CONCENTRATION OF MANGANESE ORES. 1199 



Manganese is present in the original igneous rocks in very much 

 smaller quantity than iron, and therefore the manganese deposits are much 

 smaller than those of iron; but so far as can be made out, the segregation of 

 manganese and iron are strictly parallel processes. Thus in the secondary 

 rocks manganese is found only as traces in the sandstones and shales, but 

 in the limestones — carbonate rocks — it is more than half as abundant as in 

 the original rocks. Since the limestones are so small in mass, only a small 

 fraction of the manganese of the original rocks is in the limestones. 

 Apparently the great mass of the manganese of the original rocks has been 

 segregated in the ores of manganese and iron and in the iron formations. 



In view of all the foregoing facts it is thought to be extremely probable 

 that the manganese is largely transported as a carbonate from the original 

 rocks to lagoons, and is there precipitated as an oxide. By this process 

 bog ores of manganese are produced just as are the bog limonites. But to 

 a large extent the manganese oxide is reduced to the manganous form by 

 the organic matter, reunites with carbon dioxide, forming carbonate, and in 

 this form is built up into carbonate formations. From such carbonate 

 formations it is segregated by underground solutions which have different 

 sources and mingle in the trunk channels. Some solutions bear the manga- 

 nese carbonate and others the oxygen. When these two meet the manga- 

 nese is precipitated as hy drated oxide according to the following reaction : 



MnC0 3 +0+nH 2 0=Mn0 2 . nH 2 0+C0 2 . 



It is yet too early to state to what extent the manganese ores have been 

 concentrated on impervious basements in pitching troughs. 



IV. SPECIAL FACTORS AFFECTING THE CONCENTRATION OF ORES. 



Thus far the discussion of ores deposited from aqueous solutions has 

 not taken into account a number of the special factors which affect the 

 concentration of ores. The general discussion may need great modification 

 to adapt it to a particular district. To illustrate my meaning, it may be 

 well to consider some of the additional factors affecting the deposition of 

 ores, and to point out the more obvious possible modifications of the general 

 theory which may result from them. There will be briefly considered the 

 effect of (1) variations in porosity and structure, (2) the character of the 

 topography, and (3) physical revolutions. 



