In the removal of 

 iron as iron carbonate from one place and its deposition as oxide in another 

 place the process was doubtless as follows: AVaters having in solution carbon 



