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II.—Notes on the Diamond-bearing Rock of Kimberley, South Africa
Published online by Cambridge University Press: 01 May 2009
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It may be fairly asserted that increased geological interest attaches to the deeper workings of the Kimberley Diamond Mines as they proceed. The remarkable development of underground mining, following upon the amalgamation of interests, which was the outcome of the collapse of the surface workings in 1886, or thereabout, has offered better opportunities for observation, not only of the diamond matrix-material but also of the varying characteristics of the neighbouring rock formations.
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page 492 note 1 Reasons, in themselves very sad, have hitherto prevented the publication of the elaborate investigations of the late Prof. Carvill Lewis, to which reference has been made below. By a curious coincidence, his manuscripts and all the materials which he had used were entrusted to me by Mrs. Carvill Lewis, just as I had completed my share of the above paper. It is my intention to lose no time in preparing for publication the work of my lamented friend.—T. G. B.
page 493 note 1 This, by the way, is not clay at all when first dislodged in the underground workings, but hard rock, which has had to be “won” by means of drilling and blasting, such as is customary in rock-mining operations.
page 494 note 1 A better idea of the riches of the mines will be gleaned from the brief statement which has just publicly appeared (July, 1895), that the De Beers Company have sold the output for the year for the sum of three million and a half pounds sterling.
page 494 note 2 One in the possession of the De Beers Company weighed in the rough state as much as 428 carats.
page 495 note 1 It may be well here to quote a passage from a letter on this subject received from Mr. W. Moses, the able director of the De Beers Mines, who ventures upon a more elaborate explanation. He says: “I am inclined to stick to my theory of the formation of diamonds and the depositing of them in the present formation of Blue Ground or volcanic mud. To put it as short as possible, the diamonds were produced from carbonic acid gas under extreme pressure and changes of temperature, the oxygen being consumed by the carbon, etc., the result being the diamond. To revert to the present deposits: they were made in the crater of an extinct volcano, into which came thermal springs, which washed away the sides of the soft strata below (the volcanic rocks forming the volcanic mud of the Blue Ground), and the diamonds were introduced from below by the action of the boiling energy of the thermal springs. The blue ground or mud, if you remember, we concluded had not gone under any great heat, as the small particles of shale clearly indicated; in short, the diamonds were separately formed, and were brought into their present position from below.” It has also been supposed, and with some show of probability, that with the opportune presence of carbon, under conditions of intense heat and perchance pressure, crystallization took place, the colours of the stones varying as they were affected by the presence of metals or oxides of metals.
page 495 note 2 A letter recently received from Mr. W. Moses states that “the sinking of our main shaft still continues in the blue metamorphic shale, and no change has yet taken place in the appearance of the strata. We have now passed through some 200 feet of this formation, and I am in hopes of getting a change which will be very interesting.” Mr. Moses also promises to send a further box of samples when this takes place, which it is needless to say will be of additional interest under the circumstances.
page 496 note 1 These specimens were given to one of the authors by C. J. Alford, Esq., F.G.S. That brought by Sir J. B. Stone was inconveniently large for weighing.
page 496 note 2 A number of analyses of minerals are given in the paper by Prof. Maskelyne and Dr. Flight, Quart. Journ. Geol. Soc., vol. xxx, 1874, pp. 408–416. In our specimens, bronzite, which appears to be common in their material, seems to be rare, if not absent.
page 496 note 3 As was also observed by Prof. Carvill Lewis.
page 498 note 1 The biotite is variable in amount, and occasionally is wanting.
page 498 note 2 Cf. Raisin, C. A. on Variolite … of the Lleyn: Quart. Journ. Geol. Soc., vol xlix, 1893, p. 151.Google Scholar
page 499 note 1 The material which Prof.Maskelyne, and Dr.Flight, were examining was evidently “blue ground,” and was taken from depths not exceeding 180 feet.—Q.J.G.S., vol. xxx, 1874, p. 406.Google Scholar
page 499 note 2 In the museum at Owens College, Manchester, are some large lumps of partially decomposed breccia intermediate between the ordinary “blue ground” and the specimens described in this paper.
page 501 note 1 In the specimens which we have examined it happens that fragments of shale, which according to Profs. Daubrée and Carvill Lewis are sometimes abundant, are either extremely small and rare or entirely absent. At any rate, they cannot now be identified with any certainty. [Professor A. H. Green, F.R.S., since these words were written, has most kindly lent me a number of specimens of rocks and minerals which he collected when visiting Kimberley in 1882, together with sundry notes and sections, of great interest. His specimens of “blue ground” are not quite so hard as those described above, but are in better preservation than most that I have examined. They contain fragments of black shale, in one case abundantly. One specimen also includes several angular fragments (up to a good half-inch in diameter) of a compact, slightly streaky, greenish-yellow rock, apparently a rotten serpentine (microscopic examination seems hopeless). There are specimens also of other rocks which occur as fragments in the breccia: three of these must have been of large size; two are amygdaloidal, one a compact diabase, the other a reddish porphyrite; the third an olivine basalt with some flakes of brown mica. Of smaller specimens (not more than about an inch in diameter) six are from the Bloemfontein Mine: one is a diorite, very slightly foliated; the other five are particularly interesting, for they represent a fairly coarse rock, chiefly composed of sahlite and a brownish mica, indistinguishable from that described above in the breccia. There are also specimens of the sheets of doleritic or diabasic rocks of the district. Dykes, sheets, etc., of these, according to Professor Green, are very abundant, as described in his paper (Q.J.G.S., xliv, pp. 254, 264). His sections make the “neck”-like character of the diamantiferous rock very clear—T. G. B.]
page 501 note 2 As described in our paper, Geol. Mag. 1891, p. 412.
page 501 note 3 We may say that this conclusion was arrived at independently of previous writers, for we did not refresh our memory of the opinions expressed by them till our work was practically concluded.
page 501 note 4 Comptes Rendus, 1890, vol. cx, p. 18.Google Scholar
page 501 note 5 Proc. Geol. Assoc., viii, p. 65.Google Scholar
page 501 note 6 Brit. Assoc. Reports, 1886, p. 667; 1887, p. 720; and Geol. Mag. 1877, pp. 22–24.Google Scholar
page 501 note 7 Maskelyne, Prof. inclines to the opinion that the diamond was produced at or near the contact of a basic igneous rock with a carbonaceous shale, but that since then the whole mass has been affected by mechanical disturbances and thermal waters.— Q.J.G.S., vol. xxx, 1874, pp. 407, 408.Google Scholar
page 501 note 8 This was Dunn's, Mr. opinion, who supposed that the diamond was produced by metamorphism of the carbonaceous material in the shales.—Q.J.G.S., vol. xxx, 1874, p. 54Google Scholar; vol. xxxiii, 1877, p. 879; vol. xxxvii, 1881, p. 609. Prof. Green informs us that his examination led him to the conclusion that the ‘pipes’ were volcanic necks.
page 502 note 1 This may have been the last stage in a series of volcanic disturbances, of which the flows noticed by Prof. Green (loc. cit.) may have been the earliest.
page 502 note 2 Loc. cit., pp. 20–24. Prof. Lewis had already called attention to certain points of likeness in the diamantiferous rock and meteorites.