VOLUME 22 (2)/April 1992

Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

Sofia University "St. Kliment Ohridski" 15 Tsar Osvoboditel Blvd., 1504 Sofia, Bulgaria

Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
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Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria