BY H. I. JENSEN. 565 



soiiite-ti-cacliyte plugs being surrounded by soft tuffs, have, in 

 many cases, been isolated by the erosion. 



Dykes of arfvedsonite-trachyte have been met with cutting the 

 Triassic sandstones at Tannabar, behind The Spire, at Gibb's 

 Pass and many other places, so that the earliest lavas are at 

 least post-Triassic. Tuffs belonging to this series are, however, 

 associated, at Gowang and Wandiallabah Creek, with leaves of 

 Cinnamomum Leichhardtii and other leaves of Eocene appear- 

 ance. This fixes the commencement of volcanic action as 

 somewhere about the Eocene. 



Distinct sills and laccolites I have not seen anywhere. 



3. Physiographic Notes. 



An observer standing on one of the central peaks such as 

 Wombalong, Berum Buckle or Siding Spring Mountain, would 

 observe (a) that the elevation of the mountains diminishes as the 

 central group is receded from; and (b) that the watercourses 

 pursue very direct paths outwards from the central group in all 

 directions; when the streams reach the " plains " country they 

 commence to deviate from their original straight courses. They 

 are therefore " consequent." 



The central mountains are very rugged. The surrounding zone 

 of darker rocks is characterised by almost flat-topped mountains 

 and ridges, sloping gently away from the central area. The 

 watercourses are often wider inside the zone of dark trachytes 

 than in it. Thus the Castlereagh Biver at Timor (Fig.3) has a 

 wide valley, having had soft tuffs and sandstone to work in, but 

 flows in a narrow V-shaped valley thence to Coonabarabran; 

 Uargon Creek occupies a wide flat valley in the sandstone 

 country north of Black Mountain and east of Tonduron, but 

 runs in a narrow gorge between the Naman Ledges and Black 

 Mountain (Fig.5). Wandiallabah Creek and Belar Creek show 

 the same peculiarities. Where the creeks leave the inner sand- 

 stones and tuffs surrounding the light-coloured trachytes and flow 

 through the hard segirine trachytes and })honolites, erosion has 

 not been able to widen the valleys at the same rate as higher up. 



