DIAMOND: OCCURRENCE IN SOUTH AFRICA 191 



The carbon dioxide (carbonic acid) is present in nearly sufficient amount to combine 

 with the whole of the lime to form calcium carbonate; deducting this, the remainder, 

 consisting of hydrated magnesium silicate, with some of the magnesium replaced by 

 ferrous oxide, has approximately the composition of the mineral serpentine. It has thus 

 become customary to speak of the whole rock as a serpentine breccia, and this term, 

 or that of volcanic tuff or agglomerate, will be used in refeiTing to the diamantiferous 

 material. 



The blocks of foreign rock embedded in the breccia, which are often known as boulders, 

 have usually perfectly sharp edges and corners, though occasionally these may be rounded. 

 The size of these rock fragments varies from that of a small splinter to that of a block 

 several thousand cubic yards in dimensions. In the pipe of De Beer's mine there is a block 

 of olivine-basalt called " the island," which has a sectional area of 330 square yards, and has 

 been traced to a depth of 237 yards. Large masses of similar rock occur commonly in all 

 the mines, they are referred to as " floating reef," in contradistinction to the " main reef" 

 which surrounds the pipe. This "floating reef" is more frequently met with in the upper 

 than in the lower levels of the pipes. Smaller fragments of the same rock have, however, 

 been met with at the greatest depths to which the mine shafts have been sunk, and here as 

 elsewhere they form a large proportion of the material filling the pipes, through which they 

 are distributed with the greatest irregularity. 



Some of the rock fragments agree completely in character with the rocks of the main 

 reef, frequently consisting of amygdaloidal basalt (melaphyre), shales, &c. In some places, 

 the highly bituminous and carbonaceous shales are present in such large amounts that some- 

 times the presence of the fire-damp characteristic of coal-mines has been observed. It has 

 been asserted that diamonds occur only in those portions of the agglomerate in which 

 bituminous shales are present in large amount, and it has been argued from this that the 

 diamonds were actually formed from the carbonaceous matter present in these shales. 

 There is, however, reason to believe, as will be shown below, that the diamonds were formed 

 not in the pipes themselves, but at far greater depths in the interior of the earth from 

 which they have been brought up by the action of volcanic forces. 



Beside the blocks, which have evidently been detached from the reef surrounding the 

 pipes, there are others which have not been found in situ in the neighbourhood, and which, 

 therefore, must necessarily have been brought up from below. In the Kimberley mine at 

 depths below 230 feet, there are found large blocks, several cubic yards in extent, of grey or 

 greyish-white sandstone, the grains of which are bound together by a calcareo-argillaceous 

 cement. They are of much the same character as the sandstone which, in other localities, 

 forms part of the Middle Karoo formation, and which from geological considerations must 

 form part of the reef at the Kimberley diamond mines at a great depth below the surface. 

 There also occur, though not so frequently, fragments of quartzite, mica-schist, talc-schist, 

 eclogite, and granite. The last named rock is rarely found, and when met with is so 

 decomposed as to be only doubtfully recognisable as granite. It was found in numerous 

 large blocks, and in smaller fragments in the upper portion of a small mine known as Doyl's 

 Rush about a mile from Kimberley. Such rocks crop out at the surface some distance north 

 of the diamond-fields ; it is therefore probable that their southern extension lies at a gi-eat 

 depth below Kimberley, and forms the base of the reef. Rock fragments of materials not 

 found in the reef enclosing the pipes, which in all probability have been brought up from 

 below, are called " exotic fragments." 



The minerals embedded in the agglomerate are usually distributed through it with some 

 regularity, but very sparingly ; they constitute only about j-^inx of the total mass of the rock 



