Petrography of Rocks. 281 



E. SUMMARY. 



The succession of the rocks forming the walls of the diamond- 

 bearing pipe in the Kimberley Mine may be conveniently summarised 

 in the following table, which is compiled partly from data supplied 

 with the specimens and partly from the sections given by Gardner 

 Williams in the British Association Handbook ; where any discrepancy 

 existed, the figures given in the manuscript list of specimens have 

 been accepted. 



The table shows the depth from the surface at which the speci- 

 mens were obtained, the original numbering as supplied by the 

 Company, the approximate thickness in feet of each type of rock. 

 The right-hand column gives the accepted petrographicab description 

 of each rock, for comparison with the previous literature. The 

 ■diagram is drawn to a scale, viz., i centimetre = 100 feet, or approxi- 

 mately 250 feet to I inch. 



2. De Beers Mine. 



It appears from the sections given by Gardner Williams that 

 the general succession of rock types here is identical wdth that in the 

 Kimberley Mine, and the thicknesses also are very similar. The 

 tipper shale is a trifle thinner, 204 feet instead of 250 feet, and the 

 sedimentary series below the " melaphyre " is about 60 feet thicker, 

 so that the top of the acid volcanic series is at practically the same 

 level. It has already been mentioned more than once that the upper 

 ■surface of the granite is here met with at a much higher level, viz., 

 at 1920 feet from the surface, instead of 2500 feet, so that the 

 volcanic series is here much thinner, only 540 feet, instead of 1070 

 feet. 



Under these circumstances, since the petrographical character of 

 specimens from similar horizons in both mines is practically identical, 

 it is unnecessary to give any further details, so far as regards the 

 rocks overlying the granite. Since, however, the granite itself is 

 in much better preservation, and shows greater variety in the specimens 

 from this mine, it will here be described in detail. So far as regards 

 the other rocks, a table is appended, similar to that given for the 

 Kimberley Mine. 



A. PETROGRAPHICAL DESCRIPTION OF THE GRANITE. 



In hand-specimens (113, 113b, 114b) the granite is a pale grey 

 Tock of moderately coarse texture, even-grained, and non-porphyritic. 

 It shows clear, glassv quartz, white feldspar, and a slightly greenish 

 mica. In some specimens a few flakes of silvery white mica are also 

 to be seen. The freshest specimen of all is one from a depth of 2040 

 feet in the De Beers INIine, collected by Mr. A. Hutchinson, and from 

 this the following description is taken. The other specimens are 

 similar, except for a greater or less degree of decomposition. 



As before stated, the rock consists of quartz, various feldspars 

 and mica, with onlv a small amount of accessory minerals. The 



