Prof. Bonney and MisH Raisin — Rocks from Kimberley. 499 



structure, such as is common in slaggy ejectamenta. Still, like the 

 bands produced in some stained rocks, it might be a result of in- 

 filtration ; but though the latter may be a modifying cause, we 

 incline to adopting the former explanation. 



The " blue clay " or " blue ground," the material obtained in the 

 earlier and shallower workings, is clearly only a decomposed con- 

 dition of the rock already described,' the cementing carbonate having 

 been more or less removed ^; and the '• yellow ground " represents 

 a further stage in the decomposition. We have compared the fine- 

 grained part of the " blue ground " with some crushed and 

 decomposed serpentine from the Lizard, and find a strong likeness 

 between the dominant material in both, so that very probably the 

 dust-like fragments of serpentine (altered peridotite) enter largely 

 into the composition of the diamantiferous i-ock. The more durable 

 minerals are separated from it by washing. Two packages, given 

 some time ago to one of us by Prof. Boyd Dawkins, are divided 

 into coarser and finer washings : the former often run from about 

 one- to nearly three-eighths of an inch, the latter do not generally 

 exceed one-tenth of an inch in diameter. The materials appear to 

 be similar, though the relative quantities may diff"er in the two, and 

 among the coarser some rock fragments (in one case a fine-grained 

 peridotite, in another apparently a sedimentary rock) are present. 

 Among the minerals in these samples, and in those brought by Sir 

 J. B. Stone, which are roughly sorted but are of larger size, we find 

 the following : — 



(a) Garnets ; more than one variety being present. The majority 

 are from a wine-red to a purple-lake in colour (almandine) ; 

 a few are orange-red to reddish resin-brown, when magnified, 

 (? essonite). They appear to be always fragmental ; this is very 

 obvious in the case of the clear-coloured specimens, and even those 

 which are more rounded suggest a fragmental origin. Many of 

 the latter seem to be only dull-coloured greenish grains, but on 

 breaking them they are found to consist of a kernel of red garnet, 

 surrounded by an aggregated doubly-refracting substance. This 

 may be merely some incrusting mineral, but it suggests the 

 possibility of a decomposition zone. Some have on the exterior 

 brown mica, perhaps also a clilorite, with another mineral too minute 

 for identification, and if we may found an inference on the ease with 

 which the grains break, the same material probably occupies 

 fissures. 



(&) The fragments of sahlite are often determined by cleavage 

 planes. The colour is commonly a fairly rich celandine or oil green, 

 but two or three of the well-rolled small grains are a very rich 

 emerald green. These probably contain a considerable amount of 

 chromium, but we should expect that all would aiford traces of this 

 base. 



' The material which Prof. Maskelyne and Dr. Flight were examining was 

 evidently " bine ground," and was taken from depths not exceeding 180 feet. — ■ 

 Q.J.G.S., vol. XXX, 1874, p. 406. 



'^ In the museum at Owens College, Manchester, are some large lumps of partially 

 decomposed breccia intermediate between the ordinary ' ' blue ground ' ' and the 

 specimens described in this paper. 



