364 GEOLOGY OF CAPE COLONY 



pipes ; but the association of the melilite-basalts of 

 Sutherland Commonage and Matjesfontein in the same 

 pipes with breccias containing some of the characteristic 

 minerals of kirnberlite, and their occurrence close to the 

 Silver Dam vent, which is filled with an agglomerate 

 still more like typical kirnberlite, render the supposition 

 much less improbable. The occurrence in the latter of 

 various granulites of the same types as those of the 

 kimberlite pipes is another important point of resem- 

 blance. In the remarkable agglomerates of the Salt- 

 petre Kop group of pipes we find that though the bulk 

 of the rocks is composed of the ddbris of sedimentary 

 beds, yet there are also numerous fragments of the 

 biotite, ilmenite, and hornblende characteristic of the 

 Silver Dam breccia. Moreover, in Prieska (Grenaat 

 Kop, etc.) pipes and fissures of the Saltpetre Kop type 

 are found intimately associated with dykes of kimberlite 

 and ultrabasic lamprophyre. Finally, no pipe or fissure 

 belonging to any of these types has been found to be 

 earlier than the Karroo dolerites, for inclusions of the 

 latter rock are extremely common in all of them, indeed 

 in some of the kimberlite pipes from 80 to 90 per cent, 

 of the inclusions are of dolerite. 



Eeverting once more to the question of the petro- 

 logical affinities of kimberlite, the opinion of Carvill 

 Lewis may here be cited: "the abundance of calcite 

 as a decomposition product, the high magnesia and low 

 alkali, the presence of biotite, and more especially of 

 perofskite, point to the possible presence of nepheline or 

 melilite in the ground mass 'V He therefore suggested 



1 Carvill Lewis, p. 49. 



