BY L. A. COTTON. 815 



well marked. The level of the basalt on the western side is above 

 the peneplain-surface of the granite-area. This How undoubtedly 

 represents a pre-basaltic valley, which was completely obliterated 

 by the lava-flows. Subsequent denudation has exposed the 

 granite along the eastern side, and partly on the western side. 

 (See Text-fig. 2a). 



As the pre-basaltic Gwydir has been traced directly into the 

 southern, truncated end of this flow, there can be no doubt that 

 the Gwydir originally continued in this direction. 



Moreover, it is clear that the Macintyre at Inverell has been 

 redeveloped along the course of a pre-Tertiary stream from ISTew- 

 stead to Inverell. Thus it would appear that the Macintyre- 

 Gwydir originally formed one stream-system, which has been 

 severed by the large basalt-flow extending from Copeton to 

 Inverell. 



iv. The Distribution op the Diamond-leads. 



The subject-matter of this Section will be rendered clear by 

 reference to the accompan3dng geological map (Plate xcii.). This 

 map is practically that of Mr. Anderson, published in 1887. A 

 few additions and alterations, including the contours, have been 

 made by the author. Previous workers have held the opinion 

 that the main Tertiary stream flowed west from Copeton to 

 Bingara. It has been stated, in support of this hypothesis, that 

 the size of the diamonds found in the river-gravels decreases 

 from Copeton towards Bingara. It has been argued that the 

 largest diamonds would remain nearest their source of origin, 

 and the smallest would be carried farthest down stream. This 

 argument is, no doubt, correct, but the converse is not, in this 

 case, true. The observation of numerous aneroid readings, which 

 were checked and standardised, has shown that the Tertiary lead 

 on the north side of the Gwydir has not flowed towards Bingara, 

 but in the opposite direction. Some other reason must, then, be 

 assigned to account for the fact that the diamonds are larger at 

 Copeton than at Bingara. 



The levels observed are recorded on the sketch contour-map. 

 The gaps between the remnants of the basalt-leads have been 



