162 Ernest W. Skeats .- 



in such close field relations to the other types as to suggest strongly 

 a genetic resemblance and reference to the same period of igneous 

 activity. 



One of the Port Cygnet porphyries in section shows as phenocrysts 

 big crystals of orthoclase or sanidiare, green aegirine-augite, a little 

 green hornblende, and small sphenes. In the ground-mass occur 

 needles of pale augite, and the rest consists of lath-shaped and 

 irregular alkali felspars, and probably some nepheline, and a very 

 little plagioclase. This rock is distinctly more alkalic than the 

 Kettering dyke or the Woodbridge and Petchey's Bay rocks, and 

 yet in its texture and mineral content, allowing for greater alkali 

 content, family resemblances are to l>e traced. 



It would appear that all the rocks of the district are consan- 

 guineous and members of one petrographic province. But it is 

 equally clear that differentiation was developed further in the Port 

 Cygnet and Regatta Point areas than in the more outlying districts 

 of Petchey's Bay in the S.W., and Woodbridge and Kettering in 

 the N.E. This is evident from the fact that while some of the 

 porphyries of the Port Cygnet area are in many respects allied to 

 the Kettering, AVoodbridge and Petchey's Bay rocks, there are also 

 present in the central area, as Mr. Twelvetrees has described, 

 adjoining and related rocks in Avhich the mutually incompatible 

 minerals, quartz and nepheline, are separately developed. This 

 close association in the field of quartz-l^earing augite syenites and 

 related quartz-bearing rocks with others containing the felspa- 

 thoids, nepheline and nosean or havyn, provides an interesting 

 example of what are probably nearly extreme type« of differentia- 

 tion in a magma of moderately^ alkalic character. The rocks 

 of Kettering, Woodbridge and Petchey's Bay probably represent 

 products intruded in a less differentiated form, and may quite 

 possibly approximate in composition to the parent magma. 



Comparison of the Alkali Rocks of S.E. Tasmania with 

 otiier Australasian Types. 



Hitlierto the question of the age of these alkali rocks has been 

 discussed, firstly, in the light of field evidence, particularly the 

 evidence of the dyke at Kettering, and, secondly, on the evidence 

 submitted that all the rocks of the area are consanguineous, and 

 belong to one petrographic province, and, therefore, probably to 

 one period of igneous activity. A third method of enquiry turns 

 on the evidence of age and of character of the principal alkali 



