[Proc. Koy. Soc. Victokia. 29 (N.S.), Part II., 1917]. 



Ar'I'. XV. — On the Age of the Alkali Rocks of Port Cygnet 

 and the D' Enirecasteaux Channel in k^.E. Tasmania. 



By professor ERNEST W, SKEATS, D.Sc, A.R.C.S., F.G.S. 

 [Read 14th December, 1916]. 



Introduction. 



The remarkable series of alkali rocks at Port Cygnet, Wood- 

 bridge, and other localities on the D'Entrecasteaux Channel, present 

 many features of mineralogical and petrographic interest. They 

 have been made known principally by the researches of Mr. W. H. 

 Twelvetrees, F.G.S. , Government Geologist of Tasmania, in a series^ 

 of papers in Avhich he has described their mineralogical and pet- 

 rographic characters as shown in the field and under the micro- 

 scope. He has also discussed the difficult and vexed question of 

 their age. Other geologists and petrologists who have contributed 

 to our knowledge of these rocks include the late Mr. Petterd, the 

 late Professor Rosenbusch, and Dr. F. P. Paul. During a visit to> 

 South-East Tasmania in January, 1916, I examined this district. 

 Mr. Twelvetrees was good enough to show me the chief outcrops, and 

 to discuss the problems with me in the field, while Dr. W. N. 

 Benson, of Sydney University, was with us in the earlier part of 

 our stay at Port Cygnet. 



This brief paper results from tliie discovery at Little Oyster Cove, 

 Kettering, of evidence bearing on the vexed question of the age 

 of the alkali series in this part of Tasmania. 



Previous Literature. 



(1) The earliest reference to the alkali rocks of this area,, 

 apart from their approximate distribution as shown on the Geo- 

 logical map in Johnston's Geology of Tasmania, 1888, appears to 

 be a paper by Twelvetrees and Petterd, entitled " On Havyne Tra- 

 chyte and allied rocks in the districts of Port Cygnet and Oyster 

 Cove. (Proc. Roy. Soc., Tas., 1898-9.) 



(2) In the handbook for the Aust. Assoc, for Advancement of 

 Science, Hobart, 1902, Mr. Twelvetrees gave a sketch of the 

 Geology of Tasmania, in which he referred to the elaeolite syenites,. 



