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MANGANESE DEPOSITS OF INDIA: GEOLOGY. [ PaRT 11: 
Chemical Composition of Kodurite. 
To get at the original composition of kodurite I picked out the two 
freshest examples I could find. One of these was a banded variety from 
Kotakarra in the Vizagapatam district. In this rock the garnet and 
apatite are still fresh, whilst the felspar has been entirely replaced by 
opal. Microscopic examination shows that this replacement by opal 
has been practically confined to the felspar, although a little garnet or 
apatite are perhaps occasionally affected. In the calcalations given 
later on it is assumed that the replacement is entirely confined to the 
felspar. The rock usually shows small patches of black manganese 
oxides in places, but the piece selected was free from these stains. 
After prolonged soaking in water and finally boiling for some 
hours the specific gravity of this specimen was found to be 2-85 
This treatment was necessary because the rock was evidently porous 
and showed under the microscope numerous cavities in the opal ; since 
some of these were perhaps closed, the specific gravity given above is 
probably a little lower than the value that would have been obtained 
if the specific gravity had been determined on the powdered rock. This 
was not considered desirable on account of the loss of particles through 
sliming that inevitably occurs when this course is pursued. This specimen, 
weighing 55 • 2 grammes, was then powdered up and the whole of it sent 
to Messrs. J. and H. S. Pattinson of Newcastle for analysis. 
The other specimen was from Boirani in the Ganjam district. It 
shows abundance of a light brown manganiferous garnet scattered 
through a matrix of opal in which are abundant remains of orthoclase 
felspar up to -{q inch long. The opal is of grepsh to greenish colour and 
translucent; and the apatite, since it is also of a pale greenish colour, is 
not easily distinguished in the hand-specimen from the opal. Under the 
microscope it is seen that the opal (with a little chalcedony) has been 
formed by the replacement of the felspar, and perhaps to a certain small 
extent by the replacement of the garnet, which is of a pale yellowish 
colom:. The apatite is seen to be much less important in quantity than in 
the Kotakarra rock. A photomicrograph of this rock is given in fig. 2, 
Plate 8 ; it shows orthoclase, garnet, and opal. Some pieces free from 
black alteration products were selected and their specific gravity deter- 
mined as before after prolonged boiling. This was found to be 2-65, 
which must also be a trifle lower than it would have been if determined 
^ I am indebted to Sir. K. A. K. Hallowes for this determination. 
