REEFS OF ALGAL LIMESTONE IN THE SAHARA 



89 



discussed in the next cliapter, the Blind River uraniferous deposits, 

 roughly 2 billion years old, and situated in the same general area, 

 were still formed under anoxygenic conditions. This difference in 

 absolute age of some 400 my corroborates earlier geologic correla- 

 tions, which also seem to indicate that the Gunflint is much younger 

 than Blind River. So it is quite possible that the earliest fossils from 

 the Gunflint did already live under the oxygenic atmosphere. The 

 fact that in this formation cherts and iron ore seem to be separated 

 also points in the same direction, according to the studies of Lepp 

 and Goldich reviewed in the next chapter. At present, the best we 

 can do is to state as a considered guess that the earliest fossils known 

 up to now, the primitive plants described by Tyler and Barghoom, 

 lived already under the actualistic, oxygenic atmosphere. 



REEFS OF ALGAL LIMESTONE IN THE SAHARA 



The biogenic deposits described from the pre-Cambrian of Central 

 Africa, amongst others by Gravelle and Lelubre, form a welcome 



Fig. 24. Reef of Conophyton in the Pharusian. Pre-Cambrian of the 

 Hoggar, Sahara (from Gravelle and Lelubre, 1954, Plate XXVII). 



