THE OLDEST REAL FOSSILS 



83 



moreover, quite distinct from any structure which may develop in 

 limestone through inorganic processes. 



Intricate structures in limestone may also be formed inorganically, 

 either closely following upon sedimentation or in a later geologic 

 period, through solution, pressure, or other factors, without any inter- 

 vention of organisms. But such structures, of which the well-known 

 'cone-in-cone' is the most distinctive, never even distantly resemble 

 biogenic limestone deposits with their peculiar wavy, regular — ir- 

 regular microlamination. In the Dolomite series of Southern Rhodesia 

 such inorganic structures do also occur. Consequently Young, who 

 gave an over-all appraisal of the 'algal' structures of the Dolomite 

 series (Young, 1940a), describes in a companion paper (Young, 

 1940 b) other structures of the Dolomite series which are absolutely 

 at variance with the 'algal' structures, and which are thought to have 

 nothing to do with early life. Not every abnormal structure con- 

 sequently is thought to be biogenic, and the structures described by 

 MacGregor underwent a most thorough appraisal before they were 

 offered and accepted as proof of the existence of life datable as at 

 least 2.7 billion years old. 



THE OLDEST REAL FOSSILS 



The fossils of primitive plants described by Tyler and Barghoom 

 (1954) are found in the Gunflint iron formation of the pre-Cambrian 

 of the Canadian Old Shield, in southern Ontario. They are described 

 to occur in cherts, an American synonym for the British flint, within 

 a series containing iron ores. Flints or cherts, are deposits of micro- 

 crystalline quartz, SiO-i. The Gunflint chert has evidently been used 

 for the flintlocks of early guns in colonial times. 



Such siliceous sediments are often formed in a swampy environ- 

 ment, where humic acid deriving from the swamp vegetation acidifies 

 the groundwater so that it attacks calcareous or phosphate organic 

 animal remains such as bones or shells. But cellulose and other 

 plant substances are relatively resistant to these acid waters, and 

 plant remains can consequently be fossilized in such an environment 

 through a replacement, molecule by molecule, of their original sub- 

 stances by silica. Mineral stains derived from the original organic 

 material may colour these silicified fossils, so that the original struc- 

 ture may sometimes still be observed in the minutest detail. Silicifica- 



