4o6 E. C. HARDER AND R. T. CHAMBERLIN 



The manganese deposits in the sedimentary series occur as 

 definite beds associated with the iron formation. The principal 

 bed, that on which the Wigg mine is situated, is 3 or 4 kilometers 

 in length, and at its maximum reaches a thickness of over 2 meters. 

 It strikes in an east-west direction parallel to the strike of the inclos- 

 ing sediments and corresponds with them in dip, making it apparent 

 that the manganese ore bed was laid down as a sedimentary bed 

 just as the inclosing rock. The bed at the Wigg mine is bounded on 

 one side by soft itabirite, with a contact zone of mixed soft hematite 

 and manganese oxide, and on the other side by a ferruginous schist 

 associated with the iron formation. 



The manganese bed at the Rodeio mine is of smaller horizontal 

 extent but of greater thickness than that at the Wigg mine and 

 shows less definitely its relation to the inclosing rocks. In the case 

 of both of these deposits, beds of carbonate rocks consisting of a 

 mixture of calcium, magnesium, iron, and manganese carbonates 

 are found in the vicinity. These, however, occur at different 

 horizons from the manganese beds. 



The manganese ores associated with the sedimentary rocks 

 consist of finely crystalline or amorphous manganese oxides, 

 probably largely a mixture of pyrolusite and psilomelane. From 

 their occurrence it must be assumed that they are similar in origin 

 to the associated rocks, that is, that they are original sedimentary 

 deposits of manganese oxide which have been somewhat altered 

 and recrystallized by subsequent metamorphism. The source of 

 the manganese is doubtful, but it may very well have been derived 

 from deposits of manganese ore in the basement complex such as 

 now occur to the south near Lafayette. Decomposition of such 

 deposits may have yielded a large amount of residual manganese 

 oxide which was worked over, transported, and deposited as beds 

 or lenses in the sedimentary series. Their origin would thus be 

 very similar to that of the iron ores of central Minas Geraes with 

 which they are closely associated. 



Sedimentary manganese ore beds similar to those of central 

 Minas Geraes are found in many places on other continents, prob- 

 ably the best known of them being those in northern Arkansas, those 

 in central Chile, those in the Caucasus near the Russian boundary, 

 and those in western Arabia. 



