THE RED BEDS 1 1 1 



carbon content of these graphitic slates is thought to be of biogenic 

 origin, indicating an abundance of primitive forms of Hfe at that 

 early period. This points in the same direction as the vestiges of 

 sulphur bacteria found by Ramdohr in his pyrite grains (Fig. 30). 

 Although real fossils of these early days are still extremely scarce, 

 there is no possible doubt that there was life on earth at those times, 

 in the days when the geologic environment proves the existence of an 

 early anoxygenic atmosphere. 



SEDIMENTS FORMED UNDER OXYGENIC ATMOSPHERE: 



THE RED BEDS 



From the studies reviewed above it follows that in properly preserved 

 localities, old sediments may supply indications that they were laid 

 down under reducing atmospheric conditions. The opposite is true 

 too. Sediments may, by the state of their oxidation, indicate that they 

 were formed under an oxygenic atmosphere. 



This distinction has, however, to be handled with care. Not every 

 sediment with a few grains of sulphides is indicative of a con- 

 temporaneous anoxygenic atmosphere, nor does a sediment formed 

 by grains of oxides automatically proclaim that it was formed under 

 an oxygenic atmosphere. The most common mineral of present-day 

 sands, quartz, for instance, docs not give an indication of either an 

 anoxygenic or an oxygenic atmosphere, because it is stable under 

 both types of atmosphere. Also under a reducing atmosphere it is 

 possible that sands are laid down which are so well classified, that is 

 so well sorted out during transportation in rivers or oceans, that all 

 heavier grains, like the sulphides, were laid down separately elsewhere 

 in the river bed or on the ocean bottom, resulting in the formation 

 of pure quartz sands. 



For the purpose of recognizing an oxygenic atmosphere by its 

 contemporary sediments, the ideal type is the 'red beds'. 



Red beds are fine-grained quartz sediments, which are mainly 

 silty; that is, composed of fine material with a grain size situated 

 between that of sands and of clays. They normally carry finer-grained 

 intercalations of clay, and also coarser beds of sands and of con- 

 glomerates. The colour, varying between bright red and reddish 

 brown, is due to a small content of iron, normally of a small per- 

 centage by weight only. The iron is in the ferric, highly oxidized, 



