VOLUME 21 (2)/April 1991

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

Institut de Recherches gitologiques, 1505 Sofia
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Institut de Recherches gitologiques, 1505 Sofia
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lnstitut de Recherches nucléaires et d'énergie nucléaire, Acad. bul. des Sciences, 1784 Sofia

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