98 



THE ENVIRONMENT 



TABLE V 



FERROUS AND FERRIC IRON CONTENT (WEIGHT PER CENt) IN QUARTZ 



DIORITES AND DERIVED SEDIMENTS OF MIDDLE PRE-CAMBRIAN 



AGE NEAR TAMPERE, FINLAND 



(from Rankama, 1955) 



Quartz diorites 



Until that time a tentative age of 2 billion years may be provisionally 

 assigned to the reductive weathering processes described by Rankama 

 in Finland. 



Rankama's assertion that the ancient sediments are really deriv-ed 

 from that particular granitic rock, the quartz diorites, whose relative 

 abundance in ferrous and ferric iron tallies so well with that of the 

 surrounding schists, is, however, open to question. There are many 

 granites in the area, there is a regional metamorphism of the en- 

 closing sediments, and it is rather difficult to establish beyond doubt 

 the relation between granitic rock and metamorphosed sediments. 

 Rankama's study has, however, shown the way in which ancient 

 sediments can be used to assess the properties of the atmosphere of 

 the earth at the time of their deposition. Rankama's ideas have later 

 been amplified by quite independent studies by the German ore 

 specialist Ramdohr. In the latter case the material is quite different, 

 but the same basic assumption underlies the work of both Rankama 

 and Ramdohr, 



STUDIES BY ramdohr: GOLD URANIUM REEFS 



The study of Ramdohr (1958) describes ancient pre-Cambrian de- 

 posits from the old shields of South Africa, Brazil and Canada, which 



