366 THE JOURNAL OF GEOLOGY. 



ant difference in the modes of deposition of iron and manganese, 

 which also will be mentioned again. 



It will thus be seen that while some of the forms in which 

 iron and manganese are deposited are the same, others differ 

 very widely, and even similar forms are often deposited under 

 different conditions. It is doubtless to these various, forms and 

 conditions of deposition that the alternate association and sepa- 

 ration of iron and manganese in nature are due. 



CAUSES OF THE ASSOCIATION OF IRON AND MANGANESE. 



The very frequent intimate association of iron and manganese 

 in sedimentary rocks is what would be expected from a deposition 

 as oxide or carbonate in basins such as coastal lagoons or bogs, 

 where the waters moved very slowly, or not at all, for under 

 such conditions, they are often deposited together.^ Moreover, it 

 is a well-known fact that isomorphous substances have a strong 

 tendency to combine in a homogeneous mass, and to crystallize 

 together in different proportions. Carbonate of iron and of man- 

 ganese are isomorphous with each other, and this is hence a 

 possible cause of the frequent intimacy of their association, such 

 as is seen in almost all manganiferous spathic iron ores, whether 

 these ores are formed by direct precipitation or by replacement of 

 carbonate of lime. The oxidation of such a mixture would give 

 the common form of an intimately combined iron and man- 

 ganese ore. 



Since there is usually more iron than manganese in the rocks 

 from which both metals were originally derived, the surface 

 waters draining from areas of such rocks usually contain the 

 metals in a similar proportion. Hence, in cases where the depo- 

 sition of the carbonates of both occurs at the same spot, the 

 isomorphous carbonates derived from the solutions have a larger 

 percentage of carbonate of iron than of carbonate of manganese, 

 and the resulting oxides contain the two metals in the same 



' If the water moved very slowly, the deposition would probably take place approxi- 

 mately in the same spot ; if the waters moved more rapidly, the iron might be 

 deposited in one place and the carbonate in another, in the way explained on page 363. 



