188 THE UPPER SHALES 



ridges, and in not a few cases the tillite is completely 

 absent on the latter. The same variations of level of 

 the old floor and consequent change in thickness of the 

 tillite from point to point are well marked in the 

 Transvaal and in Natal. 



Lying above the boulder-beds in the south and west 

 there are some 500 to 600 feet of sediments called the 

 Upper Dwyka shales, into which the tillite passes con- 

 formably by the gradual diminution in the number and 

 size of the boulders. The lowest beds are bluish or 

 greenish sandy shales, overlain by thin sandstones, 

 which are in turn succeeded by a group of black shales 

 weathering white on exposure to the air. The black 

 shales are followed by fine-grained green beds with thin 

 beds of limestone and ferruginous rocks, and several 

 layers of chert, grey or b]ack when freshly broken, but 

 with a thin white crust on exposed surfaces. The 

 uppermost of the chert beds, usually from eight to 

 twelve inches thick, is taken as the top of the Dwyka 

 series. The black shales contain a certain amount of 

 carbonate of lime often gathered together in the form 

 of nodules, and iron pyrites. These two minerals and 

 the carbonaceous matter, that gives the black colour to 

 the shales, decompose under the influence of the air, 

 forming gypsum (sulphate of lime) and iron oxides, 

 leaving the shales white. These white rocks make 

 very conspicuous features on the southern, western, 

 and northern borders of the Karroo, where the vegeta- 

 tion is not sufficiently abundant to hide the colour of 

 the bare hill-sides. Thus the black shales near the 



