The Parent-rock of the Diamond in South Africa. 231 



variety of the last-described rock, I have preferred to leave it as an 

 intact boulder. 



6. The next fragment, measuring about 3 in. by 2\ in. by 2 in., and 

 retaining part of its smooth outer surface, is labelled " found in the 

 yellow ground of No. 2 mine,* 50 feet deep." Though it is much more 

 decomposed than the others, the purplish garnet, the emerald-green 

 pyroxene, the altered enstatite (here very rotten), and a flake or two 

 of phlogophite (?) are easily made out. It is obviously a more 

 decomposed specimen of the rock represented by the two preceding- 

 specimens. 



7. The last of this group of specimens is a rock fragment,! measuring 

 about 3 J in. by 2 in. in length and breadth, and slightly exceeding an 

 inch in greatest thickness. Its outline is irregular, being determined 

 by the fracture of the predominant diallage-like mineral. The crystals 

 of this run large, an inch or more in length, breadth, and thickness. 

 It is greyish-green in colour, having one dominant cleavage, with a 

 sub-metallic lustre, and close subordinate cleavages, giving a somewhat 

 fibrous aspect to that surface. Between these large crystalline lumps,, 

 numerous small, ill-defined garnets (pyrope) seem crowded, so as to 

 form fairly continuous partings, generally hardly 0T inch in thickness. 

 As the readiness with which the rather soft pyroxenic constituent split 

 away made it improbable that a good slice could be cut, and I was 

 reluctant to injure the specimen, I contented myself with detaching a 

 few flakes of this constituent for microscopic work, since the deter- 

 mination of its identity was sufficient 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 quite so good a condi- 

 tion, the optical picture is less distinct. Thus we may name the rock 

 from which the present specimen has been broken, a garnet-bearing 

 bastitite. 



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

 from the rest. It is a compact greenish-grey rock containing enclosures, 

 which give it the aspect, at first sight, of a pebbly mudstone. Micro- 

 scopic examination 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 con- 

 glomerate which we shall mention in a later paragraph. 



* The others come from another mine (No. 1). 



f I am informed that this was not part of a boulder, but came out of the " blue 

 ground " nearly in its present condition. 



