234 GEOLOGY OF THE WARRUMBUNGLE MOUNTAINS, 



iii. — Succession of Lavas. 



The earliest volcanic eruptions gave rise to tuff cones, into- 

 which plugs of arfvedsonite trachyte were injected. The next 

 outbursts led to flows of asgirine trachyte, which surrounded and 

 sometimes covered over the already existing cones. Subsequent 

 eruptions gave trachydolerites. Finally sodalite-basalts and 

 ordinary basalts were erupted. 



In the dissection of the conical mass thus formed, as soon as> 

 the streams found soft tuff and ash-beds, they worked down with 

 great rapidity. In this way plugs of arfvedsonite trachyte like 

 Timor Rock have been completely isolated, and stand with their 

 precipitous sides and hoary summits in broad open valleys. 

 Other plugs of trachydolerite have been isolated in the same way. 



iv. — Geological History of the District. 



1. In Permo-Carboniferous times this area lay on the coast of 

 an old Palaeozoic landmass which extended westwardly. 



2. In Triassic times it was depressed, and the sandstones were 

 formed. The subsidence of the old masses continued throughout 

 Trias-Jura and Cretaceous times. 



3. An elevatory movement in the Liverpool Range gives the 

 new sediments the prevailing N.N.W. dip. This probably took 

 place in Cretaceous times, and elevated much of the Trias-Jura 

 sea to the state of dry land. 



4. A plain or peneplain develops (the present two thousand 

 feet level). 



5. It is further elevated and dissected, perhaps, by arid- 

 agencies. 



6. Lavas are erupted in the order already stated (Eocene- 

 Pliocene). 



7. The Pleistocene pluvial period leads to the dissection of the- 

 dome-shaped volcanic mass and the accumulation of detritus at 

 lower levels. 



8. The landscape is further modified by a superimposed arid 

 cycle which is still prevailing. 



