316 Profesnor T. G. Bonney — Parent-rock of the Diamond. 



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-|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 O'l 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 determination 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 slightl}/^ moi'e fibrous, and not in quite 

 so good a condition, the optical picture is less distinct. Thus we 

 may name the rock from which the present specimen has been 

 broken, a gai'net-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 con- 

 taining enclosures, which give it the aspect, at first sight, of a pebbly 

 mudstone. Microscopic 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 conglomerate which we shall mention 

 in a later paragraph. 



The ' Blue Ground ' and Associated Eocks. 



Two areas of diamantiferous rock are now being worked at the 

 Newlands 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 underground 



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

 ground ' nearly in its present condition. 



