272 
MANGANESE DEPOSITS OF INDIA : GEOLOGY. [PaET II : 
From what has been written above it is evident that the ores of the 
The ores formed in Vizagapatam district can be divided into three 
three ways. main divisions : — 
1. Those formed by the replacement of rocks, such as quartz-felspar- 
rock, not originally containing manganese. In this case all the mangan- 
ese is of external origin. 
2. Those formed by the replacement of rocks, such as kodurite, that 
contain a fair amount of manganese siUcate, the manganese of the 
latter being added to that of the attacking solutions. In this case the 
larger proportion of the manganese is of external origin. 
3. Those formed by the decomposition in situ of rocks composed 
almost entirely of manganese silicates, to the manganese of which a 
further portion, brought by the attacking solutions, is added. In this 
case about half of the manganese is of external origin. 
It is difficult to say what proportion of the ores of this district have 
been formed by each method. In the Kodur mine ores of groups 2 and 3 
probably predominate ; whilst at Garbham a considerable proportion 
of the ore has probably been formed by method 1, although ores formed 
by 2 and 3 still predominate. The ores formed by the different methods 
are not exactly the same and are shown below ; — 
1. Pyrolusite and psilomelane. ; 
2. Psilomelane with some braunite. ^ 
3. Psilomelane with braunite as^specks and patches. 
In the equation given on page 270 it is assumed that all the ferric 
Ferric oxide in the oxide remains in the manganese-ore. The spandite 
ore. from Garbham, of which an analysis is given on 
page 168, shows 21*28 per cent, of Fe203, whilst the percentages of 
Fe203 in the three ores from the part of Kodur mentioned on page 269 are 
5*86, 6 '57, and 13-14, with a mean of 8*52. This seems to indicate a 
removal of iron in solution. That this is probably not the case is indi- 
cated by a consideration of a larger number cf analyses. Thus the 
Kotakarra garnet contains only 3*47 per cent, of Fe203, so that the 
mean of the two garnets is 12 '37 per cent. ; whilst the mean of the 
Fe203 in the 22 analyses of samples and pieces of manganese-ore from 
this district is 13*57 per cent.l 
1 See foot-note to page 271. 
