Petrography of Rocks. 271 



extends down to 2470 feet, from which depth comes another speci- 

 men of a sediment of somewhat peculiar character. At 2500 feet we 

 reach granite, and the greatest depth represented by the specimens in 

 our possession is 2520 feet. In the De Beers Mine, on the other 

 hand, granite is reached at 1920 feet, so that it appears that the 

 upper surface of the granite is here very uneven, rising 600 feet in 

 about a mile. The presence of a sedimentary rock immediately above 

 the granite suggests that this is a buried land surface, and extreme 

 unevenness is a common character of land surfaces composed of 

 denuded archsean or igneous complexes, e.g., the Lewisian gneiss, and 

 the gneissose area of Brazil, in the neighbourhood of Rio de Janeiro. 

 The other mines, Bultfontein, Wesselton, -and Dutoitspan, do not 

 reach a greater depth than 750 feet, so that they throw no light on 

 this question. 



In the latter group of Mines the upper part of the series shows 

 some variation in detail, but the general succession is very similar. 

 The differences chiefly occur in the rocks above the melaphyre ; in 

 the diagram given by Williams, quartzite is shown both at Bultfontein 

 and Dutoitspan at a much higher horizon than elsewhere. We shall 

 return to this question later on. 



Since the Kimberley Mine is the one which has up to the present 

 reached the greatest depth, it may for our present purpo.se be taken as 

 typical, and will be described first, and in considerable detail. Some 

 of the rock types, however, show better development elsewhere ; in 

 such cases description will be deferred till the mine in question is 

 treated of. 



Since the ground surface around all the mines is at practically 

 the same height above sea level, varying only a few feet on either 

 side of the contour of 3990 feet, it will be convenient in all cases to 

 use the surface as a datum line, and to speak always of depths below 

 this generalised surface. Since the depths given appear to be 

 approximate, correct to only 10 feet or so, no appreciable error will 

 be introduced, and the descriptions will be more intelligible than if 

 heights above sea-level were used. 



III.— PETROGRAPHICAL DESCRIPTIONS. 



I. Kimberley Mine. 



The present surface of the ground at the Kimberley Mine consists 

 of material which is collectively described as debris, having a total 

 thickness of 30 feet, overlying 5 feet of red soil, which is probably of 

 a lateritic nature. Below this we come to the usual basalt, which is 

 here 50 feet thick, rather below the average. The specimens of 

 basalt from this locality (201) (a) are much decomposed, while 

 excellently fresh examples of what is undoubtedly the same rock come 

 from other mines, and notably from Wesselton, so that consideration 



(a) Numbers in brackets refer throughout to the original numbering of the 

 specimens as supplied by the Company. 



