300 
MANGANESE DEPOSITS OF INDIA : GEOLOGY. [PaRT II : 
because various colleagues, on account of the plausible and probably 
correct theory that these very gneisses were formed by the metamorphism 
of impure calcareous sediments, have cast doubt on the reaUty of the 
passage of rocks similar to or identical with the quartz-pyroxene - 
gneisses into crystalline limestones. The theory outliued above shows 
that it is possible for impure calcareous sediments to have been first 
converted into very calcareous gneisses and then to have been partly 
reconverted into crystalUne limestones. Moreover, this reconciliation 
of the two theories also allows for the case of pure or comparatively 
pure sediments being converted directly into crystalline limestones 
without passing through the gneiss stage. It must be noticed, how- 
ever, that since the quartz-pyroxene-gneisses contain felspars, such 
as microcline and labradorite, both containing considerable amounts 
of alkalies, it follows that if these gneisses really were derived from 
calcareous sediments, the sediments must have contained considerable 
quantities of potash and soda. 
In the paper cited above the origin of the manganiferous Limestones 
T,, ■ c is not discussed, simply because at that time no 
The origin of the > f J 
manganiferous lime- evidence had been found to show whether their 
origin was the same or not as that of the non-manga- 
niferous limestones with which they are often intimately associated. 
Subsequent microscopic work has, however, shown that their origin is 
the same. 
These manganiferous limestones have only been found in the Nagpur 
and Chhindwara districts in the Central Provinces. In the Chhindwara 
district they contain spessartite and rhodonite, but no piedmontite, and 
are, moreover, of a dark brown to black colour owing to the secondary 
deposition of manganese oxides along the cleavage and twinning planes 
of the calcitei. At every occurrence in the Nagpur district, however, 
these limestones contain piedmontite, both in scattered grains and in 
nodules, the limestone matrix in which they are set being usually of 
a light colour and not blackened by the secondary deposition of man- 
ganese oxide, although the blackened limestones are also of frequent 
occurrence. The limestones as a rule also contain small bands and 
nodules of a brilliantly crystalline manganese-ore. Moreover, some of the 
no iules are of complex composition, consisting partly of piedmontite 
and partly of manganese-ore. The microscopic examination of these 
limestones usually shows crystals of piedmontite (Plate 10, fig. 4,) in a 
1 Loc. cit., pp. 200, 201 ; and Plate 17, %. 1 
