214 
MANGANESE DEPOSITS OF INDIA: MINERALOOY. [ PaRT I 
been a considerable production of secondary silica ; this does not 
form as quartz, but as either opal or chalcedony ; the latter is 
often impure and coloured by the presence of small amounts of 
manganese or iron oxides and is then best designated chert. Chert is 
found in large quantities in the manganese mines of the Vizagapatam 
district, its colour being as a rule of some shade of brown. Such 
brown chert is also foimd in the manganese-ore deposits of the Central 
Provinces, but not very commonly, and then only in small quantities. 
In association with the manganese-ore deposits that occur in the crystal- 
line limestones in the Chhindwara and Nagpur districts there is, however, 
often a considerable quantity of black chert, owing its colour to the 
presence of manganese oxides, and formed by the replacement of the 
J limestones by silica. Opal is also not infrequently 
formed in the manganese-ore deposits. Thus it is 
common in the Vizagapatam mines, as a replacing material for the 
felspathic portion of kodurite, the product frequently taking the form 
of manganese-garnet set in a matrix of opal. Good examples of this 
are se<in at Kotakarra (page 1097), Kodur, and at Boirani in 
Ganjam (page 1034) ; see also Plate 8, figures 1 and 2. In the Nagpur 
district opal is sometimes found filling cracks in the manganese- ores and 
associated rocks. Good examples are mentioned under Kodegaon 
(page 847) and Kandri (page 873). Opal is also sometimes found in the 
Chhindwi' .-a deposits. 
One of the most extraordinary featiires of the manganese- ore deposits 
of the gondite series, is the freedom of the manga- 
Iron -ores. • . • 
nese-ores from admixture with iron-ores. When it 
is considered that these deposits are supposed to have been formed 
either immediately or ultimately by the metamorphism of manga- 
niferous sediments, this rarity of iron-ores is all the more surpris- 
ing ; for it means that when the manganiferous sediments were 
deposited the mineralized waters from which they may be supposed to 
have been formed contained large quantities of manganese salts and 
only a comparatively small quantity of iron salts. It might be argued 
that these waters really contained considerable amounts of iron salts 
as well and that the portions that were not deposited with the manganese 
oxides (for it must be remembered that nearly all the manganese-ores 
contain a certain amount of iron) were deposited in other parts of the 
game area where the conditions were slightly different. Wore this 
the case deposits of iron-ores should be found at the same horizon as 
the manaanesp-orc deposits, i.e., on the continuation of their strike. 
