830 A TREATISE ON METAMORPHISM. 



FF.i;i::'(.i\ors miai.es. 



The ferruginous shales are produced from siliceous siderite, ankerite, or 

 parankerite, by the decomposition of the iron carbonate and the partial or 

 complete oxidation and hydration of the ferrous oxide, combined with the 

 rearrangement of the particles, and frequently the introduction of silica. 

 In the zone of katamorphism the first alteration is the oxidation and partial 

 or complete hydration of the ferrous oxide, with the liberation of carbon 

 dioxide, thus producing limonitic and hematitic slate and various gradations 

 between them. The reaction for hematite is as follows: 



2FeC0 3 +0=FeA+2C0 2 . 



For limonite it is: 



4FeC0 3 +3H 2 0+20=2Fe 2 3 .3H 2 0+4C0 2 . 



Where the constituents of the rock are not extensively rearranged the 

 original regular stratiform arrangement of the siderite is preserved, and the 

 rock is called a ferruginous shale. Where the iron oxide is mainly 

 hematite the rock may properly be called hematitic shale; where mainly 

 limonite, a limonitic shale: and where both are abundant, hematitic and 

 limonitic shale. Simultaneously with the oxidation of the iron carbonate, 

 calcium and magnesium carbonate, so far as they are present, may be 

 partially or completely dissolved. 



FERRUGINOUS CHERTS. 



At the same time the iron carbonate is altered the silica, which is 

 usually present in greater or less quantity, may become concentrated in 

 layers by exactly the same process as it becomes concentrated in layers in 

 limestones, and thus there may be produced a hematitic and limonitic chert. 

 The rock may be somewhat regularly banded, or the bands may be very 

 irregular and broken. As the rearrangement goes further, oxide of iron is 

 apt to become concentrated in bands, alternating with bands which are 

 predominantly chert. Also in this stage a concretionary nodular or geodal 

 character is apt to be developed, and then the material may be called 

 banded iron oxide and chert, or chert with bands and shots of iron oxide. 

 (PI. VII, C.) In the earlier stages frequently the silica, as in the original 

 rock, is partly opaline, and as the rearrangement becomes more nearly 

 complete the silica takes the form of finely crystalline, interlocking 



