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Transactions of the Symposium on Photo Interpretation

by E. A. Stephens
Dept, of Technical Go-operation, Overseas Geological Surveys,
Photogeological Division, Tolworth, Surrey, England
Abstract An account is given of the problems of interpretation and identification of meta-
morphic rocks encountered during the reconnaissance mapping of areas of regional meta
morphism in Tanganyika and Nyasaland.
The photogeological identification and correlation of individual rock types are more difficult
in areas of regional metamorphism than in areas of unmetamorphosed sediments, and are
often impossible. It is considered that information from reconnaissance field work is essen
tial for the successful determination of individual metamorphic rocks from air photographs.
Résumé L’auteur donne un rapport concernant les problèmes d’interprétation et d’iden
tification de roches métamorphiques rencontrées sur le terrain, lors du lever d’une carte de
reconnaissance dans les zones de métamorphisme régional du Tanganyika et du Nyasaland.
Il est plus difficile, souvent même impossible, d’établir l’identification et la corrélation photo
géologiques des différents types de roches dans les zones de métamorphisme régional que dans
celles de sédiments non métamorphisés. Les informations obtenues par le travail de recon
naissance sur le terrain sont considérées comme essentielles pour pouvoir distinguer avec
succès différentes sortes de roches métamorphiques d’après des photographies aériennes.
Zusammenfassung Probleme der Interpretation und der Identifizierung metamorpher
Gesteine werden besprochen, welche während einer Aufklärungs-Kartierung in Gebieten
regionaler Metamorphose in Tanganyika und Nyasaland auftauchten.
Die photogeologische Identifizierung und Korrelation individueller Gesteinstypen sind
schwieriger in Gebieten regionaler Metamorphose als in solchen unveränderter Absatz
gesteine. In vielen Fällen ist dies sogar unmöglich. Die durch die Aufklärung im Gelände ge
lieferten Daten werden als unbedingt nötig erachtet, um einzelne metamorphe Gesteine in den
Luftbildern einwanfrei bestimmen zu können.
This paper describes briefly the results of the reconnaissance photogeological
mapping during 1960 of approximately 2,300 square miles in the Mahenge
Area, Tanganyika (bounded by coordinates 8° 30' S, 9° 00' S, 36° 00' E, and
37° 00'E) and, during 1961, of approximately 2,500 square miles in the
Lilongwe-Salima area, Nyasaland (bounded by coordinates 13°15'S, 14°
00' S, 33° 45' E, and 34° 40' E). Field trips of four months duration were made
in both areas to check the photogeological interpretation and to provide the
geological basis for the final maps. Air photographs at scales of 1 : 35,000 and
1 : 40,000 were used. Additional photo cover at 1 : 20,000 scale was available
for part of the Lilongwe-Salima area.
Mahenge Area, Tanganyika
The Basement rocks in the Mahenge area, Tanganyika, are of Usagaran age
and comprise a group of well-foliated metasediments, predominantly horn
blende-, biotite-, and quartzo-feldspathic gneisses. These rocks are impregnated
with quartzo-feldspathic material and migmatisation is particularly well-