BY H. I. JENSEN, D.SC. 177 



Cambrian. 

 Acidic igneous rocks supposed to be of this age exist 

 in Western and Central Australia. Similar rocks were 

 not intruded in Eastern Australia till many periods later. 

 They belong mainly to the Upper Silurian period in Central 

 New South Wales, to the Carboniferous in Eastern New 

 South Wales, south of the Sydney basin, and in part at 

 least to the late Per mo- Carboniferous or early Mesozoic 

 in New England and Southern Queensland. 



Ordovician. 

 In Victoria, Professor Skeats [25] has referred the 

 Heathcotian Series to this period. The igneous rocks 

 of this series were of an intermediate to basic nature, and 

 chemically rich in magnesia. From this, one would expect 

 the Victorian area to have been undergoing subsidence 

 during Ordovician times. The evidence of the associated 

 sediments confirm such a supposition. 



In Central New South Wales we have contemporane- 

 ously interbedded andesites in the Ordovician near Orange, 

 Forbes and Mandurama. Here, too, the sedimentary 

 rocks would seem to indicate a period of subsidence. 

 Upper Silurian. 

 In Victoria some of the Snowy River porphyries and 

 felsites (all very acid rocks) may belong to this period. 



In New South Wales we have acid submarine tuffs of 

 this age at Wellington, and banded rhyolites and dacite 

 tuffs at Yass. 



The sedimentary rocks of the Upper Silurian 

 in Victoria and Central New South Wales show that a 

 shallowing of the sea had commenced. 



In North Queensland too, in the Chillagoe district, 

 we have granites and felsites of this age. 



In the New England district and the north coast of 

 N.S. Wales, where subsidence was still in progress, the 

 intrusive rocks being dioritic (blue) granites. 



Devonian. 

 Professor Skeats has demonstrated that some of the 

 Snowy River porphyry series of Victoria were erupted 

 in the Devonian. 



Many granite intrusions in N.S. Wales are supposed 

 to be of this age, to which, as well, the Snowy River 

 porphyries of N.S. Wales are referred. 



