22 Prof. H. Carvill Lewis — Genesis of the Diamond. 



Fig. 3, one-twentieth natural size. Transverse section of a large concretionary 

 mass of compact siliceous ironstone, exhibiting cone-in-cone structure upon both 

 upper and under surfaces, as well as in the interior of the mass, at a and c. The 

 band b, b, is clay ironstone crowded with leaflets of Neuropteris gigantea. 



Fig. 4, one-tenth natural size. Transverse section of a nodule of compact 

 siliceous ironstone, entirely coated with cone-in-cone structure. 



Fig. 5, one-tenth natural size. Transverse section of a large concretionary mass 

 of close-grained ferruginous sandstone exhibiting cone-in-cone structure upon upper 

 and under surfaces, and also a peculiar double arrangement of a spherulitic formation 

 surrounding the cone-in-cone areas. 



Fig. 6, one-twentieth natural size. Shows the appearance, on the surface of 

 nodule in Fig. 5, of the spherulitic structures occupying the areas surrounding the 

 cone-in-cone masses. 



IV. OjST a DlAMANTIFEEOirS PeeIDOTITE, AJSl) THE GenESIS OE THE 



Diamond. 



By Prof. H. Carvill Lewis, M.A., F.G.S. 



(Abstract of a Paper read at the Birmingham Meeting of the British Association for 



the Advancement of Science, September, 1886.) 



THE discovery of diamonds at Kimberley, South Africa, has proved 

 to be a matter, not only of commercial, but of much geologiial 

 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 river. This led to the "river diggings" on the Orange and 

 Yaal 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. Eour of these 

 pipes or necks, all rich in diamonds, and of similar geological structure, 

 were found close together. They have been proved to 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 50 feet became harder and 

 darker, finally acquiring a slaty blue or dark green 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 disinte- 

 grates, and is then washed for its 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 diamond has been well described by several observers. As 

 Grriesbach, Stow, Shaw, Rupert Jones, and others have shown, the 

 diamond-bearing pipes penetrate carbonaceous strata of Triassic age, 

 which are known as the Karoo formation. 



