162 THE BUILDING OF EASTERN AUSTRALIA 



older than the great basic flows, there is strong evidence 

 in the Moreton and Fassifern districts of Queensland, and 

 in the Canoblas, Nandewars, and Mittagong trachyte 

 series in New South Wales, that the trachytic eruptive 

 rocks were preceded by basic and ultrabasic intrusives, and 

 flows quite distinct from the Miocene and Pliocene basalts. 

 The age of this post-Jurassic volcanic period is probably 

 not older than late Cretaceous, nor newer than Eocene. 



The Cretaceous was a period of relative quiet, and 

 the great continental rim elevated in the late Permian, 

 Triassic and Trias-Jura periods was being worn down to 

 base level. The formation of the Bolivia and Mole pene- 

 plains was completed in this period. (Andrews— Tertiary 

 History of New England). An early Tertiary uplift followed, 

 and this was accompanied by renewed degradation until 

 the great Miocene peneplain of Eastern Australia was 

 formed (Sandon and Stannifer peneplain of Andrews op. 

 cit. [10], Monaro peneplain of Sussmilch [11] ). 



The first great basalt demonstration of the Tertiary 

 occurred before the completion of Miocene peneplanation, 

 and the alkaline lavas of Eastern Australia are usually 

 regarded as early extrusives of that eruptive cycle, a view 

 that assigns them to the Eocene or early Miocene periods. 

 The later basalts succeeded, and are best regarded as late 

 Pliocene. Their extrusion was followed or accompanied 

 by great upheavals in some areas, such as New England. 



The igneous activity of the Mesozoic periods has not 



left such striking evidence as the Tertiary volcanic action 



just referred to. The following clues are, however, afforded : 



(a) The chocolate shales of early Trias (Narrabeen) 



age contain augite, and are probably andesitic tuffs. 



(6) The Wianamatta shales do in many places near 

 Penrith pass into tuffy shales. The nature of the shales 

 themselves is indicative of derivation from basic volcanic 

 rock. Some shaly beds in the Clarence series are likewise 

 tuffy. 



(c) The alkaline rocks of Eastern Australia were 

 preceded by basic eruptions from which they are separated 

 by at least one geological period. The magnesites of the 

 Desert Sandstone may be related to these basic eruptives. 

 [12] 



