WORKING GROUP 2 
STEPHENS 
105 
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.