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



The Acid granite has been shown to be intimately associated 

 with the tin-deposits of the New England plateau. It is a coarse- 

 grained granite, containing quartz-grains from 2 to 5 mm. in dia- 

 meter, and tabular crystals of pink orthoclase up to as much as 

 15 em. in length. A small amount of biotite is usually present, 

 and the rock is not infrequently tourmaline-bearing. It thus has 

 close affinities with the Alaskite of Spurr. The whole Terti- 

 ary system of diamond-bearing gravels at Copeton has this rock 

 as its channel-base, with the exception of that part between the 

 Oakey Creek and the Malacca mines, which rests upon the 

 Oakey Creek granite. It is worthy of note that the portions of 

 the Copeton leads which are diamond-bearing are nowhere far 

 distant from the junction of the Acid granite with some other for- 

 mation, of a greater geological age. 



The Tingha granite has been shown to be older than tlie Acid 

 granite. It is a hornblende-biotite type, containing tabular pheno- 

 crysts of plagioclase-felspar. It occurs only at the south-east 

 portion of the area now^ under consideration. Text-fig. 3 shows 

 the relationships of the series of rocks from Staggy Creek to 

 Rider's Lead. 



The Oakey Creek granite has been further investigated since it 

 was last described, and has been found unique in the nature of its 

 inclusions. The granite itself is very coarse, containing pheno- 

 crysts of plagioclase up to three inches in length. It is intersected 

 by numerous tourmaline-bearing veins, in which this mineral 

 occurs in long pencils up to three inches in length. The most 

 striking feature, however, is the presence of a great number of 

 inclusions. These are uniformly of one type, and consist of 

 masses of dark-coloured rock containing phenocrysts of felspar 

 similar to those in the granite. The inclusions are frequently more 

 than a foot in diameter, and range in size down to small masses 

 only an inch across. The shape of these is very variable. Some 

 are spherical, others ellipsoidal, while others, again, are subangu- 

 lar. In all eases, the contact of these bodies with the granite is 

 fairly sharp and well defined. The inclusions possess a dark base, 

 consisting chiefly of biotite and quartz, in which are set pheno- 



