174 



KNOWLEDGE & SCIENTIFIC NEWS. 



[July, 1905. 



The DioLmond Mines 

 of South AfricQc. 



About forty-eight jcari ago n child picked up a 

 diamond in the gravels of the Orange River. Experts in 

 Europe were not very ready to believe the news, but 

 others were Qaund, and in 1S70 the gem was discovered 

 in the so-called " dry ground " — large patches of a 

 peculiar clayey material well away Irom any stream 

 course. This was at a spot which is now the noted 

 Uutoits Pan Mine, near Kimberley. During the next 

 year three other diamantiferous patches were detected 

 in this neighbourhood; one, the present Bultfontein 

 Mine, within a short distance to the south; another, 

 the Ue Beers, about two miles away to the north-west, 

 and the third, the Kimberley, about a mile west of it. 

 They formed low hills or kopjes, rising from a com- 

 paratively level basin; the tops of the Kimberley and 

 its neighbour being about 4,000 feet above sea-level, 

 and the others a few hundred feet lower. The country 

 rock is a dark shale, with occasional beds of hard 

 sandstone, belonging to the Karoo series of geologists, 

 which is either a little older than, or contemporary with, 

 the European Trias, and is very different from the dia- 

 mantiferous material, which was afterwards found to 

 fill large vertical pipes or funnels, descending to an 

 unknown depth. These were nearly oval in form, the 

 area of the largest, Dutoits Pan, being about 41 acres; 

 that of the smallest, the Kimberley, about nine acres. 

 Since then several other diamantiferous patches have 

 been discovered, to which we shall presently refer. 



The nature of the material and the origin of the gem 

 have long been geological problems, but the former of 

 them has at length been solved, and the latter is much 

 better understood. The difficulty partly arose from the 

 state of the material. The yellow ground, to use the 

 miners' name for that first dug up, was a rotten clayey 

 stuff in which sundry minerals and rock fragments were 

 scattered. This, they found, passed gradually down — 

 perhaps a hundred feet from the surface — into a dark 

 bluish-green material, which they called " blue 

 ground." Though more coherent than the other, it also 

 was at first ill-suited for microscopic examination, by 

 which, however, the minerals scattered through it, 

 sometimes as fragments, could be determined. The 

 more notable, besides the diamonds, were garnets of 

 one or two kinds, iron oxide, generally titaniferous, 

 a brown mica, named vaalite by Professor Story- 

 Maskelyne, a chrome-augite, enstatite, and olivine, 

 more or less converted into serpentine. Fragments of 

 rock, sedimentary and crystalline, were also present; 

 the former often seeming slightly altered. About one- 

 third of the matrix, or blue ground itself, consisted of 

 very minute fragments of these constituents; about 

 half was serpentine, and the remainder a carbonate of 

 lime, sometimes magnesian. 



It was impossible to determine the true nature of 

 this rock, or the origin of the diamond, until the blue 

 ground was hard enough to be cut into slices sufficiently 

 thin for a satisfactory examination with the micro- 

 scope, and this was not reached until the mines were 

 carried down to some hundreds of feet from the surface. 

 The exact depth cannot be precisely stated, or that of 

 the passage from the " yellow " to the " blue ground," 

 but it was not till about a dozen years ago that really 

 good specimens of the latter reached this country. The 

 Kimberley mines had by that time been carried to a 



depth of over a thousand feet, and the material brought 

 up was about as hard as an ordinary limestone.* These 

 workings also afforded sections of the rocks pierced 

 by these great pipes or shafts. First they found the 

 dark shales already mentioned, sometimes covered, 

 sometimes cut, by masses of an igneous rock allied to 

 basalt. These occupy the first few hundred feet. Beneath 

 them comes a thick mass of simikir rock, an old la\a 

 flow, often called melaphyre, resting (in the Kimberley 

 district) on a quartzite or very hard sandstone, which 

 continues, thickly interbanded with the dark shales, till, 

 at a depth of more than five hundred yards from the 

 surface, a floor of very ancient crystalline rock is 

 reached. The rock fragments in the blue ground are 

 similar to these, whether sedimentary or igneous; the 

 shales being sometimes quite unaltered, but sometimes 

 with a " baked " aspect, especially in their outer 

 part. The rock, then, is a breccia, and often bears a 

 rough resemblance to that which fills the volcanic necks 

 on the Fifeshire coast. That the pipes had been driven 

 in some way or other through the surrounding rock 

 was indisputable, but it was for long uncertain whether 

 the material in them was a true breccia, like that just 

 mentioned, or some peculiar kind of igneous rock. 

 The latter view was at first more general, and was not 

 incompatible with the presence of rock fragments. The 

 late Professor Carvill Lewis maintained the material 

 (which he named Kimberlite) to be a peculiar kind of 

 peridotile — a rock composed mainly of olivine, but with 

 a glassy matrix — in which the diamonils and other 

 minerals had formed. But farther examination showed 

 the latter to be in many cases indubitably broken, and 

 the rock is now generally admitted to be a true breccia. 

 It has, however, a volcanic or, perhaps, we should say, 

 an explosive origin since we find no signs of ordinary 

 scoria. After the pipes had been filled, steam or hot 

 water probably continued to be discharged for some 

 time, converting the ferromagnesian minerals into 

 serpentine, producing carbonates, forming a peculiar 

 coating on some of the garnets, and more or less affect- 

 ing the rock fragments. 



This, however, did not settle the question whether 

 the diamonds had originated in the pipes or elsewhere, 

 like the other larger minerals. Professor Carvill 

 Lewis, taking the Kimberlite to be an igneous rock, 

 held the former view. So did some of those who main- 

 tained it to be a breccia, for they thought the diamond 

 had been produced by the action of very hot water on 

 the carbonaceous material of the dark Karoo shales. 

 But this hypothesis is beset with insuperable dillicuities. 

 The crystals of diamond are not unfrequently broken 

 like the garnets or augites, and w hen ])erfect are often 

 in a state of strain. Either would be inexplicable had 

 they been formed in such a material as the breccia. 

 Besides this, small diamonds have been found at the 

 De Beers and the Xewlands mine (some forty miles 

 north-west of Kimberley), more or less included in 

 garnets, :ind in 1897 they were detected in two boulder- 

 like pieces of rock which had been brought to England 

 from the latter mine. One of these, when it was 

 broken, displayed in the largest fragment no fewer than 

 ten diamonds, the biggest — an ocl.iliedion — measuring 

 about three-twentieths of an inch from point to point, t 



• See Papers in the Geological Magazine for 1895, p. 492. 

 Illustrations of the material itself and of its microscopic structure 

 are given in Carvill Lewis' " The Genesis of the Diamond," 1S97. 



t This (ranment was presented l>y the Directors of the Company 

 to the British Museum. For an account of it, see Proc. Hoy. Soc, 

 Vol LXV,, p. 223 



