7.— NOTES ON SOjVIE DIAMOND BEARING AND 

 ASSOCIATED ROCKS. 



By F. P. .^iKXNELL, F.G.S. 



Almost every one is aware that the South African dianioncls 

 occur in a rock of igneous origin, popularly known as "blue ground." 

 But it does not appear to he decisively settled what is the exact nature 

 of this " bine ground "' — whether it is to be regarded as a fragniental 

 rock or as one sf)lidified from fusion after the manner of an ordinary 

 dyke, although distinguished by containing many fragments torn off 

 from the rocks through which it has passed on its way towards the 

 surface. Personally I think there can be little doubt that in the 

 main the latter conclusion is correct. The blue ground often occui's, 

 in fact, in the form of actual dykes, which seems (juite sutficient evi- 

 dence of its true nature. Parts, however, of the occurrences filling 

 the large volcanic rents usuall}- termed "pipes" have undoubtedly 

 been brecciated by subsequent explosions and movements caused by 

 uprush of further materials. It is evident, indeed, that some of the 

 inclusions in these disturbed portions have suffered from the cup and 

 ball action which writers like Prof. Bonney ha\e ridiculed. The frag- 

 ments derived from lioriz(»ns higher than now represented alongside 

 the pipes are fairh" conclusive evidence that such action must have 

 taken place. 



The rounding of so man}' of the included fragments and also of 

 the larger crj-stals of the constituent minerals is, however, to be 

 <xscribed to corrosion by the molten mass and to resorption as the 

 material rose in the pipe. Tlie altered borders of the garnets, itc, 

 are quite normal in this class of rock, as may be seen by reference 

 to a standard text-book like Hai-ker's Petrolo(jy. The}' cannot, there- 

 fore, be quoted, as is often done, in favour of the garnets being derived 

 from rocks like eclogites. 



As a decomposition product it has always been remarked that th»> 

 abundance of calcite is surprising in so basic a rock. The ultra-basic 

 class, of which the "blue ground'' is a volcanic representative, is 

 nearly always highly, magnesian, serpentine, for example, changing to 

 }nagnesite in many cases of alteration. Prof. Ciirvill Lewis long ago 

 suggested that the high lime contents of the " Kimberlite," as he 

 called the blue ground, was perhaps due to the original presence of 

 a calcium silicate like nepheline or melilite. In view of the known 

 presence of the latter mineral in such closely related fresh rocks as 

 that of the Spiegel River, the probability is greatly increased. Some 

 time ago the writer was fa\oured by Mr. A. Williams witli samples 

 of one of the so-called dvkes which ti'averse the blue ground of the 

 Kimberley mine. It was found to be in every respect similar to the 

 •ordinary blue ground, sa^■e in its greater freshness and in showing 

 little laths of a mineral which at first irlance seemed to be melilite. 



