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



before my visit, in January, 1913, some nine tons of this were 

 screened and washed. Two diamonds were shown to me, which 

 were reported to have been recovered from this test. About 

 three tons more were washed in mj'^ presence, but no diamonds 

 were recovered. 



It thus appears that, quite apart from the fact that diamonds 

 have been found in the dolerite, the evidence points to the dykes 

 as the probable source of the diamond-supply. From the great 

 variety, both in size and character, of the diamonds from different 

 parts of the leads, it is probable that the diamonds have been 

 derived from a great number of such dykes or necks. These are, 

 probably, like the Oakey Creek occurrence, still hidden by the 

 recent basalt-flows. 



vi. Petrology. 

 The Oakey Creek Granite. 



Crystal linity : holocrystalline. 



Grainsize : relative^ porphyritic; absolute, smaller crystals from 

 3 to 6 mm.; phenocrysts up to 8 cm. 



Fabric granitoid; also graphic intergrowth of quartz and 

 orthoclase in some instances. 



Minerals in order of decreasing abundance : quartz, orthoclase, 

 albite, biotite, microcline, muscovite, magnetite, apatite, and 

 fluorite. 



Secondary minerals : kaolin and chlorite. 



The felspars have crystallised out in two generations ; the 

 older ones are idiomorphic, zoned, and fairly free from decom- 

 position, while the opposite characteristics mark the later felspars. 

 These remarks apply to both the orthoclase and the albite. The 

 biotite is a dark variety, its pleochroism varying from yellowish- 

 brown to very dark brown. 



A small amount of microcline is present in subidiomorphic 

 crystals. 



The muscovite is rather rare, and is mostly included in the 

 orthoclase. It oppears to have been corroded by the magma 

 subsequently to its complete crystallisation. 



The apatite possesses its characteristic, prismatic habit. 



One or two very small allotriomorphic grains of fluorite were 

 observed in one slide of this rock. 



