E. J. DUNN ON THE SOUTH-AFRICAN DIAMOND-FIELDS. 



609 



40. Notes on the Diamond-fields, South Africa, 1880. By E. J. 

 Dunn, Esq. (Communicated by Prof. Ramsay, E.B.S., P.G.S.) 

 (Eead June 22, 1881.) 



The mining-operations carried on during the last few years at the 

 diamond-fields, South Africa, have brought to light some additional 

 facts bearing on the formation of diamonds* ; the most interesting 

 is the exposure, at all the old mines (Kimberley, De Beer's, Du Toit's 

 Pan, and Bultfontein), of considerable deposits of black carbonaceous 

 shale underlying the surface beds of grey shale. 



By the removal of the diamond-bearing ground of the old vol- 

 canic " pipes " constituting the above mines, the wall or rim is left 

 unsupported ; after rain immense masses of this shale, or " reef," 

 as miners term it, fall into the excavated gulf, leaving excellent 

 clean sections of the horizontal strata ; for the bedding of the shales 

 is horizontal, except where locally disturbed by intrusive rocks, or 

 at the sides of the " pipes," where they are turned upwards for a 

 few feet, perhaps by the gabbro in the " pipes." 



At Kimberley mine the surface-shales, grey or, in places, pink or 

 yellow, which contain remains of small Saurians, are from 40 to 50 

 feet thick ; underneath are black carbonaceous shales, for the most 

 part arenaceous, and more than 100 feet thick. So combustible 

 are these shales that in a part of the mine where they were acciden- 

 tally fired they have smouldered on for more than eighteen months 

 and are still alight. 



Time did not admit of a search for plant-remains ; but a diligent 

 search, especially in the finer and more argillaceous beds, would 

 be almost certain of success. Thin seams of very impure coal full of 

 pyrites occur in the black shales ; and here and there a long flattened 

 piece of pure coal is found, probably the stem of some plant altered 

 to coal and flattened by compression. 



At De Beer's mine, on the north side, a somewhat different sec- 

 tion is laid bare. First there is, from the surface down, about 50 feet 

 of dolerite, then about 12 feet of yellow thinly laminated shales ; 

 beneath these are the black carbonaceous shales, corresponding with 

 those in Kimberley mine, and also containing thin seams, up to 

 1 inch in thickness, of impure coal. 



The precise depth of these carbonaceous shales has yet to be 

 determined; but that they extend horizontally over the whole 

 country at no great depth below the surface there is no reason to 

 doubt ; for wherever wells have been sunk in the neighbourhood of 

 Kimberley, De Beer's, Du Toit's Pan, or Bultfontein, these black 

 shales have been encountered at depths varying from 40 to 60 feet 

 from the surface. These shales are to be traced cropping out on the 

 banks of the Modder river f , some forty miles from Kimberley, and 



* [See E. J. Dunn's previous papers, Quart. Journ. Geol. Soc. vol. xxx. 

 p. 54 ; and vol. xxxiii. p. 879.] 



t [Quart. Journ. Geol. Soc. vol. xxx. p. 582.] 

 Q. J. G. S. No. 148. 2 s 



