254 THE PENOKEE IRON-BEARING SERIES. 



The ferruginous cherts. — h\ explaining- the genesis of the second phase 

 of the second type of rocks it is necessary to account for its concretion- 

 ary and brecciated character; for the production of brown hydrated liema- 

 tite, red hematite, and magnetite; for tlie concentration of the iron oxide in 

 shots and bands; for the hirge (juantity of sihca which it contains; for 

 the somewhat coarsely crystalline character of the chert as compared with 

 that of the original carbonates; and for the presence of numerous ramify- 

 ing veins of finely crystalline quartz. 



In the microscopical description of this phase of rock (pp. 205-209) 

 the concretionary and brecciated character has already been accounted for. 

 These peculiar areas have been actually observed in all stages of their 

 formation, so tliat their historj^ is not theory, l)ut definitely observed fact, 

 and will not l)e repeated here. The oxides of iron have been produced 

 from the irf)n carbonate just as are the oxides of the first phase of rock, 

 the only difference being that here occasionally magnetite is formed. This 

 mineral, however, is readily accounted for by the decomposition of iron 

 carbonate under conditions not favoralde to complete oxidation. It is not 

 to be supposed that these rocks were highl}- heated, but it may be remarked 

 that a low degree of heat is sufficient to change iron carbonate into 

 magnetic oxide of iron with the liberation of carbon monoxide and 

 dioxide. It is not believed that any carlion monoxide has been liberated, 

 for the oxygen needed to change the protoxide of iron into the proto- 

 sesquioxide was doubtless supi)lied by oxygen in percolating water. 

 The manner of concentration of the iron oxides in bands ;ni(l shots 

 Avill l)e fully discussed in considering the origin of the iron ores, but 

 it is here necessary to mention the causes which have produced such t-oncen- 

 tration. Percolating waters bearing oxygen in solution have tlecompttsed 

 a part of the iron carbonate, the carbon dioxide passing into the water and 

 ox)-gen passing from the water into the rock, thus simultaneously forming 

 sesquioxide of iron and solutions capable of taking up other iron carlionate. 

 Such iron-bearing waters would after a time reach some crack or channel 

 in the rock. This opening in many cases would serve as a passage fi)r 

 other waters niori' directly from the surface bearing oxygen in solution, 

 and as a result of tliis mingling the iron in solution would be precipitated, 



