Tlic Parent-rock of the Diamond in South Africa. 



223 



~ The Parent-rock of the Diamond in South Africa." By Professor 

 T. G. BoNNEr, D.Sc, LL.D., VJP.RS. Eeceived May 1 — 

 Read June 1, 1899. 



So much has been written on the occurrence of diamonds in South 

 Africa that a very few words may suffice as preface to this communication. 

 References to many papers on the subject are given in ' The Genesis 

 and Matrix of the Diamond' (1897), by the late Professor H. Carvill 

 Lewis,* and others have been published since that date.f It may 

 suffice to say that the diamond, first discovered in 1867 in gravels 

 on the Orange River, was found three years later in certain pecu- 

 liar deposits, which occur locally in a region where the dominant 

 rock is a dark shale, sometimes interbedded with hard grits, or asso- 

 ciated with igneous rocks allied to basalt. These deposits occupy 

 areas irregularly circular in outline, and bearing a general resemblance 

 to volcanic necks. The diamantiferous material, near the surface, is 

 soft, yellowish in colour, and obviously much decomposed ; at a greater 

 depth it assumes a dull greenish to bluish tint, and becomes harder. 

 At the well-known De Beers Mine, near Kimberley, the works in 1898 

 had been carried to a depth of about 1,500 feet, and the diamantiferous 

 material, for at least the last 100 yards, was not less hard than an 

 ordinary limestone. It has a brecciated aspect, the dark, very minutely 

 granular, matrix being composed mainly of serpentine (about four-fifths 

 of the whole), and of a carbonate of lime (with some magnesia and a 

 little iron). In this matrix are embedded grains of the following 

 minerals : — Olivine, enstatite, smaragdite, chrome-cliopside (omphache 

 of some authors), a brown mica, garnet (mostly pyrope, but more than 

 one variety observed), magnetite, chromite, ilmenite, with several other 

 minerals much more] sparsely distributed. 



Rock fragments are also present, variable in size, but commonly not 

 exceeding about an inch in diameter, as well as in quantity. These, 

 occasionally, but not generally, are rather abundant. In some cases 

 they are chips of the neighbouring black shale, but in others they are 

 greyish-coloured with somewhat of a porcelain aspect. The latter are 

 generally sub-angular in form and externally banded or bordered with 

 a darker tint ; crystalline rocks have also been noticed, though these 

 appear to be far from common, such as granite, cliorite, and varieties of 



* Edited by the present writer. 



t Jules Cornier, ' Qeol. Soc. South Africa Trans.,' 1897, p. 91 ; H. S. Harger, 

 ibid., p. 124. See also AY. G. Atherstone, Hid., 1896, p. 76 ; L. De Launay, 

 'Compt. Eend.,' vol.125 (1897), p. 335. The last author, in 'Les Diamants du 

 Cap' (Paris, 1S97), gives a very full account of the mines, but an even better one 

 will be found in Max Bauer, ' Edelsteinkunde ' (Leipzig, 1896. p. 208). 



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