Chap. XII. ] kodurite series : petrology. 
253 
Manganmagnetite. — On page 40 vrill be found an account of the apatite- 
manganmagnetite-spandite-rock of Garbham. It is not improbable 
that this manganmagnetite is of primary origin, although the evidence 
cne way or the other is not conclusive. In a series of rocks containing 
basic and ultra-basic members one would expect the pres^ence of a 
black ore. In the ordinary basic and ultra-basic rocks such as gabbros 
and peridotites black ores are common, taking the form of ilmenite, 
magnetite, and chromite. In the case of the kodurite series we should 
expect any ore if present to contain manganese, and perhaps mangan- 
magnetite would be the most likely mineral. Hence it is not unreasonable 
to suppose that the manganmagnetite of this Garbham rock is an original 
constituent. As this mineral is often found in the manganese-ores of this 
area, the question naturally arises as to whether such manganmagnetite 
also is original, so that it is the only mineral left unaltered during the 
chemical changes by which the manganese-ores were produced; or whether 
it is, like the remainder of the manganese-ores, of secondary origin. 
There is at present no evidence available that will enable this point 
to be determined. 
I do not intend to give here any detailed description of the rocks 
Petrology of the series. A short account of the minerals 
kodurite series. has just been given and by reading this and the 
notices of the various rocks scattered through the descriptions of 
the deposits, a very good idea of the characteristics and mode of 
occurrence of these rocks will be obtained. Taken as a whole 
these rocks are of medium grain, being either of about the same degree 
of coarseness as an ordinary medium-grained granite, or of somewhat 
finer grain (constituents averaging to i inch diameter). The apatites 
and garnets are often scattered as rounded idiomorphs in the remainder 
of the rock. The rocks do not usually shew banding, but at Tai^uxu there 
is a distinct tendency to shew this characteristic, as also at Kotakarra. 
It wiU, however, be interesting to consider the typical kodurite. 
As already stated this has nowhere been found unaltered. But as the 
processes of alteration have usually acted so as to affect different consti- 
tuents at difierent places, it is easy to reconstruct the original rock. Judged 
in this way, the imaltered kodurite must have shewn red-brown garnet 
crystals, usually averaging yV to ^ inch diameter, profusely scattered 
through a light-coloured matrix of white orthoclase (averaging ^ to ^ 
inch diameter), and greyish to greenish-blue rounded apatite prisms 
(averaging yV to -^^ inch diameter), the apatite being usually much less 
