Chap. XIV.] dharwars : piedmontitk gneisses. 
361 
matrix of calcite showing no signs of derivation from any other mineral ; 
but, in one case, namely, Ghogara in the Pench river near Parsioni, I 
was fortunate enough to find an example of a piedmontite-bearing gneiss 
Picdmnntite-<'iieis- sliowing the same process of chemical alteration 
ses. of felspar into calcite as I had so often found in 
connection with the non-manganiferous limestones. (Plate 10, fig. 3). 
I had collected this specimen as an example of a piedmontite-bearing 
limestone, so that it is possible that a far larger proportion of these 
supposed limestones are really gneisses, partially converted into lime- 
stones, than would be suspected from their appearance in the field. 
Piedmontite-gneisses were also found at Pali (Plate 10, fig. 2). 
Assuming that these gneisses are only metamorphosed sediments, 
c c the formation of the manganiferous limestones 
iheory ci loritiu- . . ^ _ 
tion cf the mangani Can be explained in the following way. During 
ferous limestones. deposition of the Dharwar sediments there must 
have been periods when the sediments deposited were of a predominantly 
calcareous character. This deposition took place no doubt in water of 
some depth, such as the bottom of a sea or large lake. At times the depo- 
sition was only chemical leading to the production of pure limestones, but 
this must have been a rare case. Much more frequently a certain pro- 
portion of mechanical sediment was laid down at the same time, the 
proportion being sometimes very considerable, even as much as half 
the total deposit. In a certain number of cases, really forming but 
a small proportion of the whole, nodules, thin lenticular seams, and in 
a few rare cases small beds of manganese oxides, were deposited, probably 
in the same way as nodu)es of manganese-ore are being formed on the 
bottoms of the present-day oceans ; namely, by some process of segre- 
gation, due to the power of a particle of manganese oxide, once 
deposited, to bring about the deposition round itself as a nucleus of 
small quantities of manganese salts dissolved in the neighbouring 
waters. The nodules and patches of manganese oxide, thus laid down, 
and subsequently covered up by a further deposit of calcareous matter, 
were no doubt often rendered impure by admixture with siliceous 
materials. In other cases, manganese must have been deposited with 
some uniformity throughout a considerable thickness of calcareous 
sediment, and in this case the deposition may well have been as 
carbonate. Then came the period during which the Dharwar sediments 
were folded and metamorphosed. Those sediments that were sufficiently 
impure were converted into gneisses, of course with liberation of 
carbon dioxide and water; and in these gneisses the impurities 
