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MANGANESE DEPOSITS OF INDIA : GEOLOGY. 
[Pakt II : 
as ffir as one could judge from its relations to the other components. If 
this be the case, we must suppose that the original sediments from which 
the gondite series was produced were in some cases highly barytiferous. 
This conclusion is supported by the high percentage of this element found 
in many of the ores ; although, if the supposed original barytes were not 
present, it might be supposed that the barium had been subsequently 
introduced. There is, however, nothing contrary to the experience of the 
present day in making this supposition that the original sediments con- 
tained barium ; for we have an actual example in our own Indian seas, 
namely that described by E. J. Jonesl of some nodules obtained by 
trawling off Colombo in water of 675 fathoms. These nodules were 
found on examination to consist almost entirely of barvtes, one of them 
giving 82-5 per cent. BaS04 on analysis. One is very apt to overlook 
the presence of barytes in a rock when it is not presence in abundance. 
There are four minerals in the gondite series that require some care in dis- 
tinguishing. These are orthoclase, apatite, barytes, and arsenates, all 
colourless minerals with a low or comparatively low birefringence. 
The felspar can, however, be at once distinguished from the other three 
substances by noticing the index of refraction ; this can be rendered 
more obvious by cutting off with the hand a portion of the light coming 
from the mirror of the microscope, or, if the instrument is provided 
with an iris diaphragm, by stopping down considerably. When this 
is done the orthoclase still shows an unpitted surface, while the other 
three minerals stand up in relief with a much pitted surface. For the 
method of distinguishing apatite, arsenates, and barytes, one from 
another see page 220. 
Another variety of the rhodonite-spessartite-rock is one containing, 
in addition to barytes, a green manganesian phosphate, for which 
see page 207. It is surprising that manganiferous phosphates have not 
been detected more often in the rocks of the gondite series than in this 
solitary case. 
Both the rhodonite-rock and the rhodonite-spessartite-rock often con- 
Rhodonite-quartz- ^^^^ small quantities of quartz, which may increase in 
rock and rhodonite- quantity until the rock becomes a rhodonite-quartz- 
rock or a rhodonite-gondite, respectively. All the 
rhodonite-bearing rocks are distinguished from those free from rhodonite, 
such as gondite or spessartite-rock, by the fact that they almost invariably 
occur in layers or beds of a massive character, and do not often occur as 
J Rec. Oeol. Surv. Ind., XXI. p. 35, (IS88). 
