156 THE BUILDING OF EASTERN AUSTRALIA 



Most of the important cave-limestones of Australia 

 belong to the Silurian — as for example, those of Chillagoe 

 and Mungana in North Queensland, and those of Jenolan, 

 Wellington and Yarang-obilly in New South Wales. The 

 rugose and tabulate corals of that age apparently did not 

 build on a subsiding sea-floor as the madrepore andmillepore 

 fauna of the Pacific to-day, but were building on a slowly 

 rising seabottom, gradually extending their domain as 

 successive portions of the ocean floor were raised to the 

 zone of shallow water. 



The Devonian. 



In the Devonian Period, the ocean which in the early 

 Palaeozoic covered Eastern Australia was much reduced. 

 Here and there basins remained in which Devonian sedi- 

 ments were planked. These basins were probably trough 

 subsidence areas (senkungs-feldter) in the Silurian plat- 

 form. One such basin extended along the south-coast of 

 New South Wales, from the neighbourhood of Tilba-Tilba, 

 through Yalwal and Sassafras, then dipping under the 

 Per mo- Carboniferous coal measures of the Sydney basin, 

 it reappears at Tamworth, on the flanks of the New England. 

 Another Devonian basin now forms the Buchan and Bindi 

 limestones of Victoria ; another depression is represented 

 by the Murrumbidgee limestones of N.S.W., and extended 

 westwards to Canovvindra and Wilcannia. Another great 

 area of Devonian sedimentation was that of the Burdekin 

 beds of Queensland. [Fig. 5). 



Upper Devonian rocks also occur at Mount Lambie, 

 N.S.W., at Back Creek and Clyde Mountains near Braid- 

 wood, N.S.W., and also between Orange and Wellington, 

 N.S.W. 



The earliest rocks of the Lower Devonian period were 

 of an igneous nature, namely, the Snowy River porphyries 

 of N.S.W. 



Generally speaking, most of the igneous rocks of the 

 Devonian period were of an acid character. 



The nature of Devonian sediments (largely limestones 

 and coarse sandstones) indicates shallow water and 

 derivation from the denudation of and igneous rocks. The 

 Devonian sediments therefore suggest that the volcanic 

 ejacamentea and the igneous intrusions of the lajbe Silurian 



