106 THE ENVIRONMENT 



Fig. 28 is again from an ancient sand of Witwatersrand, which is 

 formed mainly by quartz and pyrite. In this picture too, we see that 

 the amount of sulphide grains is large compared to that of the quartz 

 grains. The pyrite grains are not a negligible minority, as is the case 

 with the 'heavy minerals' in recent sands. They form one of the main 

 constituents. Moreover, at the upper side a re-deposited lump of 

 an older conglomerate is found. The rounded borders of the older 

 grains, cemented together in the lump of the earlier conglomerate, 

 are still faintly seen. 



Fig. 29 shows a sedimentary rhythm, in all aspects comparable to 

 rhythms in sedimentation found in younger sands and gravels. On 

 top of a bed of quartz grains lies a thin horizon composed of grains 

 of pyrite and of pitchblende. The boundaries between the individual 

 quartz grains are no longer seen in the reproduction. These grains 

 are, however, much larger than those of the pyrite, whilst the pitch- 

 blende grains are not only the smallest, but moreover show a strict 

 classification of size. 



Indications were also found of the presence of life during the 

 formation of these deposits. Fig. 30 shows a polished section of a 

 pyrite grain at a higher magnification. The many spots seen are 

 thought to represent centres of deposition of the iron sulphide, 

 through the influence of early microbes. 



The conclusion to be drawn from these deposits is that they were 

 formed under a reducing, anoxygenic atmosphere. 



Not every geologist will agree with this conclusion. This is caused 

 by the fact that these reefs are very old, and consequently the pri- 

 mary structures of their sedimentation have often been obscured by 

 later secondary processes. Pyrite and the other sulphides, and also 

 gold, are easily mobilized and later re-deposited on secondary loca- 

 tion, within the long times of later geologic history. Moreover, there 

 is a constant bombardment through the radioactive decay of the 

 uranium in the pitchblende grains which also tends to obliterate 

 primary structures. If we look at the pyrite grains, for instance, we 

 see how many rounded grains were later covered by new growths of 

 pyrite now deposited along crystallographic planes, which tends to 

 render the original roundness of the pyrite sand grains unrecognizable 

 (Figs. 31 and 32). Some geologists have been led astray by these 

 secondary phenomena, which they think to be of primary character. 



