BY L. A. COTTON. 835 



It is interesting to note, however, that rocks very similar to 

 the Copeton dolerites have been found in the peridotite-necks of 

 South Africa. 



Analysis numbered iii., is of a " hard rock'' from the blue 

 ground of De Beer's mine at Kimberley. This rock closely 

 resembles the Staggy Creek dolerite, the chief difference being 

 that the South African rock possesses about 6 % more of the 

 iron oxides, and about 5 % less lime than the New South Wales 

 variety. 



In addition to this rock, I have examined microscopically an 

 olivine-dolerite stated to be from the Kimberley pipe, which, 

 apart from its relatively small percentage of olivine and absence 

 of quartz, is not unlike the Oakey Creek dolerite. 



Although these two South African rocks do not appear to repre- 

 sent the type-rocks of the South African diamond-bearing necks, 

 it is interesting to note that they do occur associated with the 

 diamond-deposits. 



vii. Summary. 



The first discovery of diamonds in the Copeton district was 

 made in 1872 or 1873, simultaneously with the discovery of tin- 

 stone in the district. The diamonds were first found in alluvial 

 workings, but these had been derived from the denudation of 

 basalt-capped leads. These leads are probably of late Tertiary 

 age. The physiographic investigation shows that the present 

 drainage-system of the district trends to the west, but the pre- 

 basaltic drainage was in a northerly direction. The original 

 course of the Gwydir was northwards from Copeton to Inverell, 

 and it then received the Macintyre as a tributary stream. The 

 present course of the Gwydir has followed the eastern and 

 northern edges of a block-fault. The Avestern boundary of this 

 block-fault has been examined by W. N. Benson, and we have 

 agreed to name the sunken area " Keera," from a Station of that 

 name situated in the area of subsidence. The basalts and river- 

 gravels (which are after Anderson) shown on the accompanying 

 map, illustrate the Tertiary river-system in its relation to the 

 present-day drainage. 



