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Abstract Cyanobacteria from five tufa‐depositing sites were studied (two in China, two in the United Kingdom, and one in the United States). Species composition reflected water relations of the sites. In wet sites, filamentous Oscillatoriaceae were dominant, but in sites subject to prolonged desiccation, coccoid forms, especially Gloeocapsa, became abundant. Sheath material consisted mainly of polysaccharide, although considerable differences in composition were evident between certain species. Estimates of cell biomass and the volume of sheath material were also made. Tufa samples, dated as being 2,125 years old, contained an organic residue; histochemical tests indicated the presence of polysaccharide and ‐NH2 groups. Petrological examination revealed an unstructured ground‐mass of micritic calcite. No preferred orientation of the crystals with respect to the cyanobacterium sheath surfaces was apparent.
Allan Pentecost (Tue,) studied this question.