Professor T. G. Bonney — Parent-rock of the Diamond. 309 



below sea-level. A very fine example of this is seen near Tiornevig. 

 Sailing northwards past the towering walls of Myling Head, we 

 turn eastwards and at once come into the basin of a great cirque 

 which falls in one grand curve two thousand feet. Myling Head, 

 like many other parts of the coasts, is a mere narrow ridge of rock, 

 separating the sea on one side from a cirque valley on the other. 



It is hardly possible to enumerate all the cirque valleys which 

 exist in the Faroes. They abound in all the larger islands, and 

 tarns occupying true rock-basins are frequently seen on their floors. 



TflE Parent - rock of the Diamond in South Africa. By 

 Professor T. G. Bonney, D.Sc, LL.D., V.P.E.S.i 



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.^ 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 peculiar deposits, which occur locally in a 

 region where the dominant rock is a dark shale, sometimes inter- 

 bedded with hard grits, or associated with igneous rocks allied to 

 basalt. These deposits occupy areas irregularly circular in outline, 

 and bearing a general resemblance to volcanic necks. The dia- 

 mantiferous 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-diopside (omphacite 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. 



1 Being a paper read before the Eoyal Society on June 1st, 1899. 



^ Edited by the present writer. 



3 Jules Gamier, Geol. Soc. South Africa Trans., 1897, p. 91 ; H. S. Harger, 

 ibid., p. 124. See also "W. G. Atherstone, ibid., 1896, p. 76; L. De Lauuay, 

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

 Cap" (Paris, 1897), gives a very lull account of the mines, but an even better one 

 will he found in Max Bauer, " Edelsteinkunde " (Leipzig, 1896, p. 208). 



