868 THE GEOLOGY OF THE NANDEWAR MOUNTAINS, 



(4) Basaltic eruptions came last, and the lavas are so rich in 

 phenocrj^sts of an extraordinary size that cooling must have pro- 

 ceeded for some time at a depth. This phenomenon is also 

 observed in the intermediate lavas (c^:>. the labradorite porphyrite, 

 N.12). 



Postbasaltic denudation has succeeded in carving the gorge- 

 like valleys and in exposing volcanic plugs and dykes on the 

 tableland, in removing crater-rings and in dissecting the semi- 

 igneous mass. The original continuity of the tops of the spurs 

 is shown by the volcanic rocks capping sandstones at the same 

 level on either side of each valley at a height of many hundred 

 or even 1,000 feet, and by the cappings of igneous rock on isolated 

 sandstone mesas. 



The irregularity of the lava-level in many places points to 

 another significant feature, namely, that the area over which the 

 lava flowed (the Mole peneplain 1) was at the time considerably 

 dissected. Absence of marine fossils shows that it was land at 

 the time of the eruptions. 



It should be here again mentioned that around Boggabri there 

 are numerous rhyolitic pinnacles which are probably of Tertiary 

 age, contemporaneous with the Nandewar rhyolitic trachytes. 

 They may, however, be older. 



In the Pilliga Scrub, between Boggabri and the Warrumbungle 

 Mountains, there are numerous conical peaks of andesite, pro- 

 bably of the same age. These igneous rocks serve to connect up 

 the two volcanic regions, and show that the fractures in early 

 Tertiary time roughly followed the border of the great Triassic 

 basin. 



(d) Stream- Developinent. — The nature of the Namoi River has 

 already been discussed in my paper on the Warrumbungle 

 Mountains. All the creeks rising in the Nandew^ar Mountains 

 have their courses determined by the original slope of the 

 conoplain, and are hence consequent streams. .The evidence 

 which they afford of a previous wet and a later arid cycle has 

 already been discussed. 



