70 THE OLD SHIELDS 



TABLE IV 



LIST OF LOCALITIES INDICATED IN FIG. 15 

 ABSOLUTE AGES GIVEN IN FIG. 36 



1. Oldest known biogenic deposists, the lime-secreting organisms de- 

 scribed by MacGregor. Compare p. 77-82. 



2. Oldest known fossils. Primitive plants and microbes, described by 

 Tyler and Barghoorn. Compare p. 83-88. 



Also, in the same general area, Blind River uranium deposits, formed 

 under pre-actualistic atmosphere. Compare p. 99-109. 



3. Lime-secreting Algae, related to Collenia, described by Gravelle 

 and Lelubre. Compare p. 89-92. 



4. Non-oxidized weathering residue of pre-Cambrian granite, formed 

 under pre-actualistic atmosphere, described by Rankama. Compare 

 p. 97, 98. 



5. Gold— uranium Dominion Reef and also Witwatersrand reefs, 

 formed under pre-actualistic atmosphere, described by Ramdohr. 

 Compare p. 99-109. 



6. Serra de Jacobina gold— uranium reefs, also formed under pre- 

 actualistic atmosphere, described by Ramdohr. Compare p. 99-109. 



7. Lake Superior iron ores, at least pardy formed under pre-actualistic 

 atmosphere, according to Lepp and Goldich. Compare p. 109, 110. 



lion. In either case primary rock structures, such as fossils, will be 

 destroyed. 



Although fossil remains are scarce for the early histoiy of life on 

 earth, we must, of course, use what is available. The first point, 

 treated in this chapter is where to look for vestiges of the oldest life 

 on earth. In the next chapter the fossils found will be described, 

 whilst in Chapter VIII the environment of the early life will be 

 studied. This is possible because, as we have seen in the preceding 

 chapter, the environment is mainly controlled by the existence of 

 either a pre-actualistic or an actualistic atmosphere, which in turn 

 influences all contemporary exogenic processes on the surface of the 

 earth. 



THE OLD SHIELDS 



The fossil remains of early life must be sought for in the oldest 

 areas of the crust of the earth; that is, in the regions technically 

 known as the 'old shields'. These are characterized by pre-Cambrian 



