870 THE GEOLOGY OF THE NANDEWAR MOUNTAINS, 



as far as I have seen, not so good in this area as in the 

 Warrumbungles. 



Beautiful veins of chalcedony and agate abound, especially in 

 connection with vesicular volcanic rocks. 



Small diamonds have been obtained at the Alpha Mine near 

 Bullawa Creek in gravel near a basalt dyke. It is possible that 

 the basalt may have been the matrix, having absorbed the carbon 

 from underlying coal seams. Rocks like the Enstatite-peridotite- 

 lamprophyre (N.18, p. 884) which are extremely basic, very rich 

 in iron and magnesia, and intruded along coal seams, might 

 easily absorb carbon and liberate it again in the form of diamond 

 on cooling. 



Coal seams have been observed in many places. In the area 

 abounding in sills, such as around Bullawa and Eulah Creeks, 

 the coal seams have in most cases been destroyed. On the 

 summit of the divide between Rocky Creek and Bobbiwaa Creek 

 there is, as already mentioned, a seam of good coal about 6 feet 

 thick. No doubt in time to come many valuable coal seams will 

 be found and worked in the area surrounding the mountains. 



I have not noticed any diatomaceous earth deposits in the 

 portions of the Nandewar Mountains which I investigated, but 

 diatomaceous earths have been recorded from the vicinity of 

 Barraba, south-east of the mountains, by Mr. E. F. Pittman.* 



Other Alkaline Areas. 



From the foregoing notes it is apparent that the two great 

 volcanic areas of the Nandewar and Warrumbungle Mountains 

 consist mainly of alkaline igneous rock varying greatly in basicity. 

 Between them there are scattered masses of rhyoiitic, phonolitic, 

 and andesitic rock which form a chain connecting the two areas. 



Similar scattered pinnacles and cones occur at intervals 

 between the south-west corner of the Warrumbungles and Dubbo. 

 This area is referred to the Upper Coal Measures on the Geological 

 Survey Map, but on closer investigation much of it will probably 



• Ann. Kept. Dept. Mines, 1881, pp. 142-143. By authority, Sydney 1882. 



