36o THE JOURNAL OF GEOLOGY. 



from areas in which iron predominates, sometimes from areas in 

 which iron and manganese are both abundant, and sometimes, 

 though rarely, on account of the scarcity of such regions, from 

 areas in which manganese largely predominates over iron. If 

 iron and manganese were always precipitated from these waters 

 in similar chemical forms and under the same conditions, it 

 would be expected that the strata deriving their iron and man- 

 ganese from surface waters would contain those substances in 

 the same relative proportions as they had existed in the rocks 

 from which they were derived, and that they would be in an 

 intimately mixed condition. Such is doubtless often the case, 

 or at least approximately so ; but it is also often the case that 

 iron and manganese occur in separate deposits, yet in close prox- 

 imity to each other and often alternating along the same horizon. 

 Besides this, the two substances frequently form parts of the 

 same deposit and yet are distinctly separate from each other. In 

 such cases the question arises as to why the iron and manganese 

 are not intimately mixed in the form of a manganiferous iron 

 ore, as would be expected if they had been precipitated together. 

 Moreover, deposits sometimes occur which are composed largely 

 of manganese ore, with little or almost no iron, and when the 

 source of the manganese is looked for, we often find that the 

 rocks which probably supplied it contained both manganese and 

 iron, and that the iron was present in a much larger proportion 

 as regards the manganese than in the new deposit. Here again 

 the question arises as to why the iron and manganese are not in 

 the same relative proportions in the new deposit as they were 

 in the rocks from which they were derived. 



Four principal causes suggest themselves in explanation of 

 this separation : 



( I ) It might be supposed that the deposits containing mostly 

 iron and those containing mostly manganese received these con- 

 stituents from waters derived from different sources, and carrying 

 iron and manganese only in the proportions in which they depos- 

 ited them. Under some conditions this explanation might suffice, 

 but in many cases, such as when iron and manganese alternate 



