100 THE ENVIRONMENT 



pitchblende, primarily UO2, in considerable quantities. 



They show all the characteristic features of deposits originally laid 

 down as superficial gravels and sands. They form placer deposits. 

 The roundness of the individual grains, the fact that they are so well 

 sorted in grain size, the differences in mineral composition and in 

 grain size between successive beds, are characteristics found in all 

 gravels and sands superficially laid down from streams and lakes, and 

 cannot be accounted for by any other process of deposition. 



In these ancient deposits, which contain grains of such a different 

 composition, and consequently of different specific weight, such as 

 quartz, pyrite and pitchblende, it is nice to see how in one single 

 horizon the lightest grains, the quartz grains, are always much larger 

 than the heavier pyrite grains, whilst the much heavier grains of 

 pitchblende are by far the smallest. This is one of the nicest examples 

 of sorting and classifying of grains according to size and weight, 

 according to physical properties, as one can find in geology. 



Moreover, Ramdohr describes evidence of erosion of earlier beds 

 of these ancient sands and gravels, and re-deposition of fragments 

 of earlier clastic horizons in newer beds. One finds rolled lumps of 

 the older deposits, re-deposited together with single grains in a 

 younger bed. 



There is a striking similarity in mode of deposition, of reworking 

 and re-deposition of these ancient gravels and sands, with their con- 

 temporary erosion channels of ancient rivers, to much younger gravels 

 and sands and to recent examples. There is but one difference, and 

 that a paramount one; namely, all newer gravels and sands are 

 formed by quartz only, whilst in these ancient def>osits we find a large 

 amount of grains of sulphides, of pitchblende and of other minerals, 

 together with grains of quartz. These other minerals are not stable 

 under the present oxygenic atmosphere. 



The structure of these reefs can be studied in Figs. 26-32. Figs. 26 

 and 27 are respectively from an ancient sand of Witwatersrand, 

 1800 my old, and from a recent black sand from the coast north of 

 Buenos Aires. The ancient sand consists mainly of pyrites, FeS, the 

 recent sand of magnetite, Fe304, which have comparable specific 

 weights. The similarity in structure is self-evident. But the pyrite 

 sand would be unstable now, whereas the magnetite sand, formed by 

 iron oxide, is stable under the present oxygenic atmosphere. 



