274 Report S.A.A. Advancement of Science. 



perowskite. The rest of the rock consists of clear quartz, with the 

 usual fluid pores. 



The cement is entirely of clear quartz, and is in crystalline con- 

 tinuity with the quartz of the grains, so that grain and cement make 

 up one individual, which extinguishes as such. The result is a 

 mosaic of irregular-shaped quartz crystals, in which the outlines of 

 the original grains are clearly to be distinguished. The rock is thus 

 a very fine example of a typical quartzite, and it bears a very strong 

 resemblance to stiperstones quartzite, t the basement bed of the Ordo- 

 vician in West Shropshire. From the appearance of the grains it is 

 evident that they have been derived from an area of metamorphic 

 crystalline rocks, probably gneisses and granulites. 



The specimen from a depth of 1200 feet (206k) is of quite 

 different character. It is composed of smaller and much more 

 angular fragments, which consist almost exclusively of quartz, 

 embedded in a large amount of interstitial material, chiefly finely- 

 divided mica, which is probably derived from the decay of feldspar 

 under the action of weathering agents. The general character of the 

 rock indicates somewhat different conditions to the last : it is probably 

 derived from a granitic or gneissose complex under such conditions 

 that the feldspar was decomposed and formed a fine argillaceous 

 sediment, in which the quartz grains were embedded. 



The specimen from, at, or near the 1300 feet level (2o6g.) is on 

 the whole very similar in character to the last, but it includes rather 

 more feldspar, together with some white and brown mica ; the chips 

 of quartz are still more angular, and suggest extremely rapid deposi- 

 tion, so that they were buried by the succeeding layers of sediment 

 before any considerable amount of attrition had taken place. When 

 examined under a high power the interstitial matter appears to be 

 almost exclusively micaceous. 



So far as regards their petrographical character, these rocks 

 were originally feldspathic sandstones or arkoses which have been 

 cemented into hard, lustrous quartzites of varying degrees of fineness. 

 In some cases a rapid alternation of coarse and fine layers produces 

 a well-marked lamination, so that they are often spoken of as shales, 

 but the so-called shales are of essentially the same composition as the 

 quartzites. 



From the small amount of evidence at our disposal, it appears 

 that this sedimentary series has been formed by the rapid denudation 

 of a series of crystalline rocks, probably of a gneissose character. 

 As we shall see later, the rocks which have been reached at the 

 deepest levels of the shafts at the Kimberley and De Beers Mines 

 are quite able to supply the kind of material here seen, and its source 

 is probably to be found in the rocks of this series. It is perhaps 

 dangerous to draw any conclusions as to the conditions under which 

 thev are formed, but what evidence there is points to deposition 

 at a rapid rate in the immediate neighbourhood of a region of great 

 denudation. 



t Harker, Petrology, p. 229, Fig. 2. 



