BY L. KEITH WARD, B.A., B.E. 



143 



No definite clue being provided in Tasmania, we look 

 to the Australian continent for signs of the existence of 

 Archaean rocks, which may be remnants of the Pre- 

 Cambrian land mass that was the source of these sedi- 

 ments. 



Brief mention is made below of the Australian occur- 

 rences of Pre-Cambrian rocks. Of these the rocks 

 which bear most directly upon the question now being 

 discussed are the gneisses, gneissose granites, and horn- 

 blendic schists of Western Australia ; and the gneisses 

 of South Australia. These bear the strongest lithological 

 resemblance to the Archaean rocks of other countries. 



The Pre-Cambrian schists of Tasmania m.ay well 

 have derived their material from these ancient crystal- 

 line gneisses, schists, and granites, and possibl}^ from 

 the more proximate southward extensions of the masses 

 referred to above now covered by the Southern Ocean. 



This question of genesis demands a much greater 

 elaboration than can here be efifected. It is sufficient 

 to state that these quartzites, argillaceous, and mica- 

 ceous schists and schistose conglomerates constitute 

 the terrigenous deposits formed on the borders of the 

 Pre-Cambrian Australis — a land mass known from its 

 exposed remnants to be competent to provide such 

 material. 



After the prolonged period of sedimentation which 

 is represented by the lower schists and conglomerates, 

 a period marked by intense dynamic metamorphism 

 must 'have ensued. 'Tlie subsequent erosion of these 

 older schists levelled the floor upon which the upper 

 sediments were deposited. 



The schistosity observed in these upper members of 

 the Pre-Cambrian may have been induced before the 

 period of deposition of the Cambrian sediments. For 

 there is a notable difiference in appearance between the 

 appearance of the upper Pre-Cambrian quartzitic schist 

 and that of any sedimentary rock of later date. 



A notable unconformity exists between the Cambrian 

 and the Pre-Cambrian sediments at the northern end of 

 the Denison Range. There is, however, not yet suffi- 

 cient evidence available upon which to base an account 

 of the physiography of Western Tasmania at the time 

 of the Cambrian sedimentation. 



