78 



THE FOSSILS 



Fig. 16. West face of a limestone quarry near Bulawayo, Southern 

 Rhodesia. Length about 5 m. Pre-Cambrian dolomite series with bio- 

 genic structures, cut by younger granitic dike (from MacGregor, 1940). 



stratigraphy of the Dolomite series. It is, however, certain that it 

 has a very great age, for it is cut by younger granitic veins, and 

 absolute dating of these veins gives 2,670 my, or roughly 2.7 billion 

 years. Consequently, the rocks of the Dolomite series are certainly 

 older than 2.7 billion years. 



The Dolomite series consists of limestone and dolomite (limestone 

 containing magnesium). In the limestone peculiarly layered struc- 

 tures occur in definite beds. They show series of domes, or of finer 

 dentate forms, on their upper surface, whilst under closer inspection 

 they are seen to be built up by sequences of fine layers which more 

 or less follow the outer surface. These layers, which may be as much 

 as 3 mm apart in the central part of the domes, do narrow con- 

 siderably outwards. Better than by words, these structures can be 

 represented by the pictures taken from the quarry wall, which in- 



