364 MANGANESE DEPOSITS OF INDTA : GEOLOGY. [ PaET II : 
conclusion that the alteration of manganese silicates to manganese- 
ores took place in Archaean times, this agreeing with the conclusions 
reached on page 357. Even if this explanation be true it does not 
follow that a certain further amount of alteration did not take place in 
post-Archsean times. Indeed it is certain that it has taken place 
at the outcrops of the gondite series since these rocks were brought to the 
surface and eroded into their present shape ; and very probable that it 
is still going on at the outcrops ; but the product of this method of altera- 
tion is not a compact crystalline ore, but a softish more or less porous 
one. 
Depths to which the Manganese-ores extend. 
If the oxygen in the attacking solutions were really that which 
was expelled from the original manganese-oxide sediments at the time 
of their metamorphism into silicates, then it is quite e\'ident that, 
even though the alteration is of the nature of an oxy-alteration, which 
is usually supposed to be especially characteristic of the surface, this 
alteration can have taken place at any depth to which the sediments 
were buried. It may be that in the deepest parts the pressure was 
not sufficiently ameliorated for the manganese silicates to have been 
reconverted into oxide. If so, then perhaps the very bottoms of the 
syncUnes have escaped this alteration and retained their siUcate 
character. From this we see that when the bodies of manganese- 
ore are mined, those that were formed by the direct compression 
of original manganese oxides without any passage througTi the silicate 
stage may be expected to extend to as great a depth as the rocks of 
the gondite series, namely to the bottoms of the sjucUnal folds. 
WTiilst those that were formed by the alteration of manganese sihcates 
may also extend to as great a depth as the rocks of the gondite series, 
except that perhaps the very bottoms of the s\Ticlines escaped being 
converted either partially or completely into manganese-ores. 
Conclusions as to the Crig-in of the Gondite Series and the 
Associated Manganese-ores. 
The conclusions at which I have arrived in the foregoing pages are 
much more open to doubt than those regarding the origin of the ores of 
the kodimte series, the reason being the smaller amoimt of e\idence 
obtained. The points that may be considered fairly certain are the 
following : — 
1. The rocks of the gondite series are the product of the metamor- 
phism of the less pure manganiferous sediments of Dharwar 
