362 MANGANESE DEPOSITS OF INDIA : GEOLOGY. [PaRT II I 
For the alteration of rhodonite similar equations to those for 
Alteration of rhcdo- spessartite Can be formed. Thus the formation 
"'t^- of braunite might be represented as follows : — 
TMnSiOs + 30 = 3Mn203.MnSi03 < 6Si02 ; 
Rhodonite. Braunite, 
whilst a somewhat more complicated equation on the lines of that on 
page 357 could easily be constructed to show the formation of psilomelane. 
It is to be noticed that these equations are independent of whether the 
attacking solutions contain COo or HoSO^. 
Thus it v.-ill be seen that either carbon dioxide or sulphtiric acid would 
be able to produce the changes noticed, the suljihuric acid accounting for 
them more completely on accotmt of its abi'ity to remove alumina in 
solution ; but if the supposition that carbon dioxide can also dissolve 
alumina under the conditions prevailing in Nature be true, then either 
reagent would do equally well. Xow there is no direct evidence 
pointing to one as the reagent more than the other. Still, we might look 
Carbon dioxide the for indirect e\adence and see if possible sources 
more probable reagent, for either of these reagents exist in the other 
rocks forming the Axchsean complex in this area. Now a most notice- 
able feattxre of this Archaean complex is the rarity of any sulphides, 
either as constituents of the rocks or as vein- forming minerals/ 
or of any rocks or minerals that may be supposed to have 
once contained sulphur. All that I have seen are barytes in the 
gondite series, copper sulphide minerals in basic dyke-rocks and in 
quartz, and pyrite in a chert vein, all of them being very rare. On the 
other hand crystalline limestones are found in great abundance and 
variety in this region. Hence we can suppose that it was carbon 
dioxide rather than sulphuric acid that brought about the alteration 
of the gondite series. 
It is to be noticed that the pyroxenic gneisses of this area are 
supposed to have been originally impure calcareous sediments ; and that 
in their formation carbon dioxide was Hbcrated ; some of tliis converted 
portions of the gneisses into calciphyres and crystalline limestones (see 
page 299) : the remainder may have brought about the alteration of 
the rocks of the gondite series. 
1 Vein-forming sulphiir-containing minerals, even if present in abundance, would 
probably have been deposited after the formation of the mnnganese-ores, if the fore- 
going theory as to the time of alteration of the gondite series be correct. 
