Chap. XVI.] gondIte series : petrology. 331 
As has been mentioned on a previous page (294), although it is thought 
Rocks of the gondite that a considerable proportion of the manganese- 
braunite. " ores associated with the rocks of the gondite series 
has been derived from these rocks by their chemical alteration, yet a 
considerable proportion of the ores has been in all probability formed 
direct from the original manganiferous sediments, whenever they were 
deposited practically free from admixture with other sediments, such as 
sand or clay. It is now not possible to determine what proportion of the 
ores have been formed in these two ways. Consequently in the lists, given 
above, of the rocks of the gondite series, the members containing original 
manganese-ores have not been included. The lists might therefore be 
considerably extended by adding many varieties containing braunite ; but 
it would not be certain that all these rocks were the direct products of the 
metamorphism of original sediments. Thus some of the rocks composed 
of braunite and spessartite may be the original products of metamor- 
phism, whilst others may have been originally composed entirely of 
Sjjessartite and owe their braunite to subsequent chemical changes. In 
the case of the gondite series in the Central Provinces the varieties con- 
taining original braunite are more likely to be those that can be formed 
by adding this mineral to the rocks given in the right-hand column on 
page 329, i.e. to those free from quartz, than those formed by adding it to 
the rocks shown in the left-hand column, i.e. to those containing quartz. 
For in the latter case the tendency during metamorphism would have 
been for the constituents that might have given rise to braunite to com- 
bine with a further quantity of quartz with the production of rhodonite. 
Thus, on account of the possibility of the reaction expressed by the 
following equation, one wou^d expect any braunite, if formed, to pass 
in the presence of free silica into rhodonite : — ■ 
3M.^203.MnSi03 + 6Si02 = TMiiSiO;, + 30. 
Braunite Quartz Rhodonite. 
If, however, during the metamorphism of the manganiferous sedi- 
ments some braunite were fojmed and enclosed in spessartite or rhodonite, 
it might be preserved ; so that one would expect to find ome cases of rocks 
containing both quartz and braunite, if only the br, unite were enclosed in 
another mineral, such as spessartite, which would protect it from the 
quartz. Such cases are frequently found. Moreover, it sometimes hap- 
pens that these manganiferous rocks contain these two minerals in direct 
contact in such a way that they must have been originally formed in tt is 
position at the time of the metamorphism of the original sediments. A 
