568 THE DISTRIBUTION, ETC., OF ALKALINE ROCKS, 



5. Calcic basalts, found mainly outside and encircling the 

 alkaline areas. 



Fossil leaves from the trachyte tuffs have been examined by 

 Mr. Henry Deane, M.A., F.L.S., and found to be Tertiary. The 

 eruptions appear therefore to have commenced in the Eocene, and 

 the freshness of the later lava-cones suggests that some may be 

 as late as Pliocene. The sequence is the same as that observed 

 in the Nandewars, in the Glass House Mountains, in the Mitta- 

 gong-Bowral group, in the Macedon area, and in fact in all 

 localities in the Australian region where alkaline rocks occur. 



At various points between the Warrum bungles and Nandewars, 

 on the Narrabri-Dubbo line, small eruptive cones and plugs of 

 alkaline rock occur, as at Scabby Rock in the Pilliga Scrub. 



In the Nandewar Mountains, about 20 miles east of Narrabri, 

 we again come upon a great series of alkaline rocks. These rocks 

 have also been described by me elsewhere.* 



Here, as in the Warrumbungles, the signs of arid erosion are 

 unmistakable, but a certain very recent rejuvenation of streams 

 in both regions shows that the arid cycle is past its maximum. 



In the Nandewars we meet with Palaeozoic (Carboniferous and 

 Permian) rhyolites and quartz porphyries, andesitic rocks of the 

 same age, and ancient tuffs and breccias just as in the Canoblas. A 

 very extended period of quiet followed. The next eruptions gave 

 dolerites, which appear to be Post-Triassic, probably late Creta- 

 ceous, as lavas of this character were erupted on the Cretaceous 

 peneplain. A period of erosion followed, when extensive faulting 

 took place and alkaline lavas were extruded from the fissures. 

 The first were essexite in composition, being probably a mixture 

 of doleritic and trachjtic magma. Then followed intrusions and 

 extrusions of pantellarite, arfvedsonite trachyte, and very fel- 

 spathic segirine trachyte. Pulaskite porphyry, akerite and allied 

 rocks were then intruded, whilst at the same time flows of segirine 

 trachyte, nepheline phonolite, and alkaline andesite took place. 



* Proc. Linn. Soc. N.S.Wales, 1907, Vol.xxxii. p. 842. 



