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fault zones, parallel joints, and zones of highly fractured rock. In one instance, 
a zone of highly fractured rock, though exhibiting no evidence of displacement 
at the surface, grades into a fault at depth. In many areas tonal lineaments form 
patterns that resemble fracture patterns of the Canadian Shield. 
Tonal lineaments mark positions of fractures and fracture zones that are 
probably formed by adjustments along lines of weakness which bound a mosaic 
of blocks in the basement complex. These adjustments, depending on their type 
(vertical or horizontal), the thickness of overlying sedimentary cover, and their 
magnitude, are reflected at the surface as individual faults, fault zones, or zones 
in which the rock is more intensely broken by joints. Underground water moves 
more freely along such fracture zones, and possibly the surface above the zone 
derives more moisture from the underground reservoir than adjoinging areas and 
hence supports a slightly denser vegetation, thereby giving rise to tonal linea 
ments that are visible on the aerial photograph. 
Linéaments tonals sur les photographies aériennes 
Les linéaments tonals sont des bandes étroites, claires ou sombres qui sont visibles sur les photo 
graphies aériennes. Nombreux linéaments sont vus d’un coup d’oeil; alors que d’autres peuvent 
être mieux vus en regardant des photographies aériennes verticales à un angle presque parallèle 
à la surface de la photographie. Le nombre de linéaments tonals depend surtout de l’échelle de la 
photographie. Les linéaments locaux qui s’étendent jusqu’à 10 miles de longueur, sont mieux vus 
sur les photographies à l’échelle d’environ 1:20 000; tandis que les lineaments subrégionaux 
s’étendant jusqu’à 40 miles de longueur, sont mieux vus sur les photographies à l’échelle d’environ 
1:200 000. Les linéaments tonals visibles sur Mars (comme les »canaux») et sur la lune s’étendent 
sur une longueur de plusieurs centaines et même plusieurs milliers de miles. 
Quelques linéaments tonals indiquent, il a été découvert, l’emplacement des failles, des zones 
de failles, des zones de jointures parallèles, ou des zones de roches grandement fracturées. Dans de 
nombreuses régions, le dessin formé par les linéaments tonals ressemble a celui du Bouclier 
Canadien formé par les fractures. 
Dans les régions de roches sédimentaires, quelques linéaments tonals indiquent l’emplacement 
des failles, et d’autres représentent probablement les zones de failles formées par les ajustages le 
long des lignes de faiblesse dans le sous-sol complexe. 
The following paper, by Professor F. A. Melton, of U.S.A., was read by title 
only, Photo-Geological Exploration in Low-dip Sedimentary Terrain: 
A major advance in geological science during the last thirty years has been 
the use of vertical aerial jthotographs for geological exploration. The results of 
their use have been especially significant in the »flatland» regions of low dip, 
and the full significance is not yet known. 
The writer first developed his techniques of geological interpretation of aerial 
photographs prior to 1930. Since then he has worked extensively in »flatland» 
regions of low dip as well as in mountainous regions. Certain basic assumptions 
should underlie the work of the geological interpreter of aerial photographs in 
the broad deeply-eroded and frequently-peneplained continental areas. Two of 
the most important ones are: 
1. The assumption of more or less universal structural control of drainage 
patterns. 
2. The assumption of almost universal superposition of drainage except for 
extremely local and minor streams. 
The proper use of these basic assumptions and other reliable generalizations in 
the interpretation of aerial photographs can yield important results in the geolo 
gical discovery of structural anomalies which are quite obscure to other means 
of exploration. 
By »flatlands» is meant those relatively low-lying areas which are, of course, 
only relatively flat. They are of regional size and are underlain by sedimentary 
rocks. Their recognition is partly due to detailed geomorphic studies with aerial 
photographs. They consist of the following main geomorphic types. 
323 
F. A. Melton: Photo- 
Geological Exploration 
in Low-dip 
Sedimentary Terrain. 
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