108 THE :marquette iko:n-bearing district. 



exteut, but trom the similarity between tlie rock associations in this and in 

 the less folded areas the author has no doubt that the ores and jaspers 

 here have the same oi'igin as in the Penokee district. 



A ffood many of the ore bodies, and more particularly some of the so-called 

 soft hematites, appear to have resulted, partly at least, from a direct oxidation of the 

 iron carbonate of some of the eherty schists. In other cases the ore bodies owe their 

 origin and general shape, we think, to processes of infiltration and replacement. 

 (P. 207.) 



Thus after the carbonates were precijiitated they were subjected to 

 metasomatic processes. 



The conclusion reached b}' Irving as the result nf his extensive 

 studies may be summarized as follows: 



(1) The original form of the beds of the iron-l)earing horizons was 

 that ot a series of thinlv bedded ferriferous carlionates iuterstratified with 

 carbonaceous slatv la^•ers, like the carbonate-bearing beds of the Coal 

 Measures. 



(2) By a process ot silicification these carbonaceous beds were trans- 

 formed into the various ferruginous rocks. The silicification varied in 

 degree, sometimes producing only a few seams of silica, ^^'hich traverse the 

 otherwise unchanged rock, at other times completely substituting the 

 original rock, in which case cherts were formed. 



(3) The iron thus removed during silicification passed into solution 

 and was redeposited as it became further oxidized, making ore bodies in 

 one place and forming the coloring matter of the jaspers in other places. 

 The hematite interlaminated with the jasper is taken to be mainly the result 

 of a secondary infiltration following the banding of the original rock, though 

 it may be imagined to have been formed at times by direct oxidation from 

 iron carbonate seams. 



(4) Sometimes, instead of leaching it out completely, the silicifying 

 waters seem to have decomposed the iron carbonate in place, producing 

 most of the actinolitic magnetite-schists. 



(5) The rich ore bodies have probably had different origins in different 

 cases. Some of the red hematites seem to have resulted from a direct oxida- 

 tion in place of the original carbonate, since in some of them pseudomorphs 



