FURTHER NOTES ON" THE DIAMOND -EI ELDS OF SOUTH AFRICA. 879 



7. Further Notes on the Diamond-fields of South Africa, with 

 Observations on the Gold-fields and Cobalt-mine in the 

 Transvaal. By E. J. Dunn, Esq. (Read June 21, 1876.) 



[Communicated by Prof. A. C. Ramsay, F.R.S., F.G.S.] 



I. Diamonds. 



A vtsit made to the Diamond-fields in December 1874 enables me 

 to make some additions to, and corrections of, my previous paper, 

 read before the Geological Society in 1873*. 



At De Beers, mining-operations have clearly established the fact 

 that the "pipes" are more recent than the sheets of dolerite and 

 other intrusive rocks surrounding them ; for on the east side of the 

 mine the rudely tabular dolerite, forming the wall of the " pipe," is 

 tilted at an angle of about 40° (fig. 7.) Included masses and nodules 

 of dolerite occurring in all the pipes hitherto opened, help to confirm 

 this view. 



In no case are the " pipes " traversed by dykes of hard rock ; and 

 the only instance of dykes of any description intersecting the 

 material forming the " pipes " is at De Beer's, where, at a depth of 

 100 feet from the surface, and at the east side of the mine, long, 

 narrow, dyke-like masses, from 2 to 5 feet wide, vertically cut 

 through the soft diamond-bearing rock ; one having the latter width 

 runs for about two hundred yards in a straight line. These dykes (?) 

 are lighter in colour and somewhat harder than the bounding rock, 

 but earthy in texture and so much altered that specimens sent to 

 the British Museum were undeterminable, though evidently quite 

 distinct from the surrounding soft rock. The general opinion of the 

 miners who have these dykes (?) cutting through their claims, is 

 that they contain no diamonds. 



The wall of De Beer's Mine, except on the north side, which is 

 shale, is formed by the sheet of dolerite (?) that commences near 

 Kimberley Mine, and continues on past Du Toit's Pan. Within this 

 mine, on the west side, an extensive crescent-shaped mass of shale is 

 exposed ; it is interpenetrated by the decomposed rock that fills the 

 "pipe" (probably it formed part of the wall at a higher level, but, 

 being undermined by the intrusive rock, fell in bodily) ; the adjacent 

 wall is of dolerite (the miners call this a " reef"). 



Kimberley Mine (Colesberg Kopje, fig. 6) is entirely bounded by 

 shale, though a sheet of intrusive rock occurs within about 300 feet 

 of the edge of the mine. The shales lie nearly horizontal as a rule ; 

 they are faulted on the north side. 



Du-Toit's-Pan Mine is bounded by the same sheet of intrusive 

 rock that forms the wall of De Beer's. Shale forms the wall on 

 the south. 



* Quart. Journ. Geol. Soc. vol. xxx. p. 54. 



