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



611 



Should it prove to be the case that these carbonaceous shales pro- 

 vided the carbon for the diamonds, the conclusion is forced on us 

 that the original source of the diamond is the atmosphere ; for the 

 plants absorbed carbonic acid gas from the air, and in course of 

 time were entombed, and thus provided a store of carbonaceous 

 matter in the shales. Later on these shales were shattered and 

 engulfed in the molten rock. The carbon was then liberated in the 

 state of vapour by the intense heat ; but, being under great pressure 

 in the " pipes," instead of escaping, it crystallized out as sparkling 

 diamonds. 



The disclosure of such extensive deposits of carbonaceous shale 

 has other bearings quite as important as on the formation of dia- 

 monds. Hitherto no such deposits were known to exist in the Karroo 

 beds or Dicynodon series of rocks that cover such an immense area 

 in Cape Colony and the Free State. Vast plains broken by hills, 

 either isolated in groups or in long ranges, characterize the country 

 occupied by these rocks ; the river-beds are generally shallow ; and 

 thus it happens that until the southern limit of the Karroo beds is 

 reached, which is 200 miles direct from Kimberley, no section, even 

 to a moderate depth below the general level is obtainable. 



' At their southern termination these horizontal beds abruptly 

 cease, exposing their edges through a thickness of more than 3000 

 feet, and form the Nieuwveldt, Camdeboo, and Winterberg ranges 

 of mountains. 



No seam of coal has been found along these well-exposed edges, 

 although well searched for. Very insignificant thin seams of black 

 shale do exist, also occasional plant-remains in the form of Glosso- 

 pteris and Equisetum, but no such shales as at the diamond-fields. 

 The remarkably pure anthracite found vertically intersecting the 

 rocks (Karoo beds) at Buifel's Kloof, Camdeboo* has been proved 

 by boring to be the result of distillation, perhaps ensuing from the 

 action of intrusive rocks on the vegetable remains included in the 

 shales. At the above locality a large dyke underlies the outcrop 

 of anthracite, though at the surface it is several hundred yards 

 distant. 



The trough of the great basin occupied by the Karroot beds lies 

 south of the Orange river, and about east and west. Is there not a 

 possibility that these carbonaceous shales cropping out near the 

 northern limit of the Karroo beds may develop into true coal-seams 

 further south, or nearer the centre of the basin ? A couple of bore- 

 holes would settle the point ; and the pressing need for fuel at the 

 diamond-fields will ensure the trial being made. 



In previous notes some dyke-like masses at De Beer's mine were . 

 described as noticeable near the surface ; now that the workings 

 have reached a depth of over 100 feet these dykes are less equivo- 

 cal, as the rock forming them appears in almost its original con- 

 dition, and not decomposed as at the surface ; it is of dark bluish- 



* [See E. J. Dunn's Report to the Colonial Parliament, 1879.] 

 t [This name, as used by Mr. Dunn, excludes the Uppermost Karroo or Storm- 

 berg beds.] 



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