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PRELIMINARY NOTE ON THE GEOLOGICAL 

 HISTORY OF THE WARRUMBUNGLE MOUNTAINS. 



By H. I. Jensen, B.Sc, Linnean Macleay Fellow of the 

 Society in Geology. 



(Plate xix.) 

 i. — Introduction. 



While carrying on geological investigations in the Warrum- 

 bungle Mountain district during October and November last, I 

 was very forcibly struck with the vast differences in physiography 

 between this district and that of the Glass House Mountains. 



In both regions the chief rocks represented are Trias-Jura 

 sandstones and the volcanic series ; the latter here consisting of 

 alkaline trachytes, trachydolerites, and alkaline basalts, associated 

 with tuffs, breccias, and diatomaceous earths. 



Granite inclusions have been brought to the surface by the 

 lavas at Tundebrine, and Carboniferous slates standing on end 

 surround the trachytic mass known as Scabby Rock in the 

 Pilliga Scrub. These facts show that rocks as old as Carbonifer- 

 ous underlie the Triassic rocks at no great depth to the north and 

 west of the Warrumbungles; whilst to the south and east, rocks 

 supposed to belong to the Upper Coal Measures and dipping 

 south-west, constitute the dominant formation. 



ii. Physiography. 



The Warrumbungle Mountains occupy a roughly circular area, 

 having a diameter of about 30 miles. They must be distinguished 

 from the Warrumbungle Range, which is merely a low ridge 

 (averaging 2,000 feet in altitude) which forms the watershed 

 between the Namoi and Castlereagh Rivers. 



