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Transactions of the Symposium on Photo Interpretation

*) 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.)
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
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.