Alt ix.i Watson, Manganese Ore-Deposits of Georgia. 195 



iron. More or less stain from the oxides had extended beyond 

 the Hmits of the original manganese-bearing silicate, discoloring 

 the inclosing clays. 



The source of the manganese in the Crystalline area has 

 been mostly, if not entirely, from the various manganese-bearing 

 silicates, which enter largely into the composition of most of 

 the rocks. By the decomposition of the minerals composing 

 these rocks, the base-forming elements either form soluble salts 

 of inorganic and organic acids promoting the decay, and are 

 removed in solution, or else the whole or a part of certain of the 

 base-elements are converted into insoluble oxides and retained, 

 in situ, to form the mantle of residual decay. In either case the 

 process may result in the removal by solution of only a part of 

 certain of the base-elements, iron and manganese, in the form 

 of soluble salts, while the remainder of the same elements is re- 

 tained as insoluble oxides in the residual decay. Both reactions 

 are common in the surface zone of oxidation, as proved by re- 

 cent work in rock-weathering. 



Recent investigations in rock-weathering show that iron is 

 frequently retained in the residual clays in amounts larger than 

 that of any other constituent in proportion to the percentage 

 amount present in the fresh rock. A loss, however, by removal 

 in solution, in the form of a soluble salt on decay of the rock, 

 is often shown in the iron. So far as investigation has gone, 

 the iron is not entirely removed in any case, but a part of it 

 remains in the form of the insoluble oxide. A like tendency 

 is indicated for manganese when present in those rocks so far 

 investigated. 



Upon subsequent chemical and physical changes, the man- 

 ganese is further concentrated in the residual clays, and the ac- 

 cumulation of the oxide is sometimes in quantity sufficiently 

 large to be of commercial value. 



Some of the manganese-deposits of the Crystalline area in- 

 dicate that the ore has been leached from the surrounding rocks 

 upon decay and concentrated in the clays along the contact-zones 

 of certain formations. In only one locality in the Crystalline 

 area, namely, the Darketown district of Haralson and Paulding 



