BY H. I. JENSEN. 23 1 



mountains proper. East and south of the mountains there is 

 also a poor sandy soil, except on the talus slopes below volcanic 

 hills, and on alluvial river flats. The plains to the west, on the 

 other hand, have a rich black soil, which in general character 

 resembles that met with on alluvial flats in the mountains, where 

 debris from various volcanic rocks and sandstone has accumu- 

 lated. These black soil plains have, in fact, been formed out of the 

 detritus and rock-waste of the Warrumbungles brought down by 

 means of water at a time when the rainfall there was much 

 greater than at present. 



The Castlereagh River is a peculiar stream. It rises in the 

 heart of the Warrumbungles and subsequently sweeps round the 

 group, describing in so doing three-parts of a circle. There can 

 be little doubt that prior to the great volcanic outbursts, the 

 drainage was more direct, taking a " consequent " direction to 

 the north-west. The volcanic mass blocked effectually the old 

 course and made the waters seek a new (" insequent ") course 

 round the obstacle. 



Whilst the Namoi is a running stream, the Castlereagh is quite 

 dry in its course through the plains. It is running at Coona- 

 barabran, but further south becomes a mere string of waterholes, 

 and further west it is usually quite dry. On the plains it has 

 shifted its position from time to time, giving rise to billabongs. 

 Often both thepresent course and the billabongs are distinguishable 

 only as sand ridges known as " monkeys." The river, instead of 

 being marked by a depression, stands in relief, its bed having 

 been filled with wind-blown sand. When floods occur the wateis 

 flow across the plain in a sheet, as the river no longer serves its 

 purpose. The fact that proper river courses exist shows that 

 once the rainfall in the mountains was much greater than at 

 present. 



The tributaries of the Castlereagh flow in definite watercourses, 

 and have eroded gorges in the mountains where they are fed by 

 springs. On reaching the plains, at any rate on the western side 

 of the mountains, they dry up and can hardly be said to occupy 

 any beds at all. This vanishing of the streams is also observed 



