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Transactions of the Symposium on Photo Interpretation

' ;•.: : A
by E. A. Stephens
Dept, of Technical Co-operation, Overseas Geological Surveys,
Photogeological Division, Tolworth, Surrey, England
The Lilongwe-Salima area in central Nyasaland is composed of regionally
metamorphosed sediments comprising gneisses, schists, and granulites. The
area had not been mapped geologically before 1961, but it was known that
graphitic gneiss occurred within the metamorphic series from the evidence of
road cuttings and isolated observations. In 1961 the author made a recon
naissance photogeological map of the area checked by four months’ field work,
during which he was asked to investigate the possibility of tracing the zones
of graphitic gneiss photogeologically.
Examination of the air photographs (scale approximately 1 : 40,000) showed
that the graphitic gneiss could not be distinguished from the other meta
sediments. The graphite occurs as small disseminated flakes within the gneiss
over a wide area and, even where the graphite is abundant in the gneiss, it
does not appear to have any recognisable effect on the vegetation or topo
Abstract An account is given of the photogeological mapping of graphitic gneiss and zones
of pyrite impregnation during a photogeological reconnaissance in central Nyasaland. The
beds of graphitic gneiss could not be distinguished photographically from other metasediments,
and their successful mapping depended on the assumption that foliation of the metasediments
represents original bedding. It is argued that metasedimentary foliation and original bedding
coincide when the regional metamorphism took place under conditions of confining pressure,
and that air photographs can provide evidence as to whether confining pressure or directed
stress was dominant during metamorphism.
Résumé L’auteur fait un exposé sur le lever d’une carte photogéologique de gneiss graphi
tique et de zones d’imprégnation pyritique pendant une reconnaissance photogéologique au
Nyasaland central. Il n’a pas été possible de distinguer sur les photographies les couches de
gneiss graphitique des autres métasédiments et l’on s’est basé, pour cartographier ces gneiss,
sur le fait que la schistosité des métasédiments correspond à leur stratification originelle.
On pense que la schistosité métasédimentaire et la stratification originelle coïncident lorsque
le métamorphisme régional s’est produit sous des conditions de pression restreinte et que les
photographies aériennes peuvent prouver si une pression restreinte ou au contraire une force
dirigée prédominait pendant le métamorphisme.
Zusammenfassung Es wird über die photogeologische Kartierung von graphitischem
Gneiss und von Zonen mit Pyrit-Impregnation während einer photogeologischen Unter
suchung in Zentral-Nyasaland berichtet. Der graphitische Gneiss konnte in den Luftbildern
nicht von anderen Metasedimenten geschieden werden und die Möglichkeit die Gneiss zu
kartieren beruhte auf der Annahme, dass die Schieferung der Metasedimente der ursprüng
lichen Schichtung entspricht. Es werden Gründe angeführt, dass Schieferung und ursprüng
liche Schichtung sich decken, wenn der regionale Metamorphismus unter den vom Gebirgs-
druck beherrschten Bedingungen stattfindet und dass Luftbilder darüber Aufschluss geben
können, ob der Gebirgsdruck, oder tektonische Druck während der Metamorphose vor