264 
MANGANESE DEPOSITS OF INDIA : GEOLOGY. 
[ Part TL : 
apparently mucli more frequently as the former, however, for large masses 
of chert, usually brown in colour, are to be found in almost every quarry 
(see pages 1065, 1079, 1089, for examples). The re-deposition of the 
silica seems often to have taken place by the metasomatic replacement of 
some or all of the constituents of portions of the kodurite mass. Where 
chert has been formed the replacement seems usually to have been com- 
plete : but where opal has been formed the replacement has been as a 
rule only partial, some chalcedony being usually formed as well. Exam- 
ple of replacement by opal occur at Kodur (see fig. 1, Plate 8; fresh 
spandite in a matrix of opal and chalcedony formed by the replacement 
of felspar, and of black manganese-ore probably representing original 
pyroxene), at Kotakarra (page 256, felspar replaced, garnet and apatite 
unaltered), and at Boirani (page 256, garnets fresh, felspar partly fresh 
Potash removed in partly replaced ; see fig. 2, Plate 8). The 
solution as carbonate, potassium carbonate formed from the potash of the 
felspar must have been completely removed by the waters. 
Though many of the microscope slides of the rocks from various 
Alteration and re- localities show that apatite suffers replacement 
placement of apatite. by manganese oxide during the formation of the 
manganese-ores, yet no evidence has been found to show what becomes 
of the phosphorus and fluorine thus removed. From the evidence of 
the microscope there is no doubt that in many cases the phosphorus of 
the manganese-ores is at least partly due to the presence of residual 
apatite that has escaped replacement. But there is no evidence that 
this is always the case, and it is possible that some of the phosphoric 
oxide that must be set free when apatite is replaced, is taken up by the 
manganese-ore as oxide ; or this oxide may be removed in solution and 
deposited in the ores at another spot. Apatite is, however, often a very 
abundant constituent of kodurite, sometimes forming as much as 10 to 
30 per cent, of the rock. The manganese-ores of this area are, it is true, 
very phosphoric from the commercial point of view, since they range in 
phosphorus contents from about 0'25 to 0"50 per cent. But this, if 
present as fluorapatite, would correspond to a proportion of only 1-35 to 
2-71 percent, of apatite; whilst even 1 per cent, of phosphorus only 
means 5-42 per cent, of apatite, Hence it seems probable that all the 
phosphorus in the original apatite has not remained in the ores, either 
as apatite or oxide. Some of it must have been removed in solution 
together with the potassium carbonate derived from the felspar. 
The fate of the lime of the apatite must have been the same as 
