110 THE ENVIRONMENT 



An example which still has some economical value today, is found 

 in the Siegerland in West Germany. In veins of iron ore the iron is 

 deposited from solutions originating in deeper levels of the crust. 

 Formation of v^ins of iron ore is an endogenic process, in contrast to 

 that of the iron formations, which are formed by exogenic processes. 



The latter are called lateritization, but this term covers a wide 

 variety of different chemical reactions. Lateritization leads to a 

 relative enrichment in iron of a parent rock, mainly through solution 

 of its other components. The iron formations thus are residual ores, 

 or, to put it in an oversimplified form, they are the left-overs from 

 parent rocks through the influence of lateritization processes. 



According to Lepp and Goldich, there is a marked and consistent 

 distinction between the iron formations of the pre-Cambrian and 

 younger ones. Those of the pre-Cambrian are siliceous, whilst in the 

 later deposits iron and silica were separated by the lateritization 

 processes operating during the later history of the earth. If we find 

 younger siliceous iron formations, such as occur intercalated in the 

 minette ores, these are clearly formed by a secondary silicification 

 process which was later than the original lateritization. Lepp and 

 Goldich now postulate that this difference between pre-Cambrian 

 iron formations and all later deposits is due to a relative deficiency in 

 oxygen of the pre-Cambrian atmosphere. This is an extremely 

 cautious way of saying that the pre-Cambrian atmosphere was an- 

 oxygenic. 



So the conclusion of Lepp and Goldich is in good agreement with 

 the results of studies by Rankama and Ramdohr. It is hoped that the 

 intensive studies of the pre-Cambrian of Minnesota and adjacent 

 regions by Professor Goldich and his associates will provide much 

 more detailed data in the near future. An important point will be the 

 absolute ages of these pre-Cambrian iron formations, because this 

 will yield a number of fixes for dates when it may be assumed that 

 the earth still had its anoxygenic atmosphere. The newest work of 

 Goldich et. al. (1961) already gives the ages for the main stratigra- 

 phical groups but does not contain detailed information about the 

 genesis of the various iron formations found in the Canadian Shield. 



The results of Lepp and Goldich are also of interest for our topic 

 in a more direct way, because they note that graphitic slates are 

 commonly associated with the pre-Cambrian iron formations. The 



