ANCIENT SANDS 99 



arc of economic importance for their gold — uranium content. But 

 quite apart from their economic importance, they have a more general 

 significance in that they are formed by ancient sands and gravels. 

 They represent sediments laid down on the surface of the earth in 

 an exogenic process. Consequently their composition was influenced 

 by that of the ancient contemporary atmosphere. These deposits are 

 formed not only by grains of quartz, but also by sulphides and by 

 pitchblende. Pitchblende now is a complex mineral with a composi- 

 tion somewhere between UO2 and UsOs. This is due, however, to 

 later oxidizing. Originally, it consisted of the mineral uraninite, UO2, 

 the least oxidized of the uranium oxides. The South African deposits 

 are gold reefs, already mined for their gold, to which the uranium 

 has added an appreciable factor. The Brazilian and Canadian de- 

 posits, although formed by quite similar reefs, are so low in gold that 

 they only recently acquired economic importance as uranium ores. 

 The deposits described by Ramdohr belong to the following districts: 

 Witwatersrand and Dominion Reef, South Africa; Serra de Jacobina, 

 Bahia, Brazil; and Blind River, Ontario, Canada. There is thus a 

 wide variation in their geographical position, whilst there is also, as 

 we will see, a very marked difference in age. 



Notwithstanding this separation in geographical position, or the 

 difference in age, their composition is strikingly similar. So similar 

 that even the experts often cannot tell samples of one district from 

 those of any of the other three. The mineral composition shows, of 

 course, quite strong variations from bed to bed. Also lateral varia- 

 tions within a single horizon may occur in the relative abundance of 

 the constituent minerals. But the over-all picture is remarkably con- 

 sistent. The variations all fall within the same sharp limits. And — 

 most important — this over-all picture, which is so strikingly similar 

 for these ancient deposits, although they are so far removed in space 

 and time, this over-all picture of these ancient deposits contrasts 

 strongly with that of all younger gravel and sand of the later history 

 of the earth. 



SANDS WITH PYRITES, PITCHBLENDE AND OTHER MINERALS 



The deposits are formed by ancient quartz conglomerates and quartz 

 sands, cemented to a very hard rock which forms the reefs. But apart 

 from the quartz they carry pyrites, FeS, ilmenite, FeTiO:i and 



