THE KARROO SYSTEM 189 



top of the Dwyka series are known as the "White 

 band ". 



The dark colour of these shales has led to their being 

 prospected for coal at many places, but although the 

 percentage of carbonaceous matter rises to 12 per cent, 

 inclusive of some volatile hydrocarbons, nothing that can 

 fairly be called coal has been found in them in Cape 

 Colony. On the banks of the Camdini Kiver near 

 Loeries Fontein the black shales are very well exposed, 

 and they are traversed by dykes of dolerite, which! have 

 brought about the formation of graphite in minute 

 scales, filling cracks in the immediate vicinity of the 

 igneous rock. 



The Upper Dwyka shales change slightly in character 

 in the north of the Colony ; they cover wide areas in 

 Prieska, Carnarvon, Hopetown, Britstown, and Griqua 

 land West, where they have been called the " Kimberley 

 shales ". They have a thickness of from 250 to 500 feet, 

 and at the top are found the white-weathering black 

 shales corresponding to the " White band ". The chert 

 band is absent, but the ferruginous layers and limestones 

 are represented ; their thickness, including some ordinary 

 shales, ranges from 25 to 50 feet or perhaps more. In 

 Gordonia there are shales with a small amount of car- 

 bonaceous matter above the tillite, but no definite limit 

 between these shales and what are possibly Ecca beds 

 has been recognised. 



The distribution of the Dwyka series can be seen at 

 a glance on the geological map of the Colony. The 

 boulder-beds are probably almost everywhere present at 



