Summary: The volatilities of SiO2, Na2O, and Fe2O31 increase with increasing thermal function within the range 1600 to 2100 °C during the melting of a sandy-silty clay. The remaining melt is to some extent enriched in Al2O3, K2O, CaO, and MgO. Chemical analyses of the sandy-silty clay, of the glasses melted from the latter, of the silicate material evaporated and newly condensed and their correlation with increasing thermal function and bulk density of glasses, as well as with the hitherto known quantitative data on moldavites, testify to the fact that in moldavites evaporated and newly condensed silicate material substantially prevails over material that passed directly from a solid to a fluid state. Evaporation of the silicate material from the rocks of the impact crater caused a significant change in relations between major oxides compared to the original relations in the source rocks. Thus, the hypothesis that tektites are directly remelted rocks of the Earth's crust, based on over-all chemical similarities of tektites and some rocks in major oxides, appears incorrect.