Chap. XVII.] gondite series : alteration. 
353 
quartz-rock made up of equal parts of the two minerals by weight and 
volume, respectively, is shown below : — 
Equal parts by Eq\ial parts hy 
weight. volume, 
MnO . . . 27-02 31-25 
SiOa . . . 72-98 08-75 
Mn . . . 20-93 2421 
In the same way the composition of given mixtures of quartz, rhodo- 
nite and spessartite, forming different varieties of rhodonite gondite, 
could be calculated. 
Alteration of the Gondite Series. 
Evidence pointing to the Fopmation of Manganese-ore by the Alteration 
of Spessartite and Rhodonite. 
I have already mentioned that a portion of the ores of the deposits 
associated with the rocks of the gondite series has been formed by the 
alteration of various rocks belonging to this series. I propose to give 
here the evidence on which this statement is based and to consider the 
way in which the alteration took place. 
In many places the quarrying operations have laid bare masses of 
gondite or spessartite-rock traversed by a black 
The Mansar deposit. ° , , r • i • „ , r 
network of manganese oxide m all stages or con- 
solidation into hard manganese-ore. An especially good locality for this 
is Mansar, where, in December 1906, there were some good exposures at 
the western end of the deposit. The solid manganese-ore was 
separated from the yellow spessartite-rock by very irregular 
boundaries, which had no relation to the bedding planes of the rock. 
From these masses of manganese-ore, veinlets of ore were seen to 
branch out into the yellow rock and there form anastomosing 
networks, which in some places ran together to form small masses of 
ore similar to the larger masses from which the veinlets came. In 
other places the rock was almost entirely black, with small irregular 
yellow patches, which looked as if they were the residuum of a rock 
that was once all yellow. The deduction that the original rock was 
all yellow and composed of spessartite, with a certain amount of 
quartz in places, is confirmed by a study of these rocks undei the 
II I 
