266 
MANGANESE DEPOSITS OF INDIA : GEOLOGY. [PaRT II : 
Now the alteration of the garnet must have been effected by the same 
agency, namely, carbonated water, as converted the orthoclase into 
kaolin. The action of such waters on a garnet of the above composition 
would be to remove the manganese and calcium, and the little magnesium, 
as bicarbonates, and to leave behind the alumina as kaolin, mixed with 
ferric oxide, the two forming the ochreous spots referred to above. The 
ailica not required for the formation of the kaolin must, as in the case 
of the felspar, have been removed in solution and may also have contri- 
buted to the formation of the chert. It is shown on page 167 that a small 
portion of the iron oxide of the garnet was probably in the ferrous condi- 
tion. This portion, 2*29 per cent, would also be removed as 
bicarbonate. 
These manganiferous solutions circulated through the mass of kodurite 
rocks until perhaps they had taken up as much of the bicarbonates of 
manganese and calcium as the carbon dioxide in the solution would allow 
of. On now coming in contact with either fresh or decomposing 
kodurite they would no longer take up any more manganese. 
Now an examination of such a deposit as Kodur shows that although 
Formation of man- portions of the mass of rocks exposed are undergoing 
ganese-ores by replace- alteration on the Hnes explained above, yet there 
ment of the kodurite . r ^ ^ ■ • i • i i 
rocks. are other portions or the deposit m which the rocks 
are not being to any great extent kaolinized, but are instead suffering 
replacement by manganese-ore. The rocks that suffer this replacement 
may be divided into two groups, those, such as the quartz-felspar-rocks, 
that contain no manganese -bearing minerals, and those, such as kodurite, 
that contain a manganese silicate. I am of opinion that it is the saturated 
manganiferous solutions referred to above that cause the replacement 
of these rocks. 
Let us deal first with the non-manganiferous rocks. It is evident, 
Replacement of the both from an examination of the rocks in situ and 
non-mangamferous ^^^^ ^ Specimens of the rocks macro- 
varieties 
scopically and under the microscope, that the whole mass of rock is under- 
going a more or less complete replacement with the production of masses 
of manganese-ore, the purity of which depends on the extent to which 
this replacement has taken place. In some cases the rock to suffer 
replacement is the more or less decomposed quartz-felspar-rock or 
felspar-rock, the partial kaolinization of the rock having been no doubt 
effected by non-manganiferous solutions before the advent of the mangan- 
iferous solutions. The manganese-ore that first appears in this must 
