WORKING GROUP 2 
ALLUM 
77 
collect all the evidence; but this is incorrect. Rocks covered by residual soil fre 
quently indicate their presence and their structures by the vegetation, and the 
distribution of that vegetation, which they support. This variation in vegeta 
tion, although frequently producing easily recognised and clearly defined 
lineaments on the aerial photographs, is of a statistical nature. If the field geol 
ogist were able to make a statistical study of the vegetation, he would produce 
a pattern similar to that of the lineaments of the photographs; he would not 
normally be aware, however, unless he had aerial photographs, that the ano 
malous distribution of vegetation, justifying a statistical study existed. Thus the 
residual-soil-covered structure would be overlooked and remain unmapped. 
It has been suggested above that geological information which, in practice 
is unobtainable in any other way, can sometimes be obtained from aerial photo 
graphs. The question now arises as to what should be the geologist’s attitude if, 
because of paucity of outcrops or some other reason, his field work produces 
no evidence either for or against the existence of the structures indicated by the 
photographs. In the writer’s opinion, the photogeological data have a status, 
value, and reliability in their own spheres fully equal to those obtained from 
other geological sources. It follows that, if the geologist, after considering the 
data obtained from the aerial photographs, postulates the existence of certain 
structures, these structures should be plotted with suitable qualifications on the 
geological map, unless positive field evidence is found which proves that the 
postulated structures are inaccurate or non-existent. The geologist has no right 
to ignore structures “seen” on aerial photographs merely on the grounds that 
he has found no positive evidence for them in the field; if he does so, he is 
rejecting much of the most valuable assistance which aerial photographs can 
offer him. It is in those areas where field observations are sometimes incon 
clusive that the geologist should pay increased attention to, and put increased 
reliance on, the evidence provided by the aerial photographs. 
References 
Allum, J. A. E. Photogeological interpretation of areas of regional metamorphism. Trans. 
Inst. Min. Metall., London, Vol. 70, 1960-61, Part 9, 521-543. (Bull. Inst. Min. Metall., 
Lond., No. 655, June, 1961). 
Hemphill, W. R. Small-scale photographs in photogeologic interpretation. Photogramm. 
Engng. 24, No. 4. Sept. 1958, 562-7. 
Ray, R. G. Aerial photographs in geologic interpretation and mapping. United States Geol 
ogical Survey, Professional Paper 373 (Washington U.S. Govt. Print. Off., 1960). 
Discussion 
F. Ruellan wished to emphasize the importance of field checks in order to avoid errors. Exam 
ple : the pre-cambrian volcanic region of Rio Branco in Brasil, which from photos alone gives 
the impression of monoclinal ridges. The author agreed that field work is necessary for the 
production of a final geological map. If, however, a geological structure is visible on the aerial 
photographs and it is not possible to confirm it by field work, the structure should be plotted 
on the final map in photogeological symbols only. There will then be no ambiguity as to the 
source of the information and all data available will have been recorded.