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Fig. 30. Microphotograph of polished surface of grain of pyrite in 

 ancient sand at Witwatersrand, South Africa (x 250). Dark spots 

 indicate centres of biogenic deposition of iron sulphides by ancient 

 microbes or fungi (from Ramdohr, 1958). In the original section, 

 professor Ramdohr writes, these dark spots show a microstructure, not 

 visible in reproduction. They are seen to be formed by a central dark 

 point, surrounded by one or two small circles, which are in turn sur- 

 rounded by the more or less rectangular spots that are clearly visible 

 in this illustration. The central point and the surrounding circles are 

 thought to represent the original 'pyrite bacteria', more probably fungi, 

 which served as nuclei for further inorganic growth of pyrite crystals 

 (Ramdohr). The age of these biogenic deposits is not known exactly, 

 because they were found in polished surfaces of pebbles within the 

 Witwatersrand deposits. Taking their age at 1800 my gives us a 

 minimum age, but their original formation might have taken place much 

 earlier. Finding of such small scale remnants, with high magnification 

 ore microscopy, is greatly dependent on luck, and a systematic search 

 for such structures in earlier deposits has not yet been carried out. 



