DIAMOND: OCCURRENCE IN SOUTH AFRICA 



189 



Melaphyre' 



\ 



Vertical, or nearly vertical crevices, not more than three-eighths of an inch across and filled 

 with a mineral substance resembling talc in character, penetrate the material down to the 

 lowest dejith to which the mines have been worked. These divide the whole contents of each 

 pipe into a number of vertical 



or nearly vertical columns, each n. Xeef cgg^^r==^ ==^.= ^wt ~^^= — ^ri:' s. 



differing slightly from theothers 

 in composition, but showing no 

 difference in its own mass. 



These small variations in 

 the material filling the pipes, 

 as well as its character as a 

 wholcj^ do not depend in any 

 way upon the nature of the 

 various rocks of the surround- 

 ing reef. It was formerly con- 

 tended that the character of the 

 reef had a more or less marked 

 influence on the richness in 

 diamonds of the material filling 

 the pipes ; thus it was feared 

 that when the base of the 

 black shale in the Kimberley 

 mine had been reached, the 

 yield of diamonds would cease, 

 since the formation of diamonds 

 was supposed to have been de- 

 pendent on some way on the 

 presence of carbon in these 

 shales. In consequence of this 

 belief, the value of mining 

 claims for a time fell ; but 

 the yield of diamonds at lower 

 levels, where the pipe is sur- 

 rounded by. melaphyre, turned 

 out to be just as good as at 

 the higher levels in the shale. 



Between the material fil- 

 ling the "pipes and the enclosing 

 i-ocks or reef there is always a 

 sharp line of demarcation and 

 never a gradual transition. 

 Usually the two sets of rocks are in immediate contact, but not infrequently they are 

 separated by a space, sometimes of considerable width, into which project beautiful 

 crystals of calcite. Other secondary minerals are also found in the numerous crevices by 

 which, in addition to the vertical cracks, the rock is penetrated. 



The actual diamond-bearing rock itself which fills the pipes must now be considered- 

 In the upper portion of the pipes it consists of a light yellow, soft, sandy or friable 

 material known to the diamond miners as " yellow ground " or " yeUow stuff.'' This upper 

 portion, which has a thickness of from 50 to 60 feet, has now, in the Kimberley mine, 



Fig. 30. 



Diagrammatic section through the Kimberley mine. 

 (Scale, 1 : 4,000.) 



