THE OLDEST BIOGENIC DEPOSITS 77 



lurally preserved remnants of early organisms. Instead, it is a secre- 

 tion of lime by early organisms, comparable to the crusts of travertine 

 limestone, secreted at present by Algae during their metabolism. But 

 even as such, it remains up till now the oldest indication of the 

 existence of life on earth. 



A more recent discovery, on the contrar)-, is of real fossils. These 

 are the silicified primitive plants described by Tyler and Barghoorn 

 from the old shield of Canada, which have been structurally pre- 

 served in microscopic detail. Their absolute age is, however, very 

 much smaller than that of the organic deposits found by MacGregor 

 in South Africa. 



From Central Africa too, there are many other finds of limestone 

 deposits formed by early organisms. This group shows a great re- 

 semblance, and is probably related to a type which extends into much 

 younger time. This is Collenia, known from several old shields, 

 which forms large colonies, and even bioherms (reefs) from late pre- 

 Cambrian to Silurian times. Reference will here be made to the 

 description of Gravelle and Lelubre, but there are many other finds 

 of similar remains. In this case, although again we have no real 

 fossils but deposits of lime secreted by early organisms, the structure 

 and large size of individual colonies, together with their rather com- 

 mon occurrence, indicate a prolific life at the time of their forma- 

 tion. The absolute dating of these remains still rests, however, on the 

 v^aguest of indications and before we know if these forms are really 

 that ver)^ old, much more reliable dates will have to be supplied. 



THE OLDEST BIOGENIC DEPOSITS 



The setting of the oldest remains formed by organic activity knovvTi 

 up to now is South Africa, and more particularly Southern Rhodesia 

 in the Bulawayo area. The pre-Cambrian series there are mainly 

 formed by crystalline schists and metamorphosed ultrabasic rocks, by 

 granitic batholites and dikes and by old volcanics — all rocks in 

 which no vestiges of early life will ever be found. But there are also 

 remains of relatively unmetamorphosed sediments, also laid down in 

 pre-Cambrian times. Of these the Dolomite series, which is quarried 

 for lime in the Bulawayo area, is of particular interest. The areas 

 where these old sediments are still preserved are separated by meta- 

 morphic and igneous rocks, so it is difficult to establish a detailed 



