TRANSACTIONS OF SECTION C. 667 



stratified -with the Laurentian gneiss, and in one of them to merge imperceptibly 

 into it. The original relations of the rocks are, of course, much obscured by the 

 effects of subsequent heat and pressure. The evidence at present, however, seems 

 to indicate that these anorthosites are the result of some kind of extravasation, 

 which in those early times corresponded to what in modern times we call volcanic 

 eruption. 



4. On a Diamantiferous Peridotiie and the Genesis of tJie Diamond. 

 By Professor H. Carvill Lewis, M.A., F.G.8. 



The discovery of diamonds at Kimberley, South Africa, has proved to be a 

 matter, not only of commercial, but of much geological interest. The conditions 

 under which diamonds here occur are unlike those of any other known locality, and 

 are worthy of special attention. 



The first diamond found in South Africa was in 1867, when a large diamond 

 was picked out of a lot of rolled pebbles gathered in the Orange Kiver. This led to 

 the ' river diggings ' on the Orange and Vaal Rivers, which continue to the present 

 time. In 1870, at which time some ten thousand persons had gathered along the 

 banks of the Vaal, the news came of the discovery of diamonds at a point some fifteen 

 miles away from the river, where the town of Kimberley now stands. These were 

 the so-called ' dry diggings,' at first thought to be alluvial deposits, but now proved 

 to be volcanic pipes of a highly interesting character. Four of these pipes or necks, 

 all ricli in diamonds and of similar geological structure, were found close together. 

 They go down vertically to an unknown depth, penetrating the surrounding strata. 

 The diamond-bearing material at first excavated was a crumbling yellowish 

 earth, which at a depth of about fifty feet became harder and darker, finally 

 acquiring a slaty blue or dark gi-een colour and a greasy feel, resembling certain 

 varieties of serpentine. This is the well-known ' blue ground ' of the diamond 

 miners. It is exposed to the sun for a short time, when it readily disintegrates, 

 and is then washed for iis diamonds. This ' blue ground ' has now been penetrated 

 to a depth of 600 feet, and is found to become harder and more rock-like as the 

 depth increases. 



Quite recently, both in the Kimberley and De Beers mines, the remarkable rock 

 has been reached which forms the subject of the present paper. 



The geological structure of the district and the mode of occurrence of the dia- 

 mond has been well described by several observers. As Griesbach, Stow, Shaw, 

 Rupert Jones, and others have shown, the diamond-bearing pipes penetrate strata of 

 Carboniferous and Triassic age, the latter being known as the Karoo formation. 

 The Karoo beds contain numerous interstratified sheets of dolerite and melaphyr, 

 also of Triassic age, the whole reposing upon ancient mica schists and granites. 

 The careful investigations of Mr. E. J. Dunn demonstrate that the diamond-bearing 

 pipes enclose fragments of all these rocks, which fragments show signs of alteration 

 by heat. Where the pipes adjoin the Karoo shales, the latter are bent sharply 

 upwards, and the evidence is complete that the diamond-bearing rock is of volcanic 

 origin aud of post-Triassic age. 



The diamonds in each of the four pipes have distinctive characters of their own, 

 and are remarkable for the sharpness of their crystalline form (octahedrons and 

 dodecahedrons), and for the absence of any signs of attrition. These facts taken 

 in connection with the character of the 'blue ground' indicate, as Mr. Dunn has 

 pointed out, that the latter is the original matri.x: of the diamond. 



Maskelyne and Flight have studied the microscopical and chemical characters 

 of the ' blue ground,' and have shown that it is a serpentinic substance containing 

 bronzite, ilmenite, garnet, diallage, aud ' vaalite ' (an altered mica), and is probably 

 an altered igneous rock, the decomposed character of the material examined pre- 

 venting exact determinations of its nature. They showed that the diamonds were 

 marked by etch figures analogous to those which Professor Gustav Rose had pro- 

 duced by the incipient combustion of diamonds, and that the ' blue ground ' was 

 essentially a silicate of magnesium impregnated with carbonates. 



The 'blue ground' often contains such numerous fragments of carbonaceous 



