I 



president's address. 563 



IIL Silurian Period. 



Whereas the su|)erficial area occupied by Cambrian rocks in 

 Australia and Tasmania does not perhaps exceed about 10,000 

 square miles, or about -^^, in round numbers, of the whole com- 

 bined area (2,944,600 + 24,500 = 2,969,100 square miles), the 

 rocks which have been pro\isionally referred to the Silurian 

 Period cover perhaps ten times as much area, or approximately 

 ^ of the whole area of Australia.* 



A. Lower Silurian. — Rocks of Lower Silurian Age have been 

 identified in Tasmania, Victoria, and South Australia, and also in 

 New Zealand. 



(1) In Tasmania Mr. Johnston has described! the Lower Silu- 

 rian rocks of Tasmania, which he has referred to two groups in the 

 following descending order :— 



(a) Gordon River Group. 

 (bj Auriferous Slate Group. 



East of the Tamar in Tasmania Gould estimated that 1000 

 square miles were covered by Lower Silurian rocks. J The lower 

 series consists of soft black and blue slates, grits, sandstones, and 

 conglomerates, and in the western area at Middle Arm Creek, 

 Beaconsfield, are estimated by Gould to be at least 2000 feet 

 thick. § They contain Licrophycus and Graptolites. Mr. Thureau 

 has referred II remains of Graptolites from the Beaconsfield slates 

 to Didymograptus nodosus. 



A species of Orthis is also stated to have been found in these 

 rocks. At Cabbage Tree Gorge in the Eastern Lower Silurian 

 area in Tasmania, Mr. Johnston recordsll the occurrence of Licro- 

 phycus sp. almost identical in appearance with L. Ottaioensis from 



* The above areas, especially that given for the Cambrian, are only 



extremely approximate. The area quoted for the Silurian rocks may be 



too much or too little by about one-quarter of the area suggested. 



t Loc. cit. pp. 54-63. 



X Quoted by Johnston, loc. cit. p. 53. 



§ Quoted by Johnston, loc. cit. p. 60. 



II Quoted by Johnston, loc. cit. p. 52. 



IT Loc. cit. p. 52. 



