224 



NA TURh 



[Ianuaky 9, 1908 



cutta gardens from Mauritius, and the unpleasant re- 

 sults which ensue if their destruction is not conducted 

 on special lines. 



Let no one, however, -imagfine that an Indian life 

 has not its lighter side. What, for instance, can equal 

 the glory of its sunsets, so graphically described by the 

 author, or the luxuriance and beauty of its gardens, 

 which he brings to our notice with the aid of both 

 camer.T and pen ? As an example of what can be done 

 in the way of effect with foliage alone, we reproduce 



the accompanying illustration, with which we must, 

 reluctantly, take leave of a charming, thoughtful, and 

 instructive work. R. L. 



" KIMBERLITE " AND THE SOURCE OF THE 

 DIAMOND IN SOUTH AFRICA. 



THE origin of the volcanic pipes of South Africa 

 and the genesis of the diamonds contained in 

 their " blue-ground " filling are as productive of 

 controversy as that other geological puzzle — the 

 source of the gold in the Witwatersrand conglomerates 

 — the discussion of which was revived by Prof. 

 J W. Gregory at a recent meeting of the Institute of 

 Mining and Metallurgy; and agreement among the 

 disputants is as little likely to be arrived at in the 

 one case as in the other. Quite a crop of papers on 

 the diamond-pipes has recently appeared,' and a 



1 A. W. Rogers and A. L. du Toil : The Sutherland Volcanic Pines and 

 their Relationship 10 other Vents in S. Africa (Trans. S.A. Phil. Soc, 

 vol XV., p. 61, 1904). 



II. S. Harger : The Diamond Pipes and Fissures of South Africa (Trans. 

 Geol. Soc. S.A., vol. viii,, p. no, 1905). 



G. S. Corslorphine ; The Occurrence in Kiinberlite of Garnet-pyroxene 

 Nodules carrying Diamonds (Trans. Geol. So". S.A., vol. x., p. 65, 1907). 



K. W. Voit: Kimberlite Dykes and Pipes (Trans. Ge.,1. Soc. S.A., 



vol. X., p. 60, 1907). 



!■■. W Vo'it : Ihe Origin of Diamonds (Trans. Geol. Soc, S.A., vol. x., 

 P- 75. '9»7)- 



A. L. du Toil : Geological Survey of the Eastern Portion of Griqualand 

 West (Kleventh Ann. Rep. Geol. Com. Cape of Good Hope, p. 135, 1906). 



A. Macco ; Ueher die sudafrikanischen Diainantlaeerstatten (Zensch. 

 derdeutsch. geol. Cesellscb., vol. li.\., p. 76, 1907 1. 



R. Beck; Unteisui:hungen liber elnige siidalrikanische Diamanlenlager- 

 stritlen (Zeit»ch. der di;ulsch. geol. Gesellsch., p. 276. 1907), 



NO. 1993, VOL. yy] 



perusal of these reveals so remarkable a variation of 

 opinion that it may perhaps serve a useful purpose to 

 give in these columns a brief summary of the different 

 views as to the nature of the original matri.\ of the 

 diamond and the place in which it was produced, 

 that have been put forward from time to time since 

 the discovery of the " dry diggings " at Kimberley 

 in 1870. 



Prof. E. Cohen, who visited the diamond field in 

 1872, gave the first scientific explanation of the origin 



of the volcanic 

 pipes in a letter 

 addressed to Prof. 

 Leonhard, and 

 published in the 

 " Neues Jahrbuch 

 fijr Mineralogie " 

 (1872, p. 859). Ac- 

 cording to Cohen 

 the diamond oc- 

 currences repre- 

 sent the centres of 

 tuff- eruptions, 

 whereby the 

 greater part of the 

 erupted material 

 was provided by 

 older crystalline 

 rocks underlying 

 the present known 

 formations. These 

 furnished not only 

 the diamonds, but 

 probably most of 

 the accompanying 

 minerals. Forced 

 violently upward 

 by volcanic ex- 

 plosions, the shat- 

 tered rocks be- 

 came pulverised, 

 and the diamonds 

 either survived as 

 ,d 1 1 ,,sure, 01 Liie in Bengal. complete crystals 



1 or were broken into fragments (the " splints " of 

 the dealers). This view explains the fragmentarv 

 character of the minerals that accompany the dia- 

 mond ; it also accounts for the presence of so manv 

 fragments of crystalline rocks in the pipe-material 

 (" blue-ground "), and for the strongly brecciated 

 ; character of the latter. Cohen's view was subsc- 

 I quently somewhat modified by Chaper (Bull. Soc. 

 Miner, de France, ii., 1879, p. 195), who found it neces- 

 sar;^ to assume a repetition of explosive eruptions in 

 I order to account for the variation in one and the same 

 i pipe of large masses of the blue ground in colour, 

 ! mineral composition, richness in diamonds, &c. 



On the other hand, Dunn, who wrote in 1874 

 (Quart. Journ. Geol. Soc, xxx., p. 54), described the 

 blue ground as a " decomposed gabbro or eupho- 

 tide " ; while Maskelyne and Flight, who gave the 

 first description of the microscopic character of the 

 diamantiferous rock (Quart. Journ. Geol. .Soc, xxx., 

 1874, P- 406), considered that it " was probably the 

 original home of the diamond, possibly at the 

 places of its contact with carbonaceous shales." 

 These ideas were further developed by Carvill Lewis 

 in papers read before the British Association in 1886 

 and 1887, and subsequently published bv Prof. T. G. 

 Bonney under the title " Papers and Notes on the 

 Genesis and Matrix of the Diamond." After an ela- 

 borate microscopic investigation made in Prof. Rosen- 

 busch's laboratory in Heidelberg, Lewis pronounced 

 the rock to be a true eruptive lava — " a porphyritic 



