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val of the stereo satellite image) the domain of the aerial 
photograph. 
Using these techniques and the potential of the aeri 
al photograph to show very slight differences in pyroclastic 
rocks (fig. 5), the occurrence and extension of pyroclastic 
rocks from the Trou-au-Natron-and the Kinimi-calderas were in 
vestigated (ROLAND 1974). It could be shown that the formerly 
held hypothesis of an explosive origin of the Trou (based on 
field work !) had to be revised, and that the Trou au Natron 
represents a collapsed caldera. 
Structures 
Important linear elements like faults or lineaments 
can be determined quickly and rather completely on ERTS-1 ima 
ges. The regional fracture pattern can be mapped and analyzed 
with an overall consistency of the interpretation that is dif 
ficult to attain in aerial photographic interpretation. 
A fracture zone of regional importance with a stri 
king length of more than 100 km northeast of Tarso Toon could 
only be recognized as such on satellite images and photographs 
(Gemini, Apollo, ERTS-1). It seems to belong to the wrench- 
fault system that is decisive for the pronounced fault patterns 
of the Tibesti,and that was recognized from aerial photographic 
interpretation (LIST 1969). Relative movement along the fault 
planes had also to be determined from aerial photographs, as 
well as the tectonic relation of granitic intrusions to the 
basement (LIST & STOCK 1969). 
Circular Features 
With the availability of satellite photography and 
imagery, regional geological structures could be viewed synop- 
tically for the first time, and consequently many new structu 
res were recognized and mapped. This applies especially to the 
so-called "circular features" that seem to be much more common 
than known previously. Such circular features can also be ob 
served in quantity in the Precambrian basement of the Tibesti. 
The majority of these circular features are granitic 
intrusions which are uncovered partly or completely by erosion. 
The latter are easily recognized on the ERTS-1 images by their 
generally lighter tone. They are also well defined on aerial 
photographs by the obvious difference in material (mostly de 
composition sands). 
But there are other, sometimes very dimly outlined 
circular patterns that occur within the same lithological unit. 
The genetic significance of these structures is not yet fully 
understood, but it seems very probable that they represent iri- 
trusive bodies which have not quite reached the level of the 
present land surface. Such buried intrusives should make a pro 
mising target for mineral exploration, for instance for pegma-