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Transactions of the Symposium on Photo Interpretation

the strike of the surrounding gneisses. All the main perthite gneiss bodies were
successfully delineated during the preliminary photogeological reconnaissance,
but the identification of the rock types depended on the results of field work
and, in this case, it required the examination of microsections of rock samples
to prove the occurrence of perthitization.
Two concordant lenticular bodies of meta-amphibolite representing original
dolerite sills, were observed on air photographs as parallel hill ridges. The hills
had been cultivated, and their natural vegetation removed, which left their
photographic tone no different from the local paragneisses. The ridges were
noticed during the initial photogeological examination because of their regular
topographic form and were therefore investigated in the field.
In the Lilongwe-Salima area, Nyasaland, the main value of photogeology
lay in the rapid mapping of structures including areas of complex isoclinal
folds, and in the tracing of zones of graphitic gneiss and pyrite impregnations.
It was also possible, with the aid of a limited amount of field information, to
delineate photogeologically certain of the meta-sedimentary and meta-igneous
types. Faults, joints, and unmetamorphosed intrusives, not relevant to the
main topic of this paper were also successfully mapped.
Interpretation of metamorphic rocks: Conclusions
1. In areas of regional metamorphism, the photogeological identification and
correlation of individual rock types is generally less precise than in areas
of unmetamorphosed sediments, because the regional recrystallisation generally
reduces the lithological dissimilarities between argillaceous and arenaceous
rocks and gives them a more uniform resistance to erosion. Variations in veg
etation are also reduced for the same reason. The hardness and resistance to
erosion of monomineralic rocks, such as limestone and quartzite, are not great
ly affected by metamorphism, while the resistance to erosion of the argillaceous
and arenaceous rocks is increased. The greatly reduced erosional differentiation
of the rocks makes photogeological interpretation of individual rock types
difficult, particularly when the outcrops are small.
2. The larger the outcrop of a particular metamorphic rock, the more readily
can it be identified photogeologically, and the more precisely can its
boundaries be drawn.
3. A reconnaissance field check is essential if the photogeological map is
required to delineate outcrops of individual rock types. It is doubted
whether a map, based solely on photogeology, could indicate more than
“Probable Paragneiss” or “Probable Orthogneiss”.
4. The photogeological identification of metamorphic rock types by extra
polation from adjacent areas is less successful than similar extrapolation in
areas of sediments because slight alterations in the grade of metamorphism,
which cannot be observed on photographs, can produce a new suite of rocks.