836 DIAMOND-DEPOSITS OF COPETON, N.S.W., 



The material filling the beds of the Tertiary stream-channels 

 is grouped under four heads : (l)the wash; (2) the drifts; (3) clay- 

 deposits; (4)lignite. There is no locality in which all four are 

 developed, and, as a rule, only the first two are present. The 

 wash is the term applied to the coarser bands of material. This 

 has usually collected on the base of the stream-channel, and 

 contains the heavier minerals. Consequently, it is essentially 

 the diamond-bearing stratum of the deposit. The wash seldom 

 exceeds one foot in thickness. The drifts are aggregates of loose 

 and rather fine, sandy material, usually stained red from the leach- 

 ing of iron oxide from the overlying basalt. These deposits attain, 

 in several places, a thickness of 20-25 feet. The clay-deposits 

 consist of kaolin derived from the decomposition of the felspars 

 in the granite, and have been deposited under lacustrine con- 

 ditions. Lignite occurs rarely, and overlies the drifts. It does 

 not appear to have been much afiected by the overlying basalt. 



The geology of the district is represented on the accompanying 

 map. It will be seen that there are present (l)slates; (2)granite; 

 (3) dykes; (4) basalt. 



The slates cover a small area, and are unimportant in connec- 

 tion with this paper. The granite is represented by three types 

 — the Acid, Tingha, and Oakey Creek granites. The first is the 

 younger, and the last is probably intermediate in age to the first 

 two types. The dykes are of two varieties— (a) fine-grained 

 felsites: (6) dolerites. These dykes have intruded the granites, 

 but are older than the basalts and the basalt-capped leads. 



The basalts are the youngest rocks in the district. These 

 overlie the Tertiary gravels, and obviously cannot have been a 

 source of diamond-supply. It is also shown that it is highly 

 improbable that the diamonds can have been derived from either 

 the slate or the granites. This points to the dykes as the pro- 

 bable origin of the diamond-supply. 



This " proof by exhaustion " evidence in favour of the dyke- 

 material as the diamond-matrix has been confirmed by the dis- 

 covery of a diamond in the dolerite-dyke at Oakey Creek. A 

 Tertiary stream-channel has crossed this dyke, and diamonds 

 were found in the gravels adjacent to the dyke on the down- 



