270 Report S.A.A. Advancement of Science. 



surrounding the diamond pipes. Besides Air. Gardner Williams' 

 works, already cited, the following may be specially referred to : — 



BoNNEY. On some rock specimens from Kimberley, South 

 Africa. Geol : Mag : 1897, p. 497. 



BuTTGENBACH. Quelques observations sur les champs dia- 

 mantiferes de Kimberley. Ann. Soc. Geol: Belgiq^ue. Tome 

 XXXII. 1905. Memoires, p. 3. ■ 



Rogers. Geology of Cape Colony. 1905. Chapter IX., 



P- Z^i"^- 



Hatch and Corstorphine. Geology of South Africa. 



Harger. The Diamond Pipes and Fissures of South 

 Africa. Trans: Geol: Soc: South Africa, VIII. 1905. p. 

 112. 



Numerous further references will be found in the works above 

 cited, and in the catalogue of printed books, etc., relating to the 

 Geology of South Africa, by Miss M. Wilman (Trans: S.A.Phil. 

 Soc.) Vol. XV., part 5. 



IL— GENERAL DESCRIPTION. 



It is unnecessary to give any detailed account of the superficial 

 deposits of the area surrounding the mines : it must suffice to say that 

 the soil appears to be thin, and is described as of a red colour ; it is 

 probably of a lateritic character, derived from the immediately under- 

 lying basic igneous rocks. A layer of what is described as limestone 

 is widely distributed over the surface of the district. This appears 

 to be of the nature of travertine or calc-sinter (the calcareous tufa of 

 some authors). At all the mines except Bultfontein, the surface 

 below the soil is composed of a thick bed of a rock, which is commonly 

 spoken of as basalt, varying in thickness from 50 to 100 feet. This 

 is underlaid by some 200 or 250 feet of black carbonaceous shale, and 

 below this again comes a representative of the well-known Dwyka 

 Conglomerate, which is here very thin. These sedimentary rocks 

 scarcely require petrographical description, and no thin slices of them 

 have been prepared. 



Below the conglomerate in the Kimberley and De Beers Mines 

 we have about 400 feet of a somewhat decomposed igneous rock, 

 commonly known as melaphyre. Then comes another series of sedi- 

 ments ; 400 feet of quartzite, followed by 260 feet of shale, accord- 

 ing to Williams. As we shall see in a subsequent section, this broad 

 division scarcely holds good on close examination, and, indeed, the 

 distinction between shale and quartzite is here purely arbitrary ; both 

 rocks have very similar mineralogical composition, and the differences 

 depend chiefly on the relative sizes of the constituent particles. 



At a depth of about 1400 feet in both Mines there begins the 

 great series of acid igneous rocks, the quartz-porphyries of Williams. 

 As we shall see later, this series is by no means uniform, but includes 

 several different petrographical types. At Kimberley this series 



