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Transactions of the Symposium on Photo Interpretation

graphy. In these circumstances it was clear that the graphitic gneiss could not
be traced by photogeological methods alone and that it would be necessary
to coordinate photogeological interpretation with field observations.
Field work proved that the parallel ridges and lineaments observed on the
photographs represented the strike of the metasedimentary foliation. The
parallelism of the graphitic beds with the foliation was confirmed in the field
by following a particular graphitic zone for several miles along the strike. Once
the coincidence of foliation and graphitic bedding had been established, it
was possible to link observed outcrops photogeologically and so to indicate
the approximate position of the graphitic beds on the photogeological map.
The inability to identify the graphitic gneiss photogeologically prevented
the precise delineation of the beds, and the accuracy of the mapping depended
rather more on the concentration of field traverses than on photogeological
interpretation. The time available for field work in the area of the graphitic
gneisses was very limited and the check field traverses were necessarily widely
spaced, but it was possible to produce a reconnaissance photogeological map
which would assist future prospecting geologists by showing the probable
outcrop of the main graphitic zones over an area of approximately 800 square
One of the factors necessary for the economic exploitation of graphite is that
the gneiss should be soft and friable for the easy extraction of the mineral.
Although it was not possible to recognise on photographs those areas soft
enough for working the graphitic gneiss, it was possible to recognise areas of
ferruginous gneiss which were certainly too hard for working. The ferruginous
gneiss formed high, resistant hills which were delineated on the photogeological
map as being of secondary importance for future prospecting.
The metasediments in this area have been folded into numerous anticlines
and synclines. The beds preserved in the centre of one of the synclines form
very steep, high, and photogeologically distinct ridges over an area of approx
imately twelve square miles. The initial photogeological interpretation in
dicated that they were formed of resistant rock, but they presented a unique
feature in that some of the ridges were discontinuous, the highly resistant rock
giving way abruptly along the strike to soft unresistant gneiss. The reason for
this became obvious during the field traverse across the syncline. The ridges
were found to be formed of ferruginous micaceous gneiss with lenses and
impregnations of iron pyrite concentrated along the spine of each ridge. The
presence of these pyrite concentrations had undoubtedly made the gneiss
more resistant to erosion and had produced the steep ridges of the syncline.
The very pale photographic tone of the ridges is caused by very short grass
and bare limonitic rock. Boulders of gossan occur locally on the ridges and
contain traces of azurite and malachite.
After one reconnaissance field traverse across the area had established the
relationship between the occurrence of the mineralisation and the topographic
expression and photographic tone of the ridges, it was possible to delineate
on the photographs the occurrence of pyrite impregnations throughout the