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area with some degree of certainty. It was also possible to assess qualitatively
the degree of pyrite impregnation in the various ridges from variations in
photographic tone, and the size and steepness of the ridges. Areas of pyrite
mineralisation outside the vicinity of the syncline could also be identified
photogeologically and, as a result of this single traverse, it was possible to
indicate areas of probable pyrite mineralisation over the whole mapping area.
It has been shown that the reconnaissance mapping of the graphitic gneisses
of the Lilongwe-Salima area was successfully accomplished by photogeological
interpretation with a minimum of ground control. If it is assumed that the
graphite represents original sedimentary carbonaceous material, and that the
beds of graphitic gneiss traced in the field and on photographs represent
original bedding, then it may be concluded that the foliation of the sur
rounding gneisses, which was observed to be in the same plane as the graphitic
beds, must also coincide with the bedding.
Evidence from photographs alone may, in certain cases, indicate whether
metasedimentary foliation represents original bedding. The occurrence of
recognisable monomineralic rocks, e.g. dolomitic marble or quartzite, in a
series of long, clearly defined, and parallel ridges may be accepted as evidence
of their sedimentary origin. If the foliation in the adjacent gneisses can be
seen to be parallel to these ridges, and particularly if the parallelism is main
tained throughout a series of folds, it may be assumed that the foliation of
the gneisses represents original bedding. While monomineralic rocks provide
the clearest evidence, other lithologically distinct rocks can also be used, such
as long ridges of quartzo-feldspathic gneiss, provided that they clearly represent
lithologically recognisable beds in the original sedimentary series. The occur
rence of a single ridge, or of short, ill-defined, or non-parallel ridges would
not be accepted as good evidence of sedimentary origin.
In the Mahenge area, Tanganyika, mapped by the author in 1960, 12,000
feet of interbedded dolomitic marble and gneiss are preserved in a syncline
over an area of approximately 100 square miles. Although there is some
lensing of the gneisses and some thickening and thinning of the dolomitic
marble, many of the individual beds can be traced on photographs and on
the ground around the nose of the fold and along both limbs. Throughout the
syncline the foliation of the interbedded dolomite and gneiss appears always
to be parallel to, and in the same plane as the original bedding of the sediments
which is clearly indicated by the dolomite beds. The foliation dips gently
inwards round the nose of the fold, and there is no evidence of a second
direction of foliation parallel to the fold axis.
In the Lilongwe-Salima area (fig. 1), Nyasaland, the coincidence of original
bedding and metasedimentary foliation is clearly demonstrated by the paral-
Fig. 1. Stereotriplet Nanzeka Hill, Nyasaland
Nanzeka Hill occurs in the centre of a broad structural basin formed dominantly of micaceous
gneiss. The central hill and steep ridges are formed of ferruginous micaceous gneiss which
is more resistant to erosion. Small lenses and impregnations of iron pyrite occur locally along
the spine of each ridge (Reproduced by permission of the British Air Ministry, British Crown
Copyright reserved).
