THE OLDEST BIOGENIC DEPOSITS 



79 



dicate their general form, and from polished or etched surfaces, 

 giving their detailed structure (Figs. 16-19). Important to note is the 

 dome-like or dentate form; the fact that they occur in definite beds; 

 and their finely layered microstructure. The latter shows a certain 

 degree of regularity, which we know so well from lime deposits of 

 later Algae, or from early Coelenterata, but which is, on the other 

 hand, too irregular for any inorganic rhythmic deposit. MacGregor 

 (1940) justly concludes that these are biogenic deposits, formed by 

 lime-secreting organisms at the time the beds of the Dolomite series 

 were formed. 





Fig. 17. Series of probably biogenic pre-Cambrian limestone deposits 

 in parallel beds. Dolomite series, near Bulawayo, Southern Rhodesia 

 (x V4(.) (from Young, 1940 a). 



