362 THE JOURNAL OF GEOLOGY. 



such concentration has taken place, such as in the replacement 

 of limestone, etc. 



It will now be attempted to show how the various degrees of 

 association and separation of iron and manganese found in nature 

 may be produced by different conditions during deposition. 



THE FORMS OF IRON AND MANGANESE DEPOSITED AT ORDINARY 



TEMPERATURES. 



The mineralogical forms in which iron and manganese are 

 deposited from solution in nature at ordinary temperatures 

 depend on the conditions of air and water, whether of an oxidiz- 

 ing or a reducing nature, and on the character of the associated 

 organic and inorganic matter either in solution or on the floor 

 of the sea, lagoon or bog in which the deposition occurs. ^ 

 There are four principal methods by which iron and manganese 

 are precipitated in nature from surface waters : 



(i) By oxidation, as in the case of the precipitation of 

 hydrous oxides and in the precipitation of the carbonate by the 

 partial oxidation of more complex organic salts. ^ 



(2) By reduction, as in the precipitation of sulphide of iron 

 by the reduction of sulphate of iron. 



(3) -^y gaseous or soluble precipitants, as in the precipita- 

 tion of sulphide of iron by the action of sulphuretted hydrogen 

 or a soluble sulphide on a soluble salt of iron, and as in other 

 cases to be mentioned later. 



(4) By replacement of carbonate of lime or some other 

 substance. Different forms are precipitated by these different 

 methods. 



Iron at ordinary temperatures is usually deposited from solu- 



^ The solutions may be precipitated, as already shown, either with or without 

 admixture with mechanical sediments ; and there are in nature all gradations from 

 almost pure deposits of iron and manganese ore to beds of shale, sandstone, etc. 

 stained with iron or manganese. Subsequent concentration frequently causes decided 

 changes in the latter deposits (see p. 370). 



^ It has been suggested by A. A. Julien (Proceed. Amer. Assoc. Adv. Sci., Vol. 

 XXVIII., 1879, P- 356) that in some cases the carbonates of iron and manganese may 

 be only the fixed residue of organic compounds of more complex form once in solution 

 in surface waters. 



