J. Allan Thomson — Diamond Matrices of Australia. 493 



concentrates washed from it were forwarded by the prospector, 

 Mr. Pike, to several gentlemen who had expressed their desire to 

 examine them, and amongst others to Professor Miers. It was my 

 privilege, whilst at Oxford, to examine these specimens, and the 

 results of this examination were conveyed in a letter to Mr. Pike. 

 As he has published part of its contents in the Australian Mining 

 Standard, it is desirable to place before British readers a more 

 connected account, and at the same time to give an outline of the 

 not very easily accessible Australian literature. 



The rock in which the diamonds have been found comes from 

 Oakey Creek, Copeton, about 18 miles south-west of Inverell, New 

 South Wales. Diamantiferous alluvial of late Tertiary age is abundant 

 in the district, and has been long worked. The matrix itself has 

 been discovered in a tunnel driven in the granite floor of the alluvial. 

 It was first described by the Government Geologist and identified by 

 Mr. G. W. Card as a dolerite.' So far, only two diamonds have been 

 obtained, as the prospectors are unable to obtain water to wash the 

 decomposed rock that has been mined. Both these diamonds were 

 exhibited by Professor David at York and Mexico, and no doubt has 

 been expressed as to the genuineness of the discovery. 



The geological occurrence of the dolerite has not yet been fully 

 cleared up ; probably it is a dyke, but Mr. Pittman was inclined to 

 believe that it might be a small pipe. In one direction a drive 

 reached granite after passing through 26 ft. 6 in. of dolerite, but in 

 a direction at right angles to this it has a much greater extent. The 

 rock shows excellent spheroidal weathering. 



Professor David refers to the rock as a hornblende diabase,' but 

 the two specimens examined by me agreed in containing only pyroxene 

 as an original ferromagnesian mineral. Sections show the rock to 

 consist for the most part of elongate quadrate prisms of felspar, on. 

 which are moulded ophitically a colourless or pale bistre augite, 

 a little ilmenite and a greenish-yellow chloritic material. The felspar 

 is twinned both according to the Carlsbad and albite laws, and is 

 determined by Levy's method as a labradorite of medium basicity. 

 The yellow chloritic material is resolved, between crossed nicols, into 

 minute imperfect sphserocrystals, with positive elongation of the fibres 

 and a moderately high birefringence. It includes rare octahedra of 

 magnetite, more commonly minute globules of a yellow, isotropic 

 iron-hydrate and rod-like growths of iron-oxide, but in the majority 

 of cases it is entirely free from ferruginous inclusions. It generally 

 occurs in larger patches than the augite, but its relation to the felspar 

 is the same. The augite appears so fresh that some difficulty is found 

 in explaining the chlorite as arising from its decomposition, and it 

 appears equally probable that it represents an altered glassy base. It . 

 would be remarkable if large ophitic plates of hornblende were entirely 

 decomposed to a chlorite free of ferruginous materials, while the 

 pyroxene escaped alteration. Possibly the specimens supplied to 

 Professor David were of a different nature to those here described, 



' E. F. Pittman, Ann. Rep. Dept. Mines, N.S.W., for 1904, p. 137 (1905). 

 2 T. E. David, Sydtictj Morning Herald, January 19 and 26, 1907. 



