Farent-Rock of the Diamond in South Africa. 107 



for my purpose. These show the mineral to have one 

 easy cleavage and a rather fibrous structure ; they give 

 straight extinction parallel with this. As the usual rings 

 and brushes can be seen on the face of easy cleavage, 

 the mineral belongs to the bastite group. The same is 

 true of the enstatite in boulder (4), though, as it is slightly 

 more fibrous, and not in cjuite so good a condition, the 

 optical picture is less distinct. Thus we may . name the 

 rock from which the present specimen lias been broken, 

 a garnet-bearino; bastitite. 



8. This specimen, said to be a fragment of a boulder, 

 is very dilferent from the rest. It is a compact greenish 

 grey rock containing enclosures, which give it the aspect, 

 at first sight, of a pebbly niudstone. Microscopic ex- 

 amination shows it to be a compact felspathic diabase, 

 with vesicles, which have been filled up with calcite, 

 chlorites, and other secondary minerals (probably zeolites), 

 but not to have any special interest. Its relations appear 

 to be with the rocks occurring in a conglomerate which 

 we shall mention in a later paragraph. 



The "Blue Ground" and Associated Rochs. 



Two areas of diamantiferous rock are now being worked 

 at the JSTewlands Mines. The shape of the one which 

 supplied most of the specimens described in this paper is 

 irregular, and, so far as I know, exceptional. Its outline 

 at the surface may be roughly compared to a rounded 

 triangle into the base of which the point of a rather short 

 shuttle is thrust, the greatest breadth of the two being 

 about equal. Exploratory workings at a depth of 300 

 feet show that the former area rather quickly narrows, 

 and the latter terminates in clefts: the " blue ground," 

 in fact, appears to fill a fissure, broadening in two places 

 to vents which have been traced for some distance under- 

 ground southwards from the principal mass of diamanti- 

 ferous rock, as represented in the annexed section. 



