Chap. XIV.] dharwae series : alteration. 
293 
from quartz, the presence or absence of this constituent depending, 
of course, on the exact composition of the silt. Rhodonite is formed 
much less commonly, although it is to be found in almost every deposit 
where the degree of metamorphism has reached the point sufficient 
to bring about the formation of spessartite, the local presence of rhodonite 
at any particular point being simply determined by the actual com- 
position of the silt and adjacent or admixed ore at that point. Other 
rarer silicates have also been formed in places, probably where the sedi- 
ments or manganese oxides happen to have had an exceptional com- 
position. The manganiferous sediments must also have contained 
a varying quantity of phosphorus, in what form 
Apatite. impossible to say, but probably as apatite. 
Whatever was its original form, it is now found as apatite in the meta- 
morphosed rocks, sometimes only in very small quantities, and some- 
times, though rarely, in comparative abundance, as c.t Jothvad in 
Narukot State, Bombay Presidency. 
The Alteration of the Manganiferous Silicates. 
The evidence collected at the various deposits in the Central Provinces 
. shows that a considerable portion of this spessartite 
Re-crnversion cf i i n i i . i • , 
spessartite and rho- and rhodonite has been reconverted mto man- 
donite into oxidized ganese-ore. Many examples are to be found of 
ores (deep secondary , , f ,i •,• 
ores). rocks made up of these manganese-silicates 
traversed by veins and patches of manganese- 
ore. In some cases this alteration has become so complete that 
the fact that the ore was originally a rock made up of manganese 
silicates would not be suspected, were it not for the occasional 
find in the midst of a solid mass of ore of a residual patch of yellow 
spessartite or pink rhodonite. It might be suggested that this supposed 
residual manganese silicate is really a feature of the deposit due to the 
presence in the mass of oxides of patches of impurities, which, under 
the influence of metamorphic agencies, were converted into silicates. 
The evidence that such is not the case will be given later. Reasons 
will be given later (page 355) for supposing that this alteration of silicates 
to ore took place in Archaean times whilst the rocks were still at some 
distance from the surface. These ores can therefore be termed deep or 
Archcean secondary ores in contradistinction to the surface or recent second- 
ary ores mentioned on page 287. Accepting for the present that it is certain 
that portions of the manganese-ore bodies have been produced by the 
