184 THE BUILDING OF EASTERN AUSTRALIA 



It is convenient to consider separately the groups 

 of non- metallic minerals and metallic minerals or ores. 



I. Non-Metallic Minerals. 



The principal non-metallic mineral is coal. Of less 

 importance are oil shale, phosphatic rock, opal, diatomaceous 

 earth and graphite. 



(a.) The occurrence of coal depends on the burial in 

 some bygone epoch of the requisite thickne^ of plant re- 

 mains, considerable subsidence following so as to allow 

 a great thickness of sediments to accumulate on top of 

 the seam. Under these conditions, the weight of the 

 superincumbent strata, together with the heat appertaining 

 to the depth to which the organic bed was depressed, have 

 produced the necessary pressure and heat to compact 

 and dehydrate the deposit. 



Folding and metamorphism in the zone of flowage 

 have not affected the coalfields, for these processes or 

 even a moderate amount of igneous intrusion would com- 

 pletely turn coal into graphite. The moderate folding 

 and the measure of igneous intrusion that have affected 

 the Gympie goldfield, have turned several coal seams into 

 plumbago, which, as is Avell known, occurs interbedded 

 with Phoenix slates. 



Faulting, especially block or trough faulting, is the 

 process most frequently leaving its mark in the coal measures. 



Coal occurs in Eastern Australia in the Permo-Carbon- 

 iferous (Newcastle and lUawarra districts of N.S.W. ; 

 Bowen, Little River, Oakey Ck. and Dawson River in Q.) ; 

 in the Triassic (in the Ipswich and Barrum districts of Q., 

 the Clarence district of N.S.W. , and in the Otway and 

 Wannon and Gippsland districts of Vic. ; Leigh's Ck., 

 S.A. ; Fingal and Jerusalem, Tas.) ; and in the Tertiary 

 rocks we have hydrous coals, the best of which are the 

 lignites of Morwell, Vic. There are numerous other localities 

 of considerable economic importance, but the main point 

 for us to recollect that vast areas of Eastern Australia 

 represented by our coalfields have undergone no con- 

 tortion since the deposition of the coal. The western half 

 of the Australian continent has been free from such forces 

 even longer. Indeed there is no reason why Carboniferous 

 and Devonian coals should not be found in Western Aus- 

 tralia. 



