different, because structural features usually play only an minor role. The 
interpretation centres on a detailed study of the characteristic landforms 
developed in the various rock types. The determination of the lithology often 
is a special difficulty, which is sometimes best solved during a subsequent field 
survey. 
Another aspect which is becoming increasingly important is the utilization 
of aerial photographs in hydrogeological investigations. Water is an extremely 
important item in a great many development schemes, not only within the 
arid zone, but also elsewhere. In compiling maps of the availability of construc 
tion materials, the geologist renders services to the engineers engaged in the 
construction of roads, dams or other engineering works. Photogeology is thus at 
present rather rapidly widening its scope by covering these fields. 
The basis for the new trend is the experience gained in oil exploration since the 
nineteen thirties. The geological exploration and mapping is today carried out 
more extensively than at any time in the past. Accurate geological information 
is needed for the search for minerals and to meet the increasing oil demands. 
The need for rapidly surveying large areas, has stimulated the various methods 
of research, and photogeology is now an essential part of geological exploration 
everywhere. It should be borne in mind that the photogeologist, in most cases, 
cannot indicate immediately where exactly a certain kind of mineral deposit 
can be found. He can, however, point out adequately where the greatest 
likelihood exists for the occurrence of such a mineral deposit. It frequently 
occurs that extensive areas to be surveyed can be eliminated immediately, or 
can be reduced considerably. Sometimes areas of particular interest, where 
an intensive field survey is highly desirable, can be pointed out directly. The 
priorities of the field investigations can thus easily be established, and planning 
for the field parties can be done. The best sites for test wells can also be selected. 
The photo interpretation thus pays off during the subsequent field survey. 
The amount of geological information which can be derived from the aerial 
photographs varies considerably from one area to another. More details can 
usually be obtained from areas of sedimentary rocks than from igneous or meta- 
morphic regions. The arid zone is evidently in a more favourable position than 
the humid tropics, where geological data is easily obliterated by dense vegetation 
and thick soil cover. Intensive land use, as in western Europe, also can interfere 
considerably with geological photo interpretation. The absolute amount of 
information obtained is not always decisive for the usefulness of photogeology. 
The field geologist working in the tropical jungle will usually follow the river 
courses and his survey will be far from complete. He will be very glad, there 
fore, to have any additional information that can possibly be obtained. Though 
the absolute amount of detail provided is less, its relative value for the explora 
tion may be even higher. The economical rewards of photogeology actually 
are not in systematic mapping, but in the discovery, from the air, of unknown 
mineral deposits and promising geological structures. 
The accent of photogeological studies for oil exploration will be on the struc 
tures present and on the measurement of dip and formational thickness. Oil