BY H. T. JENSEN, D.SC. 155 



SO that they stand in the same relation to the goldbearing 

 quartz reefs of Victoria as the plumbago beds of the 

 Gympie Goldfield stand to the reefs there. 



The main areas of Ordovician Sedimentation in 

 Australia were the Cephalopodan limestone of Tasmania, 

 the Victorian graphitic shales at Sandhurst, etc., the 

 black graptolite slates of the Victorian and Snowy Alps, 

 the Myall Reefs, N.S.W., the graptolite slates at Dubbo 

 and Cadia (near Orange), N.S.W., the Mandurama 

 graptolite and radiolarian rocks (N.S.W.), and the 

 Larapintine system of the MacDonald Ranges of S.A. 



The Ordovician rocks of N. S. Wales, Victoria and 

 Tasmania were probably deposited in very deep water, 

 radiolaria being very abundant in them in the Snowy 

 Alps, at Mandurama and elsewhere. 



Contemporaneous tuffs and lavas were erupted 

 and occur in the series at Mandurama and Cadia, N.S.W. 

 These volcanic extrusions were of an andesitic nature. 

 Silurian Sedimentation. 

 Silurian sediments occur over wide areas of Eastern 

 Australia. In Victoria, Tasmania and Southern N. S. 

 Wales, where the Ordovician sea was deep, the Silurian 

 sediments follow the Ordovician, and largely overlie 

 them. Further north, however, the areas of most 

 intense Silurian sedimentation are several hundred miles 

 east of the main Ordovician belt. {Fig. 4). 



The Silurian Sea was an extensive one, and a 

 moderately shallow one. Corals played an important 

 part in the life of the time. 



Silurian rocks occur at Yass and Bowning, the 

 Jenolan Caves, Wellington, Molong, etc., in N. S. Wales ; 

 at Chillagoe in Queensland ; at Lily dale in Victoria, etc. ; 

 and at Yass (N.S.W.) they contain banded rhyolites and 

 tuffs of a dacitic nature, and submarine tuffs at Wel- 

 lington (N.S.W.) 



The shallow nature of the Silurian Sea, together 

 with the slow uplift of the sea bottom, caused a rapid 

 advance of the coastline towards the east, so that 

 it is doubtful that the various Silurian areas of AustraliB 

 are contemporaneous. They are homotaxial, and belong 

 to the same great period — but may have become elevated 

 into dry land at different times in that period. 



