Parent-Rock of the Diamond in South Africa. 109 



crystalline calcite. The last 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 fragments 

 a little darker than the matrix, with others, angular to 

 subangular, some also darker and some lighter than it. 



A brief summary of the results of microscopic examina- 

 tion 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 {Geol. Mag., 1895, p. 492, 

 and 1897, p. 448.) 



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 idio- 

 morphic outlines of a warm-brown mica, moderately 

 pleochroic, corresponding with that described {Geol. Mag., 

 1897, pp. 450, 451) 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 abundantly 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 worst preserved speci- 

 men), 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 



