188 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



of the pipe in sectional area as greater depths are reached ; thus at a depth of about 300 feet 

 the two diameters are reduced to 260 and 160 yards respectively, and the contraction is 

 continued as still low er depths are rgached^ A diagrammatic section of the Kimberley 

 mine is given in Fig. 39, an explanation of which is given below. 



The rocks composing the reef are, on the whole, much the same ever} where, still, 

 in the various mines, certain differences do exist. 



The neighbourhood for a considerable distance round Kimberley is covered with a layer 

 of red clay, 1 to 5 feet thick ; underlying this is a bed from 5 to 20 feet thick of 

 calcareous tufa, also of wide distribution. This tufa is of recent origin and has no genetic 

 connection with either the reef or the diamond-bearing pipes, since it covers both 

 indiscriminately, and to a certain extent penetrates cracks and crevices in them. Beneath 

 this tufa lie the rocks of the Karoo formation which constitute the reef. .^.^ 



The uppermost part of the reef in the Kimberley mine consists of a series of bedded 

 shales, 4<0 to 50 feet thick, greenish -grey in the upper pai't and yellowish or gi-eyish in the 

 lower ; they are of varying hardness, and at different levels in the mine are interbedded with 

 a fine-grained to compact olivine-basalt. Beneath these pale shales are about 270 feet of 

 black bituminous shales, very similar in character to the shales of the English coal-measures ; 

 certain of these beds are impregnated with iron-pyrites, and they often contain nodules of 

 clay-iron-stone, small bands of calcite, and thin layers of coal, while interbedded with them 

 near their base is a sheet of basalt one foot thick. Beneath the black shales is a hard grey 

 or green amygdaloidal diabase (melaphyre), the base of which is not exposed in the open 

 workings, but is seen in the underground shafts at a depth of 440 feet below the upper 

 surface of the mass. 



Beneath this igneous rock the shafts penetrate a bed of quartzite of about the same 

 thickness, and under this again black shales, both of which are penetrated in places by dykes 

 of eruptive rock (dolerite). The deepest shaft of the mine has not yet penetrated to the 

 base of the black shales, so that the total thickness of these beds is unknown. Probably 

 at still greater depths, as yet untouched by mining operations, there are deep-seated rocks, 

 such as granite, gneiss, or olivine-rocks, but this question will be discussed later. 



In the De Beer's mine, a sheet of basalt 47 to 61 feet in thickness is met with in the 

 upper part of the reef, otherwise the beds are the same as in the Kimberley mine. A similar 

 sheet of basalt is present in Du Toifs Pan mine, but is absent from the Bultfontein mine. 

 The walls of the pipe consist here, as far as they have been laid bare, only of shales, which 

 are much displaced, sometimes having an inclination of at least 15° to the horizon ; this 

 is also the case to a certain extent in the De Beer's mine, while inother places the beds are 

 horizontal. In the Du Toifs Pan and Bultfontein mines the shales have not yet been 

 penetrated to their base, and their thickness appears to be greater here than in the Kimberley 

 and De Beer's mines, which lie a little fuz'ther to the north. 



The material filling the pipes, like that of the surrounding rock, is essentially 

 the same in every mine, and in every part of each mine, but in all mines the upper 

 portions of the pipes to a fairly considerable depth have suffered the effects of weathering. 

 Observable differences do exist, however, and an experienced miner can sometimes i-ecognise 

 not only from which mine, but also from what part of a particular mine, any given specimen 

 of material has been taken. These small differences are usually connected with variations 

 in colour, hardness, and composition, the nature of the enclosed minerals and fragments of 

 foreign rock, &c., and are, as a rule, unimportant. 



The different kinds of rocks constituting the material which fills the pipes are 

 separated by no trace of bedding planes, but masses of rock, slightly different in character 

 have been recently observed to be separated from each other in quite another way. 



