174 T. G. BONNE Y [September 



anomalous optical character, to be in a condition of strain, and they are 

 sometimes only fragments of crystals. 



The matrix, in which the above-named minerals are rather 

 irregularly scattered, consists of serpentine, somewhat fragmentary in 

 aspect, mixed with about 16 per cent of a carbonate — calcite or dolomite, 

 granules of iron oxide and perovskite ; sometimes tiny flakes of brown 

 mica — apparently of secondary origin — are generally disseminated. To 

 some investigators the rock seems to be porphyritic, to others brecciated, 

 several of the minerals looking rather rounded. Anmilar rock fra<r- 

 ments — shales, grits, diabases, and the like (the first of these sometimes 

 apparently a little altered) — are also present, though in variable quantity. 

 The country rock is a shale, often dark, interbedded with hard grits, and 

 associated with flows or sills, and with dykes of igneous rocks, mostly 

 basalt or diabase. Dykes also occasionally cut the diamantiferous 

 rock. The latter occurs in pipes which bear a general resemblance to 

 volcanic necks. These vary in size, the largest, named Du Toit's Pan, 

 being about 45 acres in area. 



This very brief sketch of the circumstances under which the South 

 African diamonds have been hitherto found may suffice for our present 

 purposes, since so much has now been written on the subject. 1 The 

 facts which have been briefly summarised have received very diverse 

 interpretations, though all admit that the rock has been considerably 

 affected by secondary mineral changes, which have been brought about, 

 in all probability, by the action of heated water. Some writers, how- 

 ever, maintain that the rock is a breccia, and that the diamond, like 

 the garnets, pyroxenes, olivines, etc., was formed elsewhere, the parent 

 rock or rocks having been shattered by some form of explosion. 

 Others, while taking the same view as to the character of the blue 

 ground, believe that the diamond was formed in situ, probably by the 

 action of highly heated water (under considerable pressure) on the 

 carbonaceous material of the country rock (Karoo shale 2 ). Others, 

 again, agree with the late Professor Carvill Lewis in regarding the 

 " blue ground " as a serpentinised and otherwise altered peridotite of 

 somewhat peculiar form. For this he proposed the name Kimberlite, 

 thus defining it " a porphyritic volcanic peridotite of basaltic structure, 

 or, according to Eosenbusch's nomenclature, the palaeovolcanic ' Erguss 

 form' of a biotite-bronzite-dunite, being an olivine-bronzite-picrite- 

 porphyrite, rich in biotite ... it is a rock sui generis, dissimilar to 



1 I think it needless to attempt a bibliography. The earlier more important papers, 

 with some which cannot be so designated, will be found in Carvill Lewis's "The Genesis 

 and Matrix of the Diamond," 1897. Some of later date are mentioned in my paper on 

 "The Parent Rock of the Diamond in South Africa," read to the Royal Society on 1st June 

 of this year. The classic paper of Professor Maskelyne and Dr. W. Flight (Quart. Journ. 

 Geol. Soc, xxx. 1874. p. 406) contains the first thorough investigation of the associated 

 minerals, and much information will be found in De Launay, " Les Diamants du Cap," 

 Paris, 1897, and in Max Bauer, " Edelsteinkunde," Leipzig, 1896, both of them most 

 valuable works of reference. 



2 This is referred to the Triassic period. 



