The Parent-rock of the Diamond in South Africa. 233 



generally similar is more decomposed, and apparently contains some 

 fragments of shale. Specimen (v) has a stratified aspect, being a dull 

 grey faintly mottled rock, with streaky, dark, rather carbonaceous- 

 looking bands ; the origin being doubtful, till it is seen under the 

 microscope. A fourth specimen (iii) shows the mudstone traversed by 

 a vein of rather pale-coloured decomposed " blue," not exceeding an 

 inch in thickness. A fifth (ii) is from near the diabase on the western 

 side, a dark compact rock, faintly mottled, here and there presenting a 

 slight resemblance to a " blue " traversed by thin veins of a carbonate ; 

 and a sixth (iv) from a like position on the opposite side is a generally 

 similar rock, but with wider veins filled with more coarsely crystalline 

 calcite. A seventh specimen represents the " blue " in the " neck," a 

 few yards to the north and at the same level (300 feet). This, inferior 

 in preservation to the first-named, includes numerous rounded frag- 

 ments a little darker than the matrix, with others, angular to sub- 

 angular, some also darker and some lighter than it. 



A brief summary of the results of microscopic examination may 

 suffice, as these rocks do not materially differ from specimens obtained 

 in the De Beers Mine, of which I have published a full account.* 



The matrix is a mixture, in slightly variable quantities, of granules 

 of calcite or dolomite, serpentine, pyroxene, and iron oxides, in which 

 occur flakes with fairly idiomorphic outlines of a warm-brown mica, 

 moderately pleochroic, corresponding with that described! in one or 

 two specimens from De Beers Mine. The prisms are about 0'002 inch 

 in diameter, and sometimes nearly as thick. This mica, which, as 

 stated in a former paper, I consider a secondary product, occurs abun- 

 dantly in all the specimens, but in that from the interior (on the whole 

 the best preserved rock) it is locally assuming a green colour, no doubt 

 by hydration. In the specimens from the thick rib, the one last 

 named contains mineral grains and rock fragments. Except for a few 

 flakes of the usual mica, the former are a mixture of two fibrous 

 minerals, the larger part corresponding with actinolite ; the rest, 

 giving lower polarisation tints, may be serpentine. This fact, and 

 structures suggestive of the former presence of a cleavage more regular 

 than that of olivine, make it more probable that diopside was the 

 original mineral. Though iron oxide is present in specks and rods 

 (especially in the worse preserved specimen), this occurs either in the 

 outer part, or as though it had been deposited along cleavage planes. 

 In the thin rib of " blue " (iii), some of the grains are composed partly 

 of a fibrous mineral, as above described, and partly of a clear one, which 

 often affords rather rich polarisation tints, and presents some resem- 

 blance to quartz. Its precise nature is difficult to determine, owing 

 to the absence of distinctive characters, but I believe it to be of second- 



* « Geol. Mag.,' 1895, p. 492 ; and 1897, p. 448. 

 f < Geol. Mag.,' 1697, pp. 450, 451. 



