268 
MANGANESE DEPOSITS OF INDIA : GEOLOGY. 
[ Part II : 
retained in the gaseous condition in these same interspaces, the manganese 
would be deposited as one of the hydrated more highly oxidized forms, 
such as H2Mn03. If originally deposited as MnO or Mn(0H)2 it would 
speedily be changed to a more highly oxidized form. The silica 
would be removed in solution in the alkaline carbonate. According to 
the well-known fact that alkaline carbonates precipitate aluminium 
hydroxide from solutions of aluminium salts, the alumina in the above 
The removal of equation should be left behind with the precipitated 
alumina. oxide of manganese, probably in the form of kaolin 
in combination with a certain proportion of the silica set free. The 
manganese-ores thus formed in these felspathic rocks do frequently 
contain a certain amount of alumina, but nothing like the quantity 
they should contain in accordance with the equation. It seems neces- 
sary, therefore, to suppose that under the conditions of temperature, 
pre-ssure, and concentration, in which this replacement took place a 
large portion of the alumina was removed in solution in spite of the 
presence of carbonates. In this connection it must be remembered 
that all the chemical changes to which reference is being made in this 
section of this Memoir were probably effected by very dilute solutions 
and that a very slight solubility of the alumina would suffice for its 
removal. 
When the saturated manganiferous solutions hypothecated above 
RepLacement of the come in contact with rocks containing manganese 
manganifeious vane minerals, they can no longer assist in the breaking 
up of these rocks by dissolving further quantities of manganese and 
calcium from the garnet. On the other hand they probably begin to 
rid themselves of their burden of manganese by a more or less complete 
replacement by manganese oxide of the rock with which they come in 
contact. If the rock is kodurite, the commonest of the manganese- 
silicate-rocks, the felspar is replaced in the same way as in the case of the 
felspathic rocks outlined above. Sometimes the replacement seems 
to stop here and the product is manganese-ore, generally psilomelane, 
studded with bright orange-red or orange-brown garnets, as at Garbham 
and Ramabhadrapuram. As the garnet itself contains manganese 
there is probably less tendency on the part of the manganiferous solutions 
to replace it by manganese oxide. Often, however, the replacement 
seems to go further than the substitution of the felspar and apatite and 
the garnet itself suffers replacement. As the solution effecting the change 
is depositing manganese oxide in the place of every constituent of the 
