610 E. J. DUNN ON THE SOUTH- AFEICAN DIAMOND-FIELDS. 



again on the Riet river, still further away. How much further they 

 extend can only be determined by boring ; for the level nature of the 

 country prevents one from obtaining any knowledge of the beds 

 100 feet below the general level. 



That the old mines are volcanic " pipes," and that they have 

 burst through these carbonaceous shales is evident. Is it not reason- 

 able to infer that the carbon, indispensable in one form or another to 

 the formation of diamond, was supplied by these shales ? 



It is well known that at and near the surface, or while the work- 

 ings in the " pipes " were bounded by grey shale, the mines were 

 not so productive as at lower depths ; when the zone of black shales 

 was reached the diamonds were more plentiful and also of better 

 quality. 



At Kimberley the improved yield as depth was attained was well 

 recognized ; and at Bultfontein mine so notably is this the case that, 

 though the surface-ground scarcely paid for working, the yield at a 

 depth of from 60 to 80 feet is most satisfactory. Jagersfontein, in 

 the Free State, again, is an instance of a mine poor at the surface, 

 but very profitable to work lower down. 



So far as experience can be drawn on as a guide, it appears that 

 the yields of diamonds in these " pipes " are greatest when mining 

 is carried on in the portions of the " pipes " surrounded by carbo- 

 naceous shales, rendering it probable that these shales supplied the 

 element necessary to the formation of diamond. 



A practical question of serious importance as regards the diamond- 

 mining industry here suggests itself : if the black shales supplied 

 the carbon of which the diamonds are formed, it is to be expected 

 that some diamonds would be found higher in the " pipes " than 

 the black shale ; for the tendency of the molten rock would be to 

 rise, and this would also be the case with carbon in the state of 

 vapour ; but is it probable that diamonds will be met with below 

 these shales ? 



Already in three separate localities shafts sunk at the edge of 

 Kimberley mine have, at a depth of 300 feet, struck remarkable 

 intrusive rocks — amygdaloids, breccias, &c. that differ essentially 

 from any thing found penetrating the surface-shales. Whether these 

 are older intrusive rocks on which the shales were laid down, or 

 whether they are later than the shales and penetrate them is not yet 

 decided. 



That the " pipes " will continue down for a vast depth there can 

 be no doubt ; but if beneath the black shales the " pipes " have 

 traversed rocks devoid of carbon, are diamonds still to be expected 

 in them ? 



Koffyfontein mine, on the road between Kimberley and Jagers- 

 fontein mines, has so far not proved rich at the surface. Many other 

 undoubted " pipes " have been opened and prospected near Kimber- 

 ley ; but subsequently they have been abandoned as unremunerative, 

 although some diamonds were obtained. It is very probable that 

 by deeper sinking in thess localities richer yields would be found, 

 as the black shales are almost certainly below. 



