METAM0RPH1SM OF IRON-BEARING CARBONATES. 841 



be so designated, since in the Lake Superior region the presence of the 

 igneous rocks seems to have been an essential condition for the process to 

 take place on an extensive scale. 



After the griineritic or actinolitic rocks have developed in the zone of 

 anamorphism, in consequence of denudation they may pass into the zone 

 of katamorphism, or even into the belt of weathering. Then there will 

 begin the processes of oxidation, hydration, and carbonation, as a result of 

 which the magnetite is changed to hematite or limonite, and the actinolite 

 and gmnerite decompose into carbonate. However, since magnetite and 

 the iron-bearing amphiboles are very refractory, this process is exceedingly 

 slow, and usually has affected only comparatively thin layers of material 

 adjacent to the surface or adjacent to openings in the rock. Indeed, the 

 reagents of the belt of weathering and the upper part of the belt of cemen- 

 tation, which, as pointed out (pp. 830-831), may produce large iron-ore 

 bodies where they have the original iron-bearing carbonates or the hydrous 

 ferrous silicates to work upon, have nowhere in the Lake Superior region 

 formed large ore bodies where they are working upon the griineritic and 

 actinolitic rocks. This follows directly from their refractory character, 

 since the average amount of iron of the iron-bearing formation is essen- 

 tially the same in both cases. In the refractory character of these rocks 

 we have the explanation of the absence of rich workable iron-ore deposits 

 for extensive areas in each of the iron-bearing districts of the Lake Superior 

 region where the alterations of the original rocks in the zone of anamor- 

 phism developed actinolitic or griineritic magnetite rocks. 



GENERAL STATEMENTS. 



From the foregoing pag-es it is apparent that the iron-bearing carbon- 

 ates and the hydrous iron silicates are metamorphosed along two main 

 lines, depending upon whether they are in the zone of katamorphism 

 or the zone of anamorphism. Where they are altered in the zone of 

 katamorphism, and especially in the belt of weathering, the ferruginous 

 shales, ferruginous cherts, and ores result. Where they are altered in the 

 zone of anamorphism the actinolitic and griineritic marbles, or the actin- 

 olitic rocks and griineritic rocks, are produced. Further, where the 

 metamorphosed rocks produced from the iron-bearing carbonates and 

 hydrous iron silicates change from one zone to another they are further 



