574 THE GEOLOGY OF THE WARRUMBUNGLE MOUNTAINS, 



(3) The drainage is therefore disintegrated. The Castlereagh 

 River itself is a striking example, with its dry beds and billa- 

 bongs distinguished from the surrounding and more lowlying 

 country only by a ridge of wind-blown sand {monkey or moongie). 

 Many other creek-beds, no longer serving as water-courses, are 

 present. Evidently in the arid period the integrated drainage- 

 system established in the wet period has been destroyed. Old 

 age of arid erosion has been reached in the countr}^ Avest of the 

 mountains, and maturity on the Coonabarabran tableland. Only 

 in the mountains themselves, on account of the hardness of the 

 rocks, has a youthful appearance been maintained. 



(4) " Scorched plains " devoid of soil, flat-topped stony hills, 

 and slopes covered with coarse shingle instead of soil, have 

 developed in the volcanic mountains; and around Coonabarabran 

 a typical bad-land topograph}^ has been shaped. 



(5) Alluvial fans occur in the valleys where declivity lessens, 

 or where the streams reach the plains, as at Tundebrine. 



(6) There are no post-Tertiary fossils, except a few plant-remains 

 and bones of terrestrial animals, in the surrounding country. No 

 marine or lacustrine Tertiary fossils have been met with either, 

 so that there is reason to believe that throughout Cainozoic times- 

 land-conditions have prevailed. 



(7) The Coonabarabran tableland, with its buttes and mesas,, 

 has the character which Passarge terms Inselberglandschafty. 

 shaped mainly by wind-erosion. 



Some of the above facts are also characteristics of a conoplain 

 as defined by Miss Ida H. Ogilvie.* 



The main reasons for looking upon the Warrumbungles as a 

 conoplain may, hoAvever, be summarised in the following words : 



(1) The mountains form an eroded lava-dome. This consisted 

 of a high core of light grey trachytes, surrounded and capped by 

 a sheet of phonolitic trachytes, which were again covered with 

 later basalts. 



* " The High Altitude Conoplain." The American Geologist, Vol. xxxvi. 

 No.l, July, 1905. 



