Prof. H. Carnll Lewis — Genesis of the Diamond. 23 



The Karoo beds contain numerous interstratified sheets of dolerite 

 and melaphyre, also of Triassic age, and the whole reposes upon ancient 

 mica-schists and granites. The careful investigations of Mr. E. J. 

 Dunn demonstrate that the diamond-bearing pipes inclose 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, and 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 "blue ground," 

 indicate, as Mr. Dunn has pointed out, that the latter is the original 

 matrix of the diamond. 



Maskelyne and Plight have studied the microscopical and chemical 

 characters of the "blue ground," and have shown that it is a ser- 

 pentinic substance containing bronzite, ilmenite, garnet, diallage, and 

 " vaalite" (an altered mica), and is probably an altered igneous rock, 

 the decomposed character of the material examined preventing exact 

 determination of its nature. They showed that the diamonds were 

 marked by etch figures, analogous to those which Prof. Gustav Eose 

 had produced 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 shale as to resemble a breccia. Recent excavations have 

 shown that large quantities of this shale surround the mines, and that 

 they are so highly carbonaceous as to be combustible, smouldering for 

 long periods when accidentally fired. Mr. Paterson states that it is 

 at the outer portions of the pipes where the " blue ground " is most 

 heavily charged with carbonaceous shale, that there is the richest yield 

 of diamonds. 



JWr. Dunn regards the "blue ground" as a decomposed gabbro, 

 while Mr. Hudleston, Prof. Rupert Jones, and Mr. Davies regard it as 

 a sort of volcanic mud. Mr. Hudleston considers that the action was 

 hydi-othermal rather than igneous, the diamonds being the result of 

 the contact of steam and magnesian mud under pressure upon the 

 carbonaceous shales, and compares the rock to a " boiled plum- 

 pudding." 



The earlier theories as to the origin of the diamond have, in the 

 light of new facts, quite given way to the theory that the diamonds 

 were formed in the matrix in which they lie, and that the matrix 

 is in some way of volcanic origin, either in the form of mud, ashes, 

 or lava. 



The exact nature of this matrix becomes, therefore, a matter of great 

 interest. The rocks now to be described are from the deeper portions 

 of the De Beers mine, and were obtained through the courtesy of Mr. 

 Hedley. They are quite fresh, and less decomposed than any previously 

 examined. Two varieties occur, the one diamantiferous, the other 

 free from diamonds, and the lithological distinction between them is 

 suggestive. The diamantiferous variety is crowded with included 



