*) Metasediments are defined as metamorphosed sediments, of all grades of metamorphism, 
up to, but excluding, that represented by permeation gneiss. 
2 ) Foliation is defined as the ability of rocks to break along approximately parallel surfaces. 
(Billings, M. P. Structural geology, New York: Prentice-Hall, 1942, 213.) 
AERIAL PHOTOGRAPHS AS INSTRUMENTS OF RESEARCH 
IN AREAS OF REGIONAL METAMORPHISM 
by J. A. E. Allum 
Dept, of Technical Co-operation, Overseas Geological Surveys, 
Photogeological Division, Tolworth, Surrey, England 
The statements made in this paper represent opinions developed during the 
photogeological mapping (with supporting field work) of some 16,000 square 
miles (40,000 sq.km) of the African Basement. Absolute proof of these state 
ments is precluded by the nature of the subject, and discussion and controversy 
based on field and photologeological study of areas of widely differing climatic 
and erosional conditions is necessary to establish or modify them. 
In areas of regional metamorphism, aerial photographs frequently provide 
the most reliable, as distinct from the quickest, way of ascertaining the strike of 
metasediments. 1 ) One of the principles in photogeological interpretation now 
considered is that, in areas of metasediments, aerial photographs indicate the 
bedding rather than the foliation 2 ) direction. 
Much can be learned by comparing the photographic appearance to be 
Abstract It is claimed, that aerial photographs provide geological data which in practice 
are unobtainable in any other way, and thus that they should be regarded as geological research 
instruments in their own right and not merely as aids to other geological work; that geological 
information from aerial photographs has a right to be treated as geological knowledge ; and 
that geological field mapping must be consistent with photographic evidence, or must show 
positively where and why that evidence may be disregarded. 
Résumé On affirme que les photographies aériennes procurent des données géologiques 
qui, en pratique, ne peuvent être obtenues d’aucune autre façon et que, par conséquent, elles 
devraient être considérées comme un instrument de recherche géologique propre et non seule 
ment comme complément à un autre travail géologique; que l’information géologique ob 
tenue par photographies aériennes a le droit d’être traitée comme connaissance géologique et 
que la cartographie géologique faite sur place doit correspondre aux données photographiques 
ou doit alors montrer sans aucun doute où et pourquoi on ne peut pas tenir compte de ces 
données. 
Zusammenfassung Es wird behauptet, dass Luftbilder geologische Daten liefern, welche 
in keiner anderen Weise erhältlich sind und dass sie daher als ein Mittel für geologische Unter 
suchungen betrachtet werden sollen, und zwar in ihrem eigenen Recht, und nicht bloss als 
Hilfsmittel für andere geologische Arbeiten; auch dass die aus Luftbildern gewonnene Infor 
mation das Recht hat, als geologisches Wissen betrachtet zu werden und dass die geologische 
Feldkartierung mit den, den Luftbildern entnommenen Daten, übereinstimmen muss, oder 
dann unzweideutig beweisen muss, wo und wann solche Daten vernachlässigt werden können.