BY H. I. JENSEN. 229 



The Warrumbungle Mountain group is built up of volcanic 

 rock. The highest points situated between the Tannabar Split 

 Rock (Berum Buckle) and Mt. Exmouth (Wombalong) reach a 

 height of 4,000 feet, and form the centre of the system. To an 

 observer stationed on one of these peaks it is apparent the other 

 mountains north, south, east and west are of a gradually lessening 

 altitude, depending on their distance from the central bunch. 

 In the latter all the streams of the district originate, diverging 

 and flowing outwards in all directions till the surrounding plain 

 country is reached. Through the mountains they flow in deep 

 gorges which gradually widen as they reach lower parts; neverthe- 

 less they retain their youthful appearance,with steep slopes, often 

 vertical cliffs of volcanic rock capping sandstone on either side. 

 Such are the valleys of the Castlereagh between Mobara Rock 

 and Coonabarabran, Jack Hall's Creek, Belar Creek, Uargon 

 Creek, and many others. 



Rock-weathering is very slow in this region, so that many of 

 the volcanic tablelands and table-mountains have only a very 

 thin covering of soil. Some are even quite bare, and become 

 known as " scorched plains." The rock waste round the more 

 conical mountains consists largely of coarse fragments of rock 

 broken up by atmospheric agencies and fires, but only slightly, if 

 at all, decomposed. 



The central group of mountains is built up essentially of 

 arfvedsonite trachyte and phonolitic trachytes. Surrounding 

 them and surrounding masses of a similar nature elsewhere, also 

 overlying tuffs and breccias of the arfvedsonite trachytes, we 

 have the regirine trachytes and trachydolerites. The outskirts 

 of the Warrumbungle Mountains are basaltic, and basalts are 

 also met with in the central group capping trachydolerites and 

 trachytes. Mt. Exmouth has a capping of sodalite basalt about 

 50 feet thick overlying a bed of tuff and breccia of about the same 

 thickness. 



Evidently at the close of the volcanic period the region of the 

 Warrumbungle Mountains formed a huge dome-shaped volcanic 

 mass, with its apex near Berum Buckle, and consisting of 



