titic-hydrothermal deposits of tin or tungsten, as well as ra 
dioactive minerals. 
In the Tibesti a number of previously unknown circu 
lar features could be mapped on the ERTS-1 images, as shown in 
fig. 6. A prominent ring structure in the southeastern spur of 
the Tibesti, Aorounga, was examined in detail on aerial photo 
graphs. It seems that granitic diapirism offers the best ex 
planation for the various phenomena observed; that would mean 
that a Post-Devonian magmatic activity must be assumed for the 
Southern Sahara. 
CONCLUSIONS 
The visual evaluation of ERTS-1 images of the Tibesti 
Mountains has proved a very useful additional source of geolo 
gical information, complementary to aerial photographic inter 
pretation and field work. In an interpretation map based on the 
ERTS-1 MSS imagery different lithological units could partly be 
mapped with higher accuracy, correlation of sandstone outcrops 
over large distances could be revised, new intrusive bodies 
were found, and a more uniform and representative picture of 
the regional fracture and fault pattern evolved. 
For the future, more emphasis must be laid on non-vi- 
sual methods like spectral fingerprint analysis and pattern re 
cognition techniques, in order to fully utilize the wealth of 
geological information still buried in satellite imagery. 
ACKNOWLEDGEMENTS 
The autors are indebted to the National Aeronautics 
and Space Administration, Washington, D.C., for rendering possi 
ble these investigations under the NASA Contract SR 349 (Prin 
cipal Investigator: F.K.LIST), carried out through the Bundes 
ministerium für Forschung und Technologie, Bonn, and wish to 
express their thanks to the Deutsche Forschungsgemeinschaft, 
Bad Godesberg, and the Freie Universität Berlin for funding the 
work reported here. 
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1974: Interpretation of orbital photographs. Photogramm. 
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Colvocoresses, A. P. 
1972: Image resolution for ERTS, SKYLAB and GEMINI/APOLLO. 
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