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THE APPLICATION OF RECONNAISSANCE 
PHOTOGEOLOGY TO MINERAL INVESTIGATION 
IN CENTRAL NYASALAND 
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 
herrschten.