REEFS OF ALGAL LIMESTONE IN THE SAHARA 91 



occur in colonies of considerable size. The colonies may form reefs, 

 or, more technically, bioherms. Such reefs are mounds or layers 

 constructed by the organisms themselves, in contrast to normal sedi- 

 ments which have been laid down, either through setting by gravity, 

 or through secretion by chemical processes. Algal reefs of the group of 

 the Stromatolites are the earliest bioherms known in geologic history. 



The best-known of these reef-building, lime-secreting organisms 

 amongst the group of the Stromatolites is Collenia, which forms 

 colonies of globose concretions of limestone, each up to several deci- 

 meters in diameter. In the Sahara not only Collenia is found, but 

 also other remains, belonging to the form genus Conophyton. An 

 example of a reef built by the latter organisms is reproduced in 

 Figs. 24 and 25. 



The main importance of Colle?ua lies, however, not in the fact that 

 it is the textbook form of early algal limestone secretions and of 

 early algal reefs, but in the fact that it is not restricted to the pre- 

 Cambrian. It persists even into the Ordovician. It is consequently 

 contemporaneous with early representatives of all the main groups 

 of modem life. 



The fact that Collenia persists into the Ordovician proves that it 

 is a representative of modem, actualistic life. Its metabolism was 

 oxygenic. In view of the similarity between the limestone secretions 

 of Collenia and other Stromatolites, there is, consequently, no objec- 

 tion to class the group of Stromatolites with the Algae, and to speak 

 of algal limestones. 



The fact that Stromatolites belong to modern life is important for 

 a general idea of their age. Because they belong to modem life, to 

 the actualistic atmosphere, they are certainly much younger than 

 their South African counterparts described by MacGregor, which 

 still belong to the pre-actualistic atmosphere. How much younger 

 they are, is, however, difficult to say. The fact is that, beyond the 

 indication that they belong to the pre-Cambrian, we have no trust- 

 worthy dating at all to arrive at an estimate of the Sahara deposits. 

 Geologically, they belong to the formation called Phamsian. This 

 formation is distinctly younger than the strongly metamorphosed 

 rocks which form the basement in the Sahara. On the other hand, 

 the Phamsian is definitely older than late pre-Cambrian rocks, now 

 often classed as Infra-Cambrian, because they belong to the same 



