IRON FORMATIONS 109 



But the many properties of these deposits which fit the picture uf 

 ancient placer deposits, of ancient gravels and sands, together with 

 the striking similarity between these deposits so widely separated 

 geographically and in time, demonstrate that Ramdohrs interpreta- 

 tion is the correct one. A later study by Liebenberg (1960) moreover 

 confirms these results. 



Once this explanation is accepted, however, the omnipresence of 

 grains of sulphides and of pitchblende in sands and gravels is con- 

 vincing proof that their deposition took place under an atmosphere 

 of reducive character in all of the four districts mentioned. 



AGE OF SEDIMENTS FORMED UNDER ANOXYGENIC ATMOSPHERE 



The tentative age of these deposits is given in Table VI. 



Taking into account the tendency of tentative ages in absolute 

 dating to grow older when better determinations become available, 

 we may safely state that these deposits range from 2 billion to 

 3 billion years. This means that a reducing, anoxygenic atmosphere 

 on earth was still present 2 billion years ago. 



TABLE VI 



TENTATIVE AGE OF THE GOLD— URANIUM DEPOSITS FORMED UNDER AN 



ANOXYGENIC ATMOSPHERE AS STUDIED BY 



RAMDOHR (1958) 



Dominion Reef, South Africa 3000 my 

 Serra de Jacobina, Bahia, Brazil ? 



Witwatersrand, South Africa 1800 my 



Blind River, Ontario, Canada 1100 my * 



STUDIES BY LEPP AND GOLDICH: IRON FORMATIONS 



Lepp and Goldich (1959) reached conclusions quite similar to those 

 arrived at by Ramdohr from the study of iron formations. Iron 

 formations are superficially formed deposits of iron ore, such as the 

 economically important iron ores in the old shield of North America 

 found, for instance, around Lake Superior, or the minette ores of 

 Jurassic age in Lotharingen and Luxembourg. Iron ore can also be 

 found in veins, and most of the economically important iron deposits 

 of early histor)- and medieval times were formed by veins of iron ore. 



* A figure of more than 1700 my (Derry, 1959) gives a more probable 

 age. 



