864 THE GEOLOGY OF THE NANDEWAR MOUNTAINS, 



neck of a volcano,* and the high conical mass north of it is a 

 similar plug in a parasitic vent. In American phraseology, these 

 plugs may be termed buttes. 



The wide Black Soil Plains expanses of Bobbiwaa Creek and 

 Maule's Creek, with so gentle a slope towards the general plains 

 of Narrabri and Tarriaro that the country might be said to be 

 practically level, show clearl}' that a previous wet cycle cut 

 gorges in the elevated conoplain and carved wide valleys. Then 

 followed an arid cycle with disintegrated drainage, the result of 

 which was the filling of the valleys with the detritus; lastly 

 there has been a return of slightl}^ moister conditions, hence 

 there is a tendency for the drainage to become integrated again 

 and for the creeks to become rejuvenated. The formation of 

 black soil plains and the subsequent rejuvenation of the streams 

 may bear a relation to the formation of a lake in the north- 

 western districts of New South Wales and its subsequent 

 drainage through its waters finding an exit by way of the Murray 

 River. The arid period probably followed this last event. 



(b) Geological History. — The geological history might be 

 summed up as follows : — In the Carboniferous period the area of 

 the Nandewar Mountains and the country to the north-west of 

 them consisted of dry land on which Silurian, Devonian and older 

 rocks were undersoino^ corrasion and denudation. The detritus 

 was carried eastwards and southwards to a Carboniferous sea of 

 which good evidence is seen at INIaule's Creek. This sea extended 

 westwards over the Barraba, Cobbadah, and Bingera di-stricts 

 as far as New England. Volcanic eruptions of an acid nature 

 took place at the time — often in the sea or along the coast-line, 

 so that on the shore huge pebbles and boulders of rhyolite 

 accumulated. Eruptions took place at frequent intervals, so 

 that we find tuffy sandstones and conglomerates, boulder beds 

 with a volcanic ash matrix, and occasionall}^ lavas all inter- 

 bedded with one another. These eruptions were accompanied 



* Cp. ' Geology of The East xMoreton,' etc., These Proceedings, 1906, p. 97. 



