Chap. XIV.] dharwar series: metamorphism. 
289 
siliceous material that had become mixed with the manganese 
oxides combined with them to form the crystalline and com- 
paratively dense mineral, braunite, whilst the manganese oxides left 
over after all the available silica had been used up in the formation of 
braunite combined with the remaining impurities to form various man- 
ganates, which when mixed together isomorphously form the mineral 
psilomelane. This psilomelane forms the matrix in which the braunite 
granules are set, acting as a sort of cement to them. In the case, then, 
of comparatively pure manganiferous sediments, the chief difference 
between the ores produced by the less pronounced metamorphism 
noticed in the previous section and that noticed in this, lies in the fact 
that in the former case any silica mixed with the manganiferous sedi- 
ments frequently remained after metamorphism in mechanical ad- 
mixture with the manganese -ore, which was thus only able to assume 
the form of psilomelane ; whilst in the latter case the intensity of heat 
developed was almost always sufficient to cause the formation of braun- 
ite, wdth the consequent production of a denser and more crystalline ore. 
There is, of course, every gradation between these two types, 
and both may be included under the term -primary ores (see page 286). 
It is, however, in those bodies of manganese-ore in which there 
Formation 'of the were layers of sand, clay, or other sediment, defi- 
manganese silicates, j^j^g^ interstratified with the layers of man- 
spessartite and rno- ... 
donite, in the ore- ganese oxide , or in which the ore-body consisted 
of manganese oxides rendered impure by the 
admixture of considerable quantities of sand or clay ; and at the peri- 
pheries of the ore-bodies where the manganese oxides come in contact 
with the sediments forming the ' country ' or wall-rock of the ore 
deposit, that the degree of metamorphism to which the rocks have 
been subjected is fomid to produce the greatest differences. As 
has already been explained, in the less metamorphosed type both the 
interstratified layers of silt and the ' cotmtry ' of the deposits were 
simply converted into sandstone-quartzite, quartzite, slate, shale, etc., 
whilst the manganese oxides were compressed and rendered more or 
less crystalline. In the more metamorphosed type, however, the man- 
ganese oxides have interacted with any interstratified or admixed silt 
and with the 'country' or waU-rock. The product of this reaction has 
been various according to the nature of the silts and impurities. The 
chief products have been a manganese-garnet, and rhodonite, a manga- 
nese-pyroxene. The garnet approximates, as far as can be judged from 
two analyses, to the manganese-alumina-gamet, spessartite, the theoretical 
