868 Reviews — Prof. E. C. Leicis — Genesis of the Diamond. 



volcanic source as the " blue ground." Olivine is the main ingredient 

 of this rock, the principal accessories being enstatite and biotite. 

 It would have been satisfactory if a detailed examination of this 

 rock had been given, since it most probably represents, to a con- 

 siderable extent, the original peridotite, which bursting through 

 the carbonaceous shales of the Karoo beds, at intervals, has supplied 

 a large portion of the mixture, with a serpentinous base, known as 

 the diamond-rock, or as Lewis would call it " Kituberlite breccia." 



The actual diamond-rock itself, being so much of a mixture, 

 the mineralogical details given in Section IT are of less importance, 

 since it is diflScult to determine which minerals are endogenous 

 and which are derivative. Most of the minerals described, including 

 the diamonds, Lewis seems to have regarded as endogenous. We 

 can easily believe that chrome - diopside, smaragdite, bastite, 

 magnetite, chromite, ilmenite, and picotite, said to be common under 

 the microscope, represent paragenetic minerals of the orginal 

 peridotite, as also pei'ofskite, said to be the most abundant titanium 

 mineral of ultra-basic rocks. The same may also be said of the 

 peculiar brown biotite (altered to vaalite) and abundant specimens 

 of pyrope. When we bear in mind, however, that Moissan is said 

 to have detected more than eighty species of minerals in the " blue 

 ground," or diamond breccia, we can well believe that many of 

 these are derivative. 



The minerals described in Section II are said to lie, some of 

 them porphyritically, in a base which is a more or less homogeneous 

 serpentinous mass containing fragmental enclosures, of which shale 

 is the most abundant. The fragments of shale have lost their 

 shaly character together with their carbonaceous matter, a result, 

 he considers, of alteration through the heat of the lava. It should 

 be noticed, in this connection, that local writers have usually spoken 

 of the shale in the diamond-rock as not being fused at the edges. 

 As regards the composition of the ground-mass, after eliminating 

 from the calculation the very considerable quantity of calcite, he 

 concludes that one-half consists of serpentine and the bulk of the 

 remainder of olivine. 



Let us now see how far the editor assists us in comprehending 

 the diamond-rock of the Kimberley district. Plate i is an effective 

 picture of a fragment of the diamond-rock of De Beers mine, from 

 rather over 1,000 feet (obtained by Stone in 1894). This presents 

 a pseudo-porphyritic appearance : the largest inclusion, nearly 

 a couple of inches long, consists of rather decomposed compact 

 serpentinized olivines and pyroxenes, with a few flakes of mica. 

 In plate ii (1) a slice of this rock is magnified twenty-two diameters ; 

 crystals of mica and altered enstatite are seen with several grains of 

 serpentinized olivine, whilst the separate mineral grains decrease in 

 size until they become indistinguishable from the granular matrix of 

 serpentine, calcite or dolomite, iron-oxide, perofskite, etc. We may 

 well believe with the author that, in his time, there was no named 

 rock type having at once the composition and structure of the 

 Kimberley rock. But, bearing in mind that the diamond-rock 



